Showing papers by "Andy Hector published in 2010"

General stabilizing effects of plant diversity on grassland productivity through population asynchrony and overyielding

Abstract: Insurance effects of biodiversity can stabilize the functioning of multispecies ecosystems against environmental variability when differential species' responses lead to asynchronous population dynamics. When responses are not perfectly positively correlated, declines in some populations are compensated by increases in others, smoothing variability in ecosystem productivity. This variance reduction effect of biodiversity is analogous to the risk-spreading benefits of diverse investment portfolios in financial markets. We use data from the BIODEPTH network of grassland biodiversity experiments to perform a general test for stabilizing effects of plant diversity on the temporal variability of individual species, functional groups, and aggregate communities. We tested three potential mechanisms: reduction of temporal variability through population asynchrony; enhancement of long-term average performance through positive selection effects; and increases in the temporal mean due to overyielding. Our results support a stabilizing effect of diversity on the temporal variability of grassland aboveground annual net primary production through two mechanisms. Two-species communities with greater population asynchrony were more stable in their average production over time due to compensatory fluctuations. Overyielding also stabilized productivity by increasing levels of average biomass production relative to temporal variability. However, there was no evidence for a performance-enhancing effect on the temporal mean through positive selection effects. In combination with previous work, our results suggest that stabilizing effects of diversity on community productivity through population asynchrony and overyielding appear to be general in grassland ecosystems.

Analysis of variance with unbalanced data: an update for ecology & evolution

Abstract: 1. Factorial analysis of variance (anova) with unbalanced (non-orthogonal) data is a commonplace but controversial and poorly understood topic in applied statistics. 2. We explain that anova calculates the sum of squares for each term in the model formula sequentially (type I sums of squares) and show how anova tables of adjusted sums of squares are composite tables assembled from multiple sequential analyses. A different anova is performed for each explanatory variable or interaction so that each term is placed last in the model formula in turn and adjusted for the others. 3. The sum of squares for each term in the analysis can be calculated after adjusting only for the main effects of other explanatory variables (type II sums of squares) or, controversially, for both main effects and interactions (type III sums of squares). 4. We summarize the main recent developments and emphasize the shift away from the search for the 'right' anova table in favour of presenting one or more models that best suit the objectives of the analysis.

Modelling the growth of parasitic plants

Abstract: 1 Hemiparasitic plants, such as Rhinanthus species, have substantial effects on community composition and biomass For example, the presence of parasites often increases diversity but reduces the combined biomass of hosts and parasites by c 25% compared with unparasitized controls We present and test a simple model of the host-parasite interaction in which parasite growth rate is a function of host growth rate that offers a new explanation for why hemiparasitic plants reduce ecosystem productivity 2 The model predicts that the combined mass of the host-parasite system is always less than the mass of the host grown alone, because the combined biomass is dependent only on host growth rate, which is reduced by the parasite The model also predicts that the parasite should adopt an intermediate virulence to maximize its own performance, but that the optimum virulence depends on host growth characteristics 3 The key assumption of the model is that parasite growth rate and hence parasite biomass is tightly coupled to host growth rate We tested this assumption by measuring the performance of Rhinanthus alectorolophus, a widespread hemiparasitic annual plant, on nine common European grass species First, we determined size-corrected growth rates for the grasses by fitting power-law growth curves to multiple-harvest data on host individuals grown without Rhinanthus Second, we grew Rhinanthus on each of the grass species and related Rhinanthus final biomass to the grass species' growth rates 4 Rhinanthus performance was strongly correlated with the growth rate of the host grass species, thus validating a key assumption of our model However, Rhinanthus biomass on three of the nine grass species differed significantly from the value predicted based on host growth rate alone, suggesting that grass species differ in their resistance to parasitism 5 Synthesis Parameterizing such models of the host-parasite relationship could help to explain variation in Rhinanthus performance on different hosts, variation in the effects of hemiparasites in grasslands of different productivity and differences in virulence among parasite populations

Effects of seed predators of different body size on seed mortality in Bornean logged forest.

Abstract: BACKGROUND: The Janzen-Connell hypothesis proposes that seed and seedling enemies play a major role in maintaining high levels of tree diversity in tropical forests. However, human disturbance may alter guilds of seed predators including their body size distribution. These changes have the potential to affect seedling survival in logged forest and may alter forest composition and diversity. METHODOLOGY/PRINCIPAL FINDINGS: We manipulated seed density in plots beneath con- and heterospecific adult trees within a logged forest and excluded vertebrate predators of different body sizes using cages. We show that small and large-bodied predators differed in their effect on con- and heterospecific seedling mortality. In combination small and large-bodied predators dramatically decreased both con- and heterospecific seedling survival. In contrast, when larger-bodied predators were excluded small-bodied predators reduced conspecific seed survival leaving seeds coming from the distant tree of a different species. CONCLUSIONS/SIGNIFICANCE: Our results suggest that seed survival is affected differently by vertebrate predators according to their body size. Therefore, changes in the body size structure of the seed predator community in logged forests may change patterns of seed mortality and potentially affect recruitment and community composition

Mass fruiting in Borneo: a missed opportunity.

Abstract: ![Figure][1] Dipterocarp tree seedlings. Many endangered Indonesian trees rarely produce seeds. CREDIT: CAMPBELL WEBB Large-scale restoration of tropical forest is increasingly recognized as a credible option for climate change mitigation and biodiversity conservation ([ 1 ][2]–[ 3 ][3

Applied ecology: How to get even with pests.

Abstract: Organic farming supports higher biodiversity. Research involving the Colorado potato beetle shows that this increased diversity can deliver a better ecosystem service in the form of more effective pest control.

Challenges for biodiversity research in Europe

Abstract: In this advice paper the League of European Research Universities (LERU) argues for a focus on biodiversity research to feed new policies and targets.