The Unified Neutral Theory of Biodiversity and Biogeography

  • Stephen P. Hubbell
Princeton University Press: 2001. 448 pp. $29.95, £19.95 (pbk); $75, £52 (hbk)

During the past 50 years, physicists have sought a 'unified theory' from which all major categories of forces could be derived. Any physicist sufficiently intrigued by the title of Stephen Hubbell's book to wade through its pages will, however, come away disappointed. This will be partly a result of the constraints on achieving any generality and unification in the field of ecology, and partly because of the limitations of Hubbell's theory. Nevertheless, Hubbell's book contains many interesting ideas and intriguing new findings, and few practising ecologists will fail to get something out of it.

The 'unification' of the title refers to the fact that this theory seeks to predict both the number of species found in a community and the distribution of the relative abundances of those species. Hitherto, these two aspects of biological communities have been explained independently by most (but not all) theoretical biologists. The number of species and their distribution of abundances are clearly dependent on some of the same processes. An extinction reduces the number of species as well as removing one of the species previously in the category of least abundance. Nevertheless, the 'best' theory for species-abundance distributions might not effectively account for species number, or vice versa.

In any event, vast expanses of ecological endeavour are totally independent of these questions of species number and statistical distributions of abundance. A fisheries manager will find nothing here to help predict the sustainable harvest of cod, nor will an entomologist derive anything about the characteristics of an effective natural enemy of a devastating pest. The unification proposed by Hubbell has limited scope.

Attempts to construct unified theories have produced laws that, at least initially, appear to be universally applicable. Thus, our hypothetical physicist reader will be surprised to find that Hubbell's theory is based on an assumption that the vast majority of ecologists believe to be almost universally false. He assumes that every individual in every species in a biological community is identical, and that the total abundance of all species is fixed. In his favoured version of the theory, each individual of each species has a low probability of mutating into a new species. All changes in distribution and abundance occur because of purely random variation in births, deaths, migration and speciation. This is what makes the theory 'neutral'.

Biodiversity: Hubbell's predictions are surprisingly accurate for tree species. Credit: GREGORY DIMIJIAN/SPL/STEVE KAUFMAN/CORBIS/PASCAL GOETGHELUCK/SPL

Hubbell has long championed the idea that the many tree species in the tropical forests where he works coexist largely because they are competitively equivalent. Random changes in abundance occur slowly enough for speciation events to have replaced the rare, random extinctions. The Unified Neutral Theory of Biodiversity and Biogeography generalizes this idea and derives many consequences from the assumption that all individuals are equal. Unfortunately, decades of experiments studying hundreds of species pairs have identified no conclusive cases of competitive equivalence, so one would expect such an assumption to lead to nonsensical consequences.

The surprising thing about Hubbell's theory is that it is pretty accurate in its predictions of many attributes of ecological communities — particularly the distribution of abundances of tree species. On the other hand, this ability is perhaps not as surprising as it initially appears; the parameters determining the shape of this distribution — speciation rate and number of individuals in the set of connected communities — are largely immeasurable. Most of the fits are achieved by adjusting these parameters (or their product) to minimize the difference between theory and observation.

There are two main resolutions to the apparent contradiction of small-scale experiments and Hubbell's global theory: first, non-neutral theories would be capable of fitting the data at least equally well; second, the assumption of equality is close to correct. But Hubbell largely ignores the first possibility. And his arguments for the second are not presented until the final 15 pages of the book, and I at least found them unconvincing. Graham Bell, another biologist who has recently analysed 'neutral' models, also favours the second alternative. However, work by Steinar Engen and collaborators indicates that alternative models are likely to be equally capable of fitting observed species-abundance relationships.

One of the likely consequences of Hubbell's and Bell's arguments is that there will probably be studies to resolve this issue in the near future. Theory must build on an understanding of simple cases and limiting assumptions, and Hubbell's deductions from 'neutral' assumptions have provided a rigorous basis for future work. At a time when books are often bland reviews of papers published many years before, this book has a high proportion of previously unpublished results and stimulating ideas. It is unlikely to be ignored.