Latitudinal and altitudinal patterns in species richness and mortality factors of the galling sawflies on Salix species in Japan

Species richness of willow species and galling sawflies living on them were examined in latitudinal and altitudinal gradients in six Japanese river systems from Hokkaido to southern Honshu. Mortality factors of gallers including plant based mortality, parasitoids and inquilines during larval development were studied by dissecting sampled galls under a microscope. The association between environmental factors, mortality factors and local diversity of galling sawflies and their willow hosts were studied. Species richness of sawfly gallers and their host plants decreased towards the south. Species richness of gallers was lower in the delta areas at lower altitudes than at higher altitudes. Different mortality factors, plant based mortality, parasitoids or inquilines, showed no significant trends with latitude or altitude. Although some parasitoids showed a weak correlation with latitude and altitude, but overall survival of larvae was not correlated with latitude or altitude. Among sawfly gall types, Pontania proxima-type was distinct by having high plant-based mortality. The observed pattern of increasing diversity with increasing latitude is opposite to that in many other animals and plants. This pattern is unlikely explained by larval survival or different mortality factors since they showed no difference in latitudinal or altitudinal gradient. A possible explanation of the pattern may be the decreasing host plant richness with other host related factors, like increased habitat fragmentation and decreased abundance of host plants towards the south. In addition, leaf flush of host plants and egg laying of galling sawflies might be better synchronised in north with highly seasonal but predictable resource availability.