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Seed dispersal is key to the persistence and spread of plant populations. Because the majority of plant species rely on animals to disperse their seeds, global change drivers that directly affect animals can cause cascading impacts on plant communities. In this review, we synthesize studies assessing how disperser loss alters plant populations, community patterns, multitrophic interactions, and ecosystem functioning. We argue that the magnitude of risk to plants from disperser loss is shaped by the combination of a plant species’ inherent dependence on seed dispersal and the severity of the hazards faced by their dispersers. Because the factors determining a plant species’ risk of decline due to disperser loss can be related to traits of the plants and dispersers, our framework enables a trait-based understanding of change in plant community composition and ecosystem functioning. We discuss how interactions among plants, among dispersers, and across other trophic levels also mediate plant community responses, and we identify areas for future research to understand and mitigate the consequences of disperser loss on plants globally., This work was supported by the National Socio-Environmental Synthesis Center’s Postdoctoral Fellowship to E.C.F. under funding received from the National Science Foundation (DBI1639145), the Alexander von Humboldt Foundation Fellowship to I.D., and the Spanish Ministry of Science to A.T. (project CGL2017-88122-P).

Dispersal is a key process in community assembly but is often considered separately from downstream assembly processes (e.g., competition, herbivory). However, dispersal varies by species and can interact with other assembly processes through establishment as species enter communities. Here, we sought to distinguish the role of dispersal in community assembly and its interaction with two biotic assembly processes: competition and herbivory. We used a tallgrass prairie restoration experiment that manipulated the competitive and herbivore environments while allowing for natural dispersal and establishment from a diverse regional species pool into areas of low diversity. Dispersal, competition, and herbivory all influenced local communities. By tracking the spread of four target species across the plots, we found interspecific and intraspecific differences in establishment patterns, with herbivores influencing the number of individuals present and the distances species moved. At the community level, only dispersal and competition significantly influenced alpha diversity, but all three processes additively influenced community composition. There was also evidence of herbivore-competition and herbivore-colonization trade-offs in our experiment. Some species that could tolerate herbivory were less likely to establish in competitive environments, while others that could tolerate herbivory were more likely to disperse greater distances. More work is needed to understand the contexts under which dispersal variation affects community assembly and its synergy with other processes.

SummaryAssessing the impacts of invasive predators on the demography and distribution of native species is critical for understanding mechanisms of species persistence and informing the design of recovery programmes. On the oceanic island of Guam, the introduction of the predatory brown treesnake Boiga irregularis after World War II caused the near-total loss of the native forest avifauna. Localised snake control measures have been implemented since the early 1990s, yet it remains poorly understood how they have impacted Guam’s remaining native bird populations. To address this question, we combined intensive area searches of Andersen Air Force Base (AAFB) with island-wide transect surveys and opportunistic sightings to provide a comprehensive update on the distribution and abundance of Såli (Micronesian Starling, Aplonis opaca) – one of Guam’s last extant native bird species. Area searches of AAFB, where the largest remnant of the Såli population persists, revealed a 15-fold population increase since the last survey in the early 1990s, and transect surveys and opportunistic sightings indicate incipient recolonisation of other urbanised areas of northern and central Guam. We estimate the current island-wide population size at ~1,400 individuals. The population increase can likely be attributed to a combination of snake control measures and the Såli’s ability to exploit urban refugia for nesting and roosting. Although these trends demonstrate some population recovery, a skewed age ratio (>90% adults and subadults) at AAFB and a highly urbanised distribution and low abundance outside AAFB indicate that snake predation continues to strongly impact the population. More intensive snake suppression efforts, particularly in forested areas, may allow for the Såli population to attain its former distribution and abundance on Guam. More broadly, our findings reinforce the importance of urban areas as refugia for some threatened species.

Many plants rely on animal mutualists for reproduction. Quantifying how animal mutualists impact plant performance provides a foundation for modelling how change in animal communities affects the composition and functioning of plant communities. We performed a meta-analysis of 2539 experiments, 6 times more than the last comprehensive meta-analysis, examining how gut passage by frugivores influences seed germination. We simultaneously analyzed multiple predictor variables related to study methodology, location, and frugivore identity to disentangle methodological from ecological impacts on effect sizes. We found that gut passage by birds, fish, reptiles, bats, primates, and other mammals on average increased seed germination, but that the magnitude varied across vertebrate groups. The positive effects of gut passage were largely explained by the de-inhibitory effects of pulp removal rather than by the scarification of seed tissues. Some previous studies and meta-analyses that found no effect of gut passage only tested scarification or did not distinguish between these tests of scarification and pulp removal. We found that, for a typical fleshy-fruited plant species, the lack of gut passage reduces germination by 60%. From an evolutionary perspective, this indicates a large risk associated with reliance on animal mutualists that is balanced against the benefits of animal-mediated seed dispersal. From a conservation perspective, this highlights the potential for large demographic consequences of frugivore declines on plant populations. Our database and findings advance quantitative predictions for the role of fruit-frugivore interactions in shaping plant communities in the Anthropocene.

Seed dispersal is an important ecological process that structures plant communities and influences ecosystem functioning. Loss of animal dispersers therefore poses a serious threat to forest ecosystems, particularly in the tropics where zoochory predominates. A prominent example is the near-total extinction of seed dispersers on the tropical island of Guam following the accidental introduction of the invasive brown tree snake (Boiga irregularis), negatively impacting seedling recruitment and forest regeneration. We investigated frugivory by a remnant population of Såli (Micronesian starling –Aplonis opaca) on Guam and two other island populations (Rota, Saipan) to evaluate their ecological role as a seed disperser in the Mariana archipelago. Using a combination of behavioural observations, nest contents and fecal samples, we documented frugivory of 37 plant species. Native plants comprised the majority (66%) of all species and 90% of all seeds identified in fecal and nest contents. Diet was highly similar across age classes and sampling years. In addition, plant species consumed by Såli comprised 88% of bird-dispersed adult trees and 54% of all adult trees in long-term forest monitoring plots, demonstrating the Såli’s broad diet and potential for restoring native forests. Overall, we provide the most comprehensive assessment to date of frugivory by the Såli and confirm its importance as a seed disperser on Guam and throughout the Marianas.

Substantial intra‐specific trait variation exists within plant communities, and in theory this variation could influence community dynamics. Although recent research has focused on intra‐specific variation in traits themselves, it is the influence of this variation on plant performance that makes intra‐specific trait variation relevant to ecological dynamics within or among species. Understanding the links between trait and performance variation, and the role of traits in mediating relationships among multiple components of performance, is critical for assessing the importance of intra‐specific trait variation for community dynamics. Seed size is thought to affect aspects of plant performance including fecundity, seedling growth, dispersal and tolerance of natural enemies. For two tropical tree species, we assessed how seed size was related to performance variation within each species and determined whether intra‐specific trait variation mediates intra‐specific performance tradeoffs. We used field seed rain collection to characterize size‐dependent outcomes of dispersal, sowed seeds of known size in soil collected near or far from conspecifics to characterize susceptibility to soil pathogens, and monitored growth of seedlings from seeds of known size. We found that intra‐specific seed size variation caused intra‐specific performance variation. The degree of trait‐based performance variation was consistently smaller than the degree of trait variation, and seed size influenced different components of performance for each species. One species exhibited a tradeoff in which small seeds had a fecundity advantage (more seedlings per unit reproductive mass) but produced smaller seedlings, whereas the other species exhibited a tradeoff in which small seeds dispersed to areas of low conspecific density but were less tolerant of density‐responsive natural enemies. Our results indicate that a single trait can influence multiple components of performance and can mediate different tradeoffs in co‐occurring species. Complex and heterogeneous effects of a single trait in multidimensional niche space may favour inter‐specific niche differentiation and coexistence.

Public attitudes toward invasive alien species management and trust in managers' ability to effectively manage non-native species can determine public support for conservation action. The island of Guam has experienced widespread species loss and ecosystem transformation due to invasive species, most notably, the brown treesnake (Boiga irregularis). Despite Guam's long history with invasives and extensive efforts to eradicate them, we know little about the sociological context of invasive species and drivers of public support or opposition on the island. Using focused group discussions, we explore public attitudes toward invasive species management measures. Respondents were familiar with the common invasive species on Guam and recognized that they were not native. They expressed support for management activities, interest in more effective and frequent management initiatives, and desire to participate directly in conservation actions. Participants also expressed frustration with government institutions and lack of confidence in managers' ability to control invasive species. Perceptions of managers' trustworthiness, communication with managers, and positive personal experiences with managers were related to positive attitudes about management and support for existing initiatives, indicating the important role of trust and engagement for invasive species management.

Although dispersal is generally viewed as a crucial determinant for the fitness of any organism, our understanding of its role in the persistence and spread of plant populations remains incomplete. Generalizing and predicting dispersal processes are challenging due to context dependence of seed dispersal, environmental heterogeneity and interdependent processes occurring over multiple spatial and temporal scales. Current population models often use simple phenomenological descriptions of dispersal processes, limiting their ability to examine the role of population persistence and spread, especially under global change. To move seed dispersal ecology forward, we need to evaluate the impact of any single seed dispersal event within the full spatial and temporal context of a plant’s life history and environmental variability that ultimately influences a population’s ability to persist and spread. In this perspective, we provide guidance on integrating empirical and theoretical approaches that account for the context dependency of seed dispersal to improve our ability to generalize and predict the consequences of dispersal, and its anthropogenic alteration, across systems. We synthesize suitable theoretical frameworks for this work and discuss concepts, approaches and available data from diverse subdisciplines to help operationalize concepts, highlight recent breakthroughs across research areas and discuss ongoing challenges and open questions. We address knowledge gaps in the movement ecology of seeds and the integration of dispersal and demography that could benefit from such a synthesis. With an interdisciplinary perspective, we will be able to better understand how global change will impact seed dispersal processes, and potential cascading effects on plant population persistence, spread and biodiversity., Although dispersal is generally viewed as a crucial determinant for the fitness of any organism, our understanding of its role in the persistence and spread of plant populations remains incomplete. In this perspective, we provide guidance on integrating empirical and theoretical approaches that account for the context dependency of seed dispersal to improve our ability to generalize and predict the consequences of dispersal, and its anthropogenic alteration, across systems. With an interdisciplinary perspective, we will be able to better understand how global change will impact seed dispersal processes, and potential cascading effects on plant population persistence, spread and biodiversity.

Current understanding of mutualistic networks is grounded largely in data on interaction frequency, yet mutualistic network dynamics are also shaped by interaction quality—the functional outcomes of individual interactions on reproduction and survival. The difficulty of obtaining data on functional outcomes has resulted in limited understanding of functional variation among a network's pairwise species interactions, of the study designs that are necessary to capture major sources of functional variation, and of predictors of functional variation that may allow generalization across networks. In this community‐scale study, we targeted a key functional outcome in plant–frugivore networks: the impact of frugivore gut passage on seed germination. We used captive frugivore feeding trials and germination experiments in an island ecosystem, attaining species‐level coverage across all extant native frugivores and the plants they consume to (a) assess sources of functional variation, (b) separate effects of pulp removal from those of scarification via gut passage, and (c) test trait‐based correlates of gut passage effect sizes. We found antagonistic seed predation effects of a frugivore previously assumed to be a seed disperser, highlighting the need to consider functional outcomes rather than interaction frequency alone. The other frugivores each exhibited similar impacts for individual plant species, with benefits primarily caused by pulp removal rather than scarification, supporting the use of animal functional groups in this context. In contrast, plant species varied widely in impacts of gut passage on germination. Species with smaller seeds and more frugivore partners had larger benefits of gut passage, showing promise for network metrics and functional traits to predict functional variation among plants. Synthesis. Combining network and demographic approaches, we assessed the degree and sources of variation in a key functional outcome of plant–frugivore interactions across an entire network. Using a detailed study design, our work shows how simpler study designs can capture primary sources of functional variation and that functional traits and network metrics may allow generalization across networks. Efficiently measuring and generalizing sources of functional variation within mutualistic networks will strengthen our ability to model network dynamics and predict mutualist responses to global change.

Bird exclusion experiments consistently show that birds exhibit strong top-down control of arthropods, including ants and the honeydew-producing insects (HPIs) that they tend. However, it remains unclear whether the results of these small-scale bird exclosure experiments can be extrapolated to larger spatial scales. In this study, we use a natural bird removal experiment to compare the prevalence of ants and HPIs between Guam, an island whose bird community has been extirpated since the 1980s due to the introduction of the brown tree snake, and two nearby islands (Rota and Saipan) that have more intact bird assemblages. Consistent with smaller-scale bird exclosure experiments, we show that (1) forest trees from Guam are significantly more likely to host HPIs than trees from Saipan and (2) ants are nearly four times as abundant on Guam than on both Saipan and Rota. The prevalence of HPIs varied slightly based on tree species identity, although these effects were not as strong as island-level effects associated with bird loss. Ant community composition differed between Guam and the other two islands. These results corroborate past observational studies showing increased spider densities on Guam and suggest that trophic changes associated with landscape-level bird extirpation may also involve alterations in the abundance of ants and HPIs. This study also provides a clear example of the strong indirect effects that invasive species can have on natural food webs. Electronic supplementary material The online version of this article (10.1007/s00442-018-4273-5) contains supplementary material, which is available to authorized users.

Vertebrate frugivores enhance tropical forest regeneration by dispersing seeds into degraded areas. However, the importance of individual species as dispersers may vary within a community. Management and restoration would benefit from understanding which species are critical in moving native seeds into degraded habitats. We compared habitat composition of bird start and end locations for movement intervals based on mean gut passage times for the avian frugivore community on the island of Saipan. The proportion of movement intervals that began in intact, native forest varied among species, with Golden White-eyes having the highest proportion. However, this species tended to remain in intact forest and only rarely crossed into degraded habitats. Bridled White-eyes and Mariana Fruit Doves exhibited slightly higher rates of crossing from intact forest to degraded habitats, suggesting an ability to disperse native seeds to degraded areas. White-throated Ground Doves were never recorded crossing from intact forest to degraded habitats. Despite having a low proportion of movement intervals beginning in intact forest, Micronesian Starlings showed a higher proportion and absolute number of movements from intact forest to degraded habitats, due to their propensity to move frequently, across long distances, and across habitat types. In this species-poor frugivore network, seed dispersal into degraded habitats appears highly dependent on one species within the community. Regeneration of degraded lands may be severely hindered if this key disperser is lost.

Dispersal is thought to be a key process underlying the high spatial diversity of tropical forests. Just how important dispersal is in structuring plant communities is nevertheless an open question because it is very difficult to isolate dispersal from other processes, and thereby measure its effect. Using a unique situation, the loss of vertebrate seed dispersers on the island of Guam and their presence on the neighboring islands of Saipan and Rota, we quantify the contribution of vertebrate seed dispersal to spatial patterns of diversity of tree seedlings in treefall gaps. The presence of vertebrate seed dispersers approximately doubled seedling species richness within canopy gaps and halved species turnover among gaps. Our study demonstrates that dispersal plays a key role in maintaining local and regional patterns of diversity, and highlights the potential for ongoing declines in vertebrate seed dispersers to profoundly alter tropical forest composition.

Following defaunation, the loss of interactions with mutualists such as pollinators or seed dispersers may be compensated through increased interactions with remaining mutualists, ameliorating the negative cascading impacts on biodiversity. Alternatively, remaining mutualists may respond to altered competition by reducing the breadth or intensity of their interactions, exacerbating negative impacts on biodiversity. Despite the importance of these responses for our understanding of the dynamics of mutualistic networks and their response to global change, the mechanism and magnitude of interaction compensation within real mutualistic networks remains largely unknown. We examined differences in mutualistic interactions between frugivores and fruiting plants in two island ecosystems possessing an intact or disrupted seed dispersal network. We determined how changes in the abundance and behavior of remaining seed dispersers either increased mutualistic interactions (contributing to “interaction compensation”) or decreased interactions (causing an “interaction deficit”) in the disrupted network. We found a “rich-get-richer” response in the disrupted network, where remaining frugivores favored the plant species with highest interaction frequency, a dynamic that worsened the interaction deficit among plant species with low interaction frequency. Only one of five plant species experienced compensation and the other four had significant interaction deficits, with interaction frequencies 56-95% lower in the disrupted network. These results do not provide support for the strong compensating mechanisms assumed in theoretical network models, suggesting that existing network models underestimate the prevalence of cascading mutualism disruption after defaunation. This work supports a mutualist biodiversity-ecosystem functioning relationship, highlighting the importance of mutualist diversity for sustaining diverse and resilient ecosystems. This article is protected by copyright. All rights reserved.

Despite the importance of seed dispersal as a driving process behind plant community assembly, our understanding of the role of seed dispersal in plant population persistence and spread remains incomplete. As a result, our ability to predict the effects of global change on plant populations is hampered. We need to better understand the fundamental link between seed dispersal and population dynamics in order to make predictive generalizations across species and systems, to better understand plant community structure and function, and to make appropriate conservation and management responses related to seed dispersal. To tackle these important knowledge gaps, we established the CoDisperse Network and convened an interdisciplinary, NSF-sponsored Seed Dispersal Workshop in 2016, during which we explored the role of seed dispersal in plant population dynamics (NSF DEB Award # 1548194). In this Special Issue, we consider the current state of seed dispersal ecology and identify the following collaborative research needs: (i) the development of a mechanistic understanding of the movement process influencing dispersal of seeds; (ii) improved quantification of the relative influence of seed dispersal on plant fitness compared to processes occurring at other life history stages; (iii) an ability to scale from individual plants to ecosystems to quantify the influence of dispersal on ecosystem function; and (iv) the incorporation of seed dispersal ecology into conservation and management strategies., Seed dispersal is fundamental to the structure and function of plant communities, but its complexity and heterogeneity impede mechanistic understanding and quantitative prediction of seed dispersal processes and their disruption. In this Special Issue, we consider the current state of seed dispersal ecology and identify collaborative research needs. The diversity of disciplines, geographic regions and expertise represented in this Special Issue yield a range of perspectives and insights and, we hope, will stimulate further collaborations to advance seed dispersal ecology and conservation.

How can hundreds of tree species coexist in a single hectare of tropical forest when the environmental conditions, as well as the species’ basic requirements, appear so similar? A leading explanation, particularly in tropical forests, is called the Janzen–Connell hypothesis after the two ecologists who proposed it in the early 1970s (1, 2). In this mechanism, specialized predators and pathogens concentrated near adult plants differentially reduce survival and growth of conspecific offspring relative to seedlings of other species. These natural enemies limit regeneration of common species and confer a relative advantage to rare species because they have more enemy-free regeneration sites. This has driven decades of empirical work to understand whether enemies are specialized enough in nature to cause this pattern and allow coexistence by this mechanism, and a recent metaanalysis supports the prediction that seed or seedling survival is greater away from conspecific adults (3). In PNAS, Eck et al. (4) show that natural enemies can be even more specialized, specializing on individual genotypes within a wild population. Through simulation models exploring the theoretical implications of this field result, Eck et al. (4) show that this greater specialization may weaken the stabilizing effects of natural enemies on species coexistence but also may select for greater dispersal over evolutionary time. The body of theory developed since the original formulation of the Janzen–Connell hypothesis has primarily asked ( i ) How specialized must the natural enemies be? and ( ii ) At what scale must these patterns operate? With regard to natural enemies, a long-standing conclusion is that species specificity of natural enemies allows coexistence. If a rare plant species shares a generalist enemy with an abundant plant species, then this abundant generalist enemy could extirpate the rare species from the community. With regard to patterns, the spatial scale matters—both in terms of enemy … [↵][1]1To whom correspondence should be addressed. Email: haldre{at}iastate.edu. [1]: #xref-corresp-1-1

Invasive vertebrate predators are directly responsible for the extinction or decline of many vertebrate species, but their indirect impacts often go unmeasured, potentially leading to an underestimation of their full impact. When invasives extirpate functionally important mutualists, dependent species are likely to be affected as well. Here, we show that the invasive brown treesnake, directly responsible for the extirpation of forest birds from the island of Guam, is also indirectly responsible for a severe decline in plant recruitment as a result of disrupting the fruit-frugivore mutualism. To assess the impact of frugivore loss on plants, we compare seed dispersal and recruitment of two fleshy-fruited tree species on Guam and three nearby islands with intact disperser communities. We conservatively estimate that the loss of frugivorous birds caused by the brown treesnake may have caused a 61–92% decline in seedling recruitment. This case study highlights the potential for predator invasions to cause indirect, pervasive and easily overlooked interaction cascades., Invasive brown treesnakes decimated the forest bird community on the island of Guam. Now, Rogers and colleagues document the indirect effects of the snake on trees, linking snake-initiated bird loss to reduced seed dispersal and plant recruitment on Guam compared to nearby uninvaded islands.

Understanding the influence of environmental variability on population dynamics is a fundamental goal of ecology. Theory suggests that, for populations in variable environments, temporal correlations between demographic vital rates (e.g., growth, survival, reproduction) can increase (if positive) or decrease (if negative) the variability of year-to-year population growth. Because this variability generally decreases long-term population viability, vital rate correlations may importantly affect population dynamics in stochastic environments. Despite long-standing theoretical interest, it is unclear whether vital rate correlations are common in nature, whether their directions are predominantly negative or positive, and whether they are of sufficient magnitude to warrant broad consideration in studies of stochastic population dynamics. We used long-term demographic data for three perennial plant species, hierarchical Bayesian parameterization of population projection models, and stochastic simulations to address the following questions: (1) What are the sign, magnitude, and uncertainty of temporal correlations between vital rates? (2) How do specific pairwise correlations affect the year-to-year variability of population growth? (3) Does the net effect of all vital rate correlations increase or decrease year-to-year variability? (4) What is the net effect of vital rate correlations on the long-term stochastic population growth rate (λS)? We found only four moderate to strong correlations, both positive and negative in sign, across all species and vital rate pairs; otherwise, correlations were generally weak in magnitude and variable in sign. The net effect of vital rate correlations ranged from a slight decrease to an increase in the year-to-year variability of population growth, with average changes in variance ranging from -1% to +22%. However, vital rate correlations caused virtually no change in the estimates of λS (mean effects ranging from -0.01% to +0.17%). Therefore, the proportional changes in the variance of population growth caused by demographic correlations were too small on an absolute scale to importantly affect population growth and viability. We conclude that in our three focal populations and perhaps more generally, vital rate correlations have little effect on stochastic population dynamics. This may be good news for population ecologists, because estimating vital rate correlations and incorporating them into population models can be data-intensive and technically challenging. This article is protected by copyright. All rights reserved.

The Mariana islands have a species-poor but functionally diverse and largely endemic bird assemblage that varies due to biogeographic legacy and anthropogenic impacts. The largest island in the chain, Guam, is the setting for one of the most extreme examples of recent avian population declines, indicating the capacity for avifaunal collapse and loss of function in neighboring islands. We performed a systematic survey of resident land birds in remnant karst forest on the Mariana Islands' 3 largest islands following Guam to assess the status of the avifauna in this habitat, characterize inter-island heterogeneity in bird communities, and consider the resulting differences in the functional roles of birds across the archipelago's native forests. We identified significantly greater functional diversity on Rota than either Saipan or Tinian, but lower bird population densities, species richness, and Shannon diversity. We recommend continued monitoring of avian population trends across the archipelago and assessments of ecosystem functions like pollination, seed dispersal, and food web dynamics.

Seed dispersal enables plants to reach hospitable germination sites and escape natural enemies. Understanding when and how much seed dispersal matters to plant fitness is critical for understanding plant population and community dynamics. At the same time, the complexity of factors that determine if a seed will be successfully dispersed and subsequently develop into a reproductive plant is daunting. Quantifying all factors that may influence seed dispersal effectiveness for any potential seed-vector relationship would require an unrealistically large amount of time, materials and financial resources. On the other hand, being able to make dispersal predictions is critical for predicting whether single species and entire ecosystems will be resilient to global change. Building on current frameworks, we here posit that seed dispersal ecology should adopt plant functional groups as analytical units to reduce this complexity to manageable levels. Functional groups can be used to distinguish, for their constituent species, whether it matters (i) if seeds are dispersed, (ii) into what context they are dispersed and (iii) what vectors disperse them. To avoid overgeneralization, we propose that the utility of these functional groups may be assessed by generating predictions based on the groups and then testing those predictions against species-specific data. We suggest that data collection and analysis can then be guided by robust functional group definitions. Generalizing across similar species in this way could help us to better understand the population and community dynamics of plants and tackle the complexity of seed dispersal as well as its disruption., Seed dispersal is critical to plant fitness and plant community dynamics. However, measuring and tracking all factors that influence seed dispersal effectiveness for any potential seed-disperser relationship would require an unrealistically large amount of time, materials and financial resources. Building on current frameworks, we suggest that seed dispersal ecology quantify seed dispersal at the scale of plant functional groups, in order to reduce complexity to manageable levels. Based on functional group classifications, plant species can be distinguished by their level of dependence on seed dispersal and its mechanisms.

As the single opportunity for plants to move, seed dispersal has an important impact on plant fitness, species distributions and patterns of biodiversity. However, models that predict dynamics such as risk of extinction, range shifts and biodiversity loss tend to rely on the mean value of parameters and rarely incorporate realistic dispersal mechanisms. By focusing on the mean population value, variation among individuals or variability caused by complex spatial and temporal dynamics is ignored. This calls for increased efforts to understand individual variation in dispersal and integrate it more explicitly into population and community models involving dispersal. However, the sources, magnitude and outcomes of intraspecific variation in dispersal are poorly characterized, limiting our understanding of the role of dispersal in mediating the dynamics of communities and their response to global change. In this manuscript, we synthesize recent research that examines the sources of individual variation in dispersal and emphasize its implications for plant fitness, populations and communities. We argue that this intraspecific variation in seed dispersal does not simply add noise to systems, but, in fact, alters dispersal processes and patterns with consequences for demography, communities, evolution and response to anthropogenic changes. We conclude with recommendations for moving this field of research forward., Seed dispersal—the single opportunity for plants to move—is important for plant fitness, species distributions and patterns of biodiversity. Models that predict extinction risk of species, range shifts and biodiversity tend to rely on average dispersal distances. However, we know that seed dispersal is highly variable even within a single species (e.g. some seeds go very far and some barely move away from their parent plant, some seeds end up in great quality habitats and some end up on roads). This paper looks at the consequences of this variation in seed dispersal for plants and their ability to respond to future global change.

When climatic or environmental conditions change, plant populations must either adapt to these new conditions, or track their niche via seed dispersal. Adaptation of plants to different abiotic environments has mostly been discussed with respect to physiological and demographic parameters that allow local persistence. However, rapid modifications in response to changing environmental conditions can also affect seed dispersal, both via plant traits and via their dispersal agents. Studying such changes empirically is challenging, due to the high variability in dispersal success, resulting from environmental heterogeneity, and substantial phenotypic variability of dispersal-related traits of seeds and their dispersers. The exact mechanisms that drive rapid changes are often not well understood, but the ecological implications of these processes are essential determinants of dispersal success, and deserve more attention from ecologists, especially in the context of adaptation to global change. We outline the evidence for rapid changes in seed dispersal traits by discussing variability due to plasticity or genetics broadly, and describe the specific traits and biological systems in which variability in dispersal is being studied, before discussing some of the potential underlying mechanisms. We then address future research needs and propose a simulation model that incorporates phenotypic plasticity in seed dispersal. We close with a call to action and encourage ecologists and biologist to embrace the challenge of better understanding rapid changes in seed dispersal and their consequences for the reaction of plant populations to global change., Global ecological change is causing plant populations to either adapt or move in response to new environmental conditions. In many species it is thought that seed dispersal may not allow populations to move at a rate that tracks the shifting climate. However, phenotypic plasticity and rapid genetic changes in traits associated with seed dispersal may modify the response capabilities of plants and allow them to responds more quickly. We explore the evidence for rapid modification of seed dispersal traits and propose a path forward to better understand the ways in which seed dispersal may buffer the effects of global ecological change.

Interspecific interactions are important structuring forces in ecological communities. Interactions can be disturbed when species are lost from a community. When interactions result in fitness gains for at least one participating organism, that organism may experience reduced fitness as a result of interaction disturbance. However, many species exhibit traits that enable individuals to persist and reproduce in spite of such disruptions, resulting in resilience to interaction disturbance. Such traits can result in interaction generalization, phenotypic and behavioral plasticity, and adaptive capacity. We discuss examples of these traits and use case studies to illustrate how restoration practitioners can use a trait-based approach to examine species of concern, identify traits that are associated with interspecific interactions and are relevant to resilience, and target such traits in restoration. Restoration activities that bolster interaction resilience could include, for example, reintroducing or supporting specific functional groups or managing abiotic conditions to reduce interaction dependence by at-risk species (e.g. providing structural complexity offering shelter and cover). Resilience may also be an important consideration in species selection for restoration. Establishment of resilient species, able to persist after interaction disturbance, may be essential to restoring to a functioning ecological community. Once such species are present, they could help support more specialized species that lack resilience traits, such as many species of concern. Understanding the conditions under which processes linked to resilience may enable species to persist and communities to reform following interaction disturbance is a key application of community ecology to ecological restoration.

[Aim] Biogeographical comparisons of interaction networks help to elucidate differences in ecological communities and ecosystem functioning at large scales. Neotropical ecosystems have higher diversity and a different composition of frugivores and fleshy-fruited plants compared with Afrotropical systems, but a lack of intercontinental comparisons limits understanding of (a) whether plant–frugivore networks are structured in a similar manner, and (b) whether the same species traits define the roles of animals across continents., [Location] Afrotropics and Neotropics., [Time period] 1977-2015., [Taxa] Fleshy-fruited plants and frugivorous vertebrates., [Methods] We compiled a dataset comprising 17 Afrotropical and 48 Neotropical weighted seed-dispersal networks quantifying frugivory interactions between 1,091 fleshy-fruited plant and 665 animal species, comprising in total 8,251 interaction links between plants and animals. In addition, we compiled information on the body mass of animals and their degree of frugivory. We compared four standard network-level metrics related to interaction diversity and specialization, accounting for differences related to sampling effort and network location. Furthermore, we tested whether animal traits (body mass, degree of frugivory) differed between continents, whether these traits were related to the network roles of species and whether these relationships varied between continents., [Results] We found significant structural differences in networks between continents. Overall, Neotropical networks were less nested and more specialized than Afrotropical networks. At the species level, a higher body mass and degree of frugivory were associated with an increasing diversity of plant partners. Specialization of frugivores increased with the degree of frugivory, but only in the Neotropics., [Main conclusions] Our findings show that Afrotropical networks have a greater overlap in plant partners among vertebrate frugivores than the more diverse networks in the Neotropics that are characterized by a greater niche partitioning. Hence, the loss of frugivore species could have stronger impacts on ecosystem functioning in the more specialized Neotropical communities compared with the more generalized Afrotropical communities., We thank Beth A. Kaplin and Norbert J. Cordeiro for their guidance and support for P.J.D., who received a travel grant by The Center for Tropical Studies and Conservation (CTEC). L.C. and I.G. were supported by the Robert Bosch Foundation. D.M.D. (DE 2754/1‐1), F.S. (HE 3041/20‐1), M.Q., V.S., E.L.N. (Research Unit 823‐825), and K.B.G., M.S. and M.G.R.V. (FOR 1246) thank the German Research Foundation (DFG) for funding. F.A.F.J. acknowledges funding by a CAPES scholarship, N.F. and D.G.S. by the Robert Bosch Foundation, M.G., C.E., A.P. and M.A.P. by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP 2010/52315‐7; 2015/15172‐7; 2016/18355‐8) and Conselho Nacional de Desenvolvimento Científico (CNPq), M.C.M. by Doctoral Fellowships from COLCIENCIAS and Rufford, M.S.S. by Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and FONCyT (PICT2013‐2759 and PICT2016‐0608), P.G.B. by CONICET (PIP 2014‐592) and FONCyT (PICT 2013‐1280), R.A.R. by a Doctoral Fellowship from CONICET, R.H. and S.T. (IF/00441/2013) and M.C. (SFRH/BD/96050/2013) by Fundação para a Ciência e Tecnologia, Portugal, and A.T. (CGL2013‐44386‐P) and D.G. (CGL2015‐68963‐C2‐2‐R) by the Spanish government. T.

Species interactions, both mutualistic and antagonistic, are widely recognized as providing important ecosystem services. Fruit‐eating animals influence plant recruitment by increasing germination during gut passage and moving seeds away from conspecifics. However, relative to studies focused on the importance of frugivores for plant population maintenance, few studies target frugivores as ecosystem service providers, and frugivores are underappreciated as ecosystem service providers relative to other mutualists such as pollinators. Here, we use an accidental experiment to elucidate the role of seed dispersal by frugivores for maintaining a culturally and economically important plant, the donne’ sali chili (Capsicum frutescens) in the Mariana Islands. One of the islands (Guam) has lost nearly all of its native forest birds due to an invasive snake (Boiga irregularis), whereas nearby islands have relatively intact bird populations. We hypothesized that frugivore loss would influence chili recruitment and abundance, which could have economic and cultural impacts. By using video cameras, we confirmed that birds were the primary seed dispersers. We used captive bird feeding trials to obtain gut‐passed seeds to use in a seedling emergence experiment. The experiment showed that gut‐passed seeds emerged sooner and at a higher proportion than seeds from whole fruits. Consistent with our findings that birds benefit chilies, we observed lower chili abundance on Guam than on islands with birds. In a survey questionnaire of island residents, the majority of residents reported an association between the wild chili and local cultural values and traditions. In addition, we identified a thriving market for chili products, suggesting benefits of wild chilies to people in the Marianas both as consumers and producers. Our study therefore documents seed dispersal as both a cultural and a supporting ecosystem service. We provide a comprehensive case study on how seed‐dispersed plants decline in the absence of their disperser, and how to apply mixed‐methods in ecosystem service assessments. Furthermore, we suggest that scientists and resource managers may utilize fruit–frugivore mutualisms concerning socially valuable plants to gather support for frugivore and forest conservation efforts.

Rewilding is an approach aiming at restoring ecosystems to a self-sustaining state by restoring ecological function through active reintroductions or passive management. Locations for most rewilding-through-reintroduction efforts today are selected based on the suitability of the habitat for the reintroduced species, often with little consideration of where the ecological function is most needed. We developed the Spatial Planning of Rewilding Effort (Spore) framework to identify priority locations for rewilding projects through simultaneous consideration of habitat suitability and provisioning of ecological function. We use the island of Guam as a case study for a potential rewilding project, as the island has functionally lost all native seed dispersers as a result of the invasive brown treesnake ( Boiga irregularis ). The Såli (Micronesian starling, Aplonis opaca ) is a good candidate for rewilding to restore ecological function, because it is an effective seed disperser with a localized remnant population. Using Spore, we identify three priority areas for the restoration of seed dispersal, each subdivided into management units. By recognizing the influence of landscape structure and the behaviour of the reintroduced species on the spatial pattern of the function provided by that species, this approach should lead to improved ecological outcomes.

Extinctions beget further extinctions when species lose obligate mutualists, predators, prey, or hosts. Here, we develop a conceptual model of species and community attributes affecting secondary extinction likelihood, incorporating mechanisms that buffer organisms against partner loss. Specialized interactors, including 'cryptic specialists' with diverse but nonredundant partner assemblages, incur elevated risk. Risk is also higher for species that cannot either evolve new traits following partner loss or obtain novel partners in communities reorganizing under changing environmental conditions. Partner loss occurs alongside other anthropogenic impacts; multiple stressors can circumvent ecological buffers, enhancing secondary extinction risk. Stressors can also offset each other, reducing secondary extinction risk, a hitherto unappreciated phenomenon. This synthesis suggests improved conservation planning tactics and critical directions for research on secondary extinctions.

Seed dispersal sets the stage for the suite of biotic and abiotic interactions that determine the fate of individual seeds. In this review, we first focus on how dispersal influences the ‘seedscape’, or the combination of abiotic and biotic factors that affect the probability of recruitment once a seed has reached its final location. We review recent papers that examine the effect of different dispersal vectors on (1) the quality of the habitat in which a seed lands; (2) the distance seeds are dispersed from the parent tree; and (3) the density and composition of plants within the neighborhood of a seed following deposition. Next, we explore methods used to scale these processes up to the level of populations. We highlight demographic models that integrate across multiple life history stages and predict the impact of dispersal in variable environments on population growth. We also review studies that analyze existing spatial patterns of trees within large forest plots and use various strategies to infer the processes that led to those patterns. We continue to scale up from populations to communities, and discuss approaches that have been taken to understand how dispersal may affect diversity and abundance in the community. We then turn to human disturbances and discuss the implications of frugivore defaunation for plant communities. We finish by highlighting several areas of research that are particularly promising for future directions of study.

Humans are the dominant ecological and evolutionary force on the planet today, transforming habitats, polluting environments, changing climates, introducing new species, and causing other species to decline in number or go extinct. Th ese worrying anthropogenic impacts, collectively termed global change, are often viewed as a confounding factor to minimize in basic studies and a problem to resolve or quantify in applied studies. However, these ‘ accidental experiments ’ also represent opportunities to gain fundamental insight into ecological and evolutionary processes, especially when they result in perturbations that are large or long in duration and diffi cult or unethical to impose experimentally. We demonstrate this by describing important fundamental insights already gained from studies which utilize global change factors as accidental experiments. In doing so, we highlight why accidental experiments are sometimes more likely to yield insights than traditional approaches. Next, we argue that emerging environmental problems can provide even more opportunities for scientifi c discovery in the future, and provide both examples and guidelines for moving forward. We recommend 1) a greater fl ow of information between basic and applied subfi elds of ecology and evolution to identify emerging opportunities; 2) considering the advantages of the ‘ accidental experiment ’ approach relative to more traditional approaches; and 3) planning for the challenges inherent to uncontrolled accidental experiments. We emphasize that we do not view the accidental experiments provided by global change as replacements for scientifi c studies quantifying the magnitude of anthropogenic impacts or outlining strategies for mitigating impacts. Instead, we believe that accidental experiments are uniquely situated to provide insights into evolutionary and ecological processes that ultimately allow us to better predict and manage change on our human-dominated planet.

Compiled supplementary information for "Contrasting ecological roles of non-native ungulates in a novel ecosystem." File includes a map of field study sites in Guam, descriptive information justifying selection of paired seedling plot sites, and information on scat counts and vegetation transects accounting for points that appeared to drive trends. Analyses were rerun without the points in question, and general trends were not significantly different from what was reported in the main manuscript.

The global decline of mutualists such as pollinators and seed dispersers may cause negative direct and indirect impacts on biodiversity. Mutualistic network models used to understand the stability of mutualistic systems indicate that species with low partner diversity are most vulnerable to coextinction following mutualism disruption. However, existing models have not considered how species vary in their dependence on mutualistic interactions for reproduction or survival, overlooking the potential influence of this variation on species' coextinction vulnerability and on network stability. Using global databases and field experiments focused on the seed dispersal mutualism, we found that plants and animals that depend heavily on mutualistic interactions have higher partner diversity. Under simulated network disruption, this empirical relationship strongly reduced coextinction because the species most likely to lose mutualists depend least on their mutualists. The pattern also reduced the importance of network structure for stability; nested network structure had little effect on coextinction after simulations incorporated the empirically derived relationship between partner diversity and mutualistic dependence. Our results highlight a previously unknown source of stability in mutualistic networks and suggest that differences among species in their mutualistic strategy, rather than network structure, primarily accounts for stability in mutualistic communities.

Environmental challenges are often global in scope and require solutions that integrate knowledge across disciplines, cultures, and organizations. Solutions to these challenges will come from diverse teams and not from individuals or single academic disciplines; therefore, graduate students must be trained to work in these diverse teams. In this article, we review the literature on training graduate students to cross these borders. We then present a National Science Foundation Integrative Graduate Education and Research Traineeship Program at the University of Washington as a model of border-crossing graduate training focused on interdisciplinary, international, and interorganizational (I3) collaborations on environmental challenges. Finally, we offer recommendations from this program to those considering similar I3 training programs, including strategies for maintaining faculty buy-in, for scaffolding student training to cross borders, and for conducting focused group trips that give the students structu...

A growing understanding of the ecology of seed dispersal has so far had little influence on conservation practice, while the needs of conservation practice have had little influence on seed dispersal research. Yet seed dispersal interacts decisively with the major drivers of biodiversity change in the 21st century: habitat fragmentation, overharvesting, biological invasions, and climate change. We synthesize current knowledge of the effects these drivers have on seed dispersal to identify research gaps and to show how this information can be used to improve conservation management. The drivers, either individually, or in combination, have changed the quantity, species composition, and spatial pattern of dispersed seeds in the majority of ecosystems worldwide, with inevitable consequences for species survival in a rapidly changing world. The natural history of seed dispersal is now well-understood in a range of landscapes worldwide. Only a few generalizations that have emerged are directly applicable to conservation management, however, because they are frequently confounded by site-specific and species-specific variation. Potentially synergistic interactions between disturbances are likely to exacerbate the negative impacts, but these are rarely investigated. We recommend that the conservation status of functionally unique dispersers be revised and that the conservation target for key seed dispersers should be a population size that maintains their ecological function, rather than merely the minimum viable population. Based on our analysis of conservation needs, seed dispersal research should be carried out at larger spatial scales in heterogenous landscapes, examining the simultaneous impacts of multiple drivers on community-wide seed dispersal networks. © 2011 Elsevier Ltd.

Frugivores play differing roles in shaping dispersal patterns yet seed dispersal distance is rarely quantified across entire communities. We model seed dispersal distance using gut passage times and bird movement for the majority (39 interactions) of known bird–tree interactions on the island of Saipan to highlight differences in seed dispersal distances provided by the five avian frugivores. One bird species was found to be a seed predator rather than a disperser. The remaining four avian species dispersed seeds but differences in seed dispersal distance were largely driven by interspecific variation in bird movement rather than intraspecific variation in gut passage times. The median dispersal distance was at least 56 m for all species-specific combinations, indicating all species play a role in reducing high seed mortality under the parent tree. However, one species—the Micronesian Starling—performed 94% of dispersal events greater than 500 m, suggesting this species could be a key driver of long-distance dispersal services (e.g. linking populations, colonizing new areas). Assessing variation in dispersal patterns across this network highlights key sources of variation in seed dispersal distances and suggests which empirical approaches are sufficient for modelling how seed dispersal mutualisms affect populations and communities.

Two new species of Volvocalean green snow algae are described from Upstate New York, Chloromonas chenangoensis and Chloromonas tughillensis. According to rbcL sequence analysis, these species belong to a subclade of five snow species of Chloromonas that includes Cr. brevispina, Cr. nivalis and Cr. pichinchae. These species have zoosporangia/gametangia (cell packs) that are propelled by flagella. The life cycles of Cr. chenangoensis and Cr. tughillensis include a change in cell shape from oblong to spherical before the loss of the cell wall. Using 8-h laboratory experiments, a peak in the change of cell shape occurred at 4 h for Cr. chenangoensis and between 2 and 4 h for Cr. tughillensis after the onset of light, and this was followed by a decline in change of cell shape for both species. Spherical cells peaked between 6 and 8 h for Cr. chenangoensis and at 8 h for Cr. tughillensis. Maximum total matings occurred at 4 h for Cr. chenangoensis and 6 h for Cr. tughillensis, which was followed by a p...

Seed dispersal is considered a key mechanism through which the structure and function of forests is maintained. Testing this can be difficult because the large scale over which dispersal operates makes it difficult to examine in a meaningful way. Using the near complete loss of native vertebrate seed dispersers from the island of Guam we examine the importance of seed dispersal for maintaining forest seedbanks. We find that seed dispersers have a strong influence on the species composition of seedbanks. Without seed dispersers seedbanks no longer serve to increase the species pool for tree regeneration following disturbance., The accumulation of seeds in the soil (the seedbank) can set the template for the early regeneration of habitats following disturbance. Seed dispersal is an important factor determining the pattern of seed rain, which affects the interactions those seeds experience. For this reason, seed dispersal should play an important role in structuring forest seedbanks, yet we know little about how that happens. Using the functional extirpation of frugivorous vertebrates from the island of Guam, together with two nearby islands (Saipan and Rota) that each support relatively intact disperser assemblages, we aimed to identify the role of vertebrate dispersers in structuring forest seedbanks. We sampled the seedbank on Guam where dispersers are absent, and compared this with the seedbank on Saipan and Rota where they are present. Almost twice as many species found in the seedbank on Guam, when compared with Saipan and Rota, had a conspecific adult within 2 m. This indicates a strong role of vertebrate dispersal in determining the identity of seeds in the seedbank. In addition, on Guam, a greater proportion of samples contained no seeds and overall species richness was lower than on Saipan. Differences in seed abundance and richness between Guam and Rota were less clear, as seedbanks on Rota also contained fewer species than Saipan, possibly due to increased post-dispersal seed predation. Our findings suggest that vertebrate seed dispersers can have a strong influence on the species composition of seedbanks. Regardless of post-dispersal processes, without dispersal, seedbanks no longer serve to increase the species pool of recruits during regeneration.

The effects of premating light regimes on sexual reproduction and the production of spherical cells in Chloromonas sp.-D, a unicellular green snow alga, were studied using cross-mating strains 582C and 582D isolated from snowpacks associated with mixed hardwood-softwood forests in Whetstone Gulf State Park, Tughill Plateau, NY. Two preacclimation regimes were used, Vita-Lite as controls (530-700 nm peak) and blue light as experimentals (430-460 nm peak) prior to the mating experiments. In blue light, an increase in the number of matings and spherical cells (spheres) produced in the life cycle was observed as the photoperiod increased. This implies that longer photoperiods of blue light are more optimal for sexual reproduction in Chloromonas sp.-D than shorter ones. Under Vita-Lite, there was a significant increase in the number of matings and spheres with the extended 20 : 4 photoperiod compared with the shorter 14 : 10 photoperiod. Under blue light, significantly more matings and spheres occurred than under Vita-Lite using the same irradiance level of 95 μmol photons m -2 s -1 (photosynthetically active radiation [PAR] of 400-700 nm) for the 14: 10 and 20: 4 photoperiods. The results of these experiments suggest that Chloromonas sp.-D. known only from the Tughill Plateau, NY, is not reproducing optimally at this site where it grows and reproduces under an approximate 14: 10 photoperiod in early April. However, in the upper 10 cm of snow in the Tughill Plateau, a blue light irradiance level of 95 μmol photons m 2 s -1 occurs, which is optimal for this species. When these conditions are combined with a 14: 10 photoperiod, the Tughill Plateau appears to be sub-optimal for mating and production of spherical cells. Since Chloromonas sp.-D does not appear to have a dependence on a dark cycle, this would allow it to expand its geographical distribution. It may reproduce more optimally under blue light (95 μmol photons m -2 s -1 ) with an extended photoperiod (>20:4 h, light:dark) in high latitude field sites such as Lake Bienville, Quebec, in eastern North America where other species of Chloromonas are known in snow associated with coniferous forests.

Specialised natural enemies maintain forest diversity by reducing tree survival in a density- or distance-dependent manner. Fungal pathogens, insects and mammals are the enemy types most commonly hypothesised to cause this phenomenon. Still, their relative importance remains largely unknown, as robust manipulative experiments have generally targeted a single enemy type and life history stage. Here, we use fungicide, insecticide and physical exclosure treatments to isolate the impacts of each enemy type on two life history stages (germination and early seedling survival) in three tropical tree species. Distance dependence was evident for five of six species-stage combinations, with each enemy type causing distance dependence for at least one species stage and their importance varying widely between species and stages. Rather than implicating one enemy type as the primary agent of this phenomenon, our field experiments suggest that multiple agents acting at different life stages collectively contribute to this diversity-promoting mechanism.

In healthy forests, vertebrate frugivores move seeds from intact to degraded forests, aiding in the passive regeneration of degraded forests. Yet vertebrate frugivores are declining around the world, and little is known about the impact of this loss on regeneration of degraded areas. Here, we use a unique natural experiment to assess how complete vertebrate frugivore loss affects native seed rain in degraded forest. All native vertebrate frugivores (which were primarily avian frugivores) have been functionally extirpated from the island of Guam by the invasive brown tree snake (Boiga irregularis), whereas the nearby island of Saipan has a relatively intact vertebrate frugivore community. We captured seed rain along transects extending from intact into degraded forest and compared the species richness, density and condition of the seed rain from native bird-dispersed tree species between the two islands. Considering seeds from native bird-dispersed species, approximately 1.66 seeds landed per 26 days in each square meter of degraded forest on Saipan, whereas zero seeds landed per 26 days per square meter in degraded forest on Guam. Additionally, on Saipan, 69% of native bird-dispersed seeds in intact forest and 77% of seeds in degraded forest lacked fleshy fruit pulp, suggesting ingestion by birds, compared to 0% of all seeds on Guam. Our results show an absence of seed rain in degraded forests on Guam, correlated with the absence of birds, whereas on Saipan, frugivorous birds regularly disperse seeds into degraded forests, providing a mechanism for re-colonization by native plants. These results suggest that loss of frugivores will slow regeneration of degraded forests on Guam.

There were errors in Figure 1 and Figure 3. The correct Figure 1 can be viewed here: . The correct Figure 3 can be viewed here: In the Materials and Methods section, under the heading Insectivorous birds, the scientific name for the Mariana Swiftlet is listed incorrectly twice in the following sentences: Only two of the 10 native insectivorous forest bird species remain on Guam: the Micronesian Starling (Aplonis opaca), and the Marianas Swiftlet (Acrocephalus luscinia). Aplonis opaca has a localized remnant population of less than 400 birds (D. Vice, J. Quitagua and L. Obra, pers.comm.) covering an area of less than 50 km2, and Acrocephalus luscinia has remnant populations numbering around 1100 in three caves in Southern Guam(A. Brooke, pers.comm.) The correct name is Aerodramus bartschi

The combination of small-scale manipulative experiments and large-scale natural experiments provides a powerful approach for demonstrating the importance of top-down trophic control on the ecosystem scale. The most compelling natural experiments have come from studies examining the landscape-scale loss of apex predators like sea otters, wolves, fish and land crabs. Birds are dominant apex predators in terrestrial systems around the world, yet all studies on their role as predators have come from small-scale experiments; the top-down impact of bird loss on their arthropod prey has yet to be examined at a landscape scale. Here, we use a unique natural experiment, the extirpation of insectivorous birds from nearly all forests on the island of Guam by the invasive brown tree snake, to produce the first assessment of the impacts of bird loss on their prey. We focused on spiders because experimental studies showed a consistent top-down effect of birds on spiders. We conducted spider web surveys in native forest on Guam and three nearby islands with healthy bird populations. Spider web densities on the island of Guam were 40 times greater than densities on islands with birds during the wet season, and 2.3 times greater during the dry season. These results confirm the general trend from manipulative experiments conducted in other systems however, the effect size was much greater in this natural experiment than in most manipulative experiments. In addition, bird loss appears to have removed the seasonality of spider webs and led to larger webs in at least one spider species in the forests of Guam than on nearby islands with birds. We discuss several possible mechanisms for the observed changes. Overall, our results suggest that effect sizes from smaller-scale experimental studies may significantly underestimate the impact of bird loss on spider density as demonstrated by this large-scale natural experiment.