Wednesday, May 27, 2009

No animals, no trees

A couple of recent papers show how seed dispersal is harmed when forest vertebrates disappear - it's not just bees and microbes that provide ecosystem services, we need the big stuff too.

In Biotropica, Stephen Blake of the WCS and colleagues look at seeds in the dung of forest elephants in the Congo. There are a lot:

Analysis of 855 elephant dung piles suggested that forest elephants disperse more intact seeds than any other species or genus of large vertebrate in African forests, while GPS telemetry data showed that forest elephants regularly disperse seeds over unprecedented distances compared to other dispersers. ... Our results suggest that the loss of forest elephants (and other large-bodied dispersers) may lead to a wave of recruitment failure among animal-dispersed tree species, and favor regeneration of the species-poor abiotically dispersed guild of trees.

And in Ecological Applications a US/Thai team look at the impact of bushmeat hunting on the dispersal of the hog plum, Choerospondias axillaris which, as the name suggests needs mammals (although not just pigs) to move it about. The clue is in the paper's title 'Bushmeat poaching reduces the seed dispersal and population growth rate of a mammal-dispersed tree.' "Extinction of C. axillaris is a real possibility, but may take many decades," they add. "Recent and ongoing extirpations of vertebrates in many tropical forests could be creating an extinction debt for zoochorous trees whose vulnerability is belied by their current abundance."

In Guanacaste, Costa Rica, they (especially Dan Janzen) got round this problem by introducing cattle, replacing the large herbivores that had gone extinct in prehistory. The trees thrived as a result. But Guanacaste is a dry forest. To paraphrase the music hall song about the horse and the lighthouse, I imagine it'd be harder to keep a cow in a jungle.

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