Land snail diversity in tropical rainforests

Tropical rainforests are ecosystems with an extraordinarily high diversity of animal and plant species. But while insects, plants and other taxa living in tropical rainforests have been studied frequently, land snails remained a relatively neglected group in rainforest research. One reason for this was that researchers believed snails wouldn't be common nor reach a large species diversity in tropical rainforests.

It was thought that the lack of available nutrients, the scarcity of litter to hide in, and the abundance of predators make tropical rainforests an unfavourable place for the relatively sluggish land snails. Furthermore, acidic rainforests are poor in calcium, and calcium is very important for snails to build a shell. Because of their need for calcium, higher diversities of shelled snail species are expected (quite correctly) on limestone rocks.

However, the numbers of snail species which can thrive in tropical rainforests does not seem to be as low as expected. From about the mid-nineties onwards a couple of studies have been published, which show that tropical rainforests can in fact harbour an amazing diversity of land snail species. The tropical rainforest of Cameroon in Central Africa proved to be particularly rich in snails: In a study in the rainforest of Cameroon (De Winter and Gittenberger, 1998), researchers found 97 snails within a single square kilometre.

Research in the pioneering field of tropical rainforest malacology is not only demanding in terms of field work. A major problem is that the taxonomical literature, which researchers would need to determine species, is scattered. Many species aren't described yet, but also the taxonomic position of newly described findings may be hard to determine. If literature from earlier studies is available, it is often very old, such that taxonomic keys drawn from it might require a major revision.

But how do researchers estimate species diversity in a habitat as varied as highly structured tropical rainforests? A recent study from the malacologist Menno Schilthuizen and his co-workers in Sabah (Borneo, Malaysia) gives some information on modern methods. The study aimed to determine land snail diversity in the tropical rainforest of Borneo.

To count snails in a comparable manner, the Bornean researchers have chosen a square kilometre of rainforest. The collectors counted all snails found by eye, while paying particular attention to habitats favoured by snails, such as rotten logs or the undersides of large leaves. Thinner plants were beaten over an inverted umbrella, such that snails hidden in the thicket would fall down into the umbrella. Also, a defined amount of litter was collected from different micro-habitats, which was then filtered in the laboratory in order to find minute snails hidden in the material.

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