No Pain, No Gain…In Species Diversity

Close your eyes and imagine it is 200 hundred years ago. What do you think about? Perhaps a horse and buggy heading down Main Street? Maybe even sepia tone, old timey photos? Something that may or may not have come to mind is the change in the Midwestern landscape. In the early 1800s, Iowa was made up of mostly tallgrass prairie with an abundance of animals and diverse vegetation. Today, the countryside no longer resembles the scenic prairie, as the vast majority of the land has been converted into cropland. Before you begin to cry, don’t worry! All is not lost. Ecologists have been working to restore the tallgrass prairie at different sites in Iowa. Interestingly, while the restorations have been successful, it has been difficult to increase the diversity of vegetative species to the level of diversity achieved by the remnant native prairie. I’m particularly curious about how a disturbance, such as soil compaction, affects the diversity of species. Given the lack of research that has been done, I secretly (and now not so secretly) wonder if this could be the key that unlocks greater diversity in restored prairie.

A visual comparison of Iowa land cover in (A) the mid-1800s and (B) as of 2001.
Photo credit: http://armi.usgs.gov/story/story.php?contentid=1630

 

I acted on my detective impulses and found a paper that could shed light on the effect of soil compaction on the tallgrass prairie via bison wallows. The paper is entitled, “Potential Impacts of Bison Wallows on a Restored Tallgrass Prairie Community” by Miller et al., 2014. The researchers sought to investigate the effects of bison wallows on “weedy” versus “non-weedy” vegetation. Weedy plants have been defined in previous literature as more tolerant to disturbance, whereas non-weedy plants have been defined as less tolerant to disturbance. Bison wallows are a natural disturbance that leave large “bald patches”, as a result of the large animal rolling around on the ground and compact the soil in the process. The research was conducted in Jasper County Iowa at the Neal Smith Wildlife Refuge where there are a heard of Bison present. They randomly sampled 7 wallows and 7 non-wallows along a transect, while controlling for elevation, proximity to each other, and proximity to nearby perennials. Of particular interest to me is their findings related to the vegetation inside versus outside the wallows. Ultimately, the authors found that the results support the presence of a disturbance gradient. The gradient favored weedy species near disturbance and vice versa. Given that the adjacent prairie tended to have lower weedy plant cover, it is possible that the soil compaction of the bison wallow enables weedy-er species to flourish. On the flip side, the adjacent prairie enabled the success of the non-weedy species. This suggests that different degrees of soil compaction are necessary to have both weedy and non-weedy species thrive. 

A bison in the process of leaving a wallow.
Photo credit: Mary Jane Gibson

This raises certain questions about prairie restoration. Specifically, how feasible is larger scale soil compaction and would it help increase diversity enough to justify the cost? How important is increasing prairie diversity?! Should we even try restore the prairie to how it was 200 years ago?!! The larger questions are current conundrums to not only me, but also all restoration ecologists today. Restoration ecology is challenging in that even if the goal is to restore the land back to the way it was hundreds of years ago or more, it simply may not be possible. What factors contribute to the possible impossibility? Of course, humans are culpable for disrupting and modifying certain ecosystems, but this is only one piece of the puzzle. In fact, there are many factors that can contribute to changing landscapes, such as cyclical climate change and natural extinction of various species. In attempting to convert cropland back into tallgrass prairie, the impossibility of increasing species diversity exists, even if we know all there is to know about restored prairie. Similarly, we have ethical dillemas to consider as well, in thinking about how much humans should alter current ecosystems to try to achieve restoration goals. There may come a time when we need accept failure of restoration to historical standards and perhaps modify ecological restoration goals.    

 

Reference

Miller, Kimran, et al. "Potential Impacts of Bison Wallows on a Restored Tallgrass Prairie Community." Proceedings of the North American Prairie Conference. Vol. 23. 2014.