Inconspicuous impacts: Widespread marine invader causes subtle but significant changes in native macroalgal assemblages

Graham Epstein,Andrew Foggo,Dan A Smale

doi:10.1002/ecs2.2814

Abstract

AbstractInvasive species are a major cause of global biodiversity decline; however, under certain environmental settings, some invaders can co‐exist with native species with little detectable impact. Even so, in many cases the realized impact of invasive species may be underestimated due to procedural or temporal constraints related to observation or experimentation. The invasive kelp, Undaria pinnatifida, is considered to have limited impact on macroalgal assemblages on rocky reefs of the northeast Atlantic, although this is largely based on correlative or observational findings. Here, a high intensity press‐removal manipulation was maintained for two years at a heavily invaded, Undaria dominated study site to improve current understanding of the potential impacts of Undaria on native macroalgal assemblages. Population and community effects as well as organismal performance effects (biochemical measures of condition and stress) were examined to investigate the potential for cryptic impacts. Where Undaria was removed, there was no difference in understory macroalgal assemblages; however, for three native kelp species, significant increases in abundance, biomass, and condition were recorded. The two perennial native kelps (Laminaria digitata and Saccharina latissima) exhibited small and inconsistent increases in all impact metrics where Undaria was removed, and therefore, the overall effects of Undaria on their populations are likely to be negligible. However, the native annual kelp, Saccorhiza polyschides, was consistently and significantly higher (3–6 times when compared to controls) in abundance and biomass under reduced competition from Undaria and exhibited significant changes in organismal‐level responses which indicated improved condition of sporophytes. Whether the potential replacement of this native species could alter ecosystem functioning requires further investigation. Targeted long‐term manipulative experiments can identify previously undetected impacts of invasive species in coastal ecosystems. Caution must be used when broadly classifying invasive species as having limited ecological impacts on recipient communities. Subtle impacts manifesting at the organism, population, and community levels should be considered before robust management prioritizations can be made.

Highlights

Human-mediated introductions of species into areas outside of their native range have caused, and continue to cause, irreversible changes to the natural environment (McKinney and Lockwood 1999, Gallardo et al 2016)
Quantifying the per capita effect of an invasive species across a given ecosystem is highly challenging because (1) perceived impacts vary across both space and time; (2) perceived impacts vary between studies conducted within the same area due to different methodological approaches or chosen response variables (Thiele et al 2010, Hulme et al 2013, Thomsen et al 2014)
Population- and community-level responses At the start of the study, prior to the first manipulation, the abundance of Undaria, the abundance and percent cover of native canopy-forming macroalgae and the percent cover of understory macroalgae did not differ between treatments

Summary

Introduction

Human-mediated introductions of species into areas outside of their native range have caused, and continue to cause, irreversible changes to the natural environment (McKinney and Lockwood 1999, Gallardo et al 2016). Many invasive species do not drive ecosystem change and some can co-exist with native species with little detectable impact (Williamson and Fitter 1996, Parker et al 1999, Ricciardi and Cohen 2007, Simberloff 2011, Schlaepfer et al 2012). It can, be argued that all species introductions must have some impact on recipient communities by their presence—reducing the availability of space, food, water, or other resources for co-occurring species (Simberloff et al 2013, Russell and Blackburn 2017). Persistent and prolonged effects at the organism level could translate to population- and community-level changes; considering a range of invasive species impacts across biological scales is important for developing evidence-based approaches to conservation and management

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