Assembly of non-native fishes in North Patagonian streams: Species occurrence and abundance respond to different spatial scales

Abstract

Ecological communities are structured by combinations of biotic and abiotic factors acting at different spatial scales. However, the relative influence of the different scale-related variables on assemblage composition is poorly understood, despite being key to the effective and efficient management of fluvial ecosystems. We took advantage of the relatively recent and well-studied history of salmonid introductions in Patagonia to evaluate if non-native species show different patterns of association with abiotic factors depending on the spatial scale of the environmental filter. We used a hierarchical approach to characterised environmental variables at the basin and reach scales to assess their influence on the presence, abundance and structure of the salmonid assemblages in breeding streams. We saw no evidence that presence/absence patterns of salmonid distribution were driven by landscape variables, except for those basins with physical environmental barriers to colonisation. However, we did find evidence for relative abundances being influenced by climatic and geomorphological variables (e.g., precipitation and relief). Our results do not support a scenario in which any of the salmonid species modulates the distribution of the other species, suggesting that interference has played only a minor role in determining current fish distribution in fluvial systems of the region. Instead, current patterns of presence and abundance of salmonids are best explained as the product of environmental filters. Our findings contribute to our understanding of the ecology of individual species and provide insight into the mechanisms structuring fish assemblages in Southern Hemisphere's lotic systems.