Assessment of factors and contexts in previous ERH syntheses
Categorising meta-analyses by the contexts and factors they examine
reveals reasons for previous inconsistencies (Table S1). We examined 16
ERH meta-analyses and meta-syntheses to ascertain whether existing
syntheses capture the component factors of the ERH and the extent to
which they consider context. Our results revealed that individual
syntheses provide only partial tests of the ERH, and many cannot be
directly compared (Fig. 5; Table S1). For example, Mitchell & Power
(2003) tested how enemy diversity changes after invasion, using the
metric of species richness, whereas González-Browne et al. (2016)
tested whether enemy impact varies between exotic and native plants,
using the metric of reproductive potential (Fig. 5). Both meta-analyses
provide valuable information but analyse quite different things. Several
meta-analyses combine different metrics into a single effect size (and,
in two cases, different factors into a single effect size: Lamarqueet al. 2011; Felker-Quinn et al . 2013; Fig. 5), likely
increasing uncertainty and variance around that effect size due to
underlying methodological differences, which can result in apparent
context dependence (Catford et al. 2022).
Ecological context seems to have been under-explored in the 16
meta-analyses (Fig. 5). Not all contexts are relevant for all factors,
and some meta-analyses implicitly account for certain contexts in their
design by, for example, accounting for resource availability by only
including common garden comparisons. However, explicit tests were
included in just 17 of 88 cases (19%) where context could affect
meta-analysis results (Fig. 5). In 15 of those 17 cases (88%), they
were found to be a significant moderator of effect sizes (Fig. 5).
Ecological context therefore has huge potential to explain variation
both within and between meta-analyses yet is rarely accounted for.