Guilty as charged? Invasive species may not be that bad for biodiversity

behindbarsInvasive 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?

sarWhat 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.

Elizabeth Schultheis

About Elizabeth Schultheis

I have always been fascinated by invasive species and their ability to outcompete native species while taking over and transforming habitats. The number of invasive species is growing year-by-year, as plants, animals, and microbes are introduced into habitats where they did not historically occur. Invasive species are often destructive, costing billions in damages to native ecosystems and human interests around the world annually. Yet, despite all the problems they cause, we still do not know what causes some species to be invasive and not others. My research addresses this question by testing whether invasive species are those that are not strongly controlled by competitors, predators, and herbivores outside their native range. That is, they are successful invaders because they have left their natural enemies behind.
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4 Responses to Guilty as charged? Invasive species may not be that bad for biodiversity

  1. Elizabeth SchultheisLiz Schultheis says:

    Great article sent to me by Mridul about how invasive species cause extinction debts for natives by disrupting their meta-population dynamics – reducing size and quality of habitable patches, and making spaces between the patches more difficult to disperse through:
    Gilbert & Levine (2013) “Plant invasions and extinction debts” PNAS 110(5) 1744-1749

  2. You mention that people despise invasives, which got me thinking about why that is. For ecologists, who spend a lot of time thinking about biodiversity, I think you’re right in that they are concerned with effects of invasives on native species. For the general public though, I wonder if that’s such a big concern. It seems like most invaders that people really care about are “pest” species that affect people’s daily lives. Zebra mussels cut open your foot when you go swimming. Kudzu overgrows your house and takes it back for nature. I wonder how many of the invasive species of concern have a big effect on people’s lives, which may or may not be correlated with their effect on diversity.

  3. Matt Chew says:

    Species introductions are a lot like weather events. Some are welcome (e.g., most of our foods), many go unnoticed, others are unwelcome, and there is little an ‘average person’ can meaningfully do about them.

    Our transportation infrastructure was neither conceived nor designed to prevent entrainment of plants and animals in ‘the currents of commerce.’ Nor could it be; the costs are prohibitive. Avoiding species introduction would require a radical reconfiguration of global commerce; we just aren’t that organized. It’s easier to blame the biota and label them bad actors than it is to initiate effective action to preclude their redistribution. Many thousands are already well established. Efforts to keep ‘natural’ places natural or ‘wild’ places wild by removing introduced species ironically require ongoing intervention: in effect, domestication.

    Governments love calling species ‘invasive’ because in doing so they only rarely encounter political resistance. Only a few charismatic species like mute swans develop a ‘pro’ constituency. Those folks, as vocal and passionate as the anti-alien activists, are usually dismissed as crackpots regardless of how principled their positions may be.

    High productivity and high ‘beta’ diversity (the difference in species rosters between any two places) are bandied about as if they were self-evident, inarguable ‘goods’. They are certainly common preferences, but they are neither universal nor objective. And they’re often mutually exclusive. So “fasten your seat belts. It’s going to be a bumpy night”.

  4. Pingback: Are invaders bigger and better in their introduced ranges? - Big Science, Little Summaries

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