Changes in rain forest tree diversity, dominance and rarity across a seasonality gradient in the Western Ghats, India

Rainfall and seasonality are important predictors of alpha diversity in tropical rain forests. Why this relationship exists, however, is not clearly understood (Phillips et al., 1994; Clinebell et al., 1995; Leigh, 1999; Pitman et al., 2002; ter Steege et al., 2003). Do wet aseasonal forests support more tree species because they have higher stem density (ter Steege et al., 2003), because trees on wetter soils are shorter-lived (Phillips et al., 1994), because understorey plants live longer where there is no drought (Huston, 1994; Pitman et al., 2002), because light gaps are more frequent (Phillips et al., 1994) or because a greater abundance of species-specific pests and pathogens allow more species to coexist than in drier forests (Janzen, 1970; Connell, 1971; Givnish, 1999; Wright, 2002)? The Janzen–Connell hypothesis (Janzen, 1970; Connell, 1971) suggests that tropical rain forests are diverse because speciesspecific pests cause mutual repulsion among conspecifics Salim Ali School of Ecology and Environmental Sciences, Pondicherry University, Kalapet, Pondicherry, India, Smithsonian Tropical Research Institute, Balboa, Republic of Panama, Centre ValBio, PB 33 Ranomafana, Ifanadiana, Fianarantsoa, Madagascar and Department of Ecology and Evolution, State University of New York at Stony Brook, Stony Brook, NY, USA