Navendu Page

Falsifying Neutral Theory by Partitioning Beta Diversity

Ecologists have long debated over the mechanisms that govern biodiversity at local and regional scales (Ricklefs & Schluter 1994). There are two contrasting group of thoughts that are commonly cited to explain the nature of organization of ecological communities. Niche based theory states that communities are structured by interacting species whose presence, absence and even abundance is governed by the functional role or ecological niche (McArthur 1970, Diamond 1975). The Dispersal assembly theory on the other hand asserts that species assemblages are a result of chance, history, random dispersal and stochastic extinction (e.g. IBT of McArthur and Wilson, 1967). The classical dispersal assembly theory has been recently revived by Hubbel (2001) by taking the neutrality mechanism further, from level of species to the level of an individual. The theory has once again challenged the niche assembly theory by proposing that ecological drift is the sole process regulating species coexistence.

There are a good number of studies which provide evidence in favor of both niche and dispersal assembly and we now know beyond doubt that both mechanisms operate to shape the ecological communities. The focus of the research therefore has now shifted to understanding the relative importance of these theories.  Studying species distribution and deciphering the mechanisms that generate species turnover has proved to be useful in testing and separating niche and dispersal assembly theories. Hubbel’s neutral theory (2001) has stimulated a lot of research in this field and attracted considerable attention to this issue. As mentioned above the most common approach has been to partition the variation in beta diversity along a gradient into percentage explained by the habitat /environmental variable and by space/ distance alone. The chain of CTFS plots has proved especially useful for such studies. Spatially referenced data on species diversity and composition has helped a great deal to disentangle the contribution of each of the above mentioned processes.

A study carried out by Condit et al (2002) is one of the influential studies which generating considerable controversy. The study tested the validity of neutral theory in predicting species turnover rates with distance. The results of the study were consistent with the predictions of the neutral theory at intermediate distances (0.2 to 50 km) between plots. This shows that dispersal limitation does play an important role in structuring forest communities. However the theory does not consider habitat, environmental and historical processes which also play a role in shaping species distributions. Therefore at scales when role of environment becomes prominent, the theory fails to predict the similarity in species composition. Even when environmental variables were used together with geographic distance it could only explain a small fraction of variable in species similarity. Duivenvoorden et al (2002) reanalyzed the data and finally concluded that dispersal may have small effect on beta diversity in tropical forests. This conclusion however was based on the Panama plot data which have a very high habitat heterogeneity and rather sharp gradient in rainfall.

Contrary to these results, another very similar large scale study carried out in Africa provides evidence in favor of dispersal as primary factor affecting species clumping. They found this particularly true for habitat generalist species. However this questions the generality of the theory given the high number of rare species in the tropics. Chust et al (2006) provide another elegant study once again highlighting the relative importance of geographic distance in predicting species similarity although climatic and topographic variables also explained some variation.

Potts et al (2002) based on  105 plots in Borneo, tested the effect of habitat and geographical distance on local, landscape to community scale. They found that, there exists a resource threshold above which the habitat    effect weakens and similarity between sites is then dominated by geographical distance effects. They concluded that community composition is governed by interplay of habitat and distance effects. A study carried out by Legendre et al (2009) is one of the latest studies targeted at investigating the effect of habitat and space on species richness and composition. The study is unique in several ways. There entire results are based on a scale of single 24ha permanent plot which unlike the other permanent plots falls in the subtropical broad leaved vegetation category. The study showed that the niche and the neutral process both are  equally important in governing species diversity and composition. These two processes in-fact work together in regulating beta diversity.

The neutral theory is a significant theoretical advancement of our understanding of factors that govern species diversity. Most studies which have followed the variance partitioning approach indicate that dispersal limitation might be more important at least in explaining species similarity. However it is still too early to make strong conclusions  since scale seems to be an important issue in such studies and the role of different factors could vary with scale of the study. Also in most of these studies it is difficult to segregate the effect of distance alone from environmental heterogeneity since the two are often correlated. The drawback of the theory perhaps is that it’s a completely stochastic theory based on very simplistic and unrealistic assumptions. However this also is its greatest strength since it is able to make predictions regarding mean dispersal distance and species similarity in spite of having simple assumptions.  Overall the theory promises to serve as a useful tool for predicting species turnover.