Measuring restoration success in Iowa’s tallgrass prairies

In recent decades, expansion of agriculture and other human activities has jeopardized Iowa’s native tallgrass prairie habitats, which historically dominated the local landscape. At present, upwards of 90% of the North American land area once covered in tallgrass prairie has since been cultivated (Sampson & Knopf, 1994).

It is no surprise, then, that researchers have awarded much attention to the conservation and restoration of these habitats throughout the state. In a 2005 article in the Journal of Applied Ecology, Dr. Leanne Martin and colleagues explored this issue in depth. The study set out to understand ecological discrepancies between remnant prairies (those having never been cultivated or otherwise anthropogenically manipulated) and reconstructed or restored prairies (in which the natural character of the landscape had been reconciled and native communities re-introduced) using measurements of species diversity as key factors for comparison (Martin et al., 2005).

The authors note from the outset that species diversity can be measured as “evenness” or as “richness,” the former signifying abundance and distribution within species and the latter defining the number of species present in a given area. With regard to these diversity parameters, the study touches on several provocative observations present in the literature. Among the most notable of these is that “restoring spatial components of diversity is rarely recognized as a goal in restoration, even though it is an integral component of ecological systems.” Martin and her colleagues suggest that presently the rehabilitation of community structure and ecosystem process rates (i.e. nutrient transport and primary productivity) are the dominating objectives in restoration efforts. While telling and valuable, these measures of restoration success have thus far limited the potential for quantitative evaluation of success in restoration.

Given this gap between the implementation and assessment of restored habitat, the researchers set out to establish a set of measurable parameters that may be used to gauge restoration success in terms of spatial species diversity—a feature distinguishing this study from others of its kind. These parameters, stemming from several important ecological attributes, include 1) the proportion of native species, 2) ecosystem processes, 3) plant diversity at all spatial scales, and 4) animal and microbial diversity at all spatial scales.

The study zeroed in on the first three of these metrics of restoration success in the context of the largest tallgrass prairie restoration in the United States, located within southern Iowa’s Neal Smith National Wildlife Refuge. Using 400 cm² quadrats, the researchers collected above ground biomass and surface litter from eight prairie plantings at the Neal Smith Refuge during three periods (autumn, early summer, late summer)—a distinction that allowed for sampling of early- and late-growing species. Also sampled, for comparison, were three remnant prairie sites selected for their nearby location and geological similarities to the restored sites.

To facilitate meaningful comparisons, the researchers quantified spatial diversity at the “neighborhood-scale” (quadrat-to-quadrat) and the larger “prairie-scale.” Consistent with the researchers’ expectations and with the conclusions of other studies of this type, restored sites at the neighborhood-scale had substantially lower diversity and richness relative to the remnant sites. In other words, any given plot of a restored prairie contained fewer species overall than plots sampled at a remnant prairie site. Contrarily, the species evenness in the restored sites at the neighborhood-scale was not significantly different from that in remnant sites—an observation that Martin associates with grazing in the restored sites, which has been shown to increase species evenness (McNaughton, 1979).

At the prairie-scale, however, the researchers’ findings were contrary to their expectations: the proportion of species diversity and richness at this level was substantially greater in restored sites than in remnant sites. As the authors suggest, this is likely because while restored sites had fewer species in any one quadrat than in remnant sites, it was often the case that the dominant species changed more frequently between quadrats in restored sites.

So what do these findings mean for the future of tallgrass prairie management? Martin et al. suggest that in order to restore tallgrass prairies effectively to their “remnant” condition, management regimes should put especial emphasis on maintaining high species richness at the neighborhood-scale. This can be accomplished, as the authors recommend, by focusing on “local-scale restoration methodologies,” such as mowing, that allow for maintained density and richness in the rehabilitation process.

References

Martin, L.M., Moloney, K.A., and Wilsey, B.J. 2005. An assessment of grassland restoration success using species diversity components. Journal of Applied Ecology, 42. pp 327-336.

McNaughton, S.J. 1979. Grassland-herbivore dynamics Serengeti, Dynamics of an Ecosystem (eds A.R.E. Sinclair & M. Norton-Griffiths), pp 46-81. University of Chicago Press.

Sampson, F. and Knopf, F. 1994. Prairie Conservation in North America. Bioscience, 44. pp 418-421.