Understanding biodiversity
Simplicity is key
McGill biologist Graham Bell has had yet another article describing his research into biodiversity published by a major scientific journal, this time the prestigious international journal Science (September 28). Dr Graham is the director of the Redpath Museum on the Universitys downtown campus and has taken the time to describe his work in lay terms below. The article is entitled "Neutral Macroecology" and focuses on neutral theories of genetic diversity. Dr Bell explains:
"A few hectares of old-growth forest in Quebec supports hundreds of species of plants. These include 40 or 50 species of trees, but also a great diversity of understorey plants -perhaps 30 ferns, 50 kinds of grasses and sedges, and dozens of violets, goldenrods and daisies. A wetland, a lake margin or a meadow would give the same picture of a great diversity of plants, and even a cultivated field or a roadside ditch will usually yield a long list of species. The animals that live on and among and beneath these plants are even more diverse, with thousands of species of flies, snails, worms and other creatures in every hectare. Why are there so many different kinds of living things? In some cases, the answer is obvious enough: birds make a living by eating insects, which in turn eat plants, or other insects. But many organisms appear to be very similar and to follow similar ways of life. Why are there 30 species of fern in the forest, and not one or two? Or 40 species of sedge, that can be distinguished only by experts? We need to understand how diversity is maintained in natural systems, if only so that we can anticipate the effects of biodiversity loss.
"One approach to these questions has been to study carefully the patterns of diversity that have been documented in natural ecological communities. Some species are much more abundant than others, for example, and the balance between the number of common species and the number of rare species is characteristic for a particular community. There is a relationship, too, between the abundance of a species and its geographical range: you can even predict, in many cases, how widespread a species is in North America from the number of individuals that you can find in your back yard (although you need to have a fairly big back yard to do this reliably!) At the same time, it is obvious that the abundance of any particular species varies from place to place, and in general species tend to be more abundant close to the centre of their range, and become steadily more uncommon towards the edges. These and many other similar patterns, some very local but others global in extent, have been interpreted by ecologists in terms of general ecological mechanisms. For example, species may have evolved differing degrees of specialization to environmental conditions. A highly specialized species that could only live (say) on limestone pebbles in high-altitude grassland would probably be much less abundant globally than a similar species that could tolerate a wide range of altitudes and substrates. The patterns of range and abundance that we observe would then depend on the way in which all the resources and opportunities provided by the environment were shared out, so to speak, by all the species in the community.
"In a sense, theories like this are obviously true. Organisms are indeed rather well adapted to the circumstances in which they usually grow, and are often quite unsuited to other circumstances. If you planted coconuts in a peat bog, then of course they would not grow, any more than spruce trees would grow on a tropical beach. It is not quite as obvious, however, that this will apply to the 30 species of fern or the 40 species of sedge that we find in a single tract of forest. Is every species of fern or sedge -- or snail or centipede -- exquisitely adapted to precisely the places where it is found, so that to explain the diversity of natural communities we must know the intimate details of how each species lives?
"The opposite approach to the problem is to ask, what is the simplest theory that might, in principle, enable us to understand diversity? It is to assume that, far from being completely distinct, all species are exactly the same. Not that snails are the same as ferns, of course: they are different kinds of organisms. Rather, it is assumed that ecologically similar organisms, such as species of ferns, are all the same. They are the same in a very precise sense: each individual has exactly the same demographic properties (rates of birth, death and migration), in all circumstances. Which species it belongs to is merely a label, and tells us nothing at all about its ecological properties. This is the neutral theory of biodiversity. It is "neutral" because the apparent differences between species are ecologically neutral, with no effect on their demography, and therefore no effect on their success or failure relative to other species. Neutral theories of genetic diversity are well known in the field of population genetics, but they have only recently begun to be developed for the problem of species diversity in the field of community ecology. The two scientists chiefly responsible for their development are myself at McGill and Stephen Hubbell at Georgia and STRI.
"The most surprising aspect of neutral theory is that it is astonishingly successful at predicting patterns of distribution, abundance and diversity. Not only those I mentioned above, but many more of the most familiar generalizations in ecology emerge as consequences of neutral community dynamics. This strongly counterintuitive conclusion comes from the spatial patterns that are generated purely through the local dispersal of seeds or offspring, even when their success in the site they grow up in is purely a matter of chance. The article in Science describes how neutral models are able to reproduce the ecological patterns that hundreds of previous authors have described and attempted to interpret. On these grounds, it puts forward the neutral model as the basic general theory of biodiversity, that will serve as a foundation for the entire subject. The theory has its limits, of course -- coconuts in peat bogs, for example. But within these limits it will be a very powerful way of understanding the dynamics of species in natural communities."
Dr Graham Bell is a member of the Department of Biology, Director of the Redpath Museum, and North American Editor of the Journal of Evolutionary Biology.
