Assessing Ecological Integrity: An Example from Grazed and Ungrazed Nachusa Old-Growth Prairie   

Story and Photos by Ecologist Dan Carter
July 7, 2025

Figure 1. Yellow star grass (Hypoxis hirsuta, Wisconsin C = 8), a relatively conservative native species of old-growth herbaceous vegetation in prairies, oak woods, savannas, and fens.

One way we measure the extent to which an ecosystem has the abiotic and biotic elements needed to perpetuate itself—an ecosystems’ ecological integrity—is by assessing floristic quality. In particular, we assign “coefficients” of conservatism to vascular plant species on a scale of zero to ten based on how faithful or “conservative² ” they are to old growth (remnants). Species assigned a value of ten are the most conservative. These numbers have ecological meaning. Relatively conservative species tend to be the most specialized to their abiotic and biotic environments. They often have more or stronger symbioses with other plants, fungi, insects, and other organisms, and they are usually part of communities structured by limiting nutrients or water compared to communities structured by competition for light in the presence of abundant available nutrients and water. For example, conservative species like prairie dropseed (Sporobolus heterolepis, Wisconsin coefficient of conservatism 10) are associated with and dependent on mycorrhizal fungi,^3,4 have more co-evolved relationships with consumers (e.g., invertebrates like the red-tailed prairie leafhopper, Aflexia rubranura⁵ ) and are strongly associated with old-growth prairies (or efforts that re-create their conditions). In contrast, weedy or opportunistic native plants like mare’s tail (Conyza canadensis, Wisconsin coefficient of conservatism = 0) are less mycorrhizal,⁶ interact with relatively few consumers and are largely restricted to disturbed environments where nutrients are more available. If you bring me a specimen of mare’s tail, I won’t know if it came from your vegetable garden, a fallow agricultural field or the soil disturbance associated with a mammal burrow out on an old-growth prairie. I will know that it came from a place where competition has been removed or suppressed and where light and nutrients are readily available. Most of our native flora (~84%) is at least moderately conservative, with coefficients of four or higher, and these species also largely comprise our old-growth ecosystems. Plant communities with mean coefficients of conservatism (mean C) among constituent species above 4.5 are typically of natural area quality (Fig. 1). Species with lower coefficients aren’t bad, but when abundant, the land is convalescent.

In plant community ecology, mainstay metrics like species richness,⁷ evenness⁸ and diversity⁹ give us information about how many species are present and how equitable their abundances are, but they do not provide the vital context of what kinds of species they are. Are the species present associated with disturbed and degraded land? Do they indicate that an ecosystem is changing states from one type to another? Are they species associated with old-growth, intact ecosystems with their many biotic and abiotic relationships? The mean C of the flora in a place gives us that context. Despite that, it’s still often omitted from studies of prairie and other old-growth plant communities, and without it, it can be difficult or impossible to judge whether community changes are associated with loss or gain of ecological integrity.

A recent study by Chakravorty et al. reported responses to five years of bison grazing at Nachusa Grasslands.¹⁰ They included areas of old-growth prairie subject to bison grazing and exclosures that kept out bison activity from portions of those prairies. They also included prairie plantings, wetlands and degraded savannas. Given the pre-European paucity of bison on eastern prairies, and effects I observed on more western prairies as a graduate student, I was interested in what this study had to report (see “Stability Part Two: Why I Seldom Recommend Grazing” in the Summer, 2024 The Prairie Promoter).

Chakravorty et al. found little effect of bison on diversity and composition across community types, though there was some evidence of an increase in non-native relative to native plants after five years of grazing. In other words, bison didn’t seem to be hurting things, or at least not much. However, the study did not examine ecological integrity. Were there any changes in mean C in the remnants at Nachusa? The authors provided access to the data used in their analyses online at Dryad,¹¹ so I decided to have a look.

Figure 2. Giant ragweed (Ambrosia trifida, Wisconsin C = 0), an opportunistic native species of a wide range of high light, high nutrient settings.

There are rare species’ habitat reasons and cultural reasons for bringing bison back on to the landscape. However, it may not be judicious to put bison on our precious few remaining tracts of old-growth prairie, especially if one of the objectives is to sustain ecological integrity. Fortunately, there are plenty of opportunities to do so elsewhere.

When presented with information about how plant communities respond to various practices or treatments, please ask, “how did mean C respond?” Without mean C or other assessments designed around ecological integrity concepts, it is more difficult to assess relevance to our mission—perpetuation of prairie, savanna and other associated ecosystems of the Upper Midwest. Maybe if we keep asking, we’ll start to see mean C more often reported in research, or maybe it will influence more of us to find ways to collect data to inform our work, which Nachusa and those doing research there should be applauded for. I plan to get to work in that regard this field season with the support of the philanthropy of our members.

For those interested in coefficients of conservatism and floristic quality assessment, I recommend reading Spyreas (2019)¹⁶ , and watching a presentation called Ecesis: The Nature of Nature with Justin Thomas on YouTube (most relevant discussion after the 33-minute mark) in addition to some of the other work cited above.

Figure 3. Line plot showing paired points connected by lines representing plots protected from bison within exclosures (green, ungrazed) and accessible to bison (orange, grazed). Gray dots represent mean values prior to grazing inside and outside of exclosures for reference; exclosures did not start with higher weighted mean C. Sampling and initiation of bison grazing were staggered over two years on different prairies, so sampling after five years of grazing occurred in 2019 and 2020.

References

1. Carter, D. (2024). Stability part one: Why I recommend frequent dormant season burning. Prairie Promoter, Spring: 14-19. https://theprairieenthusiasts.org/stability-part-one/

2. Swink and Wilhelm 17.

3. Ebbers, B. C., Anderson, R. C., & Liberta, A. E. (1987). Aspects of the mycorrhizal ecology of prairie dropseed, Sporobolus heterolepis (Poaceae). American Journal of Botany, 74(4), 564-573.

4. Middleton, E. L., Richardson, S., Koziol, L., Palmer, C. E., Yermakov, Z., Henning, J. A., … & Bever, J. D. (2015). Locally adapted arbuscular mycorrhizal fungi improve vigor and resistance to herbivory of native prairie plant species. Ecosphere, 6(12), 1-16. 

5. https://apps.dnr.wi.gov/biodiversity/Home/detail/animals/7015

6. Řezáčová, V., Řezáč, M., Wilson, G. W., & Michalová, T. (2022). Arbuscular mycorrhiza can be disadvantageous for weedy annuals in competition with paired perennial plants. Scientific Reports, 12(1), 20703.

7. Richness is how many species are in a defined area.

8. Evenness is how equitable the species in a defined area are in terms of their abundance.

9. Diversity is a function of both richness and evenness. 

10. Chakravorty, J., Harrington, J. A., & Bach, E. M. (2024). Bison Grazing in Eastern Tallgrass Prairie Does Not Alter Plant Diversity after Five Years. Natural Areas Journal, 44(4), 215-222.

11. https://datadryad.org/dataset/doi:10.5061/dryad.d7wm37qb2

^{12. Taft, J. B., Wilhelm, G. S., Ladd, D. M., & Masters, L. A. (1997). Floristic quality assessment for vegetation in Illinois, a method for assessing vegetation integrity. Erigenia, 14, 3-95,}

^{13. https://universalfqa.org/} 

^{14. Some taxonomic resolution was also necessary, sedges and grasses not identified to species were omitted (they were similar in abundance between grazed and ungrazed exclosures), and two taxa very unlikely to have truly been present in upland Illinois prairie that were recorded in low abundance from single plots were removed from the data (Hippuris vulgaris, an emergent plant of cold, mineral-rich water; Linum flavum, a shrub native to southern Europe and not in North America’s flora.).}  

^{15. Carter, D. L., & Blair, J. M. (2012). Recovery of native plant community characteristics on a chronosequence of restored prairies seeded into pastures in West‐Central Iowa. Restoration Ecology, 20(2), 170-179.} 

^{16. Spyreas, G. (2019). Floristic Quality Assessment: a critique, a defense, and a primer. Ecosphere, 10(8).}

This article appeared in the Spring 2025 edition of The Prairie Promoter, a publication of news, art and writing from The Prairie Enthusiasts community. Explore the full collection and learn how to submit your work here. 

Read the rest of Dan Carter’s Stability Series:

Stability Part One: Why I Recommend Frequent Dormant Season Burning

Stability Part Two: Why I Seldom Recommend Grazing

About The Prairie Enthusiasts 

The Prairie Enthusiasts is an accredited land trust that seeks to ensure the perpetuation and recovery of prairie, oak savanna, and other fire-dependent ecosystems of the Upper Midwest through protection, management, restoration, and education. In doing so, they strive to work openly and cooperatively with private landowners and other private and public conservation groups. Their management and stewardship centers on high-quality remnants, which contain nearly all the components of endangered prairie communities.