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Knowing (and Sharing) the Difference Between ‘Non-Native’ and ‘Invasive’

Jacquart, Ellen 9/13/2011

At the heart of the recent Nature article by Davis et al. (2011) is a common misconception; the authors have confused “non-native” species with “invasive” species. For those of us who work on invasive species issues, the difference between the two terms is one of the first things you explain to land managers, making the distinction by reciting the U.S. federal definition of invasive species (an invasive species is "…an alien (or non-native) species whose introduction does, or is likely to cause economic or environmental harm or harm to human health").

Most people are reassured to find out that the majority of non-native species cause no harm whatever, and that some are even beneficial. There is just this small subset of troublemakers we call invasive species — those non-native species that cause harm — to address. In case there is any lingering concern, we assure everyone that, no, we are not intent on eradicating soybeans or petunias from the United States. Those species are nonnative, not invasive.

I am not sure how this basic misconception found its way into a peer-reviewed journal of such repute, but I’m afraid its appearance there tells us we’re still not very good at talking about invasive species, even to our own colleagues. Perhaps there are practitioners out there focused on trying to control non-native species that aren’t truly causing harm, but in my experience they’re a rare exception. Most of us spend a great deal of time figuring out which non-native species are just innocent bystanders and which are real threats to conservation targets.

In fact, there are simple criteria for assessing and ranking invasive species at a site so we can focus efforts on the highest priority species. There are different systems of ranking out there, but my favorite is the one developed by the Conservancy’s Global Invasive Species Team (GIST) back in the late 1990’s in their Weed Management Plan Template. Since the loss of GIST due to budget cuts in March 2009, the University of Georgia has been gracious enough to host all the content from the former GIST website, including the Weed Management Template ( products.html).

The template poses four questions to land managers to help them prioritize among the invasive species at their sites and determine which species pose the greatest risk to conservation targets and which are a minor issue with no control needed.

The four questions are:

  1. What is the current extent of the invasive species (with high priority going to the least prevalent species)?
  2. What are the current or expected impacts of the invasive species (with high priority going to impacts that will threaten conservation targets)?
  3. What is the value of the habitat infested or that could potentially be infested (with high priority going to habitats which hold conservation targets)?
  4. How difficult is the species to control (with high priority going to species easy to control)?

These questions are specifically designed help insure optimal resource use relative to the threat posed by invasive species.

This process is a common-sense approach to strategically taking on the invasive species problem. Importantly, the approach also involves identifying invasive species that are not yet on site but have potential to invade — and the prevention of those invasions becomes the highest priority for the land manager.

Admittedly, sometimes we don’t have all the information we would like in order to answer those questions, particularly on what the current and expected impacts of a species might be. It can be difficult to know whether that little patch of teasel is going to stay put or move quickly through the prairie, displacing native plant species, decreasing habitat for grassland birds, etc. Like every other aspect of ecological management, we have to make our best guess based on all known information. Monitoring to make sure that best guess was correct is part of the Weed Management Plan Template, as is changing the management strategy if it turns out the guess was wrong.

Ultimately, I guess, I’m wholeheartedly agreeing with the Davis et al. article’s main premise. It is important to focus our attention on the non-native species that cause the most harm. Thanks to GIST, we’ve had the tools to help us do that for many years. Our challenge — one that we are failing to meet — is to share these tools internally and externally to help inform these kinds of debates.

Image: Hedgehog in New Zealand (where it is non-native). Image credit: Mouse/Flickr