23 June 2005

How does macroclimate influence land snail distribution patterns?

In my previous posts on the land snails of the family Enidae in Turkey (here and here), I concluded that macroclimate doesn’t seem to have had a major influence on the ranges of these snails.

My obvious (at least, to me) conclusion was based on the simple observation that the ranges of several species and genera extend over more than one climate zone. Lest I am accused of jumping into this conclusion without the benefit of a sophisticated analysis, I will now point out that 2 recent studies using more sophisticated methods made more or less similar determinations for the land snail faunas of nearby regions.

Kadmon, R. & Heller, J. 1998. Modelling faunal responses to climatic gradients with GIS: land snails as a case study. Journal of Biogeography, 25:527-539.
Kadmon & Heller analyzed the distribution patterns of land snails in Israel using GIS tools and determined that “Above 450 mm [mean annual rainfall], no relationship could be detected between the observed patterns of faunal variation and rainfall”. And they concluded that the “effect of rainfall on land snail distribution diminishes from the relatively arid parts of Israel towards its more rainy parts, and that factors other than rainfall are important in determining the distribution of land snails in the Mediterranean region”.

Their result makes sense to me, because I suspect that animals that normally live in arid regions require special adaptations to survive the scarcity of water that the animals living in areas that receive more precipitation don’t necessarily need. In other words, once the mean annual precipitation level is above a certain threshold (400?-450? mm), I suspect that any or most snails may survive indefinitely provided that their other ecological requirements are also satisfied.

Hausdorf, B. & Hennig, C. 2005. The influence of recent geography, palaeogeography and climate on the composition of the fauna of the central Aegean Islands. Biological Journal of the Linnean Society Volume 84:785-795.
Hausdorf & Hennig used partial Mantel tests and structural equation models to investigate the influence of recent geography, palaeogeography and climate on the composition of the fauna (land snails, land isopods, tenebrionid beetles, butterflies and reptiles) of the central Aegean Islands. They determined that “There was no significant influence of recent climatic parameters, mean annual temperatures and annual precipitation on the composition of the land snail, land isopod, tenebrionid beetle and butterfly faunas of the central Aegean Islands, if recent distances between the islands were controlled for”. According to their results “the composition of land snail island faunas has been influenced by recent and by Pliocene distances [between the islands]”.

My tentative conclusion in an earlier post regarding the Turkish Enidae was that their present day distributions had resulted mostly from Miocene and older geography.

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