Same Old, Same Old Tropics

What’s so unique about the tropics? A UBC researcher says: “Less than we thought.” The assertion, published in the journal Science, is focused on the concept of beta-diversity – a measure of the change in species composition between two sites, such as neighbouring patches of forest. High beta-diversity means that two given sites have few species in common.

Typically, beta-diversity increases as you move from the poles towards the equator, often leading ecologists to conclude that there is something inherently different about the ecology of the tropics that leads to greater turnover of tropical species from place to place.

But a group led by Nathan J.B. Kraft, a postdoctoral fellow at UBC’s Biodiversity Research Centre, challenged this interpretation, using an extensive dataset of tree inventories from around the world and archived at the University of Arizona.

Kraft and colleagues found that the crucial factor in shaping beta-diversity at large scales is how many species are present in the region in the first place. Once they accounted for these differences, the resulting beta-diversity patterns were the same in forests at tropical and temperate latitudes. They found the same consistency between high and low elevations in mountain regions.

“It was believed that something ‘extra’ must be going on in the ecology of the tropics to produce greater beta-diversity there,” says Kraft. “We now see that the patterns can all be explained not by current ecological processes, unfolding over one or two generations, but by much longer-term historical and geologic events. For decades now, ecologists have gone to the tropics to try to explain the often incredibly high diversity found there. But what our results show is that the same ecological mechanisms might operate in similar ways in Costa Rica and Calgary.”

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