Comparative suitability of binocular observation, burrow counting and excavation for the quantification of the mangrove fiddler crab Uca annulipes (H. Milne Edwards)

Insect emergence from three different sections of a small mountain brook in Germany was studied: a free-flowing section (`brook'), a section impounded by beavers (`beaver pond') and a beaver dam. The three sections had very different faunas with that of the dam being more similar to that of the brook than that of the pond. The heterogeneity of the fauna was greatest in the dam and lowest in the pond. Median emergence density in the dam was 443 specimens m^–2 d^–1. This was 3.2 times higher than median emergence density in the brook section and 5.5 times higher than median emergence density in the pond section. In particular, the density of emerging Trichoptera (especially filter-feeding species) was increased. Mean annual number of emerging EPTC species was 32.3/trap (779 cm² ground area) in the dam, 18.4/trap in the brook and 11.0/trap in the pond. Emerging biomass was highest in the dam (18.0 – 26.7 g m^–2 yr^–1; 5.0 times higher than in the free flowing section and 5.4 times higher than in the pond section). The composition of a recently broken beaver dam is described by measuring all pieces of wood used in building the dam and analysing sediment samples. The dam consisted of a framework of wood pieces almost entirely of a diameter <5 cm, in which cavities are partly filled with fine sediment (mean organic content = 20%). The `inner surface' of the dam was 2.09 times higher than the dam's ground area and 1.63 times higher than the dam's surface. Beavers, therefore, significantly increase the heterogeneity of faunal composition and productivity of small brooks.