One of the big goals of ecology is to understand how ecosystems contain so many different species – in other words, to understand the mechanisms that maintain biodiversity. This is a particularly good question to ask for trees in the tropics both because forests there can have hundreds of tree species in just a single hectare and because trees use pretty much the same resources. How is it that one tree species doesn’t do slightly better than the rest at capturing light or using nutrients and crowd out the other species from the forest? One of our best explanations is that herbivores, predators, and pathogens keep individual tree species from becoming too abundant within the forest. The idea is that the more common a tree species is, the bigger target it becomes for specialized predators and pathogens. These ‘natural enemies’ limit the abundance of plant species, leaving room for other species to stay in the community. Ecologists see the signature of this occurring in tropical and temperate forests all over the world – plants experience distance- or density-dependent mortality, surviving better when away from members of the same species.
Despite having so much evidence of distance-dependent mortality, we don’t have a great idea of the organisms that are the most important causes of it. The most likely players are probably insects, mammals, and fungi. Individual studies show that each of these natural enemy types contribute for certain plant species. In fact, the great majority of these studies show a role for fungi. This has led some folks to claim that fungal pathogens are the most important cause of the phenomenon.
So are fungi responsible for the extraordinary diversity of tropical forests? Well, using evidence from existing experiments might be misleading for a couple reasons. First, there have been way more studies targeting fungi than any other enemy type – other enemy types might seem more important if we looked harder at them too. Second, existing experiments that determine the cause of distance-dependent mortality have only targeted at single enemy type for the plant species – that sort of study can’t in principle tell us whether one enemy is more important than another for that plant species. To determine the relative importance of these natural enemy types, what we really need are studies that simultaneously manipulate multiple enemy types in the field.
And that is what we did. Here is the link to the new paper in Ecology Letters. Using fungicide, insecticide, and mesh exclosures, we determined the source and severity of distance-dependent mortality for the seed and seedling stages of three tree species on the Pacific island of Saipan. We observed distance-dependent mortality in 5 of the 6 species-stages, with survival higher in areas away from adults of the same species. But surprisingly, these experiments did not show that fungal pathogens were most important. Instead, insects caused distance dependence in three species-stages, rodents in two, and fungal pathogens in just one species-stage. Our approach provides a way forward for tackling this question about the relative importance of enemy types, and the results show in particular that the importance of insects may have been overlooked. While this is a nice step in our basic understanding of this diversity promoting mechanism, these same data give us useful information for understanding the consequence of the loss of avian seed dispersers for these same tree species on the neighboring island of Guam. No seed dispersal on Guam is great news for the predators and pathogens that hang out under the trees – not so great for the trees though.
In the interest of catching up Science Positive, I thought I would plug a paper that came out last summer while I was doing fieldwork in the Mariana Islands. Here’s the link to that paper. That work also concerns natural enemy attack and dispersal away from parent plants, and more specifically if there are benefits of dispersal when a plant can’t escape natural enemies.
This is getting to be quite a long entry, and some other folks were kind enough to talk about this elsewhere, so I’ll just include some links. Here is a more academic perspective in the Journal of Experimental Biology and one from Science. Or if you’re looking for the spoken word, some popular science outlets made little radio spots too: Scientific American and PRI’s Living On Earth. To offer a bit more of a teaser, this study sort of involves bird poop – the more popular articles (especially Science) seem to focus heavily on the poop aspect.Posted 4th March 2014 by Unkn
