Influence of beaver ponds on the macroinvertebrate benthic community in lowland brooks

Beavers (Castor fiber) alter stream ecosystems by dam building resulting in a lower stream velocity, retention of sediments and organic matter as well as modifying physical, chemical and geomorphological conditions in these streams. The effects of beaver dams on invertebrate benthic communities were examined in two semi-natural lowland brooks. For this purpose, beaver ponds and reference sections upstream and downstream of each pond were sampled. Mollusca, Crustacea and five orders of aquatic insects were analyzed according taxa richness, abundances, micro-habitat preferences and feeding types to characterize the macroinvertebrate communities. Detailed data in downstream sections and taxa-related parameters of insects (upstream and downstream sections) were analyzed first time. The abundance of Trichoptera, Plecoptera and Crustacea as well as taxa numbers of EPT taxa decreased significantly in the ponds compared to the free-flowing sections. Odonata and Ephemeroptera did not respond in abundance but in a change of the species composition, because lentic species replaced the lotic species in the impoundment section. Only the number of molluscs increased in the ponds. Regarding the microhabitat preferences, lithal dwellers dominated in all free-flowing sections, whereas its proportion decreased in the ponds. On the opposite, pelal dwellers increased in the impounded area. Significant differences were also found in proportions of shredders and passive filter feeders (decreasing in ponds), whereas predators increased in the ponds compared to the downstream section of the brooks.