Prey subsidy or predator cue? Direct and indirect effects of caged predators on aquatic consumers and resources

Abstract

The non-consumptive effects of predators on prey can affect prey phenotypes, potentially having important consequences for communities due to trait-mediated indirect interactions. Predicting non-consumptive effects and their impacts on communities can be difficult because predators can affect resources directly through nutrient cycling and indirectly by altering prey resource use, which can lead to complex interactions among resources and consumers. In this study we examined the effects of caged dragonfly predators on aquatic resources in the presence and absence of two focal herbivores, the tadpoles of Neotropical tree frogs Agalychnis callidryas and Dendropsophus ebraccatus. We crossed the presence/absence of caged dragonflies with four tadpole treatments: no tadpoles, each tadpole species alone, and both species together to examine interactions among tadpole composition, predator presence, and time on tadpole growth, resources, and zooplankton abundances. Predator effects on growth changed through ontogeny and was species-dependent. Predators initially reduced then dramatically increased A. callidryas growth, but had no effect on D. ebraccatus. Predators also increased the abundances of both periphyton and phytoplankton. However, there was no evidence of a trait-mediated trophic cascade (i.e., tadpole by predator interaction). Instead, nutrients from prey carcass subsidies likely played an increasingly important role in facilitating resources, and shaping tadpole growth, competitive interactions, and zooplankton abundances through time. In nutrient-poor aquatic systems the release of nutrients via the consumption of terrestrially derived prey items by aquatic predators may have important impacts on food webs by facilitating resources independent of the role of trait-mediated trophic cascades.

Conspecific density affects predator‐induced prey phenotypic plasticity

“…Therefore, we expect that risk assessment should be more evident in prey traits that match the types of predators faced by the prey, avoiding unnecessary defense costs at high density. Our results highlight that experiments that systematically manipulate prey conspecific density could be as effective as experiments that manipulate caged predators (Peacor andWerner 2001, Costa andVonesh 2013) or the concentration of predation cues (Fraker 2008) to generate a gradient of predation risk. The present study, therefore, has important implications for evaluating past studies and for the design of future experiments.…”

A multifaceted trophic cascade in a detritus‐based system: density‐, trait‐, or processing‐chain‐mediated effects?

“…The most direct evidence for processing chain effects arises when detritus generated by predators produces effects on microbial growth in the absence of intervening bacteriovores (prey) (Costa and Vonesh 2013). Experiment I shows that bacterial abundance increased from day 7 to day 14 in the infusion-alone controls ( IA , Fig.…”

“…Second, predator cues may stimulate costly defensive traits (e.g., behavioral changes) in prey that indirectly impact lower trophic levels (trait mediated indirect interaction, or TMII ) (reviewed by Werner and Peacor 2003). Third, predator consumption of prey may produce additional detritus in the form of predator feces, excreted nitrogenous waste, and partially eaten prey, releasing nutrients that fuel the growth of the basal trophic level, a phenomenon characterized as ‘nutrient cycling’ in systems dominated by periphyton and phytoplankton (Costa and Vonesh 2013). This effect of predator feeding is likely in detritus-based systems, where bacteria are the basal trophic level, and it is better described as a “processing chain interaction” (PCI) (Heard 1995).…”

“…The scarcity of such comprehensive studies is surprising, given that detritus-based systems would be ideal for such investigations of trophic cascades and that detritus plays a prominent role in many food webs (Moore et al 2004). Determining whether density-, trait-, and processing chain-mediated interactions affect trophic cascades requires study systems in which we can investigate all of these effects simultaneously, with and without the presence of the intermediate trophic level (Costa and Vonesh 2013). …”

“…Organic detritus, mainly decomposing plant parts and arthropod carcasses, forms the base of the food web and bacteria play a key role in converting that detritus into biomass edible by consumers (Kitching 2000, Moore et al 2004, Yee et al 2007a, b). Although it is clear that detritus-derived productivity affects abundances of decomposers and consumers within natural and artificial containers (e.g., Yee et al 2007a, b; Murrell and Juliano 2008), few studies have tested the three pathways by which predators in a bacteria-bacteriovore-predator trophic system may affect bacterial abundance (see Costa and Vonesh 2013). Human-made containers are an ideal system for investigating detritus-based trophic cascades, because they are small, increasing the potential impact of a single predator on lower trophic levels.…”