Diverging diversity patterns of vascular plants and geometrid moths during forest regeneration on Mt Kilimanjaro, Tanzania

Aim This study investigates diversity patterns of vascular plants and plant-feeding geometrid moths during montane rain forest regeneration in relation to the biogeographical and historical conditions of Mt Kilimanjaro. Location Investigations were undertaken on the south-western slopes of Mt Kilimanjaro at altitudes between 2075 and 2265 m. Methods Thirteen plots were selected for this study. Four of these were situated in the middle of large clearings (> 1000 m(2)), three in secondary forest, two in mature forest remnants surrounded by secondary forest and four plots within continuous closed mature forest. Vascular plant species were recorded in an area of 20 x 20 m(2). Geometrid moths were attracted using lamps placed inside reflective gauze cylinders. Results Ninety-three species of vascular plants were recorded on the plots. Plant diversity increased in the course of forest regeneration from clearings and secondary forest to mature forest remnants and mature forest. This increase was visible in all vegetation strata as well as in the species number of Dicotyledoneae. The diversity of geometrid moths conversely decreased from early to late successional stages. A total of 2276 Geometridae representing 114 morphospecies were included in the study. Local values of Fisher's alpha varied from 10.3 to 18.3 on clearings and in secondary forest, whereas they remained below 8.0 in mature forest and mature forest remnants. There was a significant negative correlation between the diversity of Geometridae and the number of dicots, and of plant species in the shrub layer. Main conclusions Contrary to an expected positive correlation between the diversity of vascular plants and herbivorous geometrid moths, diversity patterns of these two groups are strongly diverging due to biogeographical and ecological factors differently affecting the two groups. The increase in plant diversity can chiefly be explained with an increase in epiphyte diversity which is related to the occurrence of suitable habitats in extensive moss layers on huge Ocotea usambarensis (Engl.) trees in the mature forest. The low diversity of geometrid moths in these forests may be connected to the isolation and relatively young age of the montane rain forests on Mt Kilimanjaro. Hence only a small number of moth species adapted to the cool and perhumid conditions within moist mature forest have so far immigrated into these habitats, and time was insufficient for the evolution of many new species.