Filling gaps in the diversity and biogeography of Chilean millipedes (Myriapoda: Diplopoda)

Research on the diversity and biogeography of Chilean millipedes represents a significant gap in knowledge. To reduce this gap we conducted a study: (1) to investigate the current state of knowledge of millipede diversity, and (2) to assess the pattern and causes underlying the latitudinal diversity gradient in Chilean millipedes. In Chile, 95 native millipede species have been recorded. However, rarefaction and extrapolation curves showed that increased sampling effort will reveal more species. An asymptotic estimate of diversity predicted that millipede diversity fluctuates between 125 and 197 species. The estimate, though, was based on a limited data set. Therefore, millipede diversity is probably higher than predicted. Chilean millipedes were categorized as micro-range endemics because they all have latitudinal ranges of less than 1,000 km (with 78% of species exhibiting latitudinal ranges of only ~222 km). Millipede species richness also exhibited a bell-shaped latitudinal diversity gradient, i.e. diversity peaks in the temperate climate of central Chile and decreases towards the arid and polar climates of northern and southern Chile. A multiple regression analysis revealed that this biogeographic pattern is shaped by environmental variables related to water availability, ambient energy inputs and climatic stability. These environmental variables are proxies for two of the five biogeographic hypotheses we tested in this study, i.e. the water-energy balance hypothesis and climatic stability hypothesis. Both hypotheses suggest that millipedes need stable, humid and warm climates to grow, survive and reproduce (niche conservatism). These climatic conditions are only found in central Chile, which is consistent with the diversity peak observed in that region.