Explaining the elevational diversity pattern of geometrid moths from Borneo: a test of five hypotheses

Aim The aim of this study was to document quantitatively a hump-shaped elevational distribution of diversity for the geometrid moths of northern Borneo. Furthermore, we aimed to test rigorously a number of hypotheses that were proposed as 'general explanations' for such patterns, namely (1) the mid-domain effect, (2) overlap between lowland and montane (or endemic) fauna, (3) water and energy availability (i.e. temperature-limitation at high elevations, water-limitation at low elevations), (4) area size of elevational bands, and (5) determination by (host) plant diversity. Location Northern Borneo, Malaysia. Methods We used quantitative light-trapping data across northern Borneo (from our own field work and published literature) for 52 sites (covering > 800 species) to quantify local diversity (as Fisher's alpha). We used these data to test predictions of the five hypotheses in spatially explicit correlation analyses, using published data on climate, plant diversity and topography (Digital Elevation Model) as predictor variables. Results We documented a pronounced diversity peak at c. 650 in a.s.l. (breakpoint regression, r(2)= 0.55). We observed additional effects of other habitat parameters, sampling methodology and time on diversity only for data below 655 in a.s.l. Furthermore, we did not find unequivocal support for any of the above-stated hypotheses. The mid-domain effect and faunal overlap can be ruled out as single explanations of observed patterns, whereas the data equally fit predictions from water-energy dynamics, area and plant diversity only at higher elevations, but not in the lower part of the gradient. Of those, only effects of energy (i.e. temperature) were expected a priori to be restricted to elevations above the breakpoint. Main conclusions We tentatively conclude that temperature (i.e. energy limitation) is the best explanation for decreasing diversity towards high elevations, but we did not find a compelling explanation of lowered diversity near sea level. We discuss and present some preliminary data on further possible explanations, i,e. niche conservatism of higher taxonomic groups.