Background/Question/Methods
The latitudinal diversity gradient is one of the most general patterns in ecology, with only a handful of exceptions. Ichneumonidae, a hyperdiverse family of parasitic wasps, have been considered the most prominent example of the anomalous latitudinal gradient of diversity for the last fifty years. However, recent revisions state that available data are too sparse and biased to resolve whether Ichneumonidae follow a classic or anomalous diversity gradient. We approach this issue with a side-step, focusing instead on more accessible elevational gradient which can be considered analogous to latitudinal gradient in temperature change. We examined how a diversity of the whole parasitoid community attacking a defined guild of hosts (externally feeding caterpillars) differs along a rainforest elevational gradient of Papua New Guinea. We collected ~40,000 caterpillars from 79 tree species and reared ~4,500 parasitoids and hyperparasitoids.
Results/Conclusions
Parasitism rate was not significantly different between elevations, and ratio of species diversity between Hymenoptera and Diptera parasitoids also remained unchanged. However, the ratio of Ichneumonidae to their sister group Braconidae was strongly biased towards higher Ichneumonidae diversity at high elevation (35% vs. 13% in the low elevation). Available hosts shifted from mostly semi-concealed at low elevations to approximately half free-living half semi-concealed at higher elevations, but host use itself did not explain Ichneumonidae anomalous gradient. Further, hyperparasitism rate was higher at high elevation (6.5% vs. 0.5%) and all hyperparasitism was caused by Ichneumonidae. Our results suggest that Ichneumonidae are more species rich at higher elevations, and thus follow the anomalous diversity gradient in terms of more species in colder environment. On the other hand, higher elevations in the tropics are heavily under sampled and further studies covering a range of elevations could find standard latitudinal gradient in Ichneumonidae diversity.