CHANGES IN OCCUPANCY, NICHE BREADTH AND ABUNDANCE OF 3 GYRINUS SPECIES AS THEIR RESPECTIVE RANGE LIMITS ARE APPROACHED

Three widely distributed Gyrinus species were studied in seven differe nt habitat types over a wide range of latitudes in northern Europe. Al l three species have a broad habitat range and are opportunistic explo iters of temporary and fluctuating habitats. The distribution range of the two southern species, G. natator and G. substriatus, overlaps wit h that of the boreal-subarctic G. opacus in south-central Sweden, crea ting an inland area of sympatry, where all three species coexisted in 10% of the waters sampled. For the two southern species occupancy incr eased with distance from their respective range limits. The occupancy level of G. opacus in its southernmost inland zone was higher than tho se of the other two species in their respective northernmost zones. Le vels of maximum occupancy were similar for all species. Patterns of ha bitat occupancy partly differed within and between species. Although t emporary pools, which represented the most unpredictable habitat, were highly frequented in the south by G. substriatus, occupancy levels as sociated with them declined sharply before reaching the northern range limit. The niche breadth of the two southern species increased as the distance from their respective northern range limits increased and wa s broadest for G. substriatus in southern Sweden, north of its geograp hical range centre (southern Poland), where pollution has no doubt sev erely affected the original distribution. The habitat range of G. opac us did not change as its southern range limit was approached, but fewe r habitat types appeared to be used in the very far north. Abundance o f all three species in temporary pools increased with distance from th eir respective range limits, and there was a regional correlation betw een occupancy and abundance for G. substriatus and G. natator in this habitat.