A functional approach to patch suitability using biomass dynamics: application to a residual population of the white-clawed crayfish

摘要

In France, the white-clawed crayfish (Austropotomabius pallipes) is more and more restricted to head-water river systems and connections between residual populations are increasingly limited. In conservation biology, to address the question of habitat suitability, it is necessary to know if this species functions in a metapopulational context. At present, many studies on habitat are restricted to water parameters. This six-year study on a small forest brook in Normandy (Western France) deals with the quality of a population and its functional stability, using biomass dynamics to assess the patch suitability. A good quality of residual populations appears crucial in constructing conservation reserves, before restoring the connectivity. Upstream, the scarcity of favourable habitats limited the carrying capacity for young-of-the-year. The shallow slope accentuated the temporary deposition of sediments due to forestry activities, which lead to a temporary ecological trap effect. Although this zone should be a population sink, instead it presumably became a source of juveniles for the downstream zone. Downstream, the complexity of the area lead to three times the mean density found upstream, but with a higher inter-annual variability. The production levels were stable in time, as shown by the P/B ratios that reflected processes of compensation and functional regulation. This parameter may be the best indicator of habitat suitability and the population qualities required for conservation planning. The compensating features observed between the two zones made up for the annual variability in vital rates, i.e.: low density and better growth upstream, high density and weaker growth downstream. This leads to a certain stability of the population in the brook as a whole. Therefore, the brook seems to function in a metapopulational context at the local level, representing a patch of sufficient size to buffer local random variation and ensure preservation of the species.