Species range boundaries changes reveal mechanisms of biological responses under climate warming. We aim to estimate whether species have shifted at different rates at their leading-edges (cool boundaries) and trailing-edges (warm boundaries) in response to climate change. This research provides the first such evidence for tropical insects. We conducted a Lepidoptera resurvey in Mt Kinabalu, Sabah, Malaysia in 2007, with a 42-year interval of the original survey in 1965. Analyses were restricted to Geometridae moths. Changes in species upper and lower boundaries and assemblages at each site were assessed. We carried out randomized sub-sampling to ensure comparable datasets between years. Responses were compared for endemic versus more widespread species, and for species that reached their range limits at different elevations. Threats to species on tropical mountains are considered.
Results/Conclusions
Species have moved uphill for the past four decades and richness declined in lower elevation sites and increased in higher sites. Species that reached their upper limits at 2500–2700 m (n = 28, 20% considered, including 10, or 45% of species endemic to Mt Kinabalu) retreated at both their lower and upper boundaries, and hence showed substantial range contractions (-300 m in altitudinal extent and -45 km2 in range area), which may be associated with cloud cover changes and the presence of ecological barriers (e.g., lack of appropriate host plants on ultrabasic rocks and granodiorite) which impede uphill movement. Other than this group, most species (n = 109, 80% considered) expanded their upper boundaries (152 m) more than they retreated at their lower boundaries (77 m). This finding supports the hypothesis that, without constraints, leading margins shift uphill faster than trailing margins retreat.
Climate warming on Mt Kinabalu will potentially prove hazardous to many species: (1) the narrowly-distributed endemic species may not find cool refuges locally (2) smaller land areas are available for uphill colonization (3) altitudinal shifts are likely to be constrained by geological and ecological barriers. Habitat area for the species reaching these barriers has compressed. (4) decoupled environmental changes may cause some types of habitats to decline; in particular, if interactions between global warming and local human activities result in reduced cloud cover, there may be a substantial loss of species from the endemic-rich cloud forest biota. Temperature and moisture gradient changes are likely to interact with other constraints, and lead to extinctions much sooner than one would expect based on temperature alone.