Multiple drivers of contrasting diversity–invasibility relationships at fine spatial grains

Abstract

The diversity–invasibility hypothesis and ecological theory predict that high‐diversity communities should be less easily invaded than species‐poor communities, but empirical evidence does not consistently support this prediction. While fine‐scale experiments tend to yield the predicted negative association between diversity and invasibility, broad‐scale observational surveys generally report a positive correlation. This conflicting pattern between experiments and observational studies is referred to as the invasion paradox and is thought to arise because different processes control species composition at different spatial scales. Here, we test empirically the extent to which the strength and direction of published diversity–invasibility relationships depend on spatial scale and on the metrics used to measure invasibility. Using a meta‐analytic framework, we explicitly separate the two components of spatial scale: grain and extent, by focusing on fine‐grain studies that vary in extent. We find evidence of multiple drivers of the paradox. When we consider only fine‐grain studies, we still observe conflicting patterns between experiments and observational studies. In contrast, when we examine studies that are conducted at both a fine grain and fine extent, there is broad overlap in effect sizes between experiments and observation, suggesting that comparing studies with similar extents resolves the paradox at local scales. However, we uncover systematic differences in the metrics used to measure invasibility between experiments, which use predominantly invader performance, and observational studies, which use mainly invader richness. When we consider studies with the same metric (i.e., invader performance), the contrasting associations between study types also disappear. It is not possible, at present, to fully disentangle the effect of spatial extent and metric on the paradox because both variables are systematically associated in different directions with study type. There is therefore an urgent need to conduct experiments and observational studies that incorporate the full range of variability in spatial extent and invasibility metric.

Forest canopy resists plant invasions: a case study ofChromolaena odoratain sub-tropical Sal (Shorea robusta) forests of Nepal

“…Resident ecological communities naturally tend to resist the establishment and spread of incoming species, a phenomenon termed 'biological resistance' (Levine et al 2004, Nunez-Mir et al 2017. It has been hypothesized that species-rich communities have a lower vulnerability to invasion at the local scale (Levine et al 2004), however, the diversity resistance hypothesis is not always supported by empirical studies (Byun andLee 2018, Smith andCôté 2019). Rather there are also instances of congruence of higher diversity and higher invasion particularly at larger spatial scales (Stohlgren et al 2006).…”

Biotic resistance to invasion is ubiquitous across ecosystems of the United States

“…However, inconsistent support for the biotic resistance hypothesis raises the question as to whether efforts to target native biodiversity have any effect on non‐native species invasions, particularly in plant communities (Fridley et al ; Jeschke et al ). Experimental studies provide substantial evidence that higher native species richness results in lower invasion (Levine et al ; Fridley et al ; Byun et al ; Peng et al ; Smith & Coté ). Observational studies at the site level vary, but in the few observational studies that extend beyond a local scale to include multiple sites (e.g.…”

“…Observational studies at the site level vary, but in the few observational studies that extend beyond a local scale to include multiple sites (e.g. Stohlgren et al ; Iannone et al ), all find that higher native richness relates to higher numbers of non‐native species (Fridley et al ; Peng et al ; Smith & Coté ).…”

“…The lack of consensus among studies was previously thought to be an artefact of spatial scale: habitat quality and habitat heterogeneity drive diversity across broad spatial extents, whereas biotic interactions drive diversity within the smaller scale plant neighbourhood (Levine ; Shea & Chesson ; Davies et al ; Sandel & Corbin ; Von Holle ). However, two recent meta‐analyses suggest that the contradiction may be driven by differences in experimental vs. observational studies (Peng et al ; Smith & Coté ). While observational studies have the power of describing variation across broad spatial extents, these studies have limited control over extrinsic environmental or anthropogenic variables that affect native and non‐native plant richness (Rejmanek ; Nunez‐Mir et al ).…”

“…Over large areas, heterogeneity in these factors may drive covariation between native and non‐native richness even when biotic resistance is in effect (Levine ; Shea & Chesson ; Levine ). As large, standardised data sets become available and methods for analysing these data advance, we have an opportunity to retest foundational ecological and invasion hypotheses underlying conservation decision making (Guo et al ; Nunez‐Mir et al ; Smith & Coté ).…”