Landscape ecology of the wart-biter Decticus verrucivorus in a patchy landscape

Summary

1. Populations of wart-biter Decticus verrucivorus living on habitat islands in an intensively exploited agricultural landscape in south-eastern Norway were studied. Auditory recordings of singing males indicated that 27 (39%) of 70 habitat islands contained populations in the first year of the study, while 2 years (one wart-biter generation) later the number was 16 (23%). Dispersing male wart-biters were also observed, and the dispersal distances measured. 2. Predictors of island occupancy were identified by applying logistic regression models to the pattern of presence/absence, probing the predictive power of both landscape variables (island size and isolation) as well as various microhabitat quality variables (vegetation, aspect and slope). The isolation of habitat islands relative to surrounding wart-biter populations was measured by two types of connectivity indices: one based on the empirical probability distribution of dispersal distances, the other on the negative-exponential dispersal function. 3. Habitat area and the connectivity indices were by far the best predictors of occupancy. The probability of occupation increased with increasing area and decreasing isolation. Of the unoccupied habitat islands, the ones with observations of single males were less isolated than the ones without. From the pattern of occupancy, the average dispersal distance was estimated to be 40 m, while the observed average dispersal distance of males was 37 m. 4. In addition to the effects of area and connectivity, the probability of occupation was positively influenced by increasing slope to the south, increasing amounts of the plant Achillea millefolium, and decreasing fraction of vegetation lower than 10 cm. The probability of local population extinction increased with declining habitat area and increasing fraction of low vegetation. 5. The situation of the wart-biter in this highly fragmented study area seems to largely fit a metapopulation model: extinctions and absence of wart-biters were adequately predicted by habitat island size and isolation, and observed dispersal distances closely matched estimated dispersal distances from the model.