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Recent work in food web studies has highlighted the importance of terrestrial and aquatic linkages and the trait and density-mediated effects of predators and abiotic conditions. Many studies have focused on species with complex life histories (amphibians and aquatic insects) because they encompass a variety of trophic levels and form important linkages between terrestrial and aquatic food webs. For example, red-eyed treefrog, Agalychnis callidryas eggs are deposited on vegetation overhanging ponds and risk mortality from a variety of terrestrial predators and abiotic stressors. Red-eyed treefrog embryos, however, exhibit phenotypic plasticity in hatching time in response to these terrestrial stressors. As a result, both predators and abiotic conditions can alter the density and phenotype (size) of tadpoles entering the pond. Because tadpoles are important herbivores affecting both total algal biomass and species composition, these terrestrial sources of mortality can spill over to influence ecosystem processes in the aquatic environment. Moreover, the spill-over effects of changes in density could have markedly different effects on ecosystem processes than changes in phenotype. Here we quantify the effects of terrestrial stressors on hatchling phenotypes and survival and the relative magnitudes of the phenotypic and density spillover effects on the aquatic ecosystem. Specifically we use a combination of field monitoring, laboratory and mesocosm experiments to investigate how changes in both density and phenotype of tadpoles entering the pond affect ecosystem function (e.g., primary productivity) via consumption and competition.
Both egg predators and dehydration had strong effects on both hatching success and timing. The largest effects occurred early in the season (July), when dehydration reduced tadpole inputs by an estimated 28% and later (October) little desiccation occurred but snakes reduced inputs by 79%. Over the course of the season the mean age of hatching of embryos from dehydrated clutches was 19% earlier than those from undisturbed clutches followed by embryos from clutches attacked by snakes (12% earlier). In feeding trials per capita ingestion rates increased with tadpole size and decreased with increasing density. However, hatching early had no effect on primary productivity, or zooplankton abundance or diversity. In contrast, increasing initial tadpole densities (e.g., due to arboreal egg predators) reduced primary productivity and also zooplankton abundance. Thus, there is the potential for terrestrial predators to indirectly impact aquatic primary productivity and consumer abundance (i.e., zooplankton), primarily through their consumptive effects, on an organism with a complex life cycle.
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