Drivers of native and non-native freshwater fish richness across North America: Disentangling the roles of environmental, historical and anthropogenic factors

Aim: A better understanding of native and non-native species responses to environmental conditions, historical processes, and human pressures is crucial in the face of global environmental changes affecting biodiversity. Here, we evaluate the relative roles of environmental, historical and anthropogenic factors in influencing species richness of native and non-native freshwater fishes in watersheds across North America.

Location: North America (exclusive of Mexico).

Time period: Recent.

Major taxa studied: Freshwater fishes.

Methods: We compiled an extensive dataset of native and non-native fish richness in 2,993 watersheds across North America, together with corresponding data for environmental (climatic, geographic), historical and anthropogenic factors. We used variance partitioning and hierarchical partitioning to quantify the relative importance of environmental, historical and anthropogenic factors in explaining richness variation in native and non-native [overall, and by geographic origin (foreign/translocated) and pathway (authorized/unauthorized)] fishes, while accounting for correlations among explanatory variables and spatial autocorrelation.

Results: Overall importance of environmental and anthropogenic factors was greater than historical factors in explaining both native and non-native richness. Precipitation-related factors were more important in explaining native richness, whereas non-native richness was largely associated with temperature-related factors. However, richness related to authorized introductions was less constrained by temperature than unauthorized introductions. Dam density, road density and urbanization gradient were major anthropogenic factors related to non-native richness, yet their relative importance varied among origin- and pathway-based categories.

Conclusions: Our findings indicate different environmental drivers influence native and non-native fish richness patterns in North America. The accumulation of non-native species in watersheds depends on the interaction between environmental conditions and anthropogenic-based processes related to introduction history including geographic origin, introduction pathway, and propagule pressure, where the latter likely plays a major role. Warmer regions with high human population densities and more impoundments are more prone to fish invasions, mostly via unauthorized introductions.