How much does a tropical forest elevational gradient contribute to biodiversity? Insights from the ant communities of Mt. Wilhelm.

Mountain slopes are known to promote the diversity and endemism of most taxa. Consequently, turnover of species is expected to be increased along elevational gradients compared to more uniform lowland areas of tropical forests. Mt. Wilhelm, the highest peak of Papua New Guinea, represents one of the last complete altitudinal forest transects with high-levels of biodiversity and pristine forests all along. We studied species diversity and distribution of the Mt. Wilhelm ant communities from 200 m a.s.l. up to tree line at 3700 m a.s.l., using a wide scope of sampling techniques. For the first time, the ants were sampled across different forest strata and at equally spaced elevational bands (500 m a.s.l.) along the complete gradient (257 species in total). The ants occurred up to 2700 m a.s.l. with general decline of their abundance and species richness with altitude, but not in the same way across forest strata. While the species occurrences linearly declined on the ground, they were highest at mid elevations on the vegetation. Species richness peaked at mid altitudes in all strata. Species composition varied both with sampled stratum and elevation, and there was an increase of soil-nesting species sampled on vegetation with increasing altitude. Most species occurred only in one or two elevational sites and there was no difference in the mean elevational range between ground-nesting and arboreal-nesting species. A simple model based on the species accumulation curves suggested that the Mt. Wilhelm ant species richness is two-fold increased, if rarefied to and compared with the same number of individuals as sampled in the lowlands. Our study demonstrates that pristine rainforests of Mt. Wilhelm considerably boost species diversity and endemism, and as such they should be of high conservation priority.