First evidence for long‐term stasis in wet‐tropics land snail community composition

Abstract

While spatial turnover in species composition is well documented (Whittaker 1975, Condit et al. 2002 much less investigation has been conducted over long-time periods. Land snails provide a unique window to observe such patterns because their fossil shells not only allow for species-level identifications and quantification of abundance (Schilthuizen 2011), but can also provide accurate radioisotope dates (Pigati et al. 2010). Here we compare species turnover and accumulation patterns in comparable temperate (Slovakia and the Czech Republic) and tropical (Malaysian Borneo) land snail datasets. These analyses show that while temperate communities demonstrated significant compositional change over the modern to Late glacial maximum (LGM) transition, tropical communities did not.Dissimilarity in both datasets was driven by species turnover (0.74 versus 0.65 for temperate versus tropics) rather than by differences in richness (0.13 versus 0.07). Highly significant temporal distance decay (DD) in species composition was noted ( Fig. 1) for the temperate dataset (p << 0.0001) while no significant decay was noted over the same period in the tropics (p = 0.28). Thus while over 70% of observed similarity decay was explained by temporal distance in Slovakia/Czech Republic, less than 10% was explained in Malaysia. AICc value comparison in the temperate data indicated that a two-parameter exponential form (-44.1) was much better fit to the data than a two-parameter power-law form (-25.6). Important differences were also noted in the shape of the species-time relationship (STR). While Slovakia/Czech Republic data demonstrate a 78% cumulative increase of species over the modern-LGM axis, the Malaysian data accumulated a dozen fewer species over the same temporal extent. The tropical data also exhibited a more pronounced asymptote: while cumulative temperate species richness demonstrated a 50% increase in cumulative richness from 0.3-25 ka, tropical richness only increased 20% over the same period.These analyses demonstrate that while there has been continual turnover in land snail assemblages in the temperate system, the tropical system demonstrated compositional stasis. Observed turnover in the temperate system was expected given regional biome change from the cold and dry LGM to wet and warm modern. Such directional change in the regional species pool is supported by extensive fossil evidence (Ložek 1964) and is further documented by the exponential form of the temporal DD relationship (Nekola and McGill 2014). At least some turnover was expected from the Malaysian