Herbivory can mitigate, but not counteract, the positive effects of warming on the establishment of the invasive macrophyte Hydrilla verticillata

Abstract

Hydrilla verticillata is a submerged, rooted macrophyte native to Asia and Australia, but currently attains broad distribution across all continents. Its success as an invasive species depends on the simultaneous influence of abiotic and biotic factors on different components of its performance. We conducted a factorial experiment to test the short-term responses of Hydrilla, present since 2005 in the upper Parana River (Brazil), to a native herbivore (apple snail Pomacea canaliculata) and increased water temperature, using two different spatial arrangements of macrophyte fragments (one simulating early establishment phase and other simulating late establishment phase). Temperature, herbivory and plant spatial arrangement individually, and in some cases through their interactions, caused changes in the growth likely indicating impacts for the ecological responses of Hydrilla´s establishment. Snail herbivory decreased plant growth thus exerting biotic resistance, while higher temperature increased Hydrilla´s invasiveness. According to our results and other pieces of evidence, invasions of Hydrilla might worsen under the future climate warming scenario, but herbivores might locally mitigate invasion speed or magnitude.