Riparian Communities Highly Invasible to Exotic Plants

In a recent paper (Ecology, Vol 92, Issue 6), Anne Eschtruth and John Battles investigated the widely held belief that riparian communities are highly invasible to exotic plants. This idea is based primarily on comparisons of the extent of invasion in riparian and upland communities. However, because differences in the extent of invasion may simply result from variation in propagule supply among recipient environments, true comparisons of invasibility require that both invasion success and propagule pressure are quantified. In this study, Eschtruth quantified propagule pressure in order to compare the invasibility of riparian and upland forests and assess the accuracy of using a community’s level of invasion as a surrogate for its invasibility. Extent of invasion was found to be a poor proxy for invasibility. The higher level of invasion in the studied riparian forests resulted from greater propagule availability rather than higher invasibility. Further, this study suggests that failure to account for propagule pressure may confound our understanding of general invasion theories. Ecological theory suggests that species-rich communities should be less invasible. However, Eschtruth and Battles found significant relationships between species diversity and invasion extent, but no diversity–invasibility relationship was detected for any species. Study results demonstrate that using a community’s level of invasion as a surrogate for its invasibility can confound our understanding of invasibility and its determinants.