The Disturbing Invasion of Sweet Clover

            Rocky Mountain National Park was established in 1915. Situated in Colorado, its main goal has been to preserve areas for the enjoyment of people. However, adding people to any environment tends to create disturbances. These disturbances, caused in part by the addition of roads, trails, and human activity, are called anthropogenic disturbances. Recent research has illuminated connections between the colonization of exotic species and these human disturbances, yet the impacts of these invasions on the native ecosystem are hard to quantify. Thus, Wolf et al (2003) investigate the impacts of sweet clover (Melilotus officinalis and Melilotus alba) invasion on the montane grasslands, specifically how invasive patches differ in species richness and community structure from control patches.

Images of Melilotus officinalis (left) and Melilotus alba (right) which are invasive species under examination in Wolf et al’s (2003) research. Image courtesy of http://ohioplants.org/wp-content/uploads/2012/04/Melilotus-officinalis-and-alba.jpg

            For their experiment, Wolf et al (2003) sampled areas that had been invaded by sweet clover, measuring each invaded patch’s total size. These patches were paired with an adjacent patch of comparable size that retained the native community (no invasives). Within these patches, the researches established plots to estimate the species cover and composition, noting also the percent cover of bare ground. Additional plots were placed along the edges of the invaded patches to look at the edge effects.

           

The researchers found several interesting relationships. First, total species richness varied as a function of time. Yet native species richness varied over the growing season while the exotic species richness remained the same. Second, invaded patches had a higher percentage of cover attributed to forbs where as the native patches had a higher percentage of grass cover. Third, more perennials were present in the control plots. The invaded plots had a higher richness of annual and biennial species (Figure 1). With respect to the edge experiments, Wolf et al found a gradient, with many of the aspects characterizing invaded plots concentrated at the center and diffusing outwards. Sweet clover spread 0.8m from the original patch boundaries in 1998 and 1.8m in 1999.

Figure 1: Summary of Wolf et al (2003) results from experiments comparing control patches to sweet clover invaded patches in Rocky Mountain National Park.

            The findings of Wolf et al signify the potential for sweet clover invasion to change the community composition of native systems. These differences in species richness and composition may be due to life history characteristics of sweet clover or inherent competitive advantages in comparison to native species. These different interactions, introduced by sweet clover colonization, may induce community composition shifts. The fact that patches invaded by sweet clover were dominated by exotic species is frightening, as it may indicate that invasion fragments existing native communities. Therefore, the invasion of an exotic species may constitute a different type of anthropogenic disturbance. As land stewards plan conservation projects, understanding invasions as a type of anthropogenic disturbance will be important in ensuring any project succeeds in its goals.

Read the full article:

Wolf, J.J., Beatty, S.W., and Carey, G. (2003). Invasion by Sweet Clover ( Melilotus ) in Montane Grasslands, Rocky Mountain National Park. Ann. Assoc. Am. Geogr. 93, 531–543.