Non-native species are among the major drivers of species extinctions and thus, loss of biodiversity, especially in freshwater ecosystems. In the European Union alone, 12,122 nonnative species have been reported so far (DAISIE European Invasive Alien Species Gateway 2013). One often overlooked hotspot for non-native species are thermally influenced freshwaters (TIFs) whose thermal conditions are altered by anthropogenic discharges of heated waters from power plants and other sources. Especially in temperate regions there are growing numbers of documented examples of TIFs where non-native species of tropical origin have established reproducing populations. The main introduction pathways of these exotic species are ornamental trade and aquarium hobbyists. Thermally altered aquatic systems with temperatures above climate change projections offer a great opportunity to study the interrelatedness of climate change and biological invasions, as they may represent semi-natural “laboratories”.
Case study: Gillbach in Germany
Artificially heated aquatic systems are ideal (semi-)natural experimental grounds, as they often harbor high numbers of exotic and newly-introduced species living in temperature conditions that correspond to future climate change projections. The Gillbach/Erft river system in Germany receives warm water influx from a nearby power plant and water temperatures never drop below 19°C all year round. Here, thermotolerant native species have co-occured with neotropical guppies (Poecilia reticulata) and convict cichlids (Amatitlania nigrofasciata) for more than 40 years. During harsh German winters, tropical species in the Gillbach only find suitable temperatures in the core area and we thus expect them to recolonize the more peripheral parts once temperatures rise during spring and early summer. In theory, individuals of the migrating part of a population might exhibit a correlated suit of behaviors/phenotypes that facilitate the colonization of new habitats (‘invasion syndrome’). We predicted guppies recolonizing downstream areas should differ in their behaviors from those that reside at the core area. To test this prediction, we caught guppies along the Gillbach in early spring, summer and fall and measured several personality traits (boldness, sociability, activity). We found high consistency in both average trait expressions as well as individual variation in all traits between seasons and along the river. Although behavioral traits were correlated, guppies from more peripheral sites did not show a special ‘invasion syndrome’.