Climate change causes species range shifts and potentially alters biological invasions. The invasion of European earthworm species across northern North America has severe impacts on native ecosystems. Given the long and cold winters in that region that to date supposedly have slowed earthworm invasion, future warming is hypothesized to accelerate earthworm invasions into yet non-invaded regions. Alternatively, warming-induced reductions in soil water content (SWC) can also decrease earthworm performance. We tested these hypotheses in a field warming experiment at two sites in Minnesota, USA by sampling earthworms in closed and open canopy in three temperature treatments in 2010 and 2012. Structural equation modeling revealed that detrimental warming effects on earthworm densities and biomass could indeed be partly explained by warming-induced reductions in SWC. The direction of warming effects depended on the current average SWC: warming had neutral to positive effects at high SWC, whereas the opposite was true at low SWC. Our results suggest that warming limits the invasion of earthworms in northern North America by causing less favorable soil abiotic conditions, unless warming is accompanied by increased and temporally even distributions of rainfall sufficient to offset greater water losses from higher evapotranspiration.
Bibliographical noteFunding Information:
We thank Cindy Buschena and Susan Barrott for their help during the experiment, Kirk R. Wythers for drawing Figure S1. Nico Eisenhauer gratefully acknowledges funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation; Ei 862/1, Ei 862/ 2). Further support came from the German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, funded by the German Research Foundation (FZT 118). The B4WarmED project has been funded by the US Department of Energy (Grant No. DE-FG02-07ER64456), College of Food, Agricultural and Natural Resource Sciences (CFANS), and Wilderness Research Foundation at the University of Minnesota, and the Minnesota Environment and Natural Resources Trust Fund.