Does the strength of cross‐ecosystem trophic cascades vary with ecosystem size? A test using a natural microcosm

Abstract

Summary The cascading effect of predators on the functioning of adjacent ecosystems can occur when the life cycles of their prey include two ecosystems. However, there has been little consideration of which habitat attributes can modify the strength of these cross‐ecosystem trophic cascades. Habitat size can mediate the strength of predator–prey interactions, and thus affect within‐ecosystem trophic cascades. We hypothesise that similar effects of habitat size might affect cross‐ecosystem trophic cascades. It has been shown that terrestrial predators (e.g. spiders) can capture terrestrial adult insects as they attempt to oviposit in the waters of bromeliads. Such terrestrial predators could therefore alter the trophic structure and functioning of the aquatic food web. If spiders affect an aquatic trophic level that is influenced by bromeliad size, then the strength of the cross‐ecosystem trophic cascade will also depend on bromeliad size. To test this general hypothesis, we manipulated the presence of a funnel‐web spider (Aglaoctenus castaneus, Lycosidae), which builds a single web over water‐filled bromeliads, in bromeliads differing in size and examined effects on the aquatic invertebrate community and on ecosystem functions (decomposition, detrital nitrogen flux). The effects of spiders were largely independent of bromeliad size. Spiders did initiate changes in the trophic structure of aquatic food webs, reducing the biomass of predators, especially damselflies and dytiscid beetles. Spiders also increased decomposition despite having no effect on detritivore biomass or composition. These results are most parsimoniously explained by (i) a behaviourally mediated trophic cascade, whereby damselfly adults avoid bromeliads with spiders, and aquatic detritivores increase rates of detrital processing in the absence of damselfly larvae, and (ii) stimulation of decomposition through nutrients added from spider faeces and prey carcasses. We believe that this is the first study to show that terrestrial predators can affect decomposition by reducing the flux of keystone aquatic predators with complex life cycles.