Invasive species are accused of being one of the major causes of modern species extinctions and biodiversity loss, on par with climate change and habitat destruction. A few charismatic examples implicate invasive predators and disease in native species extinctions. For example, the brown tree snake (Boiga irregularis), introduced to Guam in 1952, is implicated in the extinction of 9 of the 11 forest bird species, 5 of which were endemic to the area.
Though there is good evidence that introduced predators and disease reduce biodiversity, there is little evidence that invasive plants cause species extinctions. In some cases the introduction of new species might actually increase biodiversity in an area, if the number of new introductions outweighs extinctions (Sax et al. 2002). If invasive plant species do not cause mass extinctions, and instead could potentially increase diversity where they invade, are they the threat to native biodiversity that we imagine?
A growing body of literature suggests that the impact of invaders on biodiversity depends upon the scale of the experiment – studies done on a small scale find that local biodiversity is lower in invaded sites when compared to non-invaded sites nearby. On a large scale, however, it seems that this effect disappears, and invasive species do not reduce biodiversity (Powell et al. 2011). What is causing this difference in effects between scales?
What they did: Researchers Powell, Chase, and Knight (2013) set out to test this question in a field experiment using three forested study sites, each dominated (>90% cover) by their own invader: Dianella ensifolia in a Florida hammock forest, Lonicera maackii in an oak-hickory forest in Missouri, and Morella faya in a tropical forest in Hawaii. The three researchers predicted that the effect of invasive species on biodiversity seemed scale dependent by affecting the species area relationship (SAR) of native species (A SAR curve predicts how many more species will be observed with an increase in sampling area, see figure).
What they found: The researchers found that population sizes of most native species were reduced 65-91% in invaded areas. Because native species are now more rare, based on pure chance they will be more difficult to detect when sampling at a small scale; this is called a sampling effect. The sampling effect reduces native species’ SAR curve intercept (less likely to find a rare species when you are sampling a small area) and increased slope (as you sample more area, you will find those rare species), supporting the researchers’ predictions. Additionally, invasive species had reduced impacts on the population numbers of rare species, meaning that at large sampling scales, biodiversity of native species was essentially back to un-invaded levels.
Why it’s cool: The results of this study could cause us to question one of the biggest reasons why invasive species are so despised. Perhaps invasive plants rarely cause species extinctions – they appear to harm biodiversity locally by reducing native population sizes, but they do not actually cause native species to go extinct. Do these results mean that we should no longer worry about invasive plants as threats to native biodiversity?
I don’t think so, and in interviews following the publication of their paper, it is clear the authors do not as well. By reducing population sizes of native species, invasions are making natives more vulnerable to other major drivers of extinction (habitat destruction, climate change, etc.). Perhaps invasive species are not the final nail in the coffin for a rare native, but they clearly make it much more vulnerable in an uncertain environmental future.
Powell, Kristin I., Jonathan M. Chase, and Tiffany M. Knight. 2013. “Invasive Plants Have Scale-Dependent Effects on Diversity by Altering Species-Area Relationships.” Science 339: 316-318.