Ecology Letters

Johnson, Edward A.; Miyanishi, Kiyoko

doi: 10.1111/j.1461-0248.2008.01173.xpmid: 18341585

Many introductory ecology textbooks illustrate succession, at least in part, by using certain classic studies (e.g. sand dunes, ponds/bogs, glacial till, and old fields) that substituted space for time (chronosequence) in determining the sequences of the succession. Despite past criticisms of this method, there is continued, often uncritical, use of chronosequences in current research on topics besides succession, including temporal changes in biodiversity, productivity, nutrient cycling, etc. To show the problem with chronosequence‐based studies in general, we review evidence from studies that used non‐chronosequence methods (such as long‐term study of permanent plots, palynology, and stand reconstruction) to test the space‐for‐time substitution in four classic succession studies. In several cases, the tests have used the same locations and, in one case, the same plots as those in the original studies. We show that empirical evidence invalidates the chronosequence‐based sequences inferred in these classic studies.

Fitze, Patrick S.; Le Galliard, Jean‐François

doi: 10.1111/j.1461-0248.2008.01158.xpmid: 18279355

Modern sexual selection theory indicates that reproductive costs rather than the operational sex ratio predict the intensity of sexual selection. We investigated sexual selection in the polygynandrous common lizard Lacerta vivipara. This species shows male aggression, causing high mating costs for females when adult sex ratios (ASR) are male‐biased. We manipulated ASR in 12 experimental populations and quantified the intensity of sexual selection based on the relationship between reproductive success and body size. In sharp contrast to classical sexual selection theory predictions, positive directional sexual selection on male size was stronger and positive directional selection on female size weaker in female‐biased populations than in male‐biased populations. Thus, consistent with modern theory, directional sexual selection on male size was weaker in populations with higher female mating costs. This suggests that the costs of breeding, but not the operational sex ratio, correctly predicted the strength of sexual selection.

Englund, Göran; Leonardsson, Kjell

doi: 10.1111/j.1461-0248.2008.01159.xpmid: 18294211

Scale transition theory is a framework for predicting regional population dynamics from local process functions and estimates of spatial heterogeneity. Using this framework, we estimated regional scale functional responses for a benthic predator–prey system in the Baltic Sea. Functional responses were based on laboratory experiments or field observations of stomach contents, and prey densities measured at a local scale (0.1 m2) or a regional scale (300 km2). Laboratory data overestimated consumption at high prey densities, whereas predictions based on local scale data tallied closely with consumption observed at the regional scale. The predicted regional functional response was different for increasing and decreasing prey densities, reflecting that predator and prey densities, as well as the covariance between them, exhibit oscillatory dynamics. We conclude that it is important to validate laboratory data with small‐scale field observations and that scale transition is a powerful tool for scaling‐up process functions in heterogeneous systems.

Guttal, Vishwesha; Jayaprakash, Ciriyam

doi: 10.1111/j.1461-0248.2008.01160.xpmid: 18279354

Empirical evidence for large‐scale abrupt changes in ecosystems such as lakes and vegetation of semi‐arid regions is growing. Such changes, called regime shifts, can lead to degradation of ecological services. We study simple ecological models that show a catastrophic transition as a control parameter is varied and propose a novel early warning signal that exploits two ubiquitous features of ecological systems: nonlinearity and large external fluctuations. Either reduced resilience or increased external fluctuations can tip ecosystems to an alternative stable state. It is shown that changes in asymmetry in the distribution of time series data, quantified by changing skewness, is a model‐independent and reliable early warning signal for both routes to regime shifts. Furthermore, using model simulations that mimic field measurements and a simple analysis of real data from abrupt climate change in the Sahara, we study the feasibility of skewness calculations using data available from routine monitoring.

Bradley, Devon J.; Gilbert, Gregory S.; Martiny, Jennifer B. H.

doi: 10.1111/j.1461-0248.2008.01162.xpmid: 18312409

Pathogens are thought to promote diversity in plant communities by preventing competitive exclusion. Previous studies have focussed primarily on single‐plant, single‐pathogen interactions, yet the interactions between multiple pathogens and multiple hosts may have non‐additive impacts on plant community composition. Here, we report that both a bacterial and a fungal pathogen maintained the diversity of a four‐species plant community across five generations; however, significant interactions between the pathogens resulted in less plant diversity when the two pathogens were present than when the fungal pathogen was present alone. Standard models predict that pathogens will maintain plant diversity when they cause a disproportionate loss of fitness in the dominant plant species. In our experiment, however, pathogens maintained plant diversity because the rare species produced more seeds through a compensatory response to pathogen infection. Finally, we found that the influence of pathogens on maintaining plant diversity was 5.5 times greater than the influence of nutrient resource heterogeneity. Pathogens may be a major factor in maintaining plant diversity, and our findings emphasize the importance of investigating the roles of pathogens in natural plant communities.

Moore, Jonathan W.; Semmens, Brice X.

doi: 10.1111/j.1461-0248.2008.01163.xpmid: 18294213

Stable isotopes are a powerful tool for ecologists, often used to assess contributions of different sources to a mixture (e.g. prey to a consumer). Mixing models use stable isotope data to estimate the contribution of sources to a mixture. Uncertainty associated with mixing models is often substantial, but has not yet been fully incorporated in models. We developed a Bayesian‐mixing model that estimates probability distributions of source contributions to a mixture while explicitly accounting for uncertainty associated with multiple sources, fractionation and isotope signatures. This model also allows for optional incorporation of informative prior information in analyses. We demonstrate our model using a predator–prey case study. Accounting for uncertainty in mixing model inputs can change the variability, magnitude and rank order of estimates of prey (source) contributions to the predator (mixture). Isotope mixing models need to fully account for uncertainty in order to accurately estimate source contributions.

Claudet, Joachim; Osenberg, Craig W.; Benedetti‐Cecchi, Lisandro; Domenici, Paolo; García‐Charton, José‐Antonio; Pérez‐Ruzafa, Ángel; Badalamenti, Fabio; Bayle‐Sempere, Just; Brito, Alberto; Bulleri, Fabio; Culioli, Jean‐Michel; Dimech, Mark; Falcón, Jesús M.; Guala, Ivan;

Fortuna, Miguel A.; García, Cristina; Guimarães Jr., Paulo R.; Bascompte, Jordi

doi: 10.1111/j.1461-0248.2008.01167.xpmid: 18318718

Gene flow in plant populations is largely determined by landscape heterogeneity. Both the shape of the pollination kernel and the spatial distribution of trees affect the distribution of pollen grains and the genotypes they harbour, but little is known about the relative contribution of each of these two factors. Using genetic markers we build a spatial network of pollination events between any two trees in a population of Prunus mahaleb, an insect‐pollinated plant. Then, we apply tools from the science of complex networks to characterize the structure of such a mating network. Although the distribution of the number of pollen donors per tree is quite homogeneous, the identity of donors is distributed heterogeneously across the population. This results in a population structured in well‐defined modules or compartments, formed by a group of mother trees and their shared pollen donors. Long‐distance pollination events decrease the modular structure by favouring mating among all available mates. This increases gene flow across the entire population, reducing its genetic structure, and potentially decreasing the role of genetic drift.

Ricketts, Taylor H.; Regetz, James; Steffan‐Dewenter, Ingolf; Cunningham, Saul A.; Kremen, Claire; Bogdanski, Anne; Gemmill‐Herren, Barbara; Greenleaf, Sarah S.; Klein, Alexandra M.; Mayfield, Margaret M.; Morandin, Lora A.; Ochieng’, Alfred; Viana, Blande F.

De Deyn, Gerlinde B.; Cornelissen, Johannes H. C.; Bardgett, Richard D.

doi: 10.1111/j.1461-0248.2008.01164.xpmid: 18279352

Plant functional traits control a variety of terrestrial ecosystem processes, including soil carbon storage which is a key component of the global carbon cycle. Plant traits regulate net soil carbon storage by controlling carbon assimilation, its transfer and storage in belowground biomass, and its release from soil through respiration, fire and leaching. However, our mechanistic understanding of these processes is incomplete. Here, we present a mechanistic framework, based on the plant traits that drive soil carbon inputs and outputs, for understanding how alteration of vegetation composition will affect soil carbon sequestration under global changes. First, we show direct and indirect plant trait effects on soil carbon input and output through autotrophs and heterotrophs, and through modification of abiotic conditions, which need to be considered to determine the local carbon sequestration potential. Second, we explore how the composition of key plant traits and soil biota related to carbon input, release and storage prevail in different biomes across the globe, and address the biome‐specific mechanisms by which plant trait composition may impact on soil carbon sequestration. We propose that a trait‐based approach will help to develop strategies to preserve and promote carbon sequestration.