Search

Moorad et al [1] reinforces and elaborates on warnings made previously [2, 3] that Williams’ “hypothesis” should be treated with scepticism. A proper hypothesis should be based upon the best information available at the time, and for the case of the evolutionary theory of senescence, Hamilton’s model of selection is superior to Williams’. Hamilton’s insights improved over Williams’ by his appreciation for how fertility and juvenile survival schedules play a critical role in defining selection. While Williams’ model is not articulated mathematically, it is clear that his hypothesis is motivated by his belief that the strength of selection against mortality specific to some age follows from the frequency of individuals that survived to that age [4]. However, Hamilton [5] was quite clear: the strength of selection is equal to the proportion of newborns that came from parents that survived to that age (see [1] or Equation 1 in the Appendix). Furthermore, Williams’ explicitly states that juvenile mortality cannot affect the evolution of senescence while Hamilton’s formulae show us that it can. It should be emphasized that Williams himself came to appreciate that Hamilton’s model describes the best way to think about how selection works [e.g., 6].

The evolutionary theory of senescence underpins research in life history evolution and the biology of aging. In 1957 G.C. Williams predicted that higher adult death rates select for earlier senescence and shorter length of life, but preadult mortality does not matter to the evolution of senescence. This was subsequently interpreted as predicting that senescence should be caused by 'extrinsic' sources of mortality. This idea still motivates empirical studies, although formal, mathematical theory shows it is wrong. It has nonetheless prospered because it offers an intuitive explanation for patterns observed in nature. We review the flaws in Williams' model, explore alternative explanations for comparative patterns that are consistent with the evolutionary theory of senescence, and discuss how hypotheses based on it can be tested. We argue that focusing on how sources of mortality affect ages differently offers greater insight into evolutionary processes.

Acoustic data can be a source of important information about events and the environment in modern cities. To date, much of the focus has been on monitoring noise pollution, but the urban soundscape contains a rich variety of signals about both human and natural phenomena. We describe the CitySounds project, which has installed enclosed sensor kits at several locations across a heavily used urban greenspace in the city of Edinburgh. The acoustic monitoring components regularly capture short clips in real-time of both ultrasonic and audible noises, for example encompassing bats, birds and other wildlife, traffic, and human. The sounds are complemented by collecting other data from sensors, such as temperature and relative humidity. To ensure privacy and compliance with relevant legislation, robust methods render completely unintelligible any traces of voice or conversation that may incidentally be overheard by the sensors. We have adopted a variety of methods to encourage community engagement with the audio data and to communicate the richness of urban soundscapes to a general audience.

Local adaptation of plants to mycorrhizal fungi helps determine the outcome of mycorrhizal interactions. However, there is comparatively little work exploring the potential for evolution in interactions with ectomycorrhizal fungi, and fewer studies have explored the heritability of mycorrhizal responsiveness, which is required for local adaptation to occur. We set up a reciprocal inoculation experiment using seedlings and soil from four populations of Scots pine (Pinus sylvestris) from Scotland, measuring seedling response to mycorrhizal inoculation after 4 months. We estimated heritability for the response traits and tested for genotype × environment interactions. While we found that ectomycorrhizal responsiveness was highly heritable, we found no evidence that pine populations were locally adapted to fungal communities. Instead, we found a complex suite of interactions between pine population and soil inoculum. Our results suggest that, while Scots pine has the potential to evolve in response to mycorrhizal fungi, evolution in Scotland has not resulted in local adaptation. Long generation times and potential for rapid shifts in fungal communities in response to environmental change may preclude the opportunity for such adaptation in this species, and selection for other factors such as resistance to fungal pathogens may explain the pattern of interactions found.

QuestionsAre communities structured on a hydrological (soil moisture) gradient? Is there spatial segregation into hydrological niches? What is the shape of the hydrological niches of individual species? Controlling for spatial autocorrelation, how much of the spatial structure in the community is due to variation in hydrology? Do annuals and perennials behave alike with respect to the above questions?LocationsLa Mina in Moscosa Farm, Salamanca, western Spain (dehesa community) and Laguna Larga in the Urbión Peaks, Soria, central‐northern Spain (alpine grassland).MethodsThe presence of plant species was sampled in two contrasting field sites, for which we also built hydrological models. First, we reduced the dimensionality of the plant distribution data (non‐metric multidimensional scaling) and measured the correlation between the resulting ordination and the hydrological gradient. Then we defined hydrological niches and tested niche segregation of plant species against null models (Pianka metrics). Finally, we characterized the hydrological niche of each species using generalised additive mixed models and partitioned the species distribution variance into (1) a hydrological component, (2) a linear trend component and (3) and a spatial component, defined through sets of spatial variables (Moran's eigenvector maps).ResultsBoth plant communities were primarily structured along hydrological gradients, and spatial segregation into hydrological niches occurred among perennial species, although not among annuals in the dehesa community. Dehesa annuals were spatially aggregated in the driest niches. Hydrological variation shaped the responses of 60% of the annual and about 70% of the perennial species in both the dehesa meadow and the alpine community. Most responses were either monotonic or hump‐shaped. Finally, spatially structured hydrological variation proved to be the main driver of spatially structured species composition in all cases.ConclusionsLinearly (gradient of slope) and topographically (at a fine scale) structured variation in hydrology is the main driver of spatially structured species composition in both communities. Our results support the ecological hypothesis that spatial niche segregation on soil moisture gradients is an important mechanism of co‐existence for perennials in both test communities, although not for the species‐rich sub‐community of annuals in the dehesa meadow.

1. The growing literature on the phylogenetic structure of plant communities places great emphasis on the role of phylogenetic niche conservatism (PNC) in community assembly. However, the patterns revealed by such analyses are difficult to interpret in the absence of independent data on niche structure. While there is increasing evidence that plant coexistence does depend upon niche differences, it is still not clear in most cases what the relevant niche axes are. 2. We address this problem by testing for PNC within the African Restionaceae (‘restios’), a clade endemic to the Western Cape where we have shown niche segregation along soil moisture gradients to be common. 3. Significant niche segregation on soil moisture gradients occurred among restios in 7 of 10 communities sampled, but PNC was detectable in only one of these and then only by one of three methods used. 4. Phylogenetic analysis of the evolution of hydrological niche traits for the species pool of 37 Restionaceae in the study showed tolerance of drought to be convergent rather than conserved. 5. Synthesis. The demonstration that clear niche segregation may occur among related species without PNC being detectable supports the hypothesis that hydrological niche responses are evolutionarily labile. More generally, the results demonstrate that phylogenetic analysis can be a poor guide to the process of community assembly. We argue that it may in future be better to apply ecological data to the interpretation of phylogenies, rather than to follow the current preoccupation with the application of phylogenies to ecology.

Background and aims Species in plant communities segregate along fine-scale hydrological gradients. Although this phenomenon is not unique to fynbos, this community regenerates after fire and therefore provides an opportunity to study the ecological genesis of hydrological niche segregation. Methods Following wildfires at two field sites where we had previously mapped the vegetation and monitored the hydrology, seeds were moved experimentally in >2500 intact soil cores up and down soil-moisture gradients to test the hypothesis that hydrological niche segregation is established during the seedling phase of the life cycle. Seedling numbers and growth were then monitored and they were identified using DNA bar-coding, the first use of this technology for an experiment of this kind. Key Results At the site where niche segregation among Restionaceae had previously been found, the size of seedlings was significantly greater, the wetter the location into which they were moved, regardless of the soil moisture status of their location of origin, or of the species. Seedling weight was also significantly greater in a competition treatment where the roots of other species were excluded. No such effects were detected at the control site where niche segregation among Restionaceae was previously found to be absent. Conclusions The finding that seedling growth on hydrological gradients in the field is affected by soil moisture status and by root competition shows that hydrological niche segregation could potentially originate in the seedling stage. The methodology, applied at a larger scale and followed-through for a longer period, could be used to determine whether species are differently affected by soil moisture

Summary 1. Volunteers have helped in scientific surveys of birds and other organisms for decades, but more recently, the use of the Internet has enormously widened the opportunity for citizen science and greatly increased its practice. There is now a need to share experience of which methods work and which do not. Here, we describe how we planned and executed the Evolution MegaLab, one of the largest surveys of polymorphism in wild species so far undertaken. 2. The aim of the Evolution MegaLab was to exploit the occasion of Charles Darwin’s double centenary in 2009 to mobilize the widest possible section of the general public in Europe to help survey shell polymorphism in the banded snails Cepaea nemoralis and Cepaea hortensis. These data were then compared with historical records to detect evolutionary change that may have taken place in the decades between samples. 3. Records of polymorphism in over 7000 populations sampled throughout the natural range of the two species were captured from published and unpublished sources and added to an online database. These data could be explored by the general public via a Google Maps interface on the project website (http://evolutionmegalab.org). The website contained a welcome page that explained what evolution is and how recent changes in climate, and the abundance of predatory birds (song thrushes Turdus philomelos) might have caused an evolutionary change in the shell patterns of banded snails. 4. A network of collaborators in 15 European countries was formed, with each country responsible for translating the website and associated materials, recruiting volunteers and raising any funds required locally. A total of 6461 users registered with the site, and 7629 records were submitted. We used an online quiz to train users and to test their ability, to recognize the correct snails and their morphs. Every user received automated, immediate feedback that compared their data with nearby records from the historical database. 5. The critical tasks achieved by the Evolution MegaLab that any citizen science project must tackle are as follows: (i) design of an appropriate project, (ii) recruitment, motivation and training of volunteers, and (iii) ensuring data quality.

Consistent, repeatable segregation of plant species along hydrological gradients is an established phenomenon that must in some way reflect a trade-off between plants' abilities to tolerate the opposing constraints of drought and waterlogging. In C3 species tissue carbon isotope discrimination (δ13C) is known to vary sensitively in response to stomatal behaviour, reflecting stomatal limitation of photosynthesis during the period of active growth. However, this has not been studied at fine-spatial scale in natural communities. We tested how δ13C varied between species and sexes of individuals in the family Restionaceae growing along a monitored hydrological gradient. Twenty Restionaceae species were investigated using species-level phylogeny at two sites in the Cape Floristic Region, a biodiversity hotspot. A spatial overlap analysis showed the Restionaceae species segregated significantly (P < 0.001) at both sites. Moreover, there were significant differences in δ13C values among the Restionaceae species (P < 0.001) and between male and female individuals of each species (P < 0.01). However, after accounting for phylogeny, species δ13C values did not show any significant correlation with the hydrological gradient. We suggest that some other variable (e.g. plant phenology) could be responsible for masking a simple response to water availability.

Long-term ecological observation affords a picture of the past that uniquely informs our understanding of present and future ecological communities and processes. Without a long-term perspective, our vision is prone to environmental myopia. Long-term experiments (LTEs) in particular can reveal the mechanisms that underlie change in communities and ecosystem functioning in a way that cannot be understood by long-term monitoring alone. Despite the urgent need to know more about how climate change will affect ecosystems and their functioning, the continued existence of LTEs is extremely precarious and we believe that dedicated funds are needed to support them. A new non-profit organization called the Ecological Continuity Trust seeks to provide a solution to this problem by establishing an endowment that will be specifically earmarked to sustain LTEs as a scientific tool for the benefit of future generations.

The extent to which divergent selection can drive genome-wide population differentiation remains unclear. Theory predicts that in the face of ongoing gene flow, population differentiation should be apparent only at those markers that are directly or indirectly (i.e. through linkage) under selection. However, if reproductive barriers limit gene flow, genome-wide population differentiation may occur even in geographically proximate populations. Some insight into the link between selection and genetic differentiation in the presence of ongoing gene flow can come from long-term experiments such as The Park Grass Experiment, which has been running for over 150 years, and provides a unique example of a heterogeneous environment with a long and detailed history. Fertilizer treatments applied in the Park Grass Experiment have led to rapid evolutionary change in sweet vernal grass Anthoxanthum odoratum, but until now, nothing was known of how these changes would be reflected in neutral molecular markers. We have genotyped ten A. odoratum populations from the Park Grass Experiment using Amplified Fragment Length Polymorphisms (AFLPs). Our data show that nutrient additions have resulted in genome-wide divergence among plots despite the high potential for ongoing gene flow. This provides a well-documented example of concordance between genomes and environmental conditions that has arisen in continuous populations across a time span of fewer than 75 generations.

1. Evidence from plant-community structure suggests that niche segregation between plant species is widespread, but the mechanisms are still generally obscure. We used experimental mesocosms to investigate the role of above- and below-ground competition in defining the distinct niche distributions of two Senecio species that separate along a water-table gradient in meadow habitats. In a target-border design, Senecio target plants were surrounded by six fence-sitting plants of Phleum pratense and fully factorial, randomised treatments for above-ground and below-ground competition, water level and nitrogen were applied. 2. Below-ground competition was found to be the most influential factor for plant biomass and seed production, whereas above-ground competition had negligible effects. Judging from their performances under different combinations of water level and nitrogen fertilization, the Senecio species showed different types of niche differentiation. Senecio aquaticus showed a preference for waterlogged over dry soils irrespective of the presence or absence of competition. Senecio jacobaea showed no preference for any hydrological condition, as long as below-ground interaction was prevented. In the presence of competing roots, it showed the expected preference for dry soils, especially under N-fertilized conditions. 3. Below-ground competition was especially intense under conditions of high supply of edaphic resources and even had the potential to entirely abolish any positive effects of increased water- or nitrogen-supply. This supports the highly debated view that the importance of below-ground competition increases rather than decreases with below-ground resource supply. A functional mechanism for the dry-habitat niche of S. jacobaea is suggested by the severe effect of competition on this species in waterlogged soil, especially when nitrogen was added. Since such conditions favoured growth of competing neighbours, the intensified depletion of other soil resources may have been the cause of the poor performance by S. jacobaea. 4. Synthesis. Niche differentiation can either be a genetically fixed preference or a result of current competition. Below-ground competition was found to be a much stronger driver of niche differentiation than generally assumed. Even in highly productive systems it can be the principal type of interaction, which is contrary to common assumptions and which might be the case when some but not all edaphic resources are in ample supply.

In theory, females that reproduce asexually should enjoy a twofold advantage in fitness over sexual females, yet sex remains the predominant mode of reproduction in virtually all eukaryotes. The evolutionary maintenance of sex is especially puzzling in clonal plants because the transition from sexual to exclusively asexual reproduction is an ever-present possibility in these species. In this article, I use published data on the genotypic diversity of populations of clonal plants to test five hypotheses about the ecological situations that limit or favor clonal reproduction in vascular plants. The data were drawn from 248 studies covering 69,000 individuals in >2000 populations of 218 species in 74 plant families. The tests showed the following: (1) the frequency of clonality increases with population age, indicating that clonal reproduction is limited by disturbance; (2) clonal reproduction is limited by dispersal; clones are more frequent in aquatic and apomictic species in which the dispersal of clonally produced propagules is less limiting; (3) clones are more frequent in populations of rare or endangered species; (4) populations of alien plants have higher frequencies of clonality; and (5) clones are more frequent at the edges of species’ geographical ranges. Thus, it appears that the ultimately successful clonal plant would be a rare, aquatic, alien apomict living in an undisturbed, geographically marginal habitat. Since this combination of circumstances is so restrictive, it is perhaps better regarded as a sign of sexual failure than as a recipe for clonal success.

I thank the authors of the three responses to my article, ‘Have Ecosystem Services Been Oversold?’ [1], for providing a range of opinions that, to varying degrees, take the debate forward. In the most critical of the three letters, Potschin et al. [2] describe my paper as a caricature of the field, but then go on to caricature the paper itself. I did not claim that the ecosystem services (ES) perspective ‘inescapably leads to monetization’, but only that it provided the conceptual basis for this to happen.

SPE GESTAD; International audience; Ecological systems comprise of individuals and species interacting with each other and their environment, and these interactions combine to form complex networks. The maintenance of biodiversity and many ecosystem functions depend upon these ecological interactions. Humans, their crops and livestock can also be considered as part of these networks of interactions making network analysis valuable for considering the resilience of ecosystem services, i.e., the benefits we gain from nature. Networks are visually appealing and visualisation can attract attention and inform, both to communicate overall messages and provide comparisons between networks. There are many different approaches and layouts for visualising networks, but there is little research to help guide best practice. Ultimately though, best practice should be to ensure that messages are supported by evidence and clearly communicated with reference to the competence of the audience. Given the appeal of visualisations and the importance of networks in communicating the interdependence of species (including humans), ecological networks and their visualisation can be used to support excellent public engagement and can be used to enhance the value of citizen science, in which people actively contribute to scientific research. Network approaches could also be valuable for engagement with decision-makers and stakeholders, including their application to complex socio-economic systems, especially where co-production of network visualisations could provide evidence-based overviews of data. In summary, ecological networks and their visualisation are an important tool for scientific inquiry, communication and engagement with even greater potential than has currently been realised.

The concept of ecosystem services (ES) neatly encapsulates the ways in whichhuman society depends upon the existence and functioning of nature, but alsodraws power by chiming with dominant neoliberal ideology. Scientific paradigmssuch as this have an inherent tendency to stop adherents from recognizingalternative approaches. It is high time to examine whether the concept isbeing oversold with potentially damaging consequences. Many authors havequestioned the monetization of ES, but the origin of the problem lies deeper inanthropocentrism. By illustration with alternatives, I attempt to show how the ESparadigm has constrained thought, particularly towards the monetization andfinancialization of nature, even when many ecologists and others oppose thistrend.

The Park Grass Experiment, begun in 1856, is the oldest ecological experiment in existence. Its value to science has changed and grown since it was founded to answer agricultural questions. In recent times the experiment has shown inter alia how: plant species richness, biomass and pH are related; community composition responds to climatic perturbation and nutrient additions; soil is acidified and corrected by liming. It also provided one of the first demonstrations of the evolution of adaptation at a very local scale and contains a putative case of the evolution of reproductive isolation by reinforcement. The application of molecular genetic markers to archived plant material promises to reveal a whole new chapter of genetic detail about the long-term dynamics of plant populations. Over the range of values observed at Park Grass, biomass (productivity) has a negative effect upon species richness. Any positive effect of species richness on productivity could only be weak by comparison. The experiment provides support for both the competitive exclusion and pool size hypotheses for determination of species density. Instantaneous comparisons of species richness between plots do not accurately reflect temporal rates of loss which may be multiplicative rather than additive. This suggests that comparisons among sites, nutrient inputs, especially N treatments, or soil acidity may in general underestimate the threat posed to plant species diversity by long-term changes in plant nutrient availability, both enrichment and depletion. Differences between plots at the community level are maintained despite a flow of propagules between plots. There is no strong evidence for a spatial mass effect. Guild (grass/legume/other) compositions of plant communities have equilibrated, but the species composition within guilds is more dynamic and continues to change over time, suggesting that species and guild abundances are independently regulated. At least some members of all the major trophic levels, including predators (spiders), herbivores (leafhoppers) and detritivores (springtails) are treatment-specific in their distributions. Plant populations on Park Grass are subdivided by treatments which, to some degree, have led to plots becoming genetically isolated from one another and decoupled demographically. This subdivision has created a metapopulation structure in each species, characterized by species-specific rates of local colonization and extinction. Inverse clines in flowering time occur in the grass Anthoxanthum odoratum across some plot boundaries. These suggest that reproductive isolation between plots has been reinforced by natural selection. Drift as well as selection may have taken place in A. odoratum, especially on plots where effective population size is restricted by population bottlenecks caused by drought. Park Grass illustrates how long-term experiments grow in value with time and how they may be used to investigate scientific questions that were inconceivable at their inception. This is as likely to be true of the future of Park Grass as it has proved to be of its past.

It is suggested that spatial niche-separation between grassland species occurs in the regeneration phase and that the differential response of species to micro-variations in leaf cover may allow coexistence at high density. A technique for measuring heterogeneity in leaf-cover on a scale of 1-cm2 units (micro-cover) and the demography of seedling cohorts of Reseda lutea and Anthyllis vulneraria in relation to this heterogeneity is described. Densities of seedlings were significantly higher in micro-sites of low micro-cover for R. lutea and for one of the three cohorts of A. vulneraria studied. No relationship was found between mortality and density of seedlings or micro-cover. The ultimate causes of these distribution patterns of seedlings and their consequences for the maintenance of species diversity in grassland are discussed.

A significant proportion of the global diversity of flowering plants has evolved in recent geological time, probably through adaptive radiation into new niches. However, rapid evolution is at odds with recent research which has suggested that plant ecological traits, including the beta- (or habitat) niche, evolve only slowly. We have quantified traits that determine within-habitat alpha diversity (alpha niches) in two communities in which species segregate on hydrological gradients. Molecular phylogenetic analysis of these data shows practically no evidence of a correlation between the ecological and evolutionary distances separating species, indicating that hydrological alpha niches are evolutionarily labile. We propose that contrasting patterns of evolutionary conservatism for alpha- and beta-niches is a general phenomenon necessitated by the hierarchical filtering of species during community assembly. This determines that species must have similar beta niches in order to occupy the same habitat, but different alpha niches in order to coexist.

Summary 1 It has been argued that niche pre-emption is not the only possible explanation for monophyly among Macaronesian endemic plants because (i) interspecific competition is diffuse, not species-specific, (ii) the radiations in question may not in fact be monophyletic, and (iii) later colonists may have hybridized with earlier ones, making a small and undetected contribution to the gene pool of lineages that appear to be monophyletic. 2 The niche pre-emption mechanism does not, however, require species-specific competitive interactions. It merely proposes that the clade created by adaptive radiation will occupy more niche space than the original colonist could on its own. Members of the clade will then collectively inhibit establishment by new colonists more effectively than can a colonist that has not radiated. 3 The monophyly of many larger radiations in the Macaronesian flora is well established and new studies tend to confirm this pattern. 4 A few later-arriving colonists may have undetectably hybridized with earlier arrivals, but this is only a genetic interpretation of the essential idea behind pre-emption, i.e. that early arrivals so outnumber later colonists that the latter cannot establish. 5 We do not therefore believe that hybridization provides an alternative explanation of why groups with multiple colonization failed to radiate in Macaronesia.

How large numbers of competing plant species manage to coexist is a major unresolved question in community ecology. The classical explanation, that each species occupies its own niche, seems at first unlikely because most plants require the same set of resources and acquire these in a limited number of ways. However, recent studies, although few in number and incomplete in many ways, do suggest that plants segregate along various environmental niche axes, including gradients of light, soil moisture and root depth, and that partitioning of soil nutrients occurs, possibly through the mediation of microbial symbionts, some of which are more species specific than was previously thought. Although it is unlikely that niche separation along environmental axes is the only mechanism of coexistence in any large community, the evidence now suggests that it plays a more significant role than has been previously appreciated. More research into the consequences of various known tradeoffs is likely to uncover further cases of niche separation facilitating coexistence.

Elasticities of matrix elements from population projection matrices are commonly used to analyze the relative contributions of different life history transitions (birth, survival, growth) to the finite rate of increase (lambda). Hitherto, comparative demography based on matrix models has relied upon decomposing elasticity matrices into blocks, each containing matrix elements deemed to represent recruitment, stasis, or progression to larger size classes. Elasticities across an entire matrix always sum to unity, and different populations and species can be compared on the basis of the relative proportions of these three variables. This method has been widely used, but it contains a weakness in that the value of matrix elements is a function of more than one vital rate. For example, transitions representing progression to larger size classes involve a survival rate as well as a growth rate. Ideally, then, demographic comparisons between populations should be made using elasticities of vital rates themselves, rather than elasticities of matrix elements that are compounds of those rates. Here, we employ the complete set of general equations for the elasticity of vital rates in an entirely new analysis of matrices for 102 species of perennial plants. The results show a surprising similarity to an earlier analysis based upon matrix element elasticity and provide important confirmation of general patterns of correlation between plant life history and demography. In addition, we show that individual vital rate elasticities cannot, on their own, predict variation in life history. Therefore, all three demographic processes (survival, growth, and reproduction) are necessary to account for life history variation. The new analysis provides a firmer foundation for comparative demography.

1 Large endemic plant taxa found on oceanic archipelagos are frequently monophyletic, indicating that they originate from a single colonization event. 2 Colonization is a two-stage process requiring both dispersal and establishment to be successful. Accordingly, once-only colonization may be explained either by dispersal barriers limiting colonization, or by the first successful colonization of an island inhibiting the establishment of later arrivals through niche pre-emption and interspecific competition. 3 Using the endemic flora of the Canaries and Macaronesia as a test case, I argue that barriers to dispersal are low and that niche pre-emption is therefore the more likely explanation for the monophyly of large endemic groups in these islands.

We report the successful amplification of microsatellite markers for the chloroplast genome from century-old samples of 2 grasses growing in the Park Grass Experiment (PGE):Anthoxanthum ordoratum andFestuca rubra. This opens the possibility of establishing a long-term genetic time series for the PGE, which began in 1856 and is believed to be the oldest ecological experiment in existence. Although the plant samples used were not originally prepared or stored with molecular analysis in mind, the hexadecyltrimethylammonium bromide (CTAB) method of DNA extraction was successfully used. Obtained DNA was degraded but could be amplified by means of PCR. It produced bands around the expected size for chloroplast microsatellite primers derived from rice. When sequenced, bands showed good homology with sequences from rice chloroplast genomes listed in GenBank (accession No X15901).

ESS models for the evolution of seed size variation assume that seedlings compete with each other for the occupancy of 'safe sites' or vegetation gaps. If mortality rates are high and/or frequency-independent, ESS models reduce to the classical model of Smith and Fretwell which predicts that a single, optimum seed size should occur. We tested whether seedlings compete with one another by following the survival of seedlings colonizing experimental gaps in a grazed grassland community. In small gaps (3 cm diameter) the proximity of established plants slightly, though significantly reduced seedling survival, but density-dependent mortality also occurred among seedlings in these gaps. In larger gaps (6 cm, 9 cm diameter) survival was significantly positively frequency-dependent. These results strongly support the validity of ESS models.

1. Despite the fundamental significance of water to plants and the persisting question of how competing species coexist, this is the first review of hydrological niches. We define hydrological niche segregation (HNS) as: (i) partitioning of space on fine-scale soil-moisture gradients, (ii) partitioning of water as a resource, and/or (iii) partitioning of recruitment opportunities among years caused by species specializing on particular patterns of temporal variance of water supply (the storage effect).\ud 2. We propose that there are three types of constraint that lead to the trade-offs that underlie HNS. (i) An edaphic constraint creates a trade-off between the supply to roots of O2 on the one hand vs. water and nutrients on the other. (ii) A biophysical constraint governs gas exchange by leaves, leading to a trade-off between CO2 acquisition vs. water loss. (iii) A structural constraint arising from the physics of water-conducting tissues leads to a safety vs. efficiency trade-off. \ud 3. Significant HNS was found in 43 of 48 field studies across vegetation types ranging from arid to wet, though its role in coexistence remains to be proven in most cases. Temporal partitioning promotes coexistence through the storage effect in arid plant communities, but has yet to be shown elsewhere. In only a few cases is it possible to unequivocally link HNS to a particular trade-off.\ud \ud 4. Synthesis. The field and experimental evidence make it clear that HNS is widespread, though it is less clear what its precise mechanisms or consequences are. HNS mechanisms should be revealed by further study of the constraints and trade-offs that govern how plants obtain and use water and HNS can be mechanistically linked to its consequences with appropriate community models. In a changing climate, such an integrated programme would pay dividends for global change research.

Summary 1 Vegetation‐environment relationships are well understood at the geographical scale, but it is still not clear what role chance and historical contigency play in the assembly of communities from the local species pool. We addressed this question experimentally by comparing the performance of congeneric species introduced into a grassland community. 2 We used the National Vegetation Classification and distribution maps to define the local pool of grassland species suited to our experimental site. From this we selected five pairs of congeneric species. One member of each pair was native to the experimental site (‘present’ species) and one member was represented in the local species pool but absent from the community (‘absent’ species). 3 If community assembly processes are deterministic, then absent species should underperform present congeners. Any other result would imply that chance, dispersal limitation or historical contingency plays a role in community assembly. Our experimental arrangement allowed us to assess the consistency of comparisons across different grazing treatment combinations. 4 Two field experiments were undertaken. The first investigated the effects of dispersal limitation by introducing seed of congeners and monitoring performance. The second investigated the survival of congeners introduced as small plants at the early stages of growth in ‘plugs’. A laboratory germination test and a combined analysis of both field experiments were also undertaken. 5 Some species showed markedly different responses from their congeners in the two field experiments. Overall, the present species did not consistently perform better than their absent congeners. This indicates that the study community bears the strong imprint of dispersal limitation or other historical factors, a finding consistent with results previously obtained for the same community using a trait-based model. 6 We conclude that the accuracy of predictions about community composition will be limited by the success with which factors such as propagule dispersal and differential effects of seed predation and herbivory can be modelled. We predict the circumstances under which modelling of community assembly is most likely to be successful.

In short-lived monocarpic perennials flowering probability depends on size and relative growth. Reproducing at a smaller size results in a higher prereproductive survival and shorter generation time but also may lead to lower fecundity. Conversely, reproducing at a larger size allows greater fecundity but leads to higher mortality during the prolonged vegetative period. Herbivory may influence the above described relationships via alterations in size at reproduction and survival. Here we use field data to explore in detail the reproduction of the short-lived monocarpic perennial C. vulgare under seasonal grazing. Vegetative plants were marked in paddocks with and without winter grazing, and their size, growth, and flowering status were recorded during a growing season in a field grazing experiment. Grazing increased both survival of vegetative plants and flowering probability, but it did not affect flowering size. The increase in flowering probability is a result of differential plant growth and size and may be related to greater resource availability, including light (necessary for flowering induction in C. vulgare) in grazed paddocks. Fil: Marco, Diana Elizabeth. Universidad Nacional de Córdoba. Facultad de Ciencias Agropecuarias; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Silvertown, Jonathan. The Open University; Reino Unido

Summary 1 Because related species tend to be ecologically similar, it has long been recognized that the phylogenetic relationships between species in a community are likely to influence its structure. Previous investigations of the influence of phylogeny on community structure have generally shown that congeneric species are more frequent than would be expected if communities were randomly assembled. Because, under most theories of coexistence, stable coexistence requires ecological differences between species, the apparent excess of coexisting congeners suggests that trait differences important to coexistence must be recently evolved, arising near the tips of phylogenetic trees. 2 We test this hypothesis in two meadow plant communities for which we have good evidence of niche segregation along axes defined by hydrological conditions. 3 Niche overlaps were calculated and compared against null models for (i) all species in the community, (ii) species within genera, (iii) genera within families, (iv) families within the eudicots and monocots, and (v) eudicots and monocots. 4 Against expectation, we found that community structure was the cumulative result of niche separation arising at a range of phylogenetic levels and therefore that niche differences have been accumulated through the evolutionary history of species.

We investigated two congeners that are biologically very similar but show differences in large- and small-scale distribution.Cirsium vulgare is a widespread species in Britain and Europe, whilstC. eriophorum is of much more restricted distribution generally restricted to soils over limestone. At the small spatial scale of tens of metres, however,C. vulgare again appears to show differences in niche occupancy, being present in agriculturally-improved grasslands whereC. eriophorum is typically lacking. We investigated niche differences in the two species using field data, and also compared plant performance under a range of different experimental conditions. The results shed light on the ecology of the two species and, more widely, on the use of species’ traits to make predictions about community assembly. The view that the twoCirsium species show niche differences is supported, and this is consistent with trait differences betweenC. eriophorum andC. vulgare. These differences are, however, only revealed by close examination. When attempting to link trait differences with differences in habitat or niche occupancy, the selection of traits for examination must be based on a detailed knowledge of the species in the field.

Subalpine forests in North America and Japan dominated by Abies spp. show an unusual pattern of regeneration in which recruitment is confined to a narrow window in time and space that just precedes the death of the largest trees. Previously, Silvertown suggested that selection in such forests should favour delayed reproduction. A graphical model supporting this prediction also suggests that trees in 'normal' forests should benefit from precocious reproduction when they are taller than their neighbours, while trees in wave populations should not. Here, we present a field test of the two hypotheses based upon a comparison of the life history of trees in adjacent wave and non-wave populations at Whiteface Mountain, New York. The results show that reproduction commences at a similar age in both kinds of forest, and that trees in the subalpine zone reproduce later and die earlier than conspecifics in lowland forests. The failure of the first hypothesis can be explained by modifying our original assumptions about how reproductive costs and benefits vary with age in the model. In our test of the second hypothesis, we find that the reproductive behaviour of individual trees in the two kinds of forest is different and consistent with our prediction. Phenotypic plasticity for age at first reproduction appears to be present only in the population where it is adaptive.

1 Gaps in the forest canopy allow establishment of many understorey species. As gaps close, their demographic behaviour is affected by changing environmental conditions. A demographic analysis of Primula vulgaris populations in forest patches with varying canopy openness was carried out to investigate the effect of forest canopy dynamics on the demography of this forest understorey herb. 2 Eight P. vulgaris populations across a range of light conditions were studied in three deciduous temperate forests in Britain. Demographic data were collected in each population during summer 1992, 1993 and 1994, and stage-based population projection matrices were built to describe their demography for two growth periods (1992–93 and 1993–94). 3 Population growth rate (λ) varied from 0.97 to 1.98 in 1992–93, and from 0.88 to 1.23 in 1993–94. There was a significant positive correlation between λ and diffuse light for the two periods studied: generally, population growth rate was lower in darker patches, implying that populations decline as the canopy closes. Lower λ-values in 1993–94 were the result of increased mortality and decreased fecundity compared to 1992–93. In most cases the observed population structures were significantly different from the calculated stable-stage distributions. 4 Sensitivity analysis showed that λ was most sensitive to changes in matrix transitions that corresponded to growth from small to adult stages, especially those that implied fast growth. Elasticity analysis revealed that the contribution of fecundity elements to λ was small, but populations in brighter patches showed comparatively higher fecundity elasticities than populations under the closed canopy. High elasticities corresponded to the entries that implied growth (especially in brighter patches) or persistence in adult categories (predominantly in darker patches). 5 The observed variation in the demography of P. vulgaris in different patches along the forest regeneration cycle stresses the importance of studying the variation in plant population dynamics across the range of habitat patches in which the plant naturally occurs.