Diversity of geometrid moths (Lepidoptera: Geometridae) along an Afrotropical elevational rainforest transect


Jan C. Axmacher, Chair of Biogeography, University of Bayreuth, Universitätsstr. 30, 95440 Bayreuth. E-mail: jan.axmacher@web.de


Geometrid moths were investigated at 26 sites on 9 elevational levels along an elevational transect at Mt. Kilimanjaro (Tanzania), stretching from the fine-grained mosaic of small agroforest plots with combined cultivation of trees, shrubs and crops at 1650 m through mountain rainforest to heathland at 3300 m. We sampled moths manually at light between 19 : 00 and 22 : 00 in the rainy seasons of March to May and October to January in the years 2000, 2001 and 2002.

Along the transect, the composition of moth communities changed from a domination by Sterrhinae and Ennominae to a dominance of Larentiinae with increasing elevation. Overall, alpha diversity was very low compared to other tropical mountain regions. Fisher's alpha showed a maximum of 30 in the agroforest mosaic at 1650 m and decreased to values around 12 in the mountain rainforest. Communities of geometrid moths within the forest belt were significantly dissimilar from communities outside the forest. The diversity patterns on Mt. Kilimanjaro can be related to the young age, island-like position and history of the mountain. These factors have led to the formation of a homogeneous upper mountain rainforest habitat which in turn houses homogeneous moth communities with a low diversity compared to habitats at lower elevations. Here, a heterogeneous habitat mosaic allowing the intrusion of savannah species into this former forest habitat may account for an increased diversity. In the heath zone above the forest, climatic conditions are very harsh, permitting only few specialists to thrive in this ericaceous woodland. Edge effects were discernible at the forest–heathland boundary where some moth species from heathland invaded the closed forest. At the boundary between agroforest and a forest mosaic of exotic Acacia and Eucalyptus forest plantations and natural mountain forest, diversity values remained low as the dominant species Chiasmia fuscataria accounted for far higher proportions than other dominant species in any of the other habitats.