Invasive Species – Passengers or Drivers of Ecosystem Change?

By Brian Mealor, UW Extension Weed Specialist

We often see a laundry list of impacts associated with invasive species when reading about them in any number of media outlets. I have discussed some widely-recognized trends in impacts associated with invasive weeds in previous Roundup articles: decreased reproduction of native plants, decreased species diversity, increased total plant production, decreased wildlife habitat, etc. 

We are able to observe and document these patterns in plant communities and the value – either neutral, negative or positive – of these patterns depends on management goals, personal perspectives or societal expectations regarding natural systems. Our willingness to undertake management actions to reduce the abundance and spread of invasive plants may depend partly on how invasive species and their impacts are discussed.

One ongoing question within the scientific literature surrounding invasive species is whether they are “passengers” or “drivers” of ecosystem changes. MacDougall and Turkington (2005) first used this terminology to describe two potential mechanisms for some of the invasive plant impacts discussed in the first paragraph. 

If a species acts as a passenger, it is a successful invader because it benefits from other changes occurring in an ecosystem, such as pollution, mismanaged grazing, increased wildfire, altered flooding regimes, etc. Although a passenger may be associated with decreased native plant diversity, it was not the direct cause of the decrease, but rather it increased in response to the same ecosystem changes that led to reduced diversity in the first place. 

Someone who says their neighbor has so many weed problems because he or she mismanages grazing on their property would be attributing their observations to a passenger model of invasion. 

Species acting as drivers, however, would directly cause the changes observed in the ecosystem. Driver species are often characterized as having unique growth characteristic, such as nitrogen-fixation where no native nitrogen fixing species occur or the ability to alter disturbance patterns, that allow them to enter a community, dominate numerically and reduce native plant species abundance without any “assistance” from accompanying environmental change. 

It may be difficult to fully determine which, if any, species act as 100 percent passenger or 100 percent driver. Interestingly, manager’s perceptions of how threatening invasive species are may depend on how they are described.

Hart and Larson (2014) evaluated how “framing” of invasive species as passengers or drivers affected public perception of risk, sense of responsibility and willingness to take management actions. 

In their study, over 400 participants – undergraduate students – were presented with “informational” material regarding invasive plants that were described with characteristics that aligned with either the passenger or driver models. Participants perceived more risk from invasive plants that were described with the driver model, or those that causeecosystem changes, and they were more likely to take management action for species described as such than were participants presented with the same species described with the passenger model. 

These results may suggest that public engagement and support may be strongly skewed toward invasive species that are drivers of potentially negative impacts. However, not all species easily fall into one or the other classification as passenger or driver.

Some species initially enter a plant community as passengers by capitalizing on resources made available by disturbance. They may remain as relatively low-abundance components of the community until they reach sufficient numbers to exert competitive pressure on the desirable native plants or they may require additional changes to occur in the system before becoming dominant. 

Once they achieve dominance, they are then capable of acting as drivers of subsequent changes in the ecosystem. 

This “backseat driver” model, as proposed by Bauer (2012), was not investigated in the behavioral study by Hart and Larson, but it has interesting implications for management. If I perceive driver species as a greater threat than passenger species and the two classifications are mutually exclusive, then my efforts largely will focus on driver species. 

However, if some passenger species are able to become drivers given the right set of conditions, then my potential list of important threats becomes larger, and I may need to be more diligent with monitoring of new and existing weed patches. 

From a practical standpoint, maintaining an active monitoring program that will provide valuable information regarding plant community condition as part of a management strategy may allow managers to determine whether they are dealing with passengers or drivers. It may also help in identifying when a backseat driver is beginning to have more of an impact in a particular setting.

The sources used in this article are as follows: 

Bauer, JT. 2012. Biological Invasions 14:1295-1304.

Hart, PS. And Larson BMH. 2014. Conservation Letters 7:545-552.

MacDougall, AS and Turkington, R. 2005. Ecology 86:42-55.

Brian A. Mealor is an assistant professor and Weed Extension Specialist at the University of Wyoming and can be reached at bamealor@uwyo.edu. More information about ongoing weed science research at the University of Wyoming can be found at weedcontrolfreaks.com.