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The effect of resource subsidies on recipient food webs has received much recent attention. The purpose of this study was to measure the effects of significant seasonal seaweed deposition events, caused by hurricanes and other storms, on species inhabiting subtropical islands. The seaweed represents a pulsed resource subsidy that is consumed by amphipods and flies, which are eaten by lizards and predatory arthropods, which in turn consume terrestrial herbivores. Additionally, seaweed decomposes directly into the soil under plants. We added seaweed to six shoreline plots and removed seaweed from six other plots for three months; all plots were repeatedly monitored for 12 months after the initial manipulation. Lizard density ( Anolis sagrei ) responded rapidly, and the overall average was 63% higher in subsidized than in removal plots. Stable-isotope analysis revealed a shift in lizard diet composition toward more marine-based prey in subsidized plots. Leaf damage was 70% higher in subsidized than in removal plots after eight months, but subsequent damage was about the same in the two treatments. Foliage growth rate was 70% higher in subsidized plots after 12 months. Results of a complementary study on the relationship between natural variation in marine subsidies and island food web components were consistent with the experimental results. We suggest two causal pathways for the effects of marine subsidies on terrestrial plants: (1) the \"fertilization effect\" in which seaweed adds nutrients to plants, increasing their growth rate, and (2) the \"predator diet shift effect\" in which lizards shift from eating local prey (including terrestrial herbivores) to eating mostly marine detritivores.

AIM: We ask empirically how the strength of the three major interactions – predation, competition and mutualism – changes with increasing island area. We review and expand current theory concerning these relationships. LOCATION: Data for evaluating this question come from several Bahamian archipelagos and involve both experiments and observations. The latter can be especially long term; in one case data were collected over a 17‐year period. METHODS: We analyse the effect size of the following interactions across a range of island areas: (1) predation by lizards on spiders, (2) competition between two lizard species, (3) competition between two spider species, (4) ant–plant mutualism, and (5) plant–pollinator mutualism. RESULTS: Effect sizes for predation and competition mostly show a hump‐shaped relationship with island area. Effect sizes for ant–plant mutualism were reduced on large islands compared with smaller islands. Germination rate showed a steady increase with island area which we infer to be caused by an increase in pollinator limitation on smaller islands. MAIN CONCLUSIONS: We argue that the effect size–area relation has rather similar aetiologies for predation, competition and to a somewhat lesser extent ant–plant mutualism. Specifically, we suggest that high species diversity, top predators, plant defences and (for predation and competition) spatial heterogeneity with respect to refuges or resource use reduce effect size on large islands, while harsh abiotic conditions, marine subsidies and stochastic events reduce effect size on small islands. Thus, for these interaction types, the greatest effect sizes are observed on intermediate‐sized islands. For plant–pollinator mutualism we suggest that the monotonic increase in effect size with increasing island area is the result of interaction strength being enhanced, rather than weakened, by diversity.

As the environment changes, will species be able to adapt? By conducting experiments in natural environments, biologists can study how evolutionary processes such as natural selection operate through time. We predicted that the introduction of a terrestrial predator would first select for longer-legged lizards, which are faster, but as the lizards shifted onto high twigs to avoid the predator, selection would reverse toward favoring the shorter-legged individuals better able to locomote there. Our experimental studies on 12 islets confirmed these predictions within a single generation, thus demonstrating the rapidity with which evolutionary forces can change during times of environmental flux.

Pulsed resource subsidies can have profound effects on recipient communities. The effects of resource pulses are often mediated by increases in the density of consumer populations. Here we investigate several mechanisms linking experimental pulses of seaweed deposition to population-level responses in the brown anole Anolis sagrei. Subsidized lizards grew approximately 30% faster than lizards in seaweed-removal plots, but there was no effect of seaweed subsidies on survival or body condition. Breeding is strongly seasonal in A. sagrei, resulting in a limited reproductive window of opportunity. Accelerated growth allows subsidized females to reach sexual maturity earlier and thereby exploit more of this window, which is projected to double fecundity in their first year of life. These results show how changes in an individual trait can translate pulses of resource input into reproductive output. Further, they highlight the importance of seasonal timing in mechanistically linking individual-, population- and community-level responses to pulsed resource subsidies.

The effect of environmental change on ecosystems is mediated by species interactions. Environmental change may remove or add species and shift life-history events, altering which species interact at a given time. However, environmental change may also reconfigure multispecies interactions when both species composition and phenology remain intact. In a Caribbean island system, a major manifestation of environmental change is seaweed deposition, which has been linked to eutrophication, overfishing, and hurricanes. Here, we show in a whole-island field experiment that without seaweed two predators--lizards and ants--had a substantially greater-than-additive effect on herbivory. When seaweed was added to mimic deposition by hurricanes, no interactive predator effect occurred. Thus environmental change can substantially restructure food-web interactions, complicating efforts to predict anthropogenic changes in ecosystem processes.

Traditionally, productivity and disturbance have been hypothesized as important determinants of food-chain length. More recently, growing empirical evidence suggests a strong role of ecosystem size. To theoretically explore the effects of basal productivity, disturbance, and ecosystem size on food-chain length, we develop and analyze a metacommunity model of intraguild predation (IGP). The model finds that, when local IGP is weak, increasing basal productivity, weakening disturbance, and increasing ecosystem size will generally increase food-chain length. When local IGP is strong, by contrast, increasing basal productivity or weakening disturbance favors intraguild predators and hinders the coexistence of intraguild predators and intraguild prey, limiting food-chain length. In contrast, increasing ecosystem size can promote coexistence even when local IGP is strong, increasing food-chain length through inserting intraguild prey and changing the degree of omnivory by intraguild predators. Intraguild prey needs to be the superior colonizer to intraguild predators for this to occur. We discuss that these theoretical predictions appear consistent with empirical patterns.

Although abiotic and biotic factors can interact to shape the spatial niche of a species, studies that explore the interactive effects of both at a local scale are rare. We demonstrate that one of the main axes (perch height) characterizing the spatial niche of a common lizard, Anolis sagrei , varies according to the interactive effects of weather and the activity of a larger predatory lizard, Leiocephalus carinatus . Results were completely consistent: no matter how favorable the weather conditions for using the ground (mainly characterized by temperature, humidity, wind speed, rain), A. sagrei did not do so if the predator was present. Hence, great behavioral plasticity enabled A. sagrei to adjust its use of space very quickly. To the best of our knowledge, these results constitute the first field demonstration for anoles (and possibly for other animals as well) of how time-varying environmental conditions and predator presence interact to produce short-term changes in utilization along a major niche axis.

Flows of energy and materials link ecosystems worldwide and have important consequences for the structure of ecological communities. While these resource subsidies typically enter recipient food webs through multiple channels, most previous studies focussed on a single pathway of resource input. We used path analysis to evaluate multiple pathways connecting chronic marine resource inputs (in the form of seaweed deposits) and herbivory in a shoreline terrestrial ecosystem. We found statistical support for a fertilization effect (seaweed increased foliar nitrogen content, leading to greater herbivory) and a lizard numerical response effect (seaweed increased lizard densities, leading to reduced herbivory), but not for a lizard diet-shift effect (seaweed increased the proportion of marine-derived prey in lizard diets, but lizard diet was not strongly associated with herbivory). Greater seaweed abundance was associated with greater herbivory, and the fertilization effect was larger than the combined lizard effects. Thus, the bottom-up, plant-mediated effect of fertilization on herbivory overshadowed the top-down effects of lizard predators. These results, from unmanipulated shoreline plots with persistent differences in chronic seaweed deposition, differ from those of a previous experimental study of the short-term effects of a pulse of seaweed deposition: while the increase in herbivory in response to chronic seaweed deposition was due to the fertilization effect, the short-term increase in herbivory in response to a pulse of seaweed deposition was due to the lizard diet-shift effect. This contrast highlights the importance of the temporal pattern of resource inputs in determining the mechanism of community response to resource subsidies.

The extent to which random processes such as founder events contribute to evolutionary divergence is a long-standing controversy in evolutionary biology. To determine the respective contributions of founder effects and natural selection, we conducted an experiment in which brown anole (Anolis sagrei) lizard populations were established on seven small islands in the Bahamas, from male-female pairs randomly drawn from the same large-island source. These founding events generated significant among-island genetic and morphological differences that persisted throughout the course of the experiment despite all populations adapting in the predicted direction—shorter hindlimbs—in response to the narrower vegetation on the small islands. Thus, using a replicated experiment in nature, we showed that both founder effects and natural selection jointly determine trait values in these populations.

Biological invasions are both a pressing environmental challenge and an opportunity to investigate fundamental ecological processes, such as the role of top predators in regulating biodiversity and food-web structure. In whole-ecosystem manipulations of small Caribbean islands on which brown anole lizards ( Anolis sagrei ) were the native top predator, we experimentally staged invasions by competitors (green anoles, Anolis smaragdinus ) and/or new top predators (curly-tailed lizards, Leiocephalus carinatus ). We show that curly-tailed lizards destabilized the coexistence of competing prey species, contrary to the classic idea of keystone predation. Fear-driven avoidance of predators collapsed the spatial and dietary niche structure that otherwise stabilized coexistence, which intensified interspecific competition within predator-free refuges and contributed to the extinction of green-anole populations on two islands. Moreover, whereas adding either green anoles or curly-tailed lizards lengthened food chains on the islands, adding both species reversed this effect—in part because the apex predators were trophic omnivores. Our results underscore the importance of top-down control in ecological communities, but show that its outcomes depend on prey behaviour, spatial structure, and omnivory. Diversity-enhancing effects of top predators cannot be assumed, and non-consumptive effects of predation risk may be a widespread constraint on species coexistence. Whole-ecosystem manipulations of Caribbean islands occupied by brown anoles, involving the addition of competitors (green anoles) and/or top predators (curly-tailed lizards), demonstrate that predator introductions can alter the ecological niches and destabilize the coexistence of competing prey species.