Species Richness Patterns Along Environmental Gradients in Island Land Molluscan Fauna

Abstract

The relationship between species richness and environmental variables may change depending on habitat structure, dispersal ability, species mixing, and community adaptation to the environment. It is crucial to know how these factors regulate the environment-diversity relationship. The land molluscan fauna of the Ogasawara Islands in the West Pacific is an excellent model system to address this question because of the high species endemicity (> 90%), small area, and simple habitat structure of the islands. I examined relationships among indigenous species composition, richness, and habitat condition, and especially productivity and forest moisture on the island of Anijima. Two major communities of snails could be distinguished by detrended correspondence analysis (DCA): one group dominated in a moist habitat with high productivity, and the other group dominated in a dry habitat with low productivity. However, species richness became highest at the intermediate condition between the habitats in which the two snail communities were dominant, so that species richness showed a hump-shaped relationship with moisture and productivity. In contrast, the species richness of the snail community in the moist habitat showed a monotonically positive correlation, and that in the dry habitat showed a monotonically negative correlation with moisture and productivity. Thus, the greater species richness in intermediate moisture and productivity resulted from the ecotone effect or community overlap at the transitional areas, where faunas with different ecologies can meet in a single site. These findings suggest that hump-shaped productivity-diversity relationships in land Mollusca would reflect the ecotone effect as a result of the mixing of species adapted to either fertile habitats or sterile habitats.

Determinants of species richness and similarity of species composition of land snail communities on Kimberley islands

“…The close association of rainfall with species composition of land snails has also been observed in many other countries (e.g. Hermida et al 1994;Kadmon and Heller 1998;Tattersfi eld et al 2001;Steinitz et al 2005), and is likely to be an indirect effect with rainfall determining the soil moisture, vegetation and productivity at a site (Nekola 2003;Chiba 2007;Slatyer et al 2007).…”

“…The high levels of endemism of land snails in the limestone karsts of Malaysia has Determinants of species richness and similarity of species composition of land snail communities on Kimberley islands been similarly explained (Clements et al 2008), as well as land snails occurring on islands or within island archipelagos (e.g. Cowie 1995;WelterSchultes and Williams 1999;Chiba 2007;Cook 2008;Overton et al 2009). …”

“…We were interested in examining if the speciesarea relationship held for the Kimberley island snail fauna as has been found on other island archipelagos (e.g. Cowie 1995;Welter-Schultes and Williams 1999;Triantis et al 2005;Chiba 2007), as well as the effect of island isolation on snail richness and composition. We expected that the high levels of local endemism of camaenid land snails observed on the adjacent mainland (Solem 1991) would be refl ected in the island fauna with a high turnover of species among islands.…”

Invasive aliens on tropical East Asian islands

“…This pattern of invasion reXects the origin of most alien bird populations in Hong Kong from Buddhist bird releases. Wild-caught birds from similar forests in southwest China are imported into Hong Kong as cage-birds and have been released in large numbers over recent decades, resulting in optimum conditions for establishment (Chan 2007). Buddhist releases have been less common in Singapore, but may be 1 C the source of the only widespread avian forest invader there, the white-crested laughingthrush (Garrulax leucolophus) (Wang and Hails 2007).…”

Species richness and diversity across rocky intertidal elevation gradients in Helgoland: testing predictions from an environmental stress model

“…The ideal approach would be to measure both traits in local communities covering the full stress range for a regional biota, but this is difficult to achieve for logistic reasons. Thus, most studies documenting diversity across stress gradients surveyed only selected taxonomic groups or restricted stress ranges (Locke 1996;Townsend et al 1997;Hacker and Bertness 1999;Ohlemüller and Wilson 2000;Attrill and Rundle 2002;Chiba 2007;Lepori and Malmqvist 2009). However, knowing trends for some groups may not allow inference of community trends (Vellend 2008).…”