Are invasive plants a threat to native biodiversity? It depends on the spatial scale

Science Daily: The phrase "invasive plant species" typically evokes negative images such as broad swaths of kudzu smothered trees along the highway or purple loosestrife taking over wetlands and clogging waterways -- and as such, invasive plants are largely viewed as major threats to native biodiversity. However, research has shown both that invasive species may be one of the most important threats to biodiversity and that plant invasions are rarely the cause for native species extinctions. How can these conflicting pieces of evidence be reconciled?

Kristin Powell, from Washington University, MO, was interested in determining whether some of the differences in the effects that invasive plant species had on biodiversity was in fact due to the spatial scales at which they were studied.

"Biological invasions are often thought to be one of the leading threats to global biodiversity," Powell comments. "However, recent studies and popular literature have begun to question this view, especially in the context of invasive plants, asking 'Are invasive species really that bad?' For example, invasive plants have never been implicated as the sole cause in driving a native plant extinct."

…"Our meta-analysis reconciles the opposing views on invaders by finding that invasive plants cause a large loss in biodiversity at small scales, but this effect essentially disappears at broader scales," explains Powell. "That is, invasive plants are much more likely to cause extinctions at local but not regional or island-level spatial scales." Furthermore, while invasive species may lead to native plant extinctions at the local-level, it may take decades, centuries, or even longer for these plant species to become extirpated at the regional or global level.

…[In] order for invasive species to drive native species extinct at the regional (or broader scale) level the model indicated that the system must have many rare species which are strongly and disproportionally influenced by invasive species relative to the more widespread, common species. In all other scenarios, invasive species would have bigger or similar impacts at the local rather than the regional scale -- which is what the meta-analysis, based on the literature, also showed...

Purple loosestrife on Chilla Moor, shot by Derek Harper, Wikimedia Commons via Geograph UK, under the Creative Commons Attribution-Share Alike 2.0 Generic license