Are Invasive Plants Really All That Bad?

TeaselThe recent story on Wyoming Public Radio by Willow Belden “As agencies control invasive species to protect diversity, some worry about side effects” poses an interesting question on whether impacts caused by invasive species have a net negative impact on invaded systems. The question – “Are invasive species really as bad as they are often perceived?” – seems to be gaining some traction these days while, at the same time, more organizations are participating in efforts to reduce invasive species distribution and abundance. The question digs deeper than the direct management of an undesirable species and begs a response different than one solely answered from biology or ecology. This discussion could quickly go many different directions – from esoteric rabbit trails about why diversity and native species are important to pragmatic challenges like water management, forage availability or toxicity to livestock.  Here, I will explore the impacts of invasive plant species instead of all types of invasive species to keep the discussion more focused.

To accurately determine how invasive plants affect natural systems requires documenting the impacts, both negative and positive, and determining the net result of exotic species invasion and dominance on a particular system. Impacts can occur at multiple levels: ecosystem, community, population — even genetic characteristics of a system may be affected by invasion.  The difficulty often arises when attempting to quantify the cumulative impacts of a single species, or suite of species, once exponential growth and dominance has occurred across highly variable ecosystems over large spatial scales.

An international research group led by Montserrat Vilà, an ecologist from Spain, analyzed almost 200 journal articles representing over 1000 field studies investigating ecological impacts of invasive plant species (Vila et al. 2011). One of the group’s primary questions was whether clear patterns exist regarding impacts of invasive plants across the globe. They found that some impacts of invasive plants vary widely across locations, depending on species and types of impact they investigated. However, 11 of 24 impact “types” showed quite consistent results across the studies. Invasive plants consistently reduced fitness (reproductive capacity) of resident plants and decreased species abundance and diversity. Overall plant production was consistently higher when invasive plants were present. Animal abundance was generally reduced in areas with invasive plants when compared to non-invaded locations. Invasive plants also enhanced soil microbial activity, increased available nitrogen, and decreased soil pH. Nitrogen-fixing invasive plants (such as Russian olive) had a more dramatic impact on soil nitrogen than non-nitrogen-fixing species. The authors conclude that, although impact magnitude and direction varied, invasive plants exert strong effects on recipient communities and ecosystems.

Some of these impacts are relatively easily observed – especially if vegetation monitoring data are routinely collected. Decreased species diversity or decreased amount of forage accompanied by an increase in a specific invasive plant could be quantified with a vegetation monitoring program, but other impacts, such as altered nitrogen dynamics or microbial populations in the soil are not as easily observed. The potential broad-reaching implications of these less-noticeable changes are probably not well understood by managers and scientists alike, but that does not mean they are less significant. Recent research in Idaho and Wyoming indicates that Russian olive presence alters nutrient (particularly nitrogen) cycling in streams where it has invaded, potentially leading to cascading impacts on multiple species (Mineau et al. 2011). Other examples of how invasive species alter nutrient cycling abound in the literature. Such effects alter how the system works, and may, in some cases, reduce the site’s suitability for desirable plant species and facilitate further invasion of other invasive plants. This ‘invasional meltdown’ as described by Simberloff and Von Holle (1999) leads to an increased effect of various invasive plants on a location – the combined effects of the multiple invaders is more severe than if a sole invasive species occurred.

Drastic changes to site characteristics, such as reduced species diversity or altered soil chemistry, can persist even after the invader is successfully removed from a site. Such ‘legacy’ effects may require efforts beyond removal of the unwanted plants including restoration practices designed to mitigate the persistent changes induced by the invaders (Corbin and D’Antonio 2012). More research is needed to understand the best combinations of restoration practices on sites dominated by invasive plants over the long-term. Setting clear goals for the long-term use of a site may assist in developing a restoration plan that may not closely approximate the ‘pristine’ pre-invasion condition, but that may still meet the goals of the individual landowner. Reestablishment of desirable plant species that improve habitat conditions over the invasive-dominated state, but not quite ‘what was there before invasion’ may be an acceptable alternative in some cases.

The question of whether invasive plants are generally a ‘bad’ thing requires attaching values to the impacts that they cause. An ecological impact, in itself, is neither good nor bad until an individual or society determines how they will be affected by such a change in the current environment. Those of us directly involved in weed science and management have likely internalized the negative impacts of invasive species from years spent in efforts to reduce their populations and, thereby, reduce associated impacts. I think the invasive plant management community emphasizes the need for control so strongly, that the overall approach can easily be misinterpreted. Complete eradication of all invasive plants from Wyoming is widely recognized as an unattainable goal. Because of this, the need to prioritize efforts on species and locations that offer high potential benefits is needed and encouraged, although opportunistic projects also occur in some cases. Do invasive plants impact natural systems, altering their characteristics to the detriment of native plants diversity, wildlife habitat, and agricultural production? Multiple examples in the literature support consistent, often long-term, impacts. Are they all bad? For now, I will leave a more thorough discussion of how individuals and society value ecosystem goods and services, and their potential loss due to plant invasion, for someone with a better understanding of the human psyche than me.

References

Corbin. J.D. and C.M. D’Antonio. 2012. Gone but not forgotten? Invasive plants’ legacies on community and ecosystem properties. Invasive Plant Science and Management 5:117-524.

Mineau, M.M., C.V. Baxter and A.M. Marcarelli. 2011. A non-native riparian tree (Eleagnus angustifolia) changes nutrient dynamics in streams. Ecosystems 14: 353-365.

Simberloff, D. and B. Von Holle. 1999. Positive interactions of nonindigenous species: Invasional meltdown? Biological Invasions 1:21-32.

Vila. M. et al. 2011. Ecological impacts of invasive alien plants: a meta-analysis of their effects on species, communities and ecosystems. Ecology Letters 14: 702-708.

This article originally appeared in the Wyoming Livestock Roundup on August 27, 2013.