Biomedical subjects

Rampal S Etienne

Publications and source records attributed to Rampal S Etienne.

7 recordsLinked to original sources

PubMed2006

The merits of neutral theory.

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.

Animals↗

PubMed2006

Theoretical biology: comparing models of species abundance.

Ecologists are struggling to explain how so many tropical tree species can coexist in tropical forests, and several empirical studies have demonstrated that negative density dependence is an important mechanism of tree-species coexistence. Volkov et al. compare a model incorporating negative density dependence with a dispersal-limited neutral model and claim that each predicts six empirical species-abundance distributions of tropical-tree communities equally well. However, we show here that their main conclusion is premature: when the two models are compared in an improved analysis, we find that the dispersal-limited model outcompetes the density-dependent model in all six cases. Hence, although density dependence is certainly an important diversity-maintaining mechanism, our improved approach indicates that the dispersal-limited model provides a more parsimonious explanation of empirical species-abundance distributions.

Biodiversity↗

PubMed2006

Independent and competing disease risks: implications for host populations in variable environments.

Disease models usually assume disease to act independently of other mortality- and morbidity-causing factors. Alternatively, disease may function as a competing risk factor, for example, killing already moribund hosts. Using tuberculosis (TB) in African buffalo as a model system, we explore consequences of competing or independent disease effects for host population dynamics. We include scenarios with density-dependent and density-independent effects of environmental variation, exemplified by variable food availability (driven by rainfall) and catastrophic droughts, respectively. Independent disease effects reduce population size linearly with prevalence, irrespective of the nature of environmental variation. Competing disease risks alter population size only if density-independent variation is present; then, disease reduces population size nonlinearly. Field data indicate that the net effect of TB on buffalo likely falls between the extremes of total independence and competition with other risk factors: TB increases mortality and decreases fecundity in some prime-aged buffalo, suggesting independent disease risks in these individuals, while similar disease effects in senescent buffalo may act as competing risks. Moreover, increased survival and fecundity of TB-negative buffalo may compensate for some disease-related losses. Model assumptions on independent or competing disease risks and environmental variability should be considered explicitly when assessing disease effects on wildlife populations.

Animals↗

PubMed2006

Comment on "Neutral ecological theory reveals isolation and rapid speciation in a biodiversity hot spot".

Latimer et al. (Reports, 9 September 2005, p. 1722) used an approximate likelihood function to estimate parameters of Hubbell's neutral model of biodiversity. Reanalysis with the exact likelihood not only yields different estimates but also shows that two similar likelihood maxima for very different parameter combinations can occur. This reveals a limitation of using species abundance data to gain insight into speciation and dispersal.

Animals↗

PubMed2004

How dispersal limitation shapes species-body size distributions in local communities.

A critical but poorly understood pattern in macroecology is the often unimodal species-body size distribution (also known as body size-diversity relationship) in a local community (embedded in a much larger regional species pool). Purely neutral community models that assume functional equivalence among species are incapable of explaining this pattern because body size is the key determinant of functional differences between species. Several niche-based explanations have been offered, but none of them is completely satisfactory. Here we develop a simple model that unites a neutral community model with niche-based theory to explain the relationship. In the model, species of similar size are assumed to belong to the same size guild. Within a size guild, all individuals are equivalent in their competition for resources, sensu Hubbell's neutral community model; they have the same speciation rate and dispersal capacities. Between size guilds, however, the total number of individuals, the speciation rate, and the dispersal capacities differ, but using known allometric scaling laws for these properties, we can describe the differences between size guilds. Our model predicts that species richness reaches an optimum at an intermediate body size, in agreement with observations. The optimum at intermediate body size is basically the result of a trade-off between, on the one hand, allometric scaling laws for the number of individuals and the speciation rate that decrease with body size and, on the other hand, the scaling law for active dispersal that increases with body size.

Biodiversity↗

PubMed2003

Ecological impact assessment in data-poor systems: a case study on metapopulation persistence.

Legislation on the protection of biodiversity (e.g., European Union Habitat and Bird Directives) increasingly requires ecological impact assessment of human activities. However, knowledge and understanding of relevant ecological processes and species responses to different types of impact are often incomplete. In this paper we demonstrate with a case study how impact assessment can be carried out for situations where data are scarce but some expert knowledge is available. The case study involves two amphibian species, the great crested newt (Triturus cristatus) and the natterjack toad (Bufo calamita) in the nature reserve the Meinweg in the Netherlands, for which plans are being developed to reopen an old railway track called the Iron Rhine. We assess the effects of this railway track and its proposed alternatives (scenarios) on the metapopulation extinction time and the occupancy times of the patches for both species using a discrete-time stochastic metapopulation model. We quantify the model parameters using expert knowledge and extrapolated data. Because of our uncertainty about these parameter values, we perform a Monte Carlo uncertainty analysis. This yields an estimate of the probability distribution of the model predictions and insight into the contribution of each distinguished source of uncertainty to this probability distribution. We show that with a simple metapopulation model and an extensive uncertainty analysis it is possible to detect the least harmful scenario. The ranking of the different scenarios is consistent. Thus, uncertainty analysis can enhance the role of ecological impact assessment in decision making by making explicit to what extent incomplete knowledge affects predictions.

Animals↗

PubMed2002

Non-equilibria in small metapopulations: comparing the deterministic Levins model with its stochastic counterpart.

In this paper, we examine, for small metapopulations, the stochastic analog of the classical Levins metapopulation model. We study its basic model output, the expected time to metapopulation extinction, for systems which are brought out of equilibrium by imposing sudden changes in patch number and the colonization and extinction parameters. We find that the expected metapopulation extinction time shows different behavior from the relaxation time of the original, deterministic, Levins model. This relaxation time is therefore limited in value for predicting the behavior of the stochastic model. However, predictions about the extinction time for deterministically unviable cases remain qualitatively the same. Our results further suggest that, if we want to counteract the effects of habitat loss or increased dispersal resistance, the optimal conservation strategy is not to restore the original situation, that is, to create habitat or decrease resistance against dispersal. As long as the costs for different management options are not too dissimilar, it is better to improve the quality of the remaining habitat in order to decrease the local extinction rate.

Animals↗