Abstract
Understanding the determinants of range location and size is fundamental to our understanding of spatial patterns in species richness. Here, we aimed to test the role of ‘climatic stability’ in determining latitudinal trends in range size and as a consequence on species richness of tropical woody plants. Using primary data from 156 (0.06 ha) plots comprising 20,400 occurrences of more than 400 species of tropical woody plants, we built a biome-wide species database that covers the entire latitudinal extent of the wet-evergreen forests of the Western Ghats (8o to 20o N), India. We consolidated this database using secondary data from other published species inventories. We then calculated the range sizes and climatic niche width of woody plants to test the predictions of the climatic stability hypothesis and examined the relationship between range position and climatic tolerance of species. Our results show a significant latitudinal gradient in species richness and turnover where local and regional species richness increase monotonically from higher latitudes to lower latitudes of the Western Ghats. We found strong support for Rapoport’s Rule with an increase in range size from lower to higher latitudes; our results are consistent with the predictions of the climatic stability hypothesis, where species at higher latitudes exhibited greater tolerance to temperature and rainfall seasonality. Contrary to earlier work, our findings suggest that Rapoport’s Rule and the climatic stability hypothesis can operate over regional scales, and even at lower latitudes. We suggest that latitude associated climatic seasonality through its influence on species ranges, can influence latitudinal patterns in species turnover as well as species richness.
Highlights
The increase in species richness from the poles to the tropics is widely documented in the field of ecology, and yet the mechanisms underlying this pattern remain poorly understood [1,2,3,4]
Species richness increased monotonically from higher to lower latitudes. This result was consistent for estimates of alpha and gamma derived from the three levels of aggregation (S2 Table and S2 Fig). This shows that the increasing trend in species richness observed for woody plants of the Western Ghats (WG) can be detected even at a spatial scale of < 1km with as few as two plots
The effect of the latitudinal diversity gradient for the woody plants of the Western Ghats was strongly evident when total regional species pool (ST) (r2 = 0.90, p < 0.001, Fig 4B) was estimated at a much larger scale of 1o latitude as well as for species richness (SR) i.e. rarefied richness (r2 = 0.91, p < 0.001, Fig 4A) which controls for differences in sampling effort or stem density (S3 Fig)
Summary
The increase in species richness from the poles to the tropics is widely documented in the field of ecology, and yet the mechanisms underlying this pattern remain poorly understood [1,2,3,4]. Climatic stability drives latitudinal diversity patterns of Western Ghats’ woody plants much detail as other competing hypothesis [10]. The idea of climatic stability is based on the fact that temperate areas experience greater seasonality (i.e. wider temperature extremes on an annual basis) compared to tropical areas. This has been shown to correspond over much larger time scales to the Milankovitch cycles that result from periodic changes in earth’s orbit, tilt and spin causing greater climatic changes in extra-tropical as compared to tropical areas [11]
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