Identifying niche and fitness dissimilarities in invaded marine macroalgal canopies within the context of contemporary coexistence theory

Graham Epstein,Stephen J Hawkins,Dan A Smale

doi:10.1038/s41598-019-45388-5

Abstract

Contemporary coexistence theory provides a framework for predicting invasiveness and impact of Invasive Non-Native Species (INNS) by incorporating differences in niche and fitness between INNS and co-occurring native species. The widespread invasive kelp Undaria pinnatifida is considered a high-risk INNS, although a robust evidence base regarding its invasiveness and impact is lacking in many regions. Invaded macroalgal canopies at nine coastal sites in the southwest UK were studied over three years to discern whether Undaria is coexisting or competing with native canopy-forming species across different habitat types. Spatial, temporal and depth-related trends in species distributions and abundance were recorded within intertidal and subtidal rocky reef as well as on marina pontoons. A primary succession experiment also examined competitive interactions between species. In rocky reef habitats, Undaria had lower fitness compared to long-lived native perennials, but was able to coexist due to niche dissimilarity between species. In contrast, Undaria was likely to be competing with short-lived native annuals on rocky reef due to large niche overlap and similar fitness. In marina habitats, Undaria dominated over all other canopy formers due to low niche diversification and higher fitness. Generalisations on INNS impact cannot be made across habitats or species, without considering many abiotic factors and biotic interactions.

Highlights

The earliest works on invasion ecology predicted that the most successful invasive non-native species (INNS) would be taxonomically or functionally distinct from the recipient communities to which they are introduced[1,2]
An Invasive Non-Native Species (INNS) will spread into a given number of microhabitats and proliferate at varying densities based on its potential niche, the niche diversity across a site and fitness differences between co-occurring species; eventually leading to a stable coexisting or dominant climax community[14]
The subtidal rocky reef had the most mixed canopy in June, with L. ochroleuca being dominant by biomass but with Undaria, S. latissima and S. polyschides being present in similar densities and only moderately lower in biomass (Fig. 2)

Summary

Introduction

The earliest works on invasion ecology predicted that the most successful invasive non-native species (INNS) would be taxonomically or functionally distinct from the recipient communities to which they are introduced[1,2]. Www.nature.com/scientificreports expand to the detriment of, or at a faster rate than, a co-occurring species, potentially leading to displacement[14] It is the balance between these two continuous variables which determines to what extent an INNS coexists with native species, is excluded by native communities, or becomes dominant[13]. An INNS will spread into a given number of microhabitats and proliferate at varying densities based on its potential niche, the niche diversity across a site and fitness differences between co-occurring species; eventually leading to a stable coexisting or dominant climax community[14]. INNS are often considered ‘passengers of environmental change’, requiring disturbance or degraded ecosystems to establish and spread[17] This concept is derivative of the storage effect whereby certain INNS require fluctuations in the abiotic environment in order to coexist or proliferate within native communities[18]. It is generally recorded at highest abundance where the cover or density of native canopy-forming macroalgae are reduced or absent, often due to abiotic factors such as depth or tidal height[22,23], wave exposure[24], reduced salinity[25] or substrate type, aspect, and stability[23,24,26,27,28]

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Results

Conclusion