Space partitioning among the wolf spiderPardosa amentata species group in Hokkaido, Japan

Summary There are 4 species belonging to the wolf spiderPardosa amentata species group in Hokkaido. The 4 species have separately occurred in Hokkaido in general, but a few species have sometimes occurred together at the same sites. Their biology at the coexisting sites, with special reference to partitioning of space was studied.P. agraria mainly inhabited fields and occurred widely in Hokkaido.P. hokkaido, P. brebivulva andP. lugubris inhabited the vicinity of forests and occurred in southern Hokkaido and on the side of the Sea of Japan in Hokkaido, all over Hokkaido excent the side of the Sea of Japan in central Hokkaido, and the lowlands and highlands in northern and eastern Hokkaido and mainly the highlands in other regions, respectively. In eastern Hokkaido whereP. brevivulva andP. lugubris have coexisted, as the number of spiders detected and the number of localities of the former were less than those of the latter, it was concluded that the former occurred mainly in southern Hokkaido and the Hidaka region. In southern Hokkaido whereP. hokkaido andP. brevivulva have coexisted, few individuals of both species were collected together at the same sites, and they were distributed allotopically. In short, the partitioning of space among theP. amentata species group occurring in Hokkaido was clear. From investigations on the coexisting sites and some other reports, it was considered that food, life cycle and diel activity periodicity amongPardosa occurring in Hokkaido were similar to one another. This may be one of the causes of the clear space partitioning amongPardosa. SincePardosa is a raptorial predator that does not weave but wanders on the ground and uses space horizontally, available prey resource forPardosa would be limited. It was therefore conjectured that the spider would be prone to space segregation, even if the spider could segregate the other niche dimensions. The partitioning of space betweenP. agraria andP. hokkaido inhabiting distinct habitats was loose when it was compared with that between species inhabiting similar habitats. As most of the spiders observed in the overlapping areas were females with egg cocoons, which required little food, the reduced feeding was considered to have allowed the loose partitioning.