Meta‐Analysis and Systematic Reviews in Ecology

Abstract

Research synthesis provides the methodological and statistical tools for the scientific integration of results of ecological research, and consists of methods and standards for systematic review and meta‐analysis. Systematic reviews use scientific approaches for searching and selecting publications, and meta‐analyses provide the statistical tools to quantitatively summarise study results. These tools can be used to synthesise the outcomes from many different kinds of ecological studies, including both experimental and observational studies taken from published literature, and the outcomes of studies conducted across different systems by coordinated research groups. Based on the rationale for such synthesis efforts, the article presents the types of effect sizes used as well as statistical analyses to test hypotheses.

Key Concepts

  • Systematic reviews and meta‐analyses are essential for generalising the outcomes of ecological, conservation and evolutionary studies.
  • Meta‐analysis is the basis for evidence‐based decisions and conclusions in conservation and fundamental ecology
  • Systematic review is a set of scientific methods for searching, inclusion and reporting results in research syntheses.
  • Meta‐analysis provides the statistical methods for the quantitative synthesis of the results of different studies.
  • Meta‐analytic methods offer ways to model and account for heterogeneity in the outcomes of different studies
  • Meta‐analyses offer the potential to resolve apparent discrepancies in study results and to reconcile longstanding scientific controversies
  • Meta‐analysis has altered the way ecologists and evolutionary biologists develop general insights beyond the specific contexts of individual studies.

Keywords: quantitative synthesis; meta‐analysis; community ecology; effect sizes; statistical methods; review techniques

Figure 1. Organisational flowchart of a meta‐analysis. Different studies (S1–S3) report means and standard deviations of a given response variable for a treatment and a corresponding control. From each study, the treatment effect size is calculated from the difference or ratio of the averages (red), whereas the variance within each study is pooled to allow the estimation of sampling variances for each study (blue). The general tendency of the experiments can then be synthesised by using unweighted or weighted meta‐analyses. The significance of the effect across studies can be tested by calculating 95% confidence intervals around the average effect size.
Figure 2. Illustration of different effect sizes along sample data set (ELSIE database on fertilisation effects on autotroph biomass as used by Elser et al., ). Effect sizes were measured as Hedges' d and ln R, showing a bow‐tie distribution reflecting the conservation of the sign (negative Hedges' d always corresponds to a negative R and vice versa) but otherwise wide variation as the same absolute change in autotroph biomass might correspond to different relative changes.
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Hillebrand, Helmut, and Gurevitch, Jessica(Aug 2016) Meta‐Analysis and Systematic Reviews in Ecology. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0003272.pub2]