REVERSED LATITUDINAL GRADIENT IN SPECIES RICHNESS OF SAWFLIES (HYMENOPTERA, SYMPHYTA)

In several taxa regional species richness typically decreases towards the poles. However, the species richness of Old World sawflies (Hymeno ptera, Symphyta) shows the opposite latitudinal trend, so that species richness is highest in the north. Among the forest-dwelling insect gr oups, aphids also make a similar exception to the general pattern. We examine the hypothesis put forward by Dixon et al. (1987) on the rever sed gradient of aphids. Both sawflies and aphids are highly host-speci fic, which is in accordance with the hypothesis. However, other life h istory traits of sawflies do not fit Dixon et al.'s hypothesis. We pro pose that the increase in sawfly species richness is due to the increa se in the diverse community of their principal host plant group, the S alix species. In general, the groups with high species richness in a p articular area should deserve special attention in an analysis of dive rsity and ecosystem properties and functioning. It is possible that sa wflies form such a group in northern boreal forests.