by Dr. Dana Blumenthal
 
Kudzu is an easy plant not to like. One look at the trees and buildings this vine engulfs gives you a feel for the problem. Statistics tell the rest of the story. Kudzu now covers 3 million hectares in the United States, killing native trees by shading and girdling them, and costing hundreds of millions of dollars annually in lost forest productivity and control efforts (Forseth and Innis 2004).
 
Kudzu is an introduced species, brought to North America from Asia in 1876 (Forseth and Innis 2004). It is a good example of the subset of introduced plants, called invasive plants, that become too abundant and cause problems. More broadly, to take the United States as an example, approximately 4,300 plant species from other countries now live in the US. Most of these are well behaved members of plant communities, politely coexisting with native species. A scant 11% (485 species), are considered to be problematic for either agricultural or natural ecosystems (http://plants.usda.gov/). And yet these species have led to a litany of problems. For example:
 
Ø   Cheatgrass increases the frequency of wildfires, killing native plants and ensuring its continued dominance.

Ø   Cordgrass overruns mudflats needed by shorebirds.

Ø   Purple loosestrife turns diverse wetlands into monocultures (Read a case study on Purple Loosestrife).

Ø   Knapweeds displace livestock forage.

Ø   Tamarisk steals precious water from western rivers.
 
In the United States alone, invasive plants are estimated to cost $25 billion annually in control costs and lost agricultural productivity (Pimentel et al. 2005). Globally, they are considered to be the one of the most important threats to biodiversity (Vitousek et al 1997).
 

Stemming plant invasion

What can be done? Where invasive plants are particularly problematic it can make sense to kill them. This is something we know how to do. Some sort of herbicide can kill most plants. And in many agricultural ecosystems, herbicides can be a cost-effective, albeit still expensive, long-term solution. Controlling invaders in natural ecosystems is more complicated. First, where invasive species occupy large areas, applying herbicides can simply be too expensive. In the Western United States, for example, cheatgrass, which increases the frequency of wildfires, is estimated to occupy more than 40 million hectares, an area the size of the state of Montana (DiTomaso 2000).
 
It can also be difficult to kill invasive species without also killing desired native species. While many herbicides are selective, influencing particular types of plants without harming others, they are rarely selective enough to kill an invader while sparing the variety of native species present in a natural ecosystem. Most importantly, however, invasive plants have generally invaded for a reason. Unless the underlying cause of the invasion is addressed they are likely to re-invade once control efforts cease. Thus achieving long-term control of invasive plants will require an understanding of why they invade.
 

State of the science

Why are some introduced species so successful? This is the central question of invasion biology. Unfortunately, scientists are far from having a complete answer. Having studied many possible causes, we have found evidence for most of them, but know little about which causes are actually most important. For example, we know that many plants escape specialized enemies (diseases and herbivores) upon moving to a new continent, and that species that escape enemies tend to be more invasive (DeWalt et al 2004, Mitchell and Power 2003). We also know that introducing missing enemies can sometimes control invasive species (Moran et al. 2005). At least as often, however, enemies appear to be unimportant to invasion (Colautti et al. 2004).
 
We know that native species can help resist invasion (Levine et al 2004). Therefore, anthropogenic (human caused) changes that make ecosystems less suitable for native species can facilitate invasion. For example, soil disturbance, logging, and nitrogen or phosphorus pollution from agriculture and industry can all lead to invasion (Davis et al 2000). And yet, even relatively undisturbed areas can also be invaded (Lonsdale 1999). Invasive plants may succeed because of novel relationships with soil microorganisms, novel competitive interactions with native plants, evolution in their new range, or an absence of closely related native plants (Mitchell et al. 2006). Finally, in some cases, none of these processes may be necessary for invasion to occur. It is possible that, just by chance, some invasive species may be exceptionally well adapted to their new environment. Thus, despite tremendous progress in understanding plant invasion in recent years, what we now know is what can cause invasion. The challenge for scientists is to figure out what does cause invasion.
 

Human answers to a human problem

One clear conclusion from invasion biology is that in many ways plant invasion is a human problem. Perhaps most importantly, we bring in the invaders — often on purpose. Kudzu, for example, was widely planted in the southeastern U.S. in the early part of the twentieth century. it was used as an ornamental, for erosion control, and for livestock forage (Forseth and Innis 2004). Its long-term costs – including hundreds of millions of dollars in lost forest productivity and control efforts – were not understood until much later.
 
Essentially, all invasive plants were originally transported by humans. Given the extent of international trade, it will not be possible to completely stop plants from moving among continents. Nevertheless, by carefully regulating which species can be imported, we could greatly reduce the number of new invaders. Because it remains difficult to predict which species are most likely become invasive (Caley and Kuhnert 2006), effective regulation would have to employ the precautionary principle, preventing movement of many species that might or might not become invasive.
 
It is also clear that when humans disturb natural ecosystems we create habitats for invasive species. By reducing disturbance where possible, and restoring natural ecosystems where necessary, we can reduce the habitat available for invasive species, thereby decreasing both population sizes and rates of spread. In this respect there is synergy between our interests in controlling invasive species and other conservation goals. Conservation aims to maintain healthy populations of native species. These same native species can compete against and prey upon invasive species (Levine et al 2004), and may be one of our most effective tools in combating invasion.
 
Dr. Dana Blumenthal works at the Agricultural Research Station-Colorado, United States Department of Agriculture. 
 

References:

Caley, P. and P.M. Kuhnert. 2006. Application and evaluation of classification trees for screening unwanted plants. Austral Ecology 31:647-655.

Davis, M.A., Grime, J.P. & Thompson, K. 2000. Fluctuating resources in plant communities: a general theory of invasibility.Journal of Ecology 88, 528-534.

DeWalt, S.J., Denslow, J.S. & Ickes, K. 2004. Natural-enemy release facilitates habitat expansion of the invasive tropical shrub Clidemia hirta. Ecology 85, 471-483.

DiTomaso, J.M. 2000. Invasive weeds in rangelands: Species, impacts, and management. Weed Science 48:255-265.

Colautti, R.I., Ricciardi, A., Grigorovich, I.A. & MacIsaac, H.J. (2004) Is invasion success explained by the enemy release hypothesis? Ecology Letters 7, 721-733.

Forseth, I.N., and A.F. Innis. 2004. Kudzu (Pueraria montana): History, Physiology, and Ecology Combine to Make a Major Ecosystem Threat. Critical Reviews in Plant Science 23:401-413.

Levine, J.M., Adler, P.B. & Yelenik, S.G. 2004. A meta-analysis of biotic resistance to exotic plant invasions. Ecology Letters. 7, 975-989.

Lonsdale, M.W. 1999. Global patterns of plant invasion and the concept of invasibility. Ecology 80:1522-1536.

Mitchell, C.E. & Power, A.G. 2003. Release of invasive plants from fungal and viral pathogens. Nature 421, 625-627.

Mitchell, C.E., A.A. Agrawal, J.D. Bever, G.S. Gilbert, R.A. Hufbauer, J.N. Klironomos, J.L. Maron, W.F. Morris, I.M. Parker, A.G. Power, E.W. Seabloom, M.E. Torchin, and D.P. Vasquez. 2006. Biotic interactions and plant invasions. Ecology Letters 9:726-740

Moran, V.C, J.H. Hoffman, and H.G. Zimmermann. 2005. Biological control of invasive alien plants in South Africa: necessity, circumspection, and success. Frontiers in Ecology and the Environment 3:77-83.

Pimentel, D., R. Zuniga and D., Morrison. 2005. Update on the environmental and economic costs associated with alien-invasive species in the United States. Ecological Economics 52:273-288.

Vitousek, P.M., C.L. D’Antonio, L.L. Loope, M. Rejmanek, and R. Westbrooks. 1997. New Zealand Journal of Ecology 21:1-16.