Are non-native species larger in invaded ranges

Background/Question/Methods

Biological invasions are a serious modern ecological problem with many negative impacts to invaded communities. However, an invasion can be considered a natural experiment and allows for testing different hypotheses. Body size is an important trait of organisms that can influence, for instance, dispersal ability, demographic parameters, and home range. Current literature indicates a tendency for invaders’ body size to be larger in non-native habitats. This tendency can be explained by an increase in resources utilization and/or adecrease inparasitism rates and competitive pressures in the non-native habitat. However, human action can also influence body size as a result of human’s propensity to introduce larger bodied individuals, making it difficult to separate human and ecological influences. The invasion of freshwater fishes in the Paraná River floodplain allows us to separate direct human influences from ecological responses. After the construction of the Itaipu Dam, the unintentional union of distinct ichthyofauna provinces occurred as species from the Lower Paraná River basin invaded theUpper Paraná River basin. Using data sets gathered during the period of 2000-2004 in the Upper Paraná River Floodplain (non-native range) and in the Manso Reservoir (native range), for fifteen species, we evaluated the support for the hypothesis stated above. Differences in body size (for each species) and environmental characteristics (conductivity, dissolved oxygen, pH, water temperature and transparency) were tested using ANCOVA and an Analysis of Similarity (ANOSIM), respectively.

Results/Conclusions

Most species were significantly larger in their native habitat. Only Potamotrygon motoro (freshwater stingray) was large in the non-native habitat and Astronotus crassipinis (oscar) and Hypophthalmus edentatus (highwaterman catfish) did not showed differences between habitats. The environmental conditions were dissimilar between native and non-native ranges (R=0.0108. p=0.001). The observed differences in body size can be attributed to differences in environmental conditions as well as possible changes in ecological interactions. However, the use of low-quality resources in the non-native habitat or fisheries pressure for some species may also influence body size.