Snails leave trail of sameness through Anthropocene

Travelers often bemoan the homogenization of the modern landscape: a strip mall outside Boston looks much the same as one outside Boise; Starbucks has spread far beyond its native Seattle to become equally ubiquitous in Seoul. Gastropod travelers may see the world similarly, a new study suggests. According to a team of scientists from Portugal, Austria, and Germany, a bland sameness is also coming to characterize the biotic world, a pattern that scientists had long suspected but that has never been rigorously examined until now.

To test this homogenization hypothesis, the researchers assembled records of 175 species of introduced land snails in 56 countries and subregions. They mapped the native distribution of each species, as well as where each snail occurs today.

In the centuries before human exploration and trade started moving species around, different regions of the globe such as Australia, Africa, and the New World tropics had different snail communities. Few shared species were found in ecosystems more than 6,500 kilometers apart, and none were shared across 11,000-kilometer distances.

But the researchers reported June 12 in Science that our activities have created a new kind of biogeography, a term that describes how communities of species are distributed in space. Today, locations up to 20,000 kilometers apart can share many snail species in common.

In the current human-dominated era, the Anthropocene, distance still makes a difference. But broadly speaking snail communities now cluster into two climate-determined biogeographic belts: temperate and tropical/subtropical. Within the limits of their climatic tolerance, snails from different continents have mixed together so that communities look pretty similar around the globe.

In addition to climate, trade links between regions help to explain the current patterns of snail distribution, the researchers found. Countries or regions with strong trade links tend to have many snail species in common, because the creatures often hitch rides on shipments of roof tiles, live plants, vegetables, and fruits.

The results help explain why, despite a widespread sense that native species are being lost and the ecology of many places is being degraded, some studies have failed to find declining biodiversity in local areas over recent decades. The new study suggests that the total number of species living in a place may stay the same, but these absolute numbers hide species turnover that has introduced species replacing organisms particular to a place.

In the future, the researchers predict, snail communities could come to be dominated by a few, widespread generalist species, the escargot equivalent of the McDonalds golden arches: seen more or less everyplace, but special to none. – Sarah DeWeerdt | 16 June 2015

Source: Capinha C. et al. 2015 The dispersal of alien species redefines biogeography in the Anthropocene. Science DOI: 10.1126/science.aaa8913

Header image:  White garden snail (Theba poison), a European species that has become widespread in temperate regions around the globe. Credit: tato grasso via Wikimedia commons.

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