Tree cover makes for lower fish diversity

Posted on March 27, 2017

A pervasive feature of lowland streams, especially in areas where land has been cleared for agriculture or urban development, is a decrease in forested cover from the headwaters to the mouth. The presence or absence of streamside tree cover has a profound influence on stream ecology. Streams running through undisturbed forests typically contain large woody debris, which creates microhabitats that vary in depth and substrate type, and the shade provided by overhanging trees minimises temperature fluctuations. In contrast, open canopy streams receive more light and often more nutrients (from farmland runoff). Light and nutrients both stimulate the growth of macrophytes and periphyton, which in turn provide food and habitat for invertebrates and fish. To explore the effects of riparian cover on the structure and composition of fish assemblages, 24 stream segments, each 50 m long and containing pool and riffle habitats, were sampled in streams running through agricultural areas in Jutland, Denmark. Half of the streams ran through riparian forest and had less than 5 % macrophyte cover, and the remainder were open streams with more than 40 % macrophyte cover. Fish were sampled in summer using multiple-pass electrofishing. On average, open-canopy streams were deeper, with lower width:depth ratios and less gravel in the sediment. Analysis revealed that open canopy streams tended to harbour smaller fish (the most common being the three-spined stickleback, Gasterosteus aculeatus) and richer, more diverse fish communities than those in closed canopy streams. Of the 21 fish species recorded in the study, 20 were found in open streams but only 10 in closed streams. By creating complex patterns of variation in velocity and habitat structure, and by boosting instream production, the higher macrophyte densities in open streams were probably mainly responsible for these results. Also, by increasing the resistance to flow they may have also helped to generate the greater depths and habitat volumes observed in open canopy streams. The findings suggest that riparian vegetation may need to be managed differently depending on the main objective (e.g., increasing fish richness or reducing nutrient inputs).

Reference: Teixeira-de Mello, F. et al. 2016.
Influence of riparian forests on fish assemblages in temperate lowland streams. Environmental Biology of Fishes 99, 133–144.