A trophic cascade causes unexpected ecological interactions across the aquatic–terrestrial interface under extreme weather

Abstract

Trophic cascades in the aquatic environment constitute important mechanisms for improving water quality. However, how the presence or non-presence of these trophic cascades may affect interactions across the aquatic–terrestrial interface remains poorly investigated. Pollinators such as bees may be especially vulnerable to changes in water resource quality induced by trophic cascades. Understanding how aquatic trophic cascades affect bees and pollination becomes even more pressing under ongoing climate change due to increased physiological demands for water under extreme weather events. In a novel field experiment combining terrestrial and aquatic mesocosms, we aimed to test how changes in water quality induced by an aquatic trophic cascade affected foraging and growth of bumblebee colonies as well as foraging of solitary bees. While we expected fish predation to reduce top–down control of zooplankton on phytoplankton and thereby, indirectly, induce increased growth of toxic cyanobacteria, we instead found the trophic cascade to induce the formation of algal surface mats that bumblebees used to access water under a severe heat wave and drought. This access to water was associated with higher bumblebee colony reproductive success, growth and weight compared to control colonies with no trophic cascade induced (and hence no algal surface mats). We also found marginal but non-significant effects on oilseed rape yield, but surprisingly with higher yields in the control treatment where bumblebees could not access water. Our results provide new insights on how aquatic trophic cascades can lead to unpredicted ecological interactions across the aquatic–terrestrial interface facilitated by climate change. Our study highlights the importance of water for the fitness of terrestrial ecosystem service providers under altered environmental conditions.