We recently completed a large multi-year restoration and management project at our Platte River Prairies. Our specific objectives were to improve habitat quality for various at-risk prairie species and evaluate the impacts of our management on at-risk butterflies – particularly regal fritillaries. The project was supported by the Nebraska Game and Parks Commission, who funded our work with two State Wildlife Grants (U.S. Fish and Wildlife Service money). Over five years, we conducted fire/grazing management in our prairies and enhanced plant diversity through overseeding and seedling plugs. We measured the results of that work by measuring changes in prairie plant communities and by looking at the use of our prairies by regal fritillaries and other butterflies.
The following is a brief summary of the major lessons we’ve gleaned from the fire/grazing component of the project, including implications for future management and restoration work. I will summarize the overseeding/seedling work in a separate upcoming post. If you want more details, you can see our entire final report to the funding agencies here. As a warning, the report is 14 pages long, with an additional 21 pages of Appendices, full of tables and graphs.
What We Did Between 2008 and 2012, we treated over 1,500 acres of prairie with varying applications of patch-burn grazing management. During that time, we altered the timing of burning and the intensity of grazing from year to year, and included years of complete rest from grazing in some prairies. For the purposes of this project, we evaluated the results of our work in two main ways:
Why is sweet clover the target of aggressive control by some prairie managers and largely ignored by others? After talking to a number of people across the Midwest and Great Plains, I think there are a couple of things happening. First, the usually biennial sweet clover can be very abundant and showy in the years it blooms, but is harder to find in other years. I think some prairie managers see those big flushes and mistake abundance for aggressiveness. However, I also think that some soil/precipitation/latitude(?) conditions may lead to real negative impacts from sweet clover on plant diversity.
One of the lessons that’s been strongly reinforced for me this summer is that it can be difficult to extrapolate successful prairie management/restoration strategies from one region to another. Just during the last several months, I’ve visited prairie managers in Nebraska, Indiana, Missouri, and South Dakota and I’ve seen tremendous variation between (and even within) those states in terms of which species are invasive and which are not. It’s dangerous to assume that just because a species like sweet clover isn’t causing problems in one prairie, it won’t cause problems in another. I hope we’ll eventually learn enough to accurately predict when to worry and when not to, but in the meantime, it behooves prairie managers to carefully evaluate species at their own sites.
I’ve been working with prairies along Nebraska’s Platte River for nearly 20 years now, and my observations have led me to conclude that sweet clover is more of a big ugly plant than a true invasive species in those prairies. Years of data collection on my plant communities support those observations. That annual monitoring work entails listing the plant species I find in each of about 100 1m2 plots across a prairie. Those plots are stratified across the prairie so the site is evenly sampled. Once I have those plotwise species lists, I calculate the floristic quality (FQI) inside each plot, a calculation that takes into account both the number of species present and the average “conservatism” value of those species. I can then look at changes in mean floristic quality over time to help me see how the plant community changes over time. I monitor a few prairies annually, and others on a periodic basis.
Those data show the same thing I’ve seen observationally – sweet clover changes in abundance from year to year (though not as much as it appears visually), but the species doesn’t increase in abundance over the long term and doesn’t appear to negatively impact floristic quality. Below are graphs from three sites that show both sweet clover frequency (% of plots occupied by sweet clover) and mean floristic quality. Two of those sites were annually grazed during the data collection period, and the other was only grazed once – toward the end of the sampling period. Cattle grazing almost certainly helps control sweet clover because it is one of their favorite plants to eat, but I don’t think sweet clover is causing me problems where I don’t graze either.
What my data don’t show is the flush of tall blooming plants that happens every other year or so. I’m just counting whether at least once sweet clover plant is present in each of my small plots – not how big it is, or whether or not it’s blooming. Nevertheless, sweet clover frequency changes from year to year but doesn’t appear to correlate at all with changes in mean floristic quality.
I feel pretty good about ignoring sweet clover and focusing on more invasive species on our prairies. Both my observations and data support that strategy. However, as I said earlier, just because the species doesn’t appear to be problematic for me doesn’t mean it isn’t an invasive species in other prairies. It’d be great if we could compare data similar to what I’m presenting here from a number of sites to see if sweet clover is acting differently in different places. Without data, it’s hard to know whether or not people are just interpreting the “invasiveness” of sweet clover in different ways. For now, my answer to the question, “Is sweet clover really invasive?” is still the same…
No, I’m not saying they do. I’m merely conducting a thought exercise, and inviting you to come along for the ride. …No, really – some of my best friends are botanists! And I’m pretty sure they have a good sense of humor…
Why is it that we define prairies and prairie quality by their plant communities? Are we making a mistake by letting botanists drive the prairie conservation bus? Let’s review the current situation:
Today’s prairies are generally categorized as high or low quality based mainly on the composition of their plant community. More specifically, prairies achieve high quality status by containing an abundance of “conservative” plants. Conservative plants are essentially defined as plant species that are rare in most of today’s prairies, don’t do well in prairies that are heavily disturbed by grazing, and don’t colonize quickly into fallowed fields, etc. Another way to think about it is that conservative plants are those deemed to be “fragile”. Whether they really are or not is another subject for another time.
So, a prairie filled with lots of fragile plants is considered to be a high quality prairie. Conversely, a prairie filled with prairie plant species that are tough and scrappy is considered to be degraded. Come to think of it, we tend to think about human society in much the same way. Speaking stereotypically, high society consists of fragile people with clean fingernails and uncalloused hands who have to hire low-society people to cook, clean, garden, and take care of their fancy cars. Those low-society people work hard to feed themselves and their families, wear functional clothes (without designer labels), and often employ double negatives and words like “ain’t”. Success in life is supposed to be measured by our ability to move from low to high, right? I suppose it makes sense that we think of prairie conservation in the same way.
Now it’s certainly understandable that people who dedicate their lives to plants would be concerned about preserving those plant species that are the most difficult to preserve. Conservative plants are important because they’re rare. Most grasslands in today’s landscapes have to earn their keep, and are managed in ways that tend to favor species that are tough and scrappy, rather than those that are fragile. In those landscapes, conservative species find hiding places on steep hillsides, in wet or sandy soils, outside fences, and in small, oddly shaped land parcels that don’t fit into agricultural systems. The question of whether conservative plants were distributed in similar ways historically or were more widespread is a topic of much debate in prairie conservation circles. Regardless, botanists today tend to focus their conservation energy on prairies that contain lots of fragile plants because they don’t want to see them disappear.
And conserving prairies full of conservative plants makes sense for the larger conservation effort anyway, right? Because prairies with lots of rare plants also have lots of rare insects, rare bird species, etc. Right? Well – maybe not. In fact, while there are a few instances in which that’s true (some rare butterflies, for example) there are many more cases where it’s not. For instance, I’ve spoken with several entomologists working in eastern tallgrass prairies who have found that large and relatively “degraded” prairies tend to have much higher numbers of rare insect species than small “high quality” prairies. In addition, two groups of Illinois entomologists have each developed their own index of prairie quality based on “conservative” insect species. You can learn more about those indices here and here. Both of them have found that there is often little correlation between the number of conservative insect species and the number of conservative plant species in a prairie. In other words, even if we saved all of the remaining prairies with “high quality” plant communities, we could still lose a lot of rare insect species.
A specific insect example, and a notable exception to the aforementioned connection between rare butterflies and high quality prairies, is the regal fritillary butterfly. States with highly fragmented grasslands, and thus a heavy emphasis on conservation of small prairies with lots of conservative plants, have very few regal fritillaries left. In contrast, regals are among the most common butterfly species found in places like eastern Nebraska and Kansas – places full of prairies scorned by many eastern botanists as having been long-ago “ruined” by cattle grazing because they don’t have abundant conservative plant species. Gorgone’s checkerspot is another butterfly with a relatively similar pattern of occurrence.
Grassland birds are rightly of great conservation concern to many people. In fact, I think it’s a requirement that ornithologists working with grassland birds have to start every paper or presentation with the phrase, “Grassland birds are the fastest declining group of birds in North America”. And it’s true. So where do we find the strongest populations of grassland birds? In landscapes full of large prairies – which typically have relatively low abundances of fragile plant species. With a few exceptions, high quality prairies – using the botanists’ definition – tend to be small. Again, they’re found in those hidden corners that have escaped having to work for a living. However, grassland birds are notoriously unsuccessful when they try to nest in small prairies, and most don’t even try because the predation risk is too high, and the prairies are often surrounded by trees and/or relatively intense human activity. Give an upland sandpiper or prairie chicken a big landscape full of nothing but cows and grass and they’re in high heaven.
What does this all mean? I’m not sure. I’m certainly not saying that prairies full of conservative plants aren’t of great value. Clearly, they contain plant species that are rare elsewhere – and some rare butterflies and other species as well. However, it’s also clear that those prairies can’t be the sole focus of conservation if we’re going to preserve the entirety of prairie species diversity. I also wonder whether at least some of those prairies (especially those larger than 40-50 acres or so) could play a larger role in prairie conservation if they were managed a little differently. For example, if some of those prairies were managed for more heterogeneous vegetation structure they might become more valuable to many insect and wildlife species. If we could improve habitat for rare wildlife and insect species while decreasing the abundance (but maintaining viable populations) of conservative plants, would that be a reasonable trade-off?
It seems to me that some of those larger prairies could accommodate some experimentation with summer fire, fire-driven grazing, and/or other less traditional management strategies by testing those strategies on a portion of each prairie. If the results of those insects showed benefits to wildlife/insects without catastrophic impacts on conservative plant populations, it might be beneficial to periodically apply those kinds of treatments to all parts of the prairie over time. Again, that management might reduce the overall abundance of conservative plant species somewhat – it would certainly periodically change the visual dominance of them. Either way, the added benefits to a wider range of prairie species might be worth the trade-off. Or, they might not. It seems important to find out, however, since we have a lot of prairie species (other than plants) that are in need of good habitat right now.
I live in work primarily in east-central Nebraska, so the prairies I’m most familiar with are those that are dismissed by some botanists as already having been ruined by grazing. It’s true that many of them have been severely degraded, not just by chronic overgrazing, but also by broadcast herbicide use. However while Nebraska prairies are rarely dominated by conservative plant species, those species aren’t absent either. Moreover, many of our restored (reconstructed) prairies have strong populations of many conservative species. Watching those species respond to disturbances like summer fire and periodic grazing has been instructive. Species like Canada milkvetch, compass plant, and leadplant, for example, that are often considered to be easily eliminated by cattle grazing, are thriving under a mixture of fire and grazing on our sites. While there are still lots of questions about how/whether to use grazing on high quality prairies, we’ve certainly busted the myth that cattle automatically pick out conservative forb species for grazing (see my report on our use of lightly-stocked patch-burn grazing for details). My hope is that the work we’re doing here can serve as a catalyst for similar experimental work in more “high quality” tallgrass prairies to the east of us. Will those prairies benefit from shaking up their management? I’m not sure. Will they be ruined by the attempt? I have a hard time believing that, but until we do some small scale experimentation we’ll never know.
Regardless of answers to the above questions, there is one thing I feel very strongly about: Good prairie managers consider more than just their favorite plant species as they think about how to manage their prairies. Yes, plant diversity is very important -a growing number of ecological functions and non-plant species needs are being tied to plant diversity as we continue to learn more about prairies. But the importance of dense populations of conservative plants versus less abundant – but still viable – numbers of those species is less clear. More importantly, we know that many species of insects (and probably other taxa) are doing better in prairies with low numbers of conservative plants. We need to learn more about whether that’s tied to the way those prairies are managed, the landscape surrounding them, or the plant composition of those sites – or (most likely) a combination of those factors.
Are botanists ruining our prairies? I don’t really think that’s the case, though it’s fun to poke them a little. Most of the botanists I know are relatively well-rounded naturalists that care deeply about the conservation of prairies and other natural areas. I do think, however, that all of us can become too attached to certain species or groups of species, to the point where it hamstrings our creativity (see my earlier post on “Calendar Prairies”). Plants are often the easiest group to become attached to for prairie managers because they’re easy to find, relatively easy to identify (especially the big showy ones), and are comforting to see every time they bloom. Birds and butterflies are also very popular, and easy to become attached to, but many small prairies don’t have many bird species, and butterflies are less familiar to most people than are plants. On the other hand, beetles, leaf hoppers, flies, micro moths, ants, and the other species that actually make up the vast majority of prairies’ biological diversity are really easy for most of us to overlook. Yet they’re really important, both for their own sake and because they play critical roles in keeping the larger prairie machine running – which supports those pretty flowers and birds.
We can all benefit from stepping outside our own comfort zone in terms of how we evaluate prairie conservation success. As I said in a recent post, looking at my prairies through the eyes of pollinators has changed my perspective considerably over the last couple of years. I’m working hard to learn more about other species like voles, beetles, and snakes so I can better think about their needs as well. If nothing else, it’s fun. But I think it’s quite a bit more important than that.
This is Part 2 of a two part series on ecological resilience in prairies. In Part 1, I interviewed Dr. Craig Allen about the basic definition of ecological resilience and then wrote about the relevance and application or resilience to prairie ecosystems. In Part 2, I explore how ecological resilience can influence the way we restore and manage prairies, and about how much we still have to learn about how to do that.
Influencing Resilience through Restoration and Management
Understanding ecological resilience should help us better design restoration and management strategies that build and maintain resilience in prairies. Using the components of resilience discussed in Part 1, it seems apparent that when restoring (reconstructing) prairies, it’s important to maximize species diversity in seed mixtures. More importantly, prairie restoration that adds to the size and connectivity of existing prairie remnants should make the entire complex of restored/remnant prairie more resilient (see earlier post on this subject). Finally, selecting and altering restoration sites, when possible, to include topographic and other habitat type variation – and multiple examples of each type – can also help ensure the resilience of the resulting restored prairie.
Designing management strategies for prairies that sustains ecological resilience is trickier because we still have much to learn. We’re far from fully understanding the various stable states prairies may exist in, or flip to, let alone where the thresholds are between those states. In addition, the level of plasticity, or range of adaptive capacity, of prairies is a subject of great debate right now among prairie ecologists – although the discussion is not usually framed in those terms. The real question is – How much can prairies change their plant and animal species composition and still remain “in the bowl”?
As an example, I manage a sand prairie that was hayed annually in the mid-summer for about 20 years before The Nature Conservancy purchased it in 2000. Over that 20 year period, the plant community in that prairie adjusted to that annual haying regime. Species such as stiff sunflower, leadplant, and sand cherry became restricted to a few steep slopes where hay equipment couldn’t go. Early summer grass and forb species became very abundant, but later season flowering plants were less so because they were mowed off around their flowering time each year. The prairie was a nice quality mixed-grass prairie, with good plant diversity, but definitely had the “look” of an annually-hayed prairie.
When we took over the management in 2000, we let the site rest for about 5 years and burned portions of it each year during that time. Then, we began introducing some combined fire and grazing treatments at different intensities and at varying times of the season. As a result, the prairie looks fairly different now. Stiff sunflower and leadplant have spread considerably through the site, re-taking lower slopes where hay equipment had earlier eliminated them. Cool-season grasses (native and non-native) change in abundance from year to year, but warm-season native grasses are certainly more dominant than they previously were. Overall forb diversity is about the same as it was, but the relative abundance of many forbs has changed, and those abundances now vary from year to year, rather than remaining fairly stable.
In the context of plasticity, or adaptive capacity, this prairie has demonstrated that the 20 years of haying was not enough to move the plant community into a new stable state from which recovery, if that’s the right word, was not possible. The community was altered by that haying regime, but upon alteration of that regime, the community composition morphed to match changing conditions – without losing plant diversity. In other words, assuming that the prairie hasn’t lost anything critical during the 11 years of our management, both the haying regime and our current management have kept the prairie “in the bowl”, though it changed appearance fairly dramatically. Its adaptive capacity is broad enough to include the “hayed” look and the “crazy Nature Conservancy management” look. The real test of this, of course, would be to reintroduce haying for another 20 years and see if the plant community reverted back to something very similar to the condition it was in when we purchased it.
Here’s another example from my own experience. We have a 45 acre restored prairie (prairie reconstruction) that was seeded in 1995 by Prairie Plains Resource Institute adjacent to a degraded remnant prairie. The seed mixture included approximately 150 species of mesic prairie plants, most of which established successfully. We managed the prairie with periodic spring fire for its first seven years, and then incorporated it into our experimental patch-burn grazing system (light stocking rate). During nine years of patch-burn grazing management, (6 of which were during a severe drought) the plant species composition in any one place has bounced around quite a bit due to the fire/intense grazing/rest cycles imposed by the patch-burn grazing management system. Overall, however, the prairie has maintained its mean floristic quality within 95% statistical confidence intervals (I collect annual data which entails calculating floristic quality within 100 1m2 plots and averaging the values across those plots). Read more about our patch-burn grazing work and results at this restored prairie and others here.
That’s not to say the prairie hasn’t changed – it has. Some plant species have increased in frequency among my annual data collection plots, some vary in frequency from year to year, and others stay fairly stable. However, no species has dramatically declined over the time period. (I wish I had data on other species, particularly insects, but I don’t.) Perhaps the most interesting, and somewhat concerning, phenomenon has been an increase in the frequency of Kentucky bluegrass in my plot data. The increase has been fairly steady over the nine years of patch-burn grazing and data collection, and bluegrass is now in about 75% of my 100 1m2 plots, though it rarely looks dominant where it occurs. To this point, that increasing frequency doesn’t seem to be impacting the overall diversity or floristic quality of the plant community, but that doesn’t mean it won’t at some point. Two possibilities are: 1) Our management is allowing bluegrass to enter the plant community but remain a minor component, or 2) Kentucky bluegrass is on a steady march of increasing dominance and will eventually turn my restored prairie into the same kind of low-diversity degraded prairie that exists in the adjacent remnant prairie. I won’t be completely shocked by either scenario, but I have hope that #2 won’t happen because of the way bluegrass is acting in the community to this point. It’s way too early to know for sure.
Interestingly, I have some large exclosures within this restored prairie that have never had grazing, only prescribed fire at a similar frequency to the grazed portion. Those exclosures have very little Kentucky bluegrass in them – probably because of both the differing management and the fact that the exclosures are on the far side of the restoration from the neighboring bluegrass-dominated remnant prairie. However, the exclosures also have much lower plant diversity and mean floristic quality than the grazed portion of the restored prairie. Visually, they are dominated by warm-season grasses and a few large forbs (e.g., perennial sunflowers). At this point, I prefer the grazed portion of the prairie because it seems to line up better with my management objectives of maintaining diverse and resilient plant communities (assuming it’s not slowly becoming a bluegrass wasteland).
To relate this example back to adaptive capacity, it appears likely that the grazed portion of the restored prairie has the adaptive capacity to retain its integrity as a prairie community through fairly wild fluctuations in species composition as a result of stresses from fire, grazing, and drought. This is, again, remembering that I’m only evaluating the plant community and that the experiment is far from over. On the other hand, it appears the exclosed portions of the prairie have lost plant diversity over time. Whether the communities in those exclosures are still “in the bowl” or in a new stable state of lower diversity is a big question. To address it, I’m going to open one of them to fire/grazing this coming year and exclude a portion of the currently-grazed prairie and see what happens. If the two plant communities trade identities to match their new management regimes, I’ll know that both were still “in the bowl” and understand more about the adaptive capacity of our prairies. If they don’t, that will be equally instructive!
Building and sustaining ecological resilience in prairies may be the most important component of prairie conservation in the coming decades. Threats from invasive species, habitat fragmentation and detrimental land management practices, compounded by climate change, will make conservation extremely difficult. Armoring prairies with ecological resilience gives us the best chance of success.
In order to build that resilience, we first have to understand it better. It is certainly more complex than the few simple examples I’ve provided. There are numerous belowground processes and systems we still know relatively little about. Even aboveground, there are many more questions than answers regarding the way species interact with each other and their environment – and what is required to maintain those interactions. To gain a better understanding of these natural systems, we have to rely on experimentation and observation. I think there are essentially two broad questions:
What is the adaptive capacity of prairies, and where are the thresholds between the desired state and other, less desired, stable states? This will certainly vary between tallgrass and mixed-grass prairies, and between sand prairies and black soil prairies, etc., but there will almost surely be some consistent themes.
How important is it to keep the ball moving within the bowl? In other words, are prairies really like our human bodies, in that the more we stress and rest them, the better prepared they are for future stresses? (Does the bowl shrink if we don’t keep pushing at the edges?) Or do we just have to keep prairies from being stressed too far in any particular direction?
We can work toward answering these questions with direct experimentation on prairies we manage (similar to my simple experiment with grazing and exclosures in the restored prairie example presented earlier). In addition, though, we can learn much from prairies that “flip” to less desirable stable states (hopefully not the ones we’re managing!) by documenting as much as we can about what happens to them and why.
Most importantly, I hope that thinking about ecological resilience with regard to prairies will make you look at the prairies you’re familiar with in a new way. Seeing prairies as balls rolling around in a bowl makes watching and managing prairies a much different experience than seeing them as a stable “climax community.” When we expect change, it’s easier for us to perceive change, and the more observant we are, the more we’ll learn. And goodness knows we have plenty to learn.
If you’re interested in learning more about ecological resilience, here are some relevant references that Craig Allen recommends (I’m pretty sure it’s just a coincidence that he’s a co-author on all of them). The Gunderson et al. book reprints a lot of the classic / foundational papers on the subject.
Sundstrom, S., C. R. Allen and C. Barichievy. Biodiversity, resilience, and tipping points in ecosystems. Conservation Biology: in review.
Gunderson, L., C. R. Allen, and C. S. Holling. 2010. Foundations of Ecological Resilience. Island Press, New York, NY. 466pp.
Allen, C. R., L. Gunderson, and A. R. Johnson. 2005. The use of discontinuities and functional groups to assess relative resilience in complex systems. Ecosystems 8:958-966.
Forys, E. A., and C. R. Allen. 2002. Functional group change within and across scales following invasions and extinctions in the Everglades ecosystem. Ecosystems 5:339-347.
Peterson, G., C. R. Allen, and C. S. Holling. 1998. Ecological resilience, biodiversity and scale. Ecosystems 1:6-18.