Bacterial communities provide important services. They break down pollutants, municipal waste and ingested food, and they are the primary means by which organic matter is recycled to plants and other autotrophs. However, the processes that determine the rate at which these services are supplied are only starting to be identified. Biodiversity influences the way in which ecosystems function, but the form of the relationship between bacterial biodiversity and functioning remains poorly understood. Here we describe a manipulative experiment that measured how biodiversity affects the functioning of communities containing up to 72 bacterial species constructed from a collection of naturally occurring culturable bacteria. The experimental design allowed us to manipulate large numbers of bacterial species selected at random from those that were culturable. We demonstrate that there is a decelerating relationship between community respiration and increasing bacterial diversity. We also show that both synergistic interactions among bacterial species and the composition of the bacterial community are important in determining the level of ecosystem functioning.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Animal Ecology. Summary 1. The preference of sheep for two temperate grassland species, ryegrass and white clover, was tested by releasing flocks (of three ewes) onto swards (0-25 ha) that contained adjacent monocultures of grass and clover, and observing their intake behaviour over 6 days. The test paddocks contained either 20, 50, or 80% clover by ground area to distinguish partial preference from indifference. 2. To test whether recent diet affected preference, separate groups of dry (nonpregnant, non-lactating) ewes grazed prior to testing on one of three diet 'backgrounds': an all-grass sward, an all-clover sward, or a 50:50 grass/clover sward by area. To consider the effects of physiological state on preference, a further group of lactating ewes, also from a 50:50 grass/clover 'background', were tested. 3. In all cases, the mean diet obtained was a mixture of grass and clover, even though greater intake rates would have been obtained eating pure clover. There was a consistent temporal pattern to preference, with ewes showing the greatest preference for clover in the morning. The results suggest that previous diet 'background' can have lasting effects on preference. Ewes that had been recently grazing monocultures initially showed an increased preference for the opposite species to the one they had been grazing. Over 6 days these animals gradually reverted to a preference for their 'background' diet. 4. Despite major differences in the energy requirement and intake behaviour of dry vs. lactating ewes, no significant effects of physiological state on preference were detected. Sheep grazing from swards of 20% clover spent a smaller proportion of time grazing clover and had a smaller proportion of clover in their diet than those grazing from 50% or 80% clover swards though, in all cases, behaviour was consistent with a preference for a high proportion of clover: sheep were not grazing at random. 5. Overall, the results demonstrate that sheep sustain a mixed diet even in situations where a monospecific diet is readily possible. Several alternative hypotheses to explain this (partial preference, novelty, rarity, 'sampling') are discussed. We suggest the results provide evidence of partial and changing preference by sheep. Our results urge caution in infering long-term grazing behaviour from short-term tests of preference and suggest why knowledge of diet preference even in common herbivores remains equivocal. Journal of Animal Ecology (1994) 63, 465-478 Diet preference of sheepanimal grazing unselectively could take its entire daily intake of dry matter from a few square m...

Lolium perenne cultivars differing in their capacity to accumulate water soluble carbohydrates (WSCs) were infected with three strains of fungal Neotyphodium lolii endophytes or left uninfected. The endophyte strains differed in their alkaloid profiles. Plants were grown at two different levels of nitrogen (N) supply in a controlled environment. Metabolic profiles of blades were analyzed using a variety of analytical methods. A total of 66 response variables were subjected to a principle components analysis and factor rotation. The first three rotated factors (46% of the total variance) were subsequently analyzed by analysis of variance. At high N supply nitrogenous compounds, organic acids and lipids were increased; WSCs, chlorogenic acid (CGA), and fibers were decreased. The high-sugar cultivar 'AberDove' had reduced levels of nitrate, most minor amino acids, sulfur, and fibers compared to the control cultivar 'Fennema', whereas WSCs, CGA, and methionine were increased. In plants infected with endophytes, nitrate, several amino acids, and, magnesium were decreased; WSCs, lipids, some organic acids, and CGA were increased. Regrowth of blades was stimulated at high N, and there was a significant endophyte 3 cultivar interaction on regrowth. Mannitol, a fungal specific sugar alcohol, was significantly correlated with fungal biomass. Our findings suggest that effects of endophytes on metabolic profiles of L. perenne can be considerable, depending on host plant characteristics and nutrient supply, and we propose that a shift in carbon/N ratios and in secondary metabolite production as seen in our study is likely to have impacts on herbivore responses.

Communities subject to disturbance are apparently vulnerable to invasion in that they contain a large fraction of the introduced plant species persisting in any given area. Still, ecologists have a poor understanding of how details of the disturbance regime influence the success with which invaders spread through a community. In this paper, we report results from an experiment that investigates how the spatial distribution of bare ground influences the rate at which offspring of an introduced invader spread through a perennial ryegrass community. Artificially created gaps of three sizes were positioned along linear transects according to one of two spatial distributions. These six transects (one for each treatment combination) were positioned as spokes on a wheel and Senecio vulgaris was introduced into the center. After controlling for the total amount of gap area, we monitored the number and position of Senecio vulgaris establishing for two subsequent generations. Our results showed that the rate of spread of Senecio vulgaris was sensitive to both the gap size and the gap distribution; plants moved a greater distance when the gaps were large and underdispersed. Simulation models confirm that this pattern follows from the way in which gaps sample from seed shadows. We derive an analytical expression to explain why the mean seed dispersal distance does not predict the spread of Senecio vulgaris adults, and show how rates of spatial spread will change under different spatial patterns of disturbance. Finally, a model of spatial point processes confirms that the distribution of gaps mediates the rate of weed spread in two dimensions as well.

We demonstrate that statistics textbooks differ in their prescription for the analysis of experiments that involve blocking factors. The differences in analysis may lead to differences in conclusions regarding the significance of experimental treatment effects. We outline the two approaches, discuss why they are different, and suggest when each approach may be applicable. We point out that simply following one's textbook may not be the best course of action for any particular situation.

Summary 1.Experiments investigating the population responses of aphids to CO 2 enrichment have yielded results suggesting that aphid populations will be both larger under elevated CO 2 and that they will be smaller under elevated CO 2 . Most studies have failed to reject the null hypothesis of no difference in population sizes due to atmospheric CO 2 concentration. This diversity of results has led some investigators to conclude that aphid responses are not general, and that every aphid-plant interaction may be unique and unpredictable a priori . We use a single, general, mathematical model to consider the population responses of cereal aphids to grass grown under different CO 2 concentrations. 2. The model shows that it is possible to explain any of the three observed results: larger populations, smaller populations, or no difference, and that which of these three outcomes arises may depend critically on the interaction between aphid nitrogen requirements and the nitrogen fertility of the soil. The model also shows that the qualitative results will depend on how sensitive the aphid species is to increases in its own density. Past studies have shown that aphids increase their production of winged offspring in response to increasing aphid density. The model predicts that, in general, aphid species that have lower nitrogen requirements and that are less sensitive to their own density will be more likely to have larger populations in elevated CO 2 compared to ambient CO 2 . 3. Differences between aphid species (and clones) in their nitrogen requirements and the strength of their density-dependent response have not been widely reported in the literature. Also, the nitrogen fertility of the soil has rarely been manipulated in experiments on aphid responses to rising CO 2 levels. The model suggests that the diversity of population responses of aphids may be both understandable and predictable in the context of such an interaction.

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Rising global carbon dioxide levels may lead to profound changes in plant composition, regardless of the degree of global warming that may result from the accumulation of this greenhouse gas. We studied the interaction of a CO2 doubling and two levels of nitrogen fertilizer on the growth and chemical composition of tall fescue (Festuca arundinacea Schreber cv. KY‐31) when infected and uninfected with the mutualistic fungal endophyte Neotyphodium coenophialum Morgan‐Jones and Gams. Two‐year‐old plants were harvested to 5 cm every 4 weeks, and after 12 weeks of growth plants grown in twice ambient CO2 concentrations: photosynthesized 15% more; produced tillers at a faster rate; produced 53% more dry matter (DM) yield under low N conditions and 61% more DM under high N conditions; the % organic matter (OM) was little changed except under elevated CO2 and high N when %OM increased by 3%; lignin decreased by 14%; crude protein (CP) concentrations (as %DM) declined by 21%; the soluble CP fraction (as %CP) increased by 13%; the acid detergent insoluble CP fraction (as %CP) increased by 12%, and in vitro neutral detergent fiber digestibility declined by 5% under high N conditions but not under low N. Plants infected with the endophytic fungus: photosynthesized 16% faster in high N compared with under low N; flowered earlier than uninfected plants; had 28% less lignin in high N compared with under low N; and had much smaller reductions in CP concentration (as %DM) and smaller increases in the soluble CP fraction (as %CP) and the acid detergent insoluble CP fraction (as %CP) under elevated CO2. Such large and varied changes in plant quality are likely to have large and significant effects on the herbivore populations that feed from these plants.