Dominance of non-native species increases over time in a historically invaded strandline community
We lack a robust understanding of whether exotic species, in addition to causing changes immediately after establishing, might continue to increase in dominance long after invasion events occur. To address this, we resurveyed strandline plant communities, which are likely to have been invaded for over two centuries.
We resurveyed the richness and cover of native and exotic plants in 2008 and 2009 at 18 sites, which had originally been surveyed in 1998. We examined whether native and exotic dominance had changed, whether native-rich sites were less likely to be impacted by exotics over time, whether changes in dominance were driven by large changes in a small number of outlier species or by small, incremental changes among many species and whether disturbance mediated any of these relationships.
Exotic dominance increased across sites. Initial native diversity was unrelated to patterns of exotic dominance during resurveys. The identity of species that were outliers with respect to changes in distribution or cover varied between resurvey years. Significant changes in exotic-to-native richness ratios at sites were detectible with or without the inclusion of outlier species, but changes in abundance ratios were only significant when outlier species were included. Disturbance across sites was not correlated with species richness, cover, or changes in dominance.
In this historically invaded community, exotics have increased in dominance over the last decade. This change is not due solely to the success of a few hyper-dominant species, but also to the cumulative effect of small changes in distribution among many species. It remains unclear whether patterns observed are due to invasion processes that are playing out very slowly through time or to some other explanation. Our findings highlight the need for a more robust understanding of the long-term dynamics of species invasions.