Structural and functional effects of global invasion pressure on benthic marine communities—patterns, challenges and priorities

Aim: Retrospective (pre- vs. post-invasion) and cross-sectional comparisons of ecosystems exposed to high and low bioinvasion pressure, provide an alternative approach to evaluate shifts in biological communities associated with non-indigenous species (NIS) introductions. In this study, we aimed to examine general patterns of change in community composition, structure and function in six well-studied and globally distributed marine ecosystems that had documented histories of biological invasions. Location: Global. Methods: By considering a range of regional datasets and different sampling approaches, we evaluated trends within and among ecosystems by comparing paired measures of community and functional structure in either space or time. Results: Our analyses revealed different patterns of structural and functional change at ecosystem scales, but direct comparisons across regions were hindered by confounding effects of study designs and other drivers of change. The most prominent shifts in community composition were observed in the retrospective studies, characterised by the greatest relative contribution of NIS. No uniform pattern of change in functional metrics was observed across study regions. However, functional evenness and dispersion showed a tendency to increase in systems under higher invasion pressure, refuting the hypothesis of selective accumulation of specific traits and functional homogenisation within ecosystems exposed to high invasion pressure. Main Conclusions: Accumulation of NIS within broader communities can be a subtle process, with inherent spatial and temporal variability. Nonetheless, not only do species' proportional contributions to communities change over time in areas subjected to high bioinvasion pressure, but trait profiles can incrementally shift, which alters the original ecology of an area. Planned, long-term studies that incorporate a range of measures of environmental drivers and ecosystem response are crucial for better understanding of cumulative, community-level and ecosystem-scale change associated with biological invasions.