by Tony Tweedale
Everyone acknowledges that invasive plants (“weeds”) can harm native ecosystems. But land stewards as the USFS, and BLM seem to see only the invasives, and disregard a crucial variable: something has to change for an invasive to become established in an ecosystem! Mankind cause the most disruptive (fast, radical) changes in tightly controlled and hyper-efficient ecosystems. For example, logging and motorized recreation lay bare soil, promoting invasive plants. Realize that ‘native’ plants were successful due to their fit in an environment.
The subtleties of ecology mean that, if one wants to protect against weeds without the broadcast of dangerous and disruptive herbicides, discernment is key. And the study of invasion dynamics is a fast-developing discipline.
Many studies confirm one big difference between native and invasive plans: invasive plants tend to put all their photosynthesis and other resources into growth and reproduction, in a bid to establish themselves in a foreign environment. Weeds also frequently have potent offensive chemical weapons to attack competitors with (allelopathy), e.g. knapweed (Bais et al.). In fact, spotted knapweed even attacks with root chemicals defensively, e.g. after insects attack it (Alper).
On the other hand, native plants are specially adapted to their tiny niche (say, a small set of hillsides in a certain climate, altitude and soil type) and the resources that area has for plants. In the once-arid interior West, native plants compete successfully under low moisture, low light and low nitrogen. Manipulation of those variables to better-favor native conditions caused native plants to take back territory that weeds had conquered! (Seabloom et al.). Similarly, the weeds (native in E. Europe/SW Asia) were preferred by herbivores over native species, at least in western interior Canada (Ecol. Letters:6:712; 2003).
Other recent research directly implies that mankind has time to observe and think before blindly rushing in with blunt, ineffective tools such as herbicides. Any type of intervention can be risky; e.g. the gall flies released to feed on knapweed seeds are a favorite meal of the field mouse that carries the human pathogen hanta virus (Pearson & Callaway). But there is much more plant hybridization preceding invasiveness than was believed until recently (Ellstrand & Schierenbeck), so we can detect weed invasions early. Similarly, and contrary to another dominant assumption, the leading edge of an invasion of weeds grew slower than the middle of the weed area, at least for the wind-pollinated weeds studied (Davis et al.). Also, while there’s some evidence that invasive plants do some adaptation once introduced to a foreign environment, much recent work presented at a conference disproved that (anonymous), so weeds appear to have vulnerabilities.
Finally, it may not be the high diversity per se that protects healthy native ecosystems from invasiveness. Rather it appears that the exquisite, efficient sharing of the available resources in a given ecologic niche causes far less resources to be available to invaders! (Tilman). However, in an example of the complexity of ecosystems, recent controlled experiments found that native diversity both encouraged and discouraged invading plants; with invading Yellow Starthistle causing more loss of native grass biomass as diversity increased, after controlling for the effects of individual species (Zavaleta & Hulvey). That may be due to the scarcity of free resources in diverse ecosystems, the authors state, reversing Tilman’s theory. Yet it was the timing of species loss in the native system that depleted resistance to invasion, because the presence of late-season annual forbs in diverse systems provided resistance to invasion by the starthistle. Unfortunately, it’s those crucial, late season broad-leaf plants that are killed by the so-called “selective” herbicides that the West is drenched in.
Such complexity of response; and the vulnerability of complex, tight (resource-efficient) ecologic niches; constitute a lesson for humans: let nature be. Our massive disturbances of ecosystems not only encourages invasive species, they cause many un-knowable consequences. It’s been said: “Complexity is easy to generate, far harder to control.”
Anonymous ‘Are Invasive Species Born Bad?’ Science:305 (20 Aug. 2004, ‘Science Selections’ section), reporting on data presented at the 2004 Ecological Society of America, Portland OR.
HP Bais, RM Callaway et al. ‘Allelopathy & Exotic Plant Invasion’ Science:301:1377-80 (3 Sept. 2003). See also the news story on p. 1337 of the issue (A Fitter, ‘Making Allelopathy Respectable’). In addition to (-)catechin’s oxidative damage directly on competing root systems, a 3 Nov. 2003 talk at the U. of Montana by a member of this team revealed that (+)catechin has potent but complex effects on nitrification and other soil macro nutrients.
RM Callaway & JM Vivanco, unpublished data; in Joe Alper ‘Wicked Weed of the West’ Smithsonian Dec. 2004.
Heather Davis et al. ‘Pollen Limitation Causes an Allee Effect in a Wind-Pollinated Invasive Grass (Spartina alterniflora)’ Proceedings Ntl. Academy of Sciences (PNAS):101:38:13804-7 (21 Sept. 2004). See also their news story on p. 13695-6 (Ingrid Parker, ‘Mating Patterns and Rates of Biological Invasion’).
N Ellstrand & Kristina Schierenbeck ‘Hybridization As a Stimulus for the Evolution of Invasiveness in Plants?’ PNAS:97:13:7043-50 (20 June 2000).
D Pearson & RM Callaway ‘Indirect Effects of Host-Specific Biological Control Agents Trends in Ecol. & Evol.:138:9:456-61 (Sept. 2003).
EW Seabloom et al. ‘Invasion, Competitive Dominance, and Resource Use By Exotic and Native California Grassland Species’ PNAS:100:23:13384-9 (11 Nov. 2003).
D Tilman ‘Niche Tradeoffs, Neutrality, and Community Structure: a stochastic theory of resource competition, invasion and community assembly’ PNAS:101:30:10854-61 (27 Jul. 2004). A description of this and other work is on p. 10851-3 in same issue.
Erika Zavaleta & Kristin Hulvey ‘Realistic Species Lossess Disproportionately Reduce Species Grassland Resistance to Biological INvaders’ Science:306:1175-7 (12 Nov. 2004).