Safety In Numbers? Not Likely in Plant Communities

The phrase ‘safety in numbers’ is familiar to you. Almost all of us have heard some variation of this idiom repeated in a biologically oriented course. All things considered, this idea often holds true in nature. In a school fish, a large body of individuals is used to essentially buffer predation to promote individual survival within the bigger group. However, a recent study conducted by Kathryn Yurkonis of the Biology Department at the University of North Dakota and her colleagues revealed that large patches of the same plant species might facilitate invasion in perennial grasslands. Their findings require us to rethink current invasion ecology and its possible effects on prairie conservation projects in Iowa.

What distinguishes a perennial grassland system?

         Perennial grasslands are usually characterized by a blended abundance of grasses and flowering plants called forbs. Historically, these prairies have contained extremely fertile soils in part due to the frequency of fires. These fires also maintained a high level of biodiversity by promoting disturbances that allowed for a variety of species to cohabit a general area. Since the soil is so rich, much of this land has been taken over by agricultural for commercial purposes. According to the Prairie Iowa Network, less than 0.1% original prairie exists in remnant patches across Iowa. Thus it is crucial that we maintain these current historic sites appropriately in conjunction with implementation of restoration projects. Fortunately, Yurkonis et al. provide novel insight as to how these projects can be initially set up to increase invasion resistance. Like stated previously, it all has to do with how species are arranged within a community assemblage.

How can plant communities differ within perennial systems?

Ecological communities can differ in two key ways: species richness, as determined by the number of species within the area; and species evenness, which is dependent upon the abundance of a species in that same area. Effectually, both species richness and evenness affect diversity maintenance. Yurkonis et al. flesh this idea out and stress how arrangement, which is driven by richness and evenness, is the ultimate driving force behind invasion resistance.

Arrangement is usually driven by a combination of external and internal factors. Examples of each may include the varying levels of resource availability amongst patches (called resource heterogeneity) and local competition strategies, respectively (Bolker & Pacala 1997). The subsequent community arrangement determines the invasibility of the area; the term invasibility emerged in the field of invasion ecology to describe the susceptibility of environments to invasion by species from other regions of the world (Davis et al. 2005). Yurkonis et al. revealed how initial species patterns affect invasion resistance, and specifically how large conspecific (that is, same species) patches encourage invasion.

Fig. 1. A comparison of invasion resistance between large conspecific patches and smaller ones. Yurkonis et al. have shown that enlarging the size of same species patches actually increases the abundance of invasive species within that plot.

The Study: Big is Bad!

          Little research has been done to characterize the effect of species patterns on competitive interactions between neighbors in perennial grasslands. That is why the work presented by Yurkonis et al. remains exciting and relevant to prairie restoration and conservation in present-day Iowa. The researchers planted community assemblages that varied in size of conspecific patches, starting with smaller patches that gradually grew in magnitude. All groups were planted at the Iowa State University Horticultural Research Station in Ames, Iowa. This ensured that soil and weather conditions were constant for each assemblage and thus any change in resistance could be attributed to differences in arrangement. Figure 1 (above) demonstrates a visual representation of their findings.To reiterate, the researchers found that invader abundance was greater in plots that were initially planted in large conspecific patches; that is, big patches are bad at keep invading species at bay!

Lessons Learned: How new approaches can aid prairie management projects

These findings do not necessarily reflect what someone might expect to occur, but it suggests a new way to think about prairie dynamics extremely relevant to current restoration and reconstruction efforts. The implications of these sorts of results are grand, and careful consideration should be taken when undergoing these sorts of ventures. It would make no sense to focus on planting large collections of the same plant species within a plot of land set aside for reconstruction, as the community would soon be degraded by invasive species. In order to sufficiently maintain historical prairie, it remains ever important that studies like the one carried out by Yurkonis et al. continue to characterize population dynamics and spatial arrangements.

Perhaps future conservation projects can aim to illustrate the relationship outlined by Yurkonis and her colleagues on a larger, more realistic scale. In doing so, we can only expect to learn more about how to adequately approach initial steps in addition to processes of maintenance required in these sorts of projects.

References

Bolker, B. & Pacala, S.W. “Using moment equations to understand

           stochastically driven spatial pattern formation in ecological systems.”

          Theoretical Population Biology 52 (1997): 179–197.

Davis, Mark A., Ken Thompson, and J. Philip Grime. “Invasiblity: the local

          mechanism driving community assembly and species diversity.”

          Ecography 28.5 (2005): 696-704.

Yurkonis, Kathryn A., Brian J. Wilsey, and Kirk A. Moloney. “Initial species

pattern affects invasion resistance in experimental grassland plots.” Ed.

Amy Symstad. Journal of Vegetation Science (2011).