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Influences of species introductions are context dependent and can extend from nutrient pools and fluxes to predator populations and behavior. Trout stocking is a classic example of a globally pervasive introduction where a single species can cause whole system changes and cross-boundary effects. Omnivorous trout consume the larvae of aquatic insects, reducing insect emergence that otherwise provides a nutrient and energy subsidy to terrestrial systems. In this dissertation I first ask whether multiple stressors (trout presence and lake temperature) have interactive effects on aquatic insect emergence. I then ask how the presence of trout affects nitrogen patterns in near-shore terrestrial zones. As a testing site for these questions I use a set of lakes in California’s Sierra Nevada where trout have been both introduced and removed over the past century, establishing a natural experiment. Across paired lakes with and without fish I examine the effects of trout presence on within-lake communities and across the aquatic-terrestrial ecotone. I conducted multiple years of field surveys in which I sampled insect communities, soil and lake sediment characteristics, and stable isotope signatures of terrestrial soils and plants, and quantified algal biomass in the littoral benthic zone of each lake. I find that insect emergence is significantly lower from lakes with trout, as expected, but also from warmer lakes – and that the impact of trout is lessened in warmer lakes. My data confirm that trout are associated with reduced insect deposition to terrestrial near-shore areas. Despite this reduction in insect-vectored nutrients, I also find a positive relationship between trout presence and nitrogen content of both soils and plants. By producing a detailed record of trout impacts 1) across climatic conditions and 2) to multiple trophic levels, in this dissertation I highlight the context dependencies of introduced species effects and expand understanding of how trout alter la

Environmental disturbances and biodiversity turnover in the Anthropocene increasingly threaten species interactions – the scaffolding of ecosystems. Plant and pollinator communities and their interactions provide ecosystem functions and services (i.e., pollination) that are biologically and economically critical. Species interactions are particularly vulnerable to environmental disturbances because each interacting group can be individually impacted with a resulting loss of the interaction. Importantly, environmental disturbances that threaten biodiversity rarely occur in isolation and, rather, occur in combination with regional and local scale effects. To protect ecosystem functions and services, and thus, the interactions between species, we must first collect a realistic understanding of the multiple stressors that can affect species, interactions, and ecosystem structure. This dissertation takes an empirical approach to understanding plant and pollinator communities, investigating: (1) the relative effects of multiple environmental attributes on plant-pollinator network structure, (2) the relative effects of regional climate and species introduction across ecosystem boundaries on plant community diversity, and (3) the bottom-up or top-down effects of regional and local stressors on plant reproduction.Through these studies, we find that the amount of available habitat and the quality of habitat is vital for the diversity of both plant and pollinator communities and the resulting complexity and frequency of their interactions. Importantly, habitat quality could be especially influential where resources are limited. These findings support the integration of habitat quality into conservation planning in order to mitigate the effects of multiple stressors on species interactions across landscapes. This work confirms conservation goals such as prioritizing habitat size in reserve planning, but also encourages a more comprehensive examination of the impacts of introduc

Dominance often indicates one or a few species being best suited for resource capture and retention in a given environment. Press perturbations that change availability of limiting resources can restructure competitive hierarchies, allowing new species to capture or retain resources and leaving once dominant species fated to decline. However, dominant species may maintain high abundances even when their new environments no longer favour them due to stochastic processes associated with their high abundance, impeding deterministic processes that would otherwise diminish them. Here, we quantify the persistence of dominance by tracking the rate of decline in dominant species at 90 globally distributed grassland sites under experimentally elevated soil nutrient supply and reduced vertebrate consumer pressure. We found that chronic experimental nutrient addition and vertebrate exclusion caused certain subsets of species to lose dominance more quickly than in control plots. In control plots, perennial species and species with high initial cover maintained dominance for longer than annual species and those with low initial cover respectively. In fertilized plots, species with high initial cover maintained dominance at similar rates to control plots, while those with lower initial cover lost dominance even faster than similar species in controls. High initial cover increased the estimated time to dominance loss more strongly in plots with vertebrate exclosures than in controls. Vertebrate exclosures caused a slight decrease in the persistence of dominance for perennials, while fertilization brought perennials' rate of dominance loss in line with those of annuals. Annual species lost dominance at similar rates regardless of treatments. Synthesis. Collectively, these results point to a strong role of a species' historical abundance in maintaining dominance following environmental perturbations. Because dominant species play an outsized role in driving ecosystem processes, thei

Compared to other animal movements, prospecting by adult individuals for a future breeding site is commonly overlooked. Prospecting influences the decision of where to breed and has consequences on fitness and lifetime reproductive success. By analysing movements of 31 satellite- and GPS-tracked gull and tern populations belonging to 14 species in Europe and North America, we examined the occurrence and factors explaining prospecting by actively breeding birds. Prospecting in active breeders occurred in 85.7% of studied species, across 61.3% of sampled populations. Prospecting was more common in populations with frequent inter-annual changes of breeding sites and among females. These results contradict theoretical models which predict that prospecting is expected to evolve in relatively predictable and stable environments. More long-term tracking studies are needed to identify factors affecting patterns of prospecting in different environments and understand the consequences of prospecting on fitness at the individual and population level.

Dominance often indicates one or a few species being best suited for resource capture and retention in a given environment. Press perturbations that change availability of limiting resources can restructure competitive hierarchies, allowing new species to capture or retain resources and leaving once dominant species fated to decline. However, dominant species may maintain high abundances even when their new environments no longer favour them due to stochastic processes associated with their high abundance, impeding deterministic processes that would otherwise diminish them. Here, we quantify the persistence of dominance by tracking the rate of decline in dominant species at 90 globally distributed grassland sites under experimentally elevated soil nutrient supply and reduced vertebrate consumer pressure. We found that chronic experimental nutrient addition and vertebrate exclusion caused certain subsets of species to lose dominance more quickly than in control plots. In control plots, perennial species and species with high initial cover maintained dominance for longer than annual species and those with low initial cover respectively. In fertilized plots, species with high initial cover maintained dominance at similar rates to control plots, while those with lower initial cover lost dominance even faster than similar species in controls. High initial cover increased the estimated time to dominance loss more strongly in plots with vertebrate exclosures than in controls. Vertebrate exclosures caused a slight decrease in the persistence of dominance for perennials, while fertilization brought perennials' rate of dominance loss in line with those of annuals. Annual species lost dominance at similar rates regardless of treatments. Synthesis. Collectively, these results point to a strong role of a species' historical abundance in maintaining dominance following environmental perturbations. Because dominant species play an outsized role in driving ecosystem processes, t

The frequency of high-severity “mega-fires” has increased in recent decades, with numerous consequences for forest ecosystems. In particular, small mammal communities are not only vulnerable to post-fire shifts in resource availability, but also play a critical role in forest function. Inconsistencies in prior observations of small mammal community response to fire severity underscore the importance of examining mechanisms regulating fire severity effects on post-fire recovery. Here, we compare small mammal abundance, diversity, and community structure among habitats burned at varying fire severity, and use vegetation characteristics and small mammal functional traits to predict community response to fire severity. We captured 544 small mammals at 27 sites in three fire severity categories (unburned, low-moderate severity, and high severity) three years after a mega-fire in the California Sierra Nevada. We measured five vegetation variables known to predict small mammal communities, and determined three small mammal functional traits associated with resource use for the captured species. Using a model-based fourth-corner analysis, we examined how interactions between vegetation variables and small mammal traits regulated post-fire small mammal community structure among fire severity categories. We found that overall small mammal abundance remained similar after fire, although diversity decreased and community structure shifted. Differences between unburned and low/moderate-severity sites were generally minimal while differences between unburned and high-severity sites were large. Three highly correlated fire-dependent vegetation variables (density of live trees, percent litter cover, and percent shrub cover) as well as volume of soft coarse woody debris were most associated with small mammal community structure. Furthermore, we found that interactions between these vegetation variables and three small mammal resource-use traits (feeding guild, primary foraging mode

Marine protected areas (MPAs), particularly large MPAs, are increasing in number and size around the globe in part to facilitate the conservation of marine megafauna under the assumption that large-scale MPAs better align with vagile life histories; however, this alignment is not well established. Using a global tracking dataset from 36 species across five taxa, chosen to reflect the span of home range size in highly mobile marine megafauna, we show most MPAs are too small to encompass complete home ranges of most species. Based on size alone, 40% of existing MPAs could encompass the home ranges of the smallest ranged species, while only < 1% of existing MPAs could encompass those of the largest ranged species. Further, where home ranges and MPAs overlapped in real geographic space, MPAs encompassed < 5% of core areas used by all species. Despite most home ranges of mobile marine megafauna being much larger than existing MPAs, we demonstrate how benefits from MPAs are still likely to accrue by targeting seasonal aggregations and critical life history stages and through other management techniques.