Showing papers by "Thomas Bell published in 2010"

Experimental tests of the bacterial distance-decay relationship.

Abstract: Community similarity declines with increasing geographic distance if species tend to be locally adapted or if they are dispersal limited. The distance–decay of similarity has been shown for bacteria previously, but distinguishing between these competing mechanisms is difficult from observational surveys. I found little evidence of a relationship between geographic distance and similarity in community composition in an aquatic bacterial community. When bacterial colonization occurred into initially identical sterile microcosms across a woodland, a strong distance–decay relationship was observed after 28 days, implying that dispersal limitation created a strong pattern in these communities in the absence of environmental differences. This conclusion was not supported by the results of a reciprocal transplant experiment. When microcosms at the extremities of the study area were transplanted to the opposite end of the study area, the communities converged on the community composition at the site to which they were transplanted. This convergence did not depend on whether colonization into the microcosms was prevented, implying a minor role for dispersal limitation over these spatial and temporal scales. Additional manipulations of colonization rates were consistent with the hypothesis that dispersal limitation structured these communities over short time scales (a few days), but that dispersal limitation had a minor role over longer time scales (>7 days).

Closely related protist strains have different grazing impacts on natural bacterial communities

Abstract: Heterotrophic protists are abundant in most environments and exert a strong top-down control on bacterial communities. However, little is known about how selective most protists are with respect to their bacterial prey. We conducted feeding trials using cercomonad and glissomonad Cercozoa by assaying them on a standardized, diverse bacterial community washed from beech leaf litter. For each of the nine protist strains assayed here, we measured several phenotypic traits (cell volume, speed, plasticity and protist cell density) that we anticipated would be important for their feeding ecology. We also estimated the genetic relatedness of the strains based on the 18S rRNA gene. We found that the nine protist strains had significantly different impacts on both the abundance and the composition of the bacterial communities. Both the phylogenetic distance between protist strains and differences in protist strain traits were important in explaining variation in the bacterial communities. Of the morphological traits that we investigated, protist cell volume and morphological plasticity (the extent to which cells showed amoeboid cell shape flexibility) were most important in determining bacterial community composition. The results demonstrate that closely related and morphologically similar protist species can have different impacts on their prey base.

Protists have divergent effects on bacterial diversity along a productivity gradient

Abstract: Productivity and predation are thought to be crucial drivers of bacterial diversity. We tested how the productivity–diversity of a natural bacterial community is modified by the presence of protist predators with different feeding preferences. In the absence of predators, there was a unimodal relationship between bacterial diversity and productivity. We found that three protist species (Bodo, Spumella and Cyclidium) had widely divergent effects on bacterial diversity across the productivity gradient. Bodo and Cyclidium had little effect on the shape of the productivity–diversity gradient, while Spumella flattened the relationship. We explain these results in terms of the feeding preferences of these predators.

Multi-species mixtures - new perspectives on models and mechanisms.

Abstract: The delivery of essential ecosystem functions (primary productivity, maintenance of soil fertility, resistance to weed invasion etc.) may be compromised by global declines in biodiversity. There is still controversy about the description of, and mechanisms behind, Biodiversity-Ecosystem Function (BEF) relationships. The Diversity-Interactions model quantified BEF relationships in terms of all the pairwise interactions between the species in a community. The model gives the contribution of two species (i and j) to the functional response in a community as _ijPiPj, where _ij reflects the potential of the two species to contribute to the response and its actual contribution depends also on Pi and Pj, the initial relative abundance of the two species in the community. This model and variants fitted well to a wide range of functional responses (biomass production, respiration) from several, but not all, experiments that examined a wide range of organisms (plants, microorganisms) and levels of species richness (1 to 72 species). A modified version introduces a more complex effect of pairwise interaction. The properties of this more flexible model and its implications for BEF relationships are discussed, particularly in the context of grass-clover contributions to sward functions.