Rainfall and hydrological stability alter the impact of top predators on food web structure and function

Abstract

Climate change will alter the distribution of rainfall, with potential consequences for the hydrological dynamics of aquatic habitats. Hydrological stability can be an important determinant of diversity in temporary aquatic habitats, affecting species persistence and the importance of predation on community dynamics. As such, prey are not only affected by drought-induced mortality but also the risk of predation [a non-consumptive effect (NCE)] and actual consumption by predators [a consumptive effect (CE)]. Climate-induced changes in rainfall may directly, or via altered hydrological stability, affect predator-prey interactions and their cascading effects on the food web, but this has rarely been explored, especially in natural food webs. To address this question, we performed a field experiment using tank bromeliads and their aquatic food web, composed of predatory damselfly larvae, macroinvertebrate prey and bacteria. We manipulated the presence and consumption ability of damselfly larvae under three rainfall scenarios (ambient, few large rainfall events and several small rainfall events), recorded the hydrological dynamics within bromeliads and examined the effects on macroinvertebrate colonization, nutrient cycling and bacterial biomass and turnover. Despite our large perturbations of rainfall, rainfall scenario had no effect on the hydrological dynamics of bromeliads. As a result, macroinvertebrate colonization and nutrient cycling depended on the hydrological stability of bromeliads, with no direct effect of rainfall or predation. In contrast, rainfall scenario determined the direction of the indirect effects of predators on bacteria, driven by both predator CEs and NCEs. These results suggest that rainfall and the hydrological stability of bromeliads had indirect effects on the food web through changes in the CEs and NCEs of predators. We suggest that future studies should consider the importance of the variability in hydrological dynamics among habitats as well as the biological mechanisms underlying the ecological responses to climate change.

Interactive effects of climate change and biodiversity loss on ecosystem functioning

“…Negative effects of drought on microbial activity have been already documented in previous studies using tank bromeliad ecosystems (Brandt et al 2015, Marino et al 2017, as well as in streams and soils (Bruder et al 2011, Vogel et al 2013. Specifically, methane concentration in the water within bromeliad tanks increased with litter diversity, but only under ambient rainfall.…”

“…For all fixed-effects models, we used bromeliad maximum volume at the end of the experiment as a covariate, since bromeliad size directly determines how rainfall affects the depth of standing water within bromeliad axils (Marino et al 2017). For all fixed-effects models, we used bromeliad maximum volume at the end of the experiment as a covariate, since bromeliad size directly determines how rainfall affects the depth of standing water within bromeliad axils (Marino et al 2017).…”

“…Tank bromeliads have been increasingly used as model ecosystems to test many ecological hypotheses (Farjalla et al 2012, 2016, 2017, Marino et al 2017. Tank bromeliads have been increasingly used as model ecosystems to test many ecological hypotheses (Farjalla et al 2012, 2016, 2017, Marino et al 2017.…”

“…We predict that (1) litter diversity will reduce the sensitivity of the bromeliad ecosystem to altered rainfall. If less frequent rainfall slows the microbial conditioning of some litter species (Jentsch et al 2011, Santonja et al 2015, Marino et al 2017, then litter diversity-decomposition relationships may weaken due to the diminishing contribution of complementarity effects. We also predict that (2) altered rainfall will shift the mechanisms underlying the litter diversity-decomposition relationship from complementarity to dominance.…”

Rainfall changes affect the algae dominance in tank bromeliad ecosystems

“…For example, increases in rainfall can raise the water level in tank bromeliad ecosystems [30] and it can favor the light entrance in the tank or speed up litter decomposition which benefits algae dominance. Controversy, rainfall itself can dilute or even overflow the water in the bromeliad tanks which may decrease nutrients concentration and algae in the water.…”