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.

The Future of Amazonia

The current New York Review of Books has an excellent piece by John Terborgh looking at the history of development in the Brazilian Amazon, and the forest's future prospects.

Terborgh's story and his arguments are too detailed to do justice here, but this, roughly, is his conclusion:

Brazil will continue to pursue its long-cherished goal of integrating the Amazon into the national economy. Much of the forest will go. But I would be surprised to see it vanish entirely because an increasing portion of the Amazon in Brazil, and in neighboring countries, is under formal, legal protection...Short of a complete breakdown of civil authority, the Amazon won't be entirely "lost".

He then sounds a note of caution: "Unforeseen developments are likely to determine the future of th Amazon... One such unforeseen development is fire, which holds the potential to be the undoing of the Amazon." Pristine tropical forest, he say, doesn't burn. Logging changes that:

Logging synergizes fire in two ways. First, cutting down trees opens the forest canopy, admitting sunlight and drying out the leaf litter on the forest floor. Second, the debris of branches, chips, and stumps left behind by logging operations serves as fuel for any subsequent fire.

The first time a tropical forest burns, the damage can hardly be detected from above because the destruction is largely confined to saplings and small trees whose crowns lie below the canopy. But the subsequent presence of large numbers of dead trees greatly increases the fuel available to stoke the next fire. Consequently, second fires burn hotter and more destructively, killing large trees as well as practically all smaller ones. And, of course, second fires generate even more fuel for the third fire. Colleagues of mine who study this subject, notably, Carlos Peres and Jos Barlow of the University of East Anglia (UK) and William Laurance of the Smithsonian Tropical Research Institute in Panama, assert that the third fire spells doom for the forest, since it kills all remaining trees.

Then, he adds, there's climate change, which will reduce rainfall, and dry the Amazon out from west to east (becasue the rain comes off the Atlantic) and threatens the 'savannaization' of the forest. In total, Terborgh's essay is all the more sobering for being balanced and unalarmist.