The question of why species assemble in different patterns with different levels of diversity around the world is one that has received much attention from ecologists. Despite the interest the question is yet to be answered. Darwin made a large jump in our understanding of the diversity of species with his theory of evolution, but little was added to our understanding for almost 100 years after the publication of The Origin of species in 1859. The Island Biogeography theory (MacArthur and Wilson 1967) attempted to explain the difference in species abundance on islands and mainland patches of the same size, and was the next leap in the quest to understand species distribution and biodiversity. Island Biogeographic theory stood as a lonely and radical theoretical explanation of spatio-temporal dynamics of whole ecological systems for the next 30 years. The major work on biodiversity for that time focused on metapopulation biology with most work being done on population dynamics of single species existing in defined habitat patches. Enter The Unified Neutral Theory of Biodiversity (UNTB), formulated by Stephen Hubbell in the mid-1990s and published in 2001, UNTB is the next big attempt at a theoretical explanation of the origin, maintenance and loss of biodiversity in a biogeographic context. The theory is mathematically non-trivial and therefore not easy to grasp at a fundamental level, however it is an important advance in our need to explain patterns in biodiversity, especially given the large scale threats imposed on diversity through anthropogenic influences.
In 2006 Alonso, Etienne and McKane published a review of neutral theory aimed at identifying the merits of the theory despite the various attempts to falsify it. The theory is the first major work in combining the fields of biogeography and biodiversity in a theoretical and testable manner. Defining features of Hubbells theory are that it is formulated entirely in term of chance, defining it as a stochastic theory; it is a sampling theory dealing with formulae that predict abundance and composition which can then be empirically tested; and it deals with dispersal. The largest criticism that the theory receives is focused on the part after which it is named: neutrality. The concept of a neutral theory in ecology focuses on individuals of the same species being ecologically equivalent; that is, although individuals might be different, there is no difference in the way the rules of life apply to each of them. According to Alonso et al (2006), in combining stochasticity, species equivalence and dispersal limitation that exists as a sampling theory, UNTB exceeds previous neutral theories, which lacked these aspects.
In research we aim to explain a unknown with as simple a theory as possible, minimising parameters that require estimation, and assumptions which often cannot be met in the real world. UNTB has strength in that it relies on only two basic observations: one, that different individuals from different species living in functionally similar ecological communities are controlled by similar birth, death and dispersal rates (the basic concept behind neutrality), and two, that ecological systems are saturated. The saturation point implies that the community is “full”, all resources are being effectively utilised and the community is in equilibrium so that new individuals can only join the community if they replace emigrated or deceased members. Although these assumptions receive a lot of criticism the neutrality is accepted as a null model for community structure, which is the equivalent of saying that we think a community could be different to the null model but we can compare it to this model to check if it is. If nothing else UNTB works as a stepping stone to better formulations of alternative models which can be similarly tested through empirical sampling.
The authors of the review praise Hubbell’s theory as one that makes the difficult jump from population-level descriptions with only a few species, to the paradigm shifted individual-based approach. In addition they point out that neutral theory is the first theory to include stochastic dynamics of species from origin to extinction, spatial formulation, and the dynamics of discrete individuals. The combination of these “ingredients” results in the ability to make quantitative predictions about general patterns, my holy grail of ecology. The desirous link between theory and empirical research extends to the fact that UNTB can be applied to a community as well as samples of the community, this feature elevates the theory above those commonly used which require additional complications in order to scale the theory from sample to community level.
By looking at developments and prospects of UNTB, Alonso et al (2006) paint a bright picture for the future of the theory, it is suggested that with some work the concept of neutrality could be relaxed. The benefits of understanding biodiversity in terms of functional diversity and redundancy are also vaguely pointed out. The idea seems to be that by understanding how different a community or assemblage is from neutral, and at what level the neutrality is functional, we can determine how at risk the community is to fatal disturbance. If all species are affected similarly (as with neutrality) will they all be wiped out by a disturbance, or will the redundancy allow for an overall resilience to change? These are questions that may one day be answered through applying Hubbell’s theory and there-by understanding the dynamics of biodiversity.
According to Hubbell (2001) our understanding of biodiversity is currently equivalent to that of a Middle Age understanding of medicine. It is important to remember that although The Unified Neutral Theory of Biodiversity and Biogeography is a major leap in an attempt to understand the complexity of biodiversity, it does not apply to all communities and situations. Its main contribution, in my opinion, will be as a reminder of how little we theoretically understand about biodiversity as a concept, and as such, as an inspiration and guidance to develop innovative theories about the mechanisms influencing biodiversity. I think it is easy to get caught up in the idea that many of the great scientific breakthroughs have been made but UNTB reminds us that it wasn't so long ago that we knew how little we know and that there is still great room to grow our understanding of the principles governing our world.
Alonso, D., Etienne, R. S., & McKane, A. J. (2006). The merits of neutral theory. Trends in Ecology & Evolution, 21(8), 451-457.
Hubbell, S. P. (2001). The unified neutral theory of biodiversity and biogeography. Princeton University Press.
MacArthur, R. H., & Wilson, E. O. (1967). The theory of island biogeography(Vol. 1). Princeton University Press.
