Provenance of invaders has scale-dependent impacts in a changing wetland ecosystem

Kathryn L Amatangelo,Lee Stevens,Dov F Sax,Douglas A Wilcox,Stephen T Jackson

doi:10.3897/neobiota.40.28914

Abstract

Exotic species are associated with a variety of impacts on biodiversity, but it is unclear whether impacts of exotic species differ from those of native species with similar growth forms or native species invading disturbed sites. We compared presence and abundance of native and exotic invaders with changes in wetland plant species diversity over a 28-year period by re-surveying 22 ponds to identify factors correlated with observed changes. We also compared communities found within dense patches of native and exotic emergent species with similar habits. Within patches, we found no categorical diversity differences between areas dominated by native or exotic emergent species. At the pond scale, the cover of the exotic grass Phragmitesaustralis best predicted change in diversity and evenness over time, likely owing to its significant increase in coverage over the study period. These changes in diversity and evenness were strongest in younger, less successionally-advanced ponds. Changes associated with cover of P.australis in these ponds were not consistent with expected diversity decreases, but instead with a dampening of diversity gains, such that the least-invaded ponds increased in diversity the most over the study period. There were more mixed effects on evenness, ranging from a reduction in evenness gains to actual losses of evenness in the ponds with highest invader cover. In this wetland complex, the habit, origin and invasiveness of species contribute to diversity responses in a scale- and context-dependent fashion. Future efforts to preserve diversity should focus on preventing the arrival and spread of invaders that have the potential to cover large areas at high densities, regardless of their origin. Future studies should also investigate more thoroughly how changes in diversity associated with species invasions are impacted by other ongoing ecosystem changes.

Highlights

The impacts of exotic species on native species and plant communities are diverse
Can species impacts be predicted based on their provenance? Do species invasions, regardless of provenance, decrease biodiversity at local scales? In this wetland complex, monodominant emergent invaders have likely altered the trajectory of pond community change over 28 years but in ways that are not fully consistent with an expectation of biodiversity loss or of categorical variation by provenance
Invader cover is only minimally correlated with changes in evenness over time and, rather than decreasing richness, invader cover (Phragmites australis) is correlated with suppression of richness gains in some ponds

Summary

Introduction

The impacts of exotic species on native species and plant communities are diverse. In many cases, exotic species have caused or contributed to extinction of native species (Ricciardi 2004; Sax and Gaines 2008; Pysek et al 2017). Exotic species have led to local extirpation of natives and declines in local biodiversity (Vellend et al 2013), whereas in other cases, no such losses in native diversity are apparent (Heard et al 2012). These disparate and often contradictory impacts of exotic species have contributed to two ongoing debates relevant to both our ecological understanding of species invasions and their implications for conservation. Eastern redcedar (Juniperus virginiana) is invading beyond its native range in central USA, where it is converting grasslands to forested habitat, with a corresponding loss of herbaceous plant diversity (Briggs et al 2002), whereas fishes in western USA have often been moved small distances beyond their historic range boundaries, but had large impacts on aquatic systems (Carey et al 2012)

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