Ecology Letters

Casula, Paolo; Wilby, Andrew; Thomas, Matthew B.

doi: 10.1111/j.1461-0248.2006.00945.xpmid: 16925648

Biodiversity–ecosystem functioning theory would predict that increasing natural enemy richness should enhance prey consumption rate due to functional complementarity of enemy species. However, several studies show that ecological interactions among natural enemies may result in complex effects of enemy diversity on prey consumption. Therefore, the challenge in understanding natural enemy diversity effects is to predict consumption rates of multiple enemies taking into account effects arising from patterns of prey use together with species interactions. Here, we show how complementary and redundant prey use patterns result in additive and saturating effects, respectively, and how ecological interactions such as phenotypic niche shifts, synergy and intraguild predation enlarge the range of outcomes to include null, synergistic and antagonistic effects. This study provides a simple theoretical framework that can be applied to experimental studies to infer the biological mechanisms underlying natural enemy diversity effects on prey.

Levine, Jonathan M.; Pachepsky, Elizaveta; Kendall, Bruce E.; Yelenik, Stephanie G.; Lambers, Janneke Hille Ris

doi: 10.1111/j.1461-0248.2006.00949.xpmid: 16925649

Plant invaders have been suggested to change soil microbial communities and biogeochemical cycling in ways that can feedback to benefit themselves. In this paper, we ask when do these feedbacks influence the spread of exotic plants. Because answering this question is empirically challenging, we show how ecological theory on ‘pushed’ and ‘pulled’ invasions can be used to examine the problem. We incorporate soil feedbacks into annual plant invasion models, derive the conditions under which such feedbacks affect spread, and support our approach with simulations. We show that in homogeneous landscapes, strong positive feedbacks can influence spreading velocity for annual invaders, but that empirically documented feedbacks are not strong enough to do so. Moreover, to influence spread, invaders must modify the soil environment over a spatial scale larger than is biologically realistic. Though unimportant for annual invader spread in our models, feedbacks do affect invader density and potential impact. We discuss how future research might consider the way landscape structure, dispersal patterns, and the time scales over which plant–soil feedbacks develop regulate the effects of such feedbacks on invader spread.

Manning, Pete; Newington, John E.; Robson, Helen R.; Saunders, Mark; Eggers, Till; Bradford, Mark A.; Bardgett, Richard D.; Bonkowski, Michael; Ellis, Richard J.; Gange, Alan C.; Grayston, Susan J.; Kandeler, Ellen; Marhan, Sven; Reid, Eileen; Tscherko, Dagmar;

Fryxell, John M.; Lynn, Denis H.; Chris, Philip J.

doi: 10.1111/j.1461-0248.2006.00960.xpmid: 16925651

Overharvesting by humans threatens a substantial fraction of endangered species. Reserves have recently received enormous attention as a means of better conserving harvested resources, despite limited empirical evidence of their efficacy. We used manipulated microcosms to test whether reserves reduce extinction risk in mobile populations of harvested Tetrahymena thermophila, a ciliate. Here we show that patterns of population distribution inside and outside reserves can be accurately predicted on the basis of simple models of diffusion coupled with logistic controls on local population growth. No extinctions occurred in eight experimental trials with reserves, whereas extinction occurred in seven of eight trials without reserves, as predicted by population viability models based on stochastic population processes. These results suggest that marine reserves may be an effective means of improving long‐term viability in heavily harvested fish species.

Schönrogge, K.; Gardner, M. G.; Elmes, G. W.; Napper, E. K. V.; Simcox, D. J.; Wardlaw, J. C.; Breen, J.; Barr, B.; Knapp, J. J.; Pickett, J. A.; Thomas, J. A.

doi: 10.1111/j.1461-0248.2006.00957.xpmid: 16925652

Habets, Michelle G. J. L.; Rozen, Daniel E.; Hoekstra, Rolf F.; de Visser, J. Arjan G. M.

doi: 10.1111/j.1461-0248.2006.00955.xpmid: 16925653

Spatial structure is thought to be an important factor influencing the emergence and maintenance of genetic diversity. Previous studies have demonstrated that environmental heterogeneity, provided by spatial structure, leads to adaptive radiation of populations. In the present study, we investigate not only the impact of environmental heterogeneity on adaptive radiation, but also of population fragmentation and niche construction. Replicate populations founded by a single genotype of Escherichia coli were allowed to evolve for 900 generations by serial transfer in either a homogeneous environment, or a spatially structured environment that was either kept intact or destroyed with each daily transfer. Only populations evolving in the structured environment with intact population structure diversified: clones are significantly divergent in sugar catabolism, and show frequency‐dependent fitness interactions indicative of stable coexistence. These findings demonstrate an important role for population fragmentation, a consequence of population structure in spatially structured environments, on the diversification of populations.

Nicholson, Emily; Westphal, Michael I.; Frank, Karin; Rochester, Wayne A.; Pressey, Robert L.; Lindenmayer, David B.; Possingham, Hugh P.

doi: 10.1111/j.1461-0248.2006.00956.xpmid: 16925654

Although the aim of conservation planning is the persistence of biodiversity, current methods trade‐off ecological realism at a species level in favour of including multiple species and landscape features. For conservation planning to be relevant, the impact of landscape configuration on population processes and the viability of species needs to be considered. We present a novel method for selecting reserve systems that maximize persistence across multiple species, subject to a conservation budget. We use a spatially explicit metapopulation model to estimate extinction risk, a function of the ecology of the species and the amount, quality and configuration of habitat. We compare our new method with more traditional, area‐based reserve selection methods, using a ten‐species case study, and find that the expected loss of species is reduced 20‐fold. Unlike previous methods, we avoid designating arbitrary weightings between reserve size and configuration; rather, our method is based on population processes and is grounded in ecological theory.

Vile, Denis; Shipley, Bill; Garnier, Eric

doi: 10.1111/j.1461-0248.2006.00958.xpmid: 16925655

We show that ecosystem‐specific aboveground net primary productivity (SANPP, g g−1 day−1, productivity on a per gram basis) can be predicted from species‐level measures of potential relative growth rate (RGRmax), but only if RGRmax is weighted according to the species’ relative abundance. This is in agreement with Grime's mass‐ratio hypothesis. Productivity was measured in 12 sites in a French Mediterranean post‐agricultural succession, while RGRmax was measured on 26 of the most abundant species from this successional sere, grown hydroponically. RGRmax was only weakly correlated (r2 = 0.12, P < 0.05) with field age when species abundance was not considered, but the two variables were strongly correlated (r2 = 0.81, P < 0.001) when the relative abundance of species in each field was taken into account. SANPP also decreased significantly with field age. This resulted in a tight relationship (r2 = 0.77, P < 0.001) between productivity and RGRmax weighted according to species relative biomass contribution. Our study shows that scaling‐up from the potential properties of individual species is possible, and that information on potential and realized species traits can be integrated to predict ecosystem functioning.

Hartley, Stephen; Harris, Richard; Lester, Philip J.

doi: 10.1111/j.1461-0248.2006.00954.xpmid: 16925656

Maps of a species’ potential range make an important contribution to conservation and invasive species risk analysis. Spatial predictions, however, should be accompanied by an assessment of their uncertainty. Here, we use multimodel inference to generate confidence intervals that incorporate both the uncertainty involved in model selection as well as the error associated with model fitting. In the case of the invasive Argentine ant, we found that it was most likely to occur where the mean daily temperature in mid‐winter was 7–14 °C and maximum daily temperatures during the hottest month averaged 19–30 °C. Uninvaded regions vulnerable to future establishment include: southern China, Taiwan, Zimbabwe, central Madagascar, Morocco, high‐elevation Ethiopia, Yemen and a number of oceanic islands. Greatest uncertainty exists over predictions for China, north‐east India, Angola, Bolivia, Lord Howe Island and New Caledonia. Quantifying the costs of different errors (false negatives vs. false positives) was considered central for connecting modelling to decision‐making and management processes.

Kardol, Paul; Martijn Bezemer, T.; van der Putten, Wim H.

doi: 10.1111/j.1461-0248.2006.00953.xpmid: 16925657

Soil abiotic and biotic factors play key roles in plant community dynamics. However, little is known about how soil biota influence vegetation changes over time. Here, we show that the effects of soil organisms may depend on both the successional development of ecosystems and on the successional position of the plants involved. In model systems of plants and soils from different successional stages, we observed negative plant–soil feedback for early‐successional plant species, neutral feedback for mid‐successional species, and positive feedback for late‐successional species. The negative feedback of early‐successional plants was independent of soil origin, while late‐successional plants performed best in late‐ and worst in early‐successional soil. Increased performance of the subordinate, late‐successional plants resulted in enhanced plant community diversity. Observed feedback effects were more related to soil biota than to abiotic conditions. Our results show that temporal variations in plant–soil interactions profoundly contribute to plant community assemblage and ecosystem development.