Biomedical subjects

Jonathan Silvertown

Publications and source records attributed to Jonathan Silvertown.

4 recordsLinked to original sources

PubMed2006

Absence of phylogenetic signal in the niche structure of meadow plant communities.

A significant proportion of the global diversity of flowering plants has evolved in recent geological time, probably through adaptive radiation into new niches. However, rapid evolution is at odds with recent research which has suggested that plant ecological traits, including the beta- (or habitat) niche, evolve only slowly. We have quantified traits that determine within-habitat alpha diversity (alpha niches) in two communities in which species segregate on hydrological gradients. Molecular phylogenetic analysis of these data shows practically no evidence of a correlation between the ecological and evolutionary distances separating species, indicating that hydrological alpha niches are evolutionarily labile. We propose that contrasting patterns of evolutionary conservatism for alpha- and beta-niches is a general phenomenon necessitated by the hierarchical filtering of species during community assembly. This determines that species must have similar beta niches in order to occupy the same habitat, but different alpha niches in order to coexist.

Base Sequence↗

PubMed2006

Phylogeny and the hierarchical organization of plant diversity.

R. H. Whittaker's idea that plant diversity can be divided into a hierarchy of spatial components from alpha at the within-habitat scale through beta for the turnover of species between habitats to gamma along regional gradients implies the underlying existence of alpha, beta, and gamma niches. We explore the hypothesis that the evolution of alpha, beta, and gamma niches is also hierarchical, with traits that define the alpha niche being labile, while those defining beta and gamma niches are conservative. At the alpha level we find support for the hypothesis in the lack of close significant phylogenetic relationship between meadow species that have similar alpha niches. In a second test, alpha niche overlap based on a variety of traits is compared between congeners and noncongeners in several communities; here, too, there is no evidence of a correlation between alpha niche and phylogeny. To test whether beta and gamma niches evolve conservatively, we reconstructed the evolution of relevant traits on evolutionary trees for 14 different clades. Tests against null models revealed a number of instances, including some in island radiations, in which habitat (beta niche) and elevational maximum (an aspect of the gamma niche) showed evolutionary conservatism.

Biodiversity↗

PubMed2004

Sustainability in a nutshell.

Sustainable exploitation is widely advocated as a strategy for reconciling economic pressures upon natural habitats with nature conservation. Two recent papers examine different aspects of the sustainability of the nut harvest on wild populations of Brazil nut trees Bertholletia excelsa in Amazonia. Peres et al. find that many populations of the Brazil nut tree lack juvenile trees and are not regenerating. In a socioeconomic study, Escobal and Aldana find that nut-gathering provides insufficient income on its own to support nut-gatherers and that their other income-raising activities damage the forest. The existence of a market for rainforest products is, therefore, not sufficient on its own to prevent habitat destruction or the overexploitation of the resource and a more sophisticated approach to sustainability is required. Development of a market in ethically traded Brazil nuts might be one solution.

Journal Article↗

PubMed1999

The Demographic Cost of Reproduction and Its Consequences in Balsam Fir (Abies balsamea).

It is an axiom of life-history theory that reproduction involves age-specific costs in terms of survival or future reproduction. The measurement of costs of reproduction in plants is difficult, and few field studies have measured these costs in terms of fitness or demographic components, thus creating a hiatus between theory and data. In this article, we describe methods for overcoming the problem, illustrated by a field study of balsam fir. We used serial correlation and a permutation test to detect growth costs of reproduction and show how these translate into demographic costs when relative tree size (and therefore growth) is critical to survival. Using chronosequences, we reconstructed the age- and size-specific dynamics of a subalpine population of Abies balsamea. A matrix model describing these dynamics was then used to estimate age- and size-specific probabilities of future survival to maturity ([Formula: see text]). By using a regression model of the relationship between tree size, age, and [Formula: see text], we were able to estimate the maximum age-specific demographic cost of reproduction for trees of all ages. The shape of the age-specific cost curve for A. balsamea may explain why, contrary to a previously published hypothesis, age at first reproduction in A. balsamea does not vary between wave-regenerating and normal populations.

Abies balsamea↗