Mais uma controvérsia no arraial de nhô Darwin!

sexta-feira, agosto 25, 2006

Isto saiu na edição de agosto de 2006 da publicação científica TREE.

A Nomenklatura científica e a Grande Mídia Internacional e Tupiniquim ainda têm a cara de pau de dizer que não há controvérsia sobre as idéias de Darwin!

Mantive o texto em inglês porque não tenho muito tempo disponível, e postado aqui tendo em mente os membros da Academia que falam, lêem e escrevem em inglês.

The merits of neutral theory

David Alonso1, , Rampal S. Etienne2 and Alan J. McKane3

1Ecology and Evolutionary Biology, University of Michigan, 830 North University Av., Ann Arbor, MI 48109-1048, USA

2Community and Conservation Ecology Group, University of Groningen, PO Box 14, 9750 AA Haren, The Netherlands

3Theory Group, School of Physics and Astronomy, University of Manchester, Manchester, M13 9PL, UK

Available online 12 June 2006.

Hubbell's neutral theory of biodiversity has challenged the classic niche-based view of ecological community structure. Although there have been many attempts to falsify Hubbell's theory, we argue that falsification should not lead to rejection, because there is more to the theory than neutrality alone. Much of the criticism has focused on the neutrality assumption without full appreciation of other relevant aspects of the theory. Here, we emphasize that neutral theory is also a stochastic theory, a sampling theory and a dispersal-limited theory. These important additional features should be retained in future theoretical developments of community ecology.

Article Outline

The entangled bank
Neutral theory is an ideal theory
Neutrality assumption
An individual-based stochastic dynamical theory
A dispersal-limited sampling theory
Recent developments and prospects
Concluding remarks

The entangled bank

‘When we look at the plants and bushes clothing an entangled bank, we are tempted to attribute their proportional numbers and kinds to what we call chance. But how false a view is this!’. In this statement, Darwin clearly summarized his philosophical position: there is no place for stochasticity in population biology [1]. In 2001, Stephen Hubbell [2], after more than 25 years working on population and community ecology of tree species in tropical forests, presented an explanatory theory that is formulated entirely in terms of chance. Given that On the Origin of Species by means of Natural Selection [1] is one of the most influential scientific books ever written, it is no wonder that Hubbell's ideas have generated so much controversy among ecologists 3, 4, 5, 6, 7, 8, 9 and 10.

Here, we do not enter into a philosophical discussion of the nature and origins of randomness in the world around us, but instead take an operational approach and argue why chance should be taken into account in any attempt to gain insight in the structure and functioning of ecological communities 11 and 12. We discuss the ability of neutral theory to generate new insights in community ecology, which, in the end, might not support neutrality. We also discuss the limitations and potential application of neutral ideas to biodiversity assessment in empirical settings.

Neutral theory is an ideal theory
Most previous articles on neutral theory highlight its failure to capture the complexity of ecological communities 5, 13, 14 and 15. However, here, we emphasize its merits and argue that neutral theory in ecology is a first approximation to reality. Ideal gases do not exist, neither do neutral communities. Similar to the kinetic theory of ideal gases in physics, neutral theory is a basic theory that provides the essential ingredients to further explore theories that involve more complex assumptions 16 and 17.

What are the essential ingredients of neutral theory? First, and foremost, it is a neutral theory in that the interactions among species are assumed to be equivalent on an individual ‘per capita’ basis [2]. Second, it is a stochastic theory, based on mechanistic assumptions about the processes controlling the origin and interaction of biological populations at the individual level (i.e. speciation, birth, death and migration). Because interactions are assumed to operate at the individual level, but the regularities that we would like to explain are truly macroscopic, this feature is reminiscent of the statistical thermodynamics approach in physics 2, 18 and 19. Third, it is a sampling theory: because it is built upon the sampling theory of selectively neutral alleles in population genetics [20], the sampling nature of the theory is guaranteed. In this way, we interact with the system under study through the sampling process and obtain measures in our sampling that are related to those in the real system in a particular way clearly specified by the theory. Fourth, and most innovatively, it is a dispersal-assembled theory [2]. This means that dispersal is assumed to have a leading role in structuring ecological communities. However, dispersal and sampling are intertwined and a nonrandom way of sampling can be formulated that incorporates dispersal limitation [17] (see Glossary).

The originality of Hubbell's neutral theory lies in the combination of the fact that it (i) assumes equivalence among interacting species; (ii) is an individual-based stochastic theory; and (iii) can be formulated as a dispersal-limited sampling theory. Previous formulations of neutral theory lacked some of these aspects and no niche-based dynamical theory for ecological communities has been formulated as a sampling theory from scratch. Here, we discuss the relevance and limitations of each of these features.

Neutrality assumption

At the beginning of the 20th century, communities were viewed as a superorganism that develops in a particular and fixed way to form a well-established climax community [21]. A community is then a group of species whose competitive interaction strengths are determined by their niche overlaps, and new species originate through adaptation to new niches. This view was challenged by MacArthur and Wilson with their equilibrium theory of island biogeography [22], which was extended by Hubbell [2].
The importance of random mutations and genetic drift was formalized as the neutral theory in population genetics by Kimura and Crow [23]. As reviewed elsewhere [24], these ideas readily found an ecological interpretation (Box 1). Although Watterson [25], Caswell [26] and Leigh and co-workers [27] had already translated neutral models from population genetics into community ecology, Hubbell's [2] original intuition was that, in addition to neutral drift, random dispersal is the main factor controlling the assembly of ecological communities. Migration had also been studied in population genetics, but had never taken such a prominent role as in Hubbell's theory.