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 similar to 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.