Abundance and species richness in natural aquatic microcosms: a test and refinement of the Niche-Limitation Hypothesis

The Energy-Limitation Hypothesis (ELH) predicts that species richness is an increasing function of abundance. In contrast, the Niche-Limitation Hypothesis (NLH) predicts that high abundances become easier to attain as species richness increases. We tested the NLH and ELH using aquatic invertebrate communities of tropical rock pools. These rock pools span a gradient from very low density pools (N <200 individuals/liter) to very high density pools (N ≯ 5001). Only in low density rock pools were species richness and abundance positively related. In intermediate abundance rock pools no relationship between species richness and abundance was observed, and in high abundance rock pools the relationship between species richness and abundance was strongly negative. These patterns are inconsistent with the predictions of the ELH, but not with predictions of the modified NLH which adds carrying capacity to the model

Effects of predation and variation in species relative abundance on the parameters of neutral models

Hubbell (2001) proposes that random demographic processes (i.e., neutral dynamics) can explain observed levels of variation in the richness and abundance of species within and among communities. Hubbell's neutral models have drawn attention because they reproduce several characteristic features of natural communities. But neutral models are criticized for ignoring nonrandom processes known to cause species densities to fluctuate. We parameterized neutral models using the population counts of 64 species of aquatic invertebrates collected from 49 discrete rock pools over a 13 year period.We used Hubbell's numerical modeling approach to evaluate the effect of natural population fluctuations on the parameter settings. We also analyzed the effect of observed variation on the species proportional abundance predicted by neutral models. We find that observed levels of variation in abundance are much higher than predicted by neutral models, forcing estimates of themigration probability, m, and fundamental biodiversity parameter, ?, to fluctuateover time. Much of the observed variation is mediated by predator-prey interactions. Low predator densities are associated with fewer species and less even relative abundances of species, resulting in lower estimates of m and ? comparedto periods of high predator densities. Our results show that by assuming an identical survival probability for all species, neutral models misrepresent substantial aspects of community dynamics

Food webs: Reconciling the structure and function of biodiversity

The global biodiversity crisis concerns not only unprecedented loss of species within communities, but also related consequences for ecosystem function. Community ecology focuses on patterns of species richness and community composition, whereas ecosystem ecology focuses on fluxes of energy and materials. Food webs provide a quantitative framework to combine these approaches and unify the study of biodiversity and ecosystem function. We summarise the progression of food-web ecology and the challenges in using the food-web approach. We identify five areas of research where these advances can continue, and be applied to global challenges. Finally, we describe what data are needed in the next generation of food-web studies to reconcile the structure and function of biodiversity. © 2012 Elsevier Ltd.Peer Reviewe

Its all about connections: hubs and invasion in habitat networks

International audienceDuring the early stages of invasion, the interaction between the features of the invaded landscape, notably its spatial structure, and the internal dynamics of an introduced population has a crucial impact on establishment and spread. By approximating introduction areas as networks of patches linked by dispersal, we characterised their spatial structure with specific metrics and tested their impact on two essential steps of the invasion process: establishment and spread. By combining simulations with experimental introductions of Trichogramma chilonis (Hymenoptera: Trichogrammatidae) in artificial laboratory microcosms, we demonstrated that spread was hindered by clusters and accelerated by hubs but was also affected by small-population mechanisms prevalent for invasions, such as Allee effects. Establishment was also affected by demographic mechanisms, in interaction with network metrics. These results highlight the importance of considering the demography of invaders as well as the structure of the invaded area to predict the outcome of invasions