Testing competing hypotheses to explain the decline of white-clawed crayfish in Switzerland
White-clawed crayfish (Austropotamobius pallipes) are an endangered species in Switzerland. The stocks are declining for unknown reasons. Anthropogenic habitat degradation and the invasion of North American crayfishes have been suggested as causal factors. These invasive exotics are highly competitive and carry the crayfish plague (Aphanomyces astaci), a fungal disease to which they are largely resistant but which is lethal to the European species that lack resistance. Reports suggest that the plague can eradicate entire populations of native crayfish within two weeks and has had a devastating impact on European crayfish stocks since its introduction to Europe in the late 19th century.
This thesis consists of two parts: 1. an experimental study, 2. an analysis of a large data set of geographical distribution data.
(1) Paradoxically, we identified one population of white-clawed crayfish in Switzerland that has persisted for at least four years in sympatry with a plague-carrying population of North American spinycheek crayfish (Orconectes limosus) in Bonfol, Canton Jura.
In order to test the hypothesis that the Bonfol white-clawed crayfish population has evolved resistance an infection-resistance experiment was carried out in which I tried to induce exotic crayfish to shed spores to infect the white-claws. None of the crayfish developed the disease and all 5 PCR-tested white-claws from Etang de Bonfol were plague negative. However, PCR-tests revealed that none of 8 tested Bonfol spinycheeks carried the disease either. Hence, it is possible that the prevalence of the disease in a carrier population is much lower than previously assumed and that none of the 9 spinycheeks that were used was carrying the plague.
(2) My analysis of crayfish distribution data from the last 15 years obtained from CSCF (Centre Suisse de la Cartographie de la Faune) indicate an increase in relative abundance of non-native crayfish between 1991 and 1997 and a subsequent stabilization at around the values of ‘95 to ‘97. Further, spinycheek and signal crayfish (Pacifastacus leniusculus), another exotic invasive, primarily occur at lower elevations whereas white-claws occur primarily at elevations above 500m. In degraded and/or warmer habitats, often found at lower elevations, exotic species seem to have a competitive advantage. Analysis of temporal changes in altitudinal distributions suggests the possibility of a stable distribution equilibrium. The data indicates that the invasion process of the exotic species has come to a halt after 1997. My results of a variance ratio analysis of distribution matrices with 5x5 km and 20x20 km grid size are consistent with the hypothesis that white-claws and spinycheeks live in direct or apparent competition and do not occur in sympatry anywhere else than in Bonfol. The sympatry in Etang de Bonfol hence appears to be a rare exception.
The invasion of exotic crayfishes and its effects on native species is discussed in the light of the Bonfol situation where sympatry of white-clawed and spinycheek crayfish seems possible. I conclude that that the crayfish plague is unlikely to be the main or the sole reason for the decline of native crayfish but one of several possibly interacting factors that mediate the outcome of competition between the species.