'Bigger data' on scale-dependent effects of invasive species on biodiversity cannot overcome confounded analyses: a comment on Stohlgren & Rejmánek (2014)

Abstract

A recent study by Stohlgren & Rejmánek (SR: Stohlgren TJ, Rejmánek M. 2014 Biol. Lett. 10. (doi:10.1098/rsbl.2013.0939)) purported to test the generality of a recent finding of scale-dependent effects of invasive plants on native diversity; dominant invasive plants decreased the intercept and increased the slope of the species-area relationship. SR (2014) find little correlation between invasive species cover and the slopes and intercepts of SARs across a diversity of sites. We show that the analyses of SR (2014) are inappropriate because of confounding causality.

Keywords: causality; invasive species; species–area relationship.

Figure 1.

Overview of the factors that influence SAR parameters, c (intercept) and z (slope). Black arrows indicate the causal effects of invaders on the SAR, while grey arrows are just a few of the many confounding variables that were controlled using the paired design by PCK, but uncontrolled by SR.

Figure 2.

Relationship between a proxy for productivity (log₁₀ per cent plant cover) and the slope and intercept of the SAR from the Modified Whittaker vegetation plots (a,b) and the USDA Forest Health Monitoring plots (c,d) from the data provided by SR. (Online version in colour.)