Systematic review and meta-analysis of the Enemy Release Hypothesis as applied to aquatic plants

Abstract

The Enemy Release Hypothesis (ERH) argues that escape from natural enemies allows exotic plants to become invasive in new habitats. This study provides a critical review of this hypothesis in aquatic plant ecosystems using a meta-analytic approach, considering diverse growth forms, multiple performance parameters, and various study methodologies. The key question posed is whether exotic macrophytes in freshwater ecosystems experience decreased enemy pressure compared to native species, and whether this translates to increased performance parameters. We hypothesize that plant growth form influences ERH expression, with the strongest effects in submerged macrophytes, and that ecological complexity obscures enemy release detection in biogeographical and community studies. A search term based on the key question was formulated and yielded 243 studies. Effects from manipulative experiments revealed significant support for the ERH under abiotic stressors such as eutrophication and temperature change, highlighting the dynamic nature of ecological interactions. However, the 53 community level effects showed contrasting findings, suggesting that native species may outperform invasive conspecifics in more stable environments. The evident complexity of ecological factors considered in the studies used here challenges the theoretical expectations of the ERH, demanding further exploration of alternative mechanisms, such as disturbance, biotic resistance, mutualism, and climate suitability.