The impact of low-head dams on fish species richness in Wisconsin, USA

Communities that live in linear systems such as streams or riparian zones may be particularly sensitive to changes in habitat connectivity. I examined the relationship between environmental variables, the number of low-head dams downstream of each reach, and fish species richness in first-order streams in Wisconsin, USA. The analysis was based on an extensive database of 13 628 localities and ∼180 000 individual collection records. The environmental variables included climate, elevation, distance from nearest town, and network-specific measures such as link number (the number of first-order tributaries upstream of the current reach), downstream link number, stream order, and fractal dimension. The link number of the downstream reach was the strongest broad-scale correlate of fish species richness in first-order streams. Path analysis of a simple directed graph suggested that, although downstream dams have a significant effect on fish species richness, this effect is small by comparison to the influence of water quantity and summer maximum temperatures. From a management perspective, the results imply that modifications of water volume and temperature are greater threats to fish communities than the decrease in connectivity that results from low-head dams.