I know I say this about a lot of different insects, but grasshoppers are truly amazing creatures. Grasshoppers have a reputation as voracious consumers of crops and forage grasses, and that reputation is well-earned. However, the pest tag is far too often and broadly applied. There are something like 400 species of grasshoppers in the western United States, and only about 20 species are categorized pests. In Nebraska, we have 108 grasshopper species, with only a handful that ever cause economic damage, and that damage occurs sporadically – mainly in years when those species have population booms.
Unfortunately, the “Grasshoppers Destroy Crops” headline tends to swamp the many and much more interesting grasshoppers stories we should be talking about. Let’s start with the numbers I’ve already presented. THERE ARE 400 SPECIES OF GRASSHOPPERS IN THE WESTERN UNITED STATES AND 108 SPECIES IN NEBRASKA ALONE! That doesn’t include katydids or crickets, by the way, just grasshoppers. I’m guessing most of you had no idea there were that many kinds of grasshoppers. Am I right?
I’m including a half dozen grasshopper photos in this post to show off just a taste of the beauty and diversity of grasshoppers in prairies. If you want to read more about this, you can read an article I wrote for the August/September issue of NEBRASKAland Magazine. While you could be forgiven for thinking there are two kinds of grasshoppers in the world – green and brown – you would be very wrong. There are grasshoppers with much more color and pattern variation than many species of birds, but nobody makes movies about people circling the globe to see more grasshopper species than anyone else, do they? Many band-winged grasshoppers show off gorgeous red or yellow wings as they fly, wings that rival those of butterflies, but you don’t hear about historically-prominent British Prime Ministers collecting grasshoppers, do you?
Now let’s discuss grasshoppers’ diet, which can include far more than just grasses. Some grasshopper species feed high up in the canopy of prairie vegetation, while others stay on the ground. Many have a very wide diet, including both grasses and wildflowers, but others are much more specialized, including grasshoppers that feed almost completely on one or a few wildflower species. Grasshoppers are often seen feeding on the pollen of flowers, especially sunflowers, providing further evidence that they are much more than just grass eaters. Most grasshopper species prefer the newest, most tender leaves of plants, but some – especially those that live mostly on the ground – make their living off of older leaves, including some dropped by their brethren living above them. Regardless of what they like to eat, grasshoppers have very sensitive organs on the tips of their antennae that help them determine the forage quality of a leaf before they eat it.
When you think about animals that have sophisticated communication systems, you probably think about creatures like apes, whales, and even prairie dogs, right? You might be surprised to learn that grasshoppers have their own complex methods of communication as well. Every grasshopper species produces its own unique set of sounds, most of them created by rubbing their hind legs against their abdomen. In addition, some employ what’s called “crepitation” – a loud clacking sound made by snapping their hind wings while flying.
Grasshoppers can also communicate visually. They can do this by rubbing their legs against their wings, flashing their wings, and making a variety of motions with their legs. Those visual signals can help them attract mates, defend breeding territories and feeding areas, and ward off unwanted suitors. I’m not saying we should make documentaries about people trying to teach grasshoppers how to communicate with American Sign Language, but I’m not NOT saying it either…
If you’re someone who doesn’t care about the beauty, diversity, or communication abilities of grasshoppers, maybe their utilitarian value will make an impression. As a major consumer of vegetation in prairies, grasshoppers play a huge role in nutrient cycling, a role that becomes even more important in prairies without large vertebrate grazers. Perhaps most importantly, though, grasshoppers are a crucial food source for many other animals, including birds and other wildlife species you (hopefully?) enjoy having around. They are large and packed with nutrients, as well as abundant and fairly easy to catch. In addition, while they aren’t a particularly popular food item among people here in North America, grasshoppers are an important source of protein for humans in other parts of the globe.
When grasshoppers start to emerge next spring, please take a little extra time to notice and appreciate them. See how many different colors and shapes of grasshoppers you can find in your neighborhood prairies (remembering that if their antennae are as long as their body or more, they are katydids, not grasshoppers.) Look for the grasshoppers with big colorful wings as they clatter noisily away from your feet. And if you’re a person of financial means, and are interested in making a movie about grasshopper watchers or people trying to teach grasshoppers how to talk to humans, call me. I know people who know people.
When converting crop land to restored prairie, it’s always hard to predict what you’re going to get. Numerous examples prove that even when you control as many variables as possible – including soil conditions and the rate, timing, and technique of planting – no two seedings turn out alike. Sometimes, you can use hindsight to explain what happened (weather conditions, herbicide carryover, etc.) but most of the time it’s clear that we just don’t understand much of what’s happening out there.
I’ve been analyzing some data from one particular restored prairie lately, and trying to puzzle out what’s going on. In this case, the results are good – which is nice. It’d be nicer, of course, if I could explain WHY things worked so well and then replicate whatever happened…
The prairie in question was seeded with a mixture of about 200 plant species onto 69 acres of disked cropland that had been in corn the previous season. The seed was planted sporadically between December 1999 and April 2000. Wetlands were added to the site by excavating down close to groundwater and recreating the kind of swale/ridge topography that is typical of nearby Platte River meadows. Those wetlands and sandy spoil piles (ridges) were seeded with appropriate seed as well.
All of the seed was broadcast onto the site – some by fertilizer spreader and some by hand (I was experimenting) and no harrowing or packing of the soil was done. Unfortunately, this was the last year BEFORE I started keeping good records of the amount of seed from each plant species I included in the mixture, so I only have a list of the species we harvested seed from that year. What I know is that my seeding rate per acre was about 15 gallons of grass seed (mostly big warm-season natives) that was harvested by combine from nearby prairies, and about 1/2 gallon of hand-harvested forbs, grasses, and sedges. That’s roughly 12 bulk pounds of grass seed and 1/2 pound of forb (wildflower) seed per acre. I have no idea what germination rates were that year, but it was a pretty light seeding rate compared to what many others around the country use. Today, our typical mix is a little lighter on grass and includes about twice the forbs.
To cut to the results, this prairie has turned into our most diverse and showy restoration we’ve ever done. You’d never know we’d used such a light seeding rate of forbs by looking at the site now – its appearance is dominated by big showy wildflowers. By every measure I use to look at the plant communities of our restored prairies, it comes out high. I’ve found 178 plant species in the site so far, which is excellent. The mean Floristic Quality (combination of species number and “conservatism values”) is high, and still climbing rapidly. It averages twelve plant species per square meter, which is higher than most other restored or remnant prairies in the area. (Yes, I know that seems like a very low number to you eastern tallgrass prairie folks, but it’s good for out here. Don’t rain on my parade, ok?) Twelve years after it was planted, tall warm-season grass species are still not very dominant. The species found at the highest frequency is big bluestem, and it was only in about 80% of 1m2 plots stratified across the site last June. In short, it’s a beautiful prairie. And I don’t know why.
I know most of you are ITCHING to see the actual data tables and graphs, but because there are a few who aren’t, I’m including them as a PDF file, which you see by clicking here. The PDF also includes a cumulative list of plant species found in the restored prairie.
It’s particularly impressive that this seeding turned out so well, because the odds seemed stacked against it early on. It was seeded right at the beginning of a 7 year drought. The first several years were dominated (as usual) by weedy species and a few colonizing native species such as Canada wild rye and common evening primrose, but in this prairie those species remained dominant for several more years than is typical. Once other plant species started breaking through, there were few legumes present – and we don’t typically have problems establishing legumes in our prairies. Those legumes are still more scarce than in other nearby sites, but they’re increasing over time. Finally, in about its eighth season, the site stopped looking like a weed patch and matured into something that most people would recognize as a prairie.
As I’ve discussed in other blog posts, I’m still struggling to define success in our overall prairie restoration efforts, but at the scale of individual seedings, there are a couple things I look for. First, I want to see a good diversity of plant species, and I want to see that diversity sustain itself over time. Second, I don’t want to see invasive species increasing at the expense of that overall plant diversity, even as the prairie is exposed to disturbances such as drought, fire, and grazing. So far, this restored prairie passes those tests with flying colors. We’re moving toward implementing some measures of invertebrate use as well, but aren’t there yet. Initial data and observations, however, show higher butterfly abundance and diversity in this site than in other nearby restored prairies – for whatever that’s worth.
So why did this restoration turn out so well? I really have no idea. It caught a couple nice rains during its first spring, but the rest of the summer was awfully dry. The overall seeding rate for forbs was considerably lower than we use now, but I don’t know how much seed we had of individual species. I wish I understood why it has taken the big grasses so long to fill in, but I don’t. I think the delayed grass dominance probably plays a role in encouraging the abundance and diversity of wildflowers at the site, but I don’t know how to replicate it. The soils at the site are a little sandier than some of our other sites, but we’ve worked on sandier soils and had very quick grass establishment, so it seems unlikely that the sand is the key.
The vast majority of our prairie restorations turn out pretty well, but this one is extraordinary, and I can’t explain it. Was it something about our technique? Something about the weather or soil conditions? I know I should probably just be happy with the results, but I want to know WHY!
Recently, there has been a lot of consternation and confusion among biologists and the public about invasive species. Much of the confusion comes from misusing the term “invasive species”, and particularly the practice of using the terms “non-native (or exotic) species” and “invasive species” interchangeably. This really needs to stop.
In North America, non-native species are generally defined as species that were not present in an ecosystem prior to European settlement. There is plenty of discussion to be had about whether or not that is a useful definition, but there you go. While definitions of invasive species vary, most ecologists use a definition similar to that used by the National Invasive Species Informational Center, which has two important parts. First, the species must be non-native to the ecosystem in question. Second, and most importantly, a non-native species must either cause – or be likely to cause – harm to the environment or human health in order to be considered invasive. That ability of the species to cause harm when it’s been introduced into an ecosystem is the key characteristic that splits invasive species from all other non-native species.
A third term often thrown into invasive species discussions is “weed.” The common definition of a weed is “a plant out of place” – meaning that any plant that shows up where someone doesn’t want it can be considered a weed. I don’t have any problem with that definition, but there are at least two problems that arise from using the term “weed” interchangeably with “invasive species”. First, of course, weeds can only be plants, and there are plenty of invasive species that aren’t plants. Second, because of the definition, a plant that is called a weed by one person might be a wonderful plant to the next. Lots of native perennial prairie wildflowers show up in weed books and websites, for example, because the authors of those sites see them as extraneous to pastures, yards, or other non-prairie habitats. Many other “weedy species” (native and non-native) are important components of prairies – temporarily occupying space abandoned by stressed perennial plants until those perennials recover from fire, grazing, or other disturbances.
Several high profile articles and opinion pieces have recently questioned the value of fighting against species simply because they are “non-native”. I think it’s a great topic of discussion, albeit one that makes many ecologists uncomfortable. However, some of the authors – and MANY of those who have subsequently reported on the original pieces – make the very dangerous error of interchanging the terms “non-native” and “invasive” as they discuss the topic. Even The Nature Conservancy’s Chief Scientist made this mistake in a blog post he wrote about a journal article by Mark Davis and his colleagues.
Here’s the thing, it’s really easy for those of us working to conserve native ecosystems to become purists about the species we want in those ecosystems. We are fortunate in North America to have ecosystems that consist largely of the same species that were present in those systems at the time of European settlement. There are advantages to having ecological systems full of those “native” species because those species have had time to develop relationships and counterbalances with/to each other. However, very few ecological communities (none that I work with) remain completely devoid of non-native species, and keeping them “pure” gets harder all the time. Drawing a line in the sand and working to prevent any non-native species from getting a foothold in ecological communities is not only impossible, it’s foolhardy as well.
Don’t get me wrong, we spend a lot of time fighting invasive species in our Platte River Prairies. It’s by far the most time-consuming and expensive part of our stewardship work – and extremely important. However, we’re as careful as we can be to prioritize those species that we work against so we aren’t spending time and effort on species that aren’t causing any problems. (If you’re interested, I wrote an earlier post on prioritizing invasive species.)
As I consider whether or not a particular species is one to attack or live with, I focus mainly on the question of whether that species adds to, or subtracts from, the species diversity, ecological resilience, or ecological function of my prairies. Does the new species outcompete others to the point where small or large scale diversity declines? Or does the species actually increase species diversity by adding itself (and its functions) to the prairie?
For example, I don’t worry about plant species like the biennial plant goatsbeard, aka salsify (Tragopogon dubious), even though it’s a non-native species. My experience with goatsbeard is that it periodically appears here and there in prairies – mainly where defoliation has weakened dominant grasses and opened space for the establishment of new plants. It certainly doesn’t appear to be eliminating other species or negatively altering habitat. Rather, it’s inserted itself into the plant community in such a way that someone who didn’t know better would assume it was native.
In contrast, I worry a great deal about species such as crown vetch and smooth brome, because both can form large monocultures, and if left unchecked, can transform a prairie plant community into one dominated by only a few plant species. Not only does that affect the plant species pushed out of the community, it severely alters habitat conditions for pollinators and other species that rely on those now missing plant species. Further, it changes the habitat structure and food availability for many animals. We try to eradicate crown vetch as soon as we find it, because it’s not (yet?) widespread on our sites. We know we can’t eradicate smooth brome because it’s already all over the place, but we do design management strategies to suppress it when possible, and to encourage other plant species to fill space left by weakened brome plants.
We also, by the way, suppress or kill native species that act aggressively and simplify either species composition or habitat structure. For example, eastern red cedars are native species that encroach upon prairies when fire is absent. We clear cedar trees that are too large to kill with fire, and use periodic prescribed fire to stop others before they grow very large. In addition, we use herbicides to kill many other native tree and shrub species, including honey locust, smooth sumac, rough-leaved dogwood, and many others when their presence works against our ecological goals for a site. Similarly, most of our fire and grazing management is designed to suppress the dominance of native warm-season grasses like big bluestem and indiangrass because those grasses can reduce plant diversity if given the chance.
There are, of course, invasive and other non-native animals too – not just plants – but for the most part, prairies in our areas have few animal species that we know to be causing problems. There are a few invasive insect species that were introduced as biocontrol for invasive thistle species but subsequently began attacking native thistles – but I really don’t know much about them. However, a couple of non-prairie birds provide good examples of non-native vs. invasive animals. European starlings are non-native birds that are a substantial reason for the decline of native cavity nesting birds such as red-headed woodpeckers. Starlings, therefore, are invasive species. In contrast, Eurasian collared-doves are a new species of bird to Nebraska, but at least to my knowledge, no one has shown any reason to be concerned about that. Collared-doves may just be a new species of bird that has successfully joined the community of species in our towns (where they are usually found).
There are, of course, some important differences between non-native invaders like crown vetch and smooth brome and native species like cedars and big bluestem. Those differences include a long history of inter-species competition that has built plant communities that are unlikely to allow any one species to become overly dominant – or that at least have the capacity to prevent that dominance under certain fire, grazing, or other conditions. Most successful invasive species are able to invade and dominate regardless of managment conditions, and require direct focused attack in order to control them. An additional difference is that with native species, we know what we’re getting. We have a pretty good idea of how they grow, how they interact with other species, and what factors give them an advantage or disadvantage within their communities. With non-native species, we don’t have that history to draw upon as we try to decide whether or not to control them. That uncertainty is a huge challenge for those of us who have decided not to be purists about native/non-native species, but to instead work against those species that decrease diversity and resilience. How do we know which new species will be problems?
Sometimes we can use information from other land managers or published journal articles that have already evaluated species which are just showing up in our prairies. However, even in those cases, it can be hard to know for sure how a new species will act when it comes in. Varying soils and precipitation rates are just two of many factors that can lead to a species becoming invasive at one site and simply non-native at others. Sweet clover is a great example of a species that is considered to be an awful invasive species in some places and simply a big ugly non-native plant in others. I don’t like sweet clover, but I haven’t seen it decrease plant diversity over time at our sites. In prairies we manage with periodic grazing, sweet clover is largely a non-issue because it is so palatable to cattle. In our other prairies, it becomes visually abundant in some years and not in others, and the surrounding plant community seems to be just the same after a flush of sweet clover as it was before it. I’ve spoken to other prairie managers around the country who see sweet clover as I do, but I also know managers who see it acting much more aggressively. We’ve made the decision not to worry about sweet clover on our prairies, but I certainly don’t blame others who spend time and effort suppressing it.
This kind of uncertainty about potential impacts creates an obvious conundrum. We can’t fight every new species that comes into our prairies, but we don’t always know which to focus on and which not to. Unfortunately, I don’t have any easy answers. We all have to make our own decisions based on the best available information and the amount of time/effort we can spare. For example, someone who owns a small prairie and has abundant resources might decide to prevent any new species from joining the prairie community to be completely safe. On the other hand, most of us are forced to make difficult decisions (guesses?) because our resources are very limited.
Maybe the key is simply to be thoughtful. I think it’s a mistake to assume that a species is bad just because it’s new. Not only are some non-native species fairly innocuous, many can provide benefits, including food sources for native animals, replacement for eradicated native species, etc. However, it’s also a mistake to look at a few prominent examples of where non-native or invasive species turn out to provide unexpected benefits and assume that we should just accept all comers. The successful nesting of endangered southwestern willow flycatchers in riparian habitats dominated by invasive salt cedar, for example, has led some to suggest that maybe salt cedar isn’t so bad after all. This inaccurate, but frustratingly common, misreading of the situation ignores all of the damage to other aspects of the environment caused by salt cedar. Clearly, there are some species that need to be controlled at all costs before they completely wreck important native habitats.
All of us who manage prairies have to make daily decisions based on best available information, past experience, and gut feelings. Decisions about invasive and other non-native species are really no different. I certainly don’t plant seeds of non-native plant species in our prairie restorations, but I also don’t worry about most of the non-native plants that establish there. Frequent conversations with other prairie managers help me keep track of which species are causing problems elsewhere, and that helps me make decisions about our sites. I’m sure I’ll make (have made?) mistakes about which species to worry about, but I hope that careful monitoring of those species and their interactions with the surrounding community will help me catch those mistakes early.
I’d love to have prairies with only native species in them. I’d also love to have prairies with millions of contiguous acres, large herds of free-roaming bison, and staggering plant, insect, and animal diversity – along with a huge staff of well-trained and well-equipped land managers. Unfortunately, the reality is that I manage relatively small and fragmented prairies with a small staff and old equipment – and our prairies contain quite a few non-native species. I try to eradicate known invasive species as they appear, suppress the ones that are entrenched, and make smart decisions about species I’m unsure of. Anyone have a better plan?
For those of you in Nebraska, there is a great invasive species site that contains current events, species lists, and other great information about invasive species. Visit the Nebraska Invasive Species Project site here.
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.
This is Part 1 of a two part series on ecological resilience in prairies. Part 1 starts with an interview with Dr. Craig Allen, the Unit Leader of the Nebraska Cooperative Fish and Wildlife Research Unit at the University of Nebraska-Lincoln. Craig has studied and written extensively about ecological resilience and I’ve been fortunate to collaborate with him on several research projects examining the role of resilience in grasslands. Following the interview with Craig, I present some of my own thoughts about how to apply the idea of ecological resilience to prairies. Part 2 builds upon that theme, exploring the relationship between resilience and prairie restoration and management. I’ve had a hard time finding much written about ecological resilience and prairies – at least not much that can be easily and directly applied to prairie conservation. This is my attempt to begin a conversation on that subject. Please chime in with your own thoughts and opinions.
Prairie Ecologist: Why don’t you start by describing “ecological” resilience?
Allen: The term resilience has a long history, and has been used in academic fields such as psychology, medicine and engineering. In psychology and engineering, the term usually refers to the return time to a state of equilibrium following disturbance. The term “ecological resilience” was forwarded in 1973 by C.S. Holling. The definition was different than earlier definitions. Ecological resilience followed from an emerging understanding of multiple stable states. When a system can occur in more than one state – for example, grasslands can remain grasslands or transition to forest – resilience is a measure of the amount of disturbance required to cause that state change. A system with low resilience can “flip” into an alternative regime very easily, but very resilient systems can be highly variable while remaining in the same regime. Resilience is not necessarily a good thing – – degraded systems (like eutrophic lakes) can be very resilient – difficult to change.
Click here to see a graphic illustration of ecological resilience.
Prairie Ecologist: So, the range of stability quantifies the ability of an ecological system to stay within one stable state. What influences that range of stability in grasslands?
Allen: First, it’s important to understand that stability in this context means the ability to stay within a regime (rather than to remain “unchanging”). In fact, highly variable systems are often more resilient, and thus the term stability is a term to be used cautiously because no ecological systems are strictly “stable”.
We think resilience is influenced by the distribution of functions, functional diversity and response diversity. A diversity of functions means that the system can cope with a wide range of perturbations. Think of function as the way an animal or plant species exploits its environment. For example, an annual nitrogen-fixing plant will respond to drought differently than a perennial. In addition, two members of the same functional group (e.g. annual nitrogen-fixing plants) may respond differently to a perturbation, so species that have been uncommon in a community may change roles and become common (or “drivers”) under some situations. In this way, resilience has a lot to do with plasticity, not just at the species level, but within communities. An ecological community with a high number of species is generally better able to adapt to changing conditions (without “flipping” to another regime) than a similar community with fewer species because of the range of functions those species are capable of performing and the high degree of redundancy and response diversity.
Prairie Ecologist: What else would you want ecologists and prairie enthusiasts to know about ecological resilience and how it should influence their thinking and strategies in prairies and prairie landscapes?
Resilience is quite different from stability, and quite different from efficiency. In fact, very efficient systems are often not very resilient at all. When we alter or manage ecological systems to try to optimize a single output (e.g., corn, annual hay, or mid-summer flowers) we are increasing efficiency but decreasing resilience. Resilience is about getting an “output” across a wide variety of conditions, while optimization and efficiency seek to maximize a narrow range of outputs.
APPLYING ECOLOGICAL RESILIENCE TO PRAIRIES
By Chris Helzer
What makes a prairie resilient?
We need to understand the components of ecological resilience in prairie ecosystems before we can design effective restoration and management strategies. Unfortunately, as Craig has told me, much of what we think about ecological resilience is still theoretical, and we are greatly in need of more experimental studies. Keep that in mind as you read this, and maybe you can help refine some of these ideas based on your own experiences or through future observations/research. Don’t take my thoughts to be the truth. I’m trying to lay out the way I understand the ecological resilience of prairies based on my own experiences and those of others I know, but there’s an awful lot I (and others) don’t understand yet. As always, your comments and thoughts are very welcome.
Clearly, species diversity plays a role in ecological diversity, and it can be one important indicator of the resilience of a prairie. However, Craig stresses that comparisons of diversity/resilience should only be made within the same ecological systems. In other words, just because a tallgrass prairie has higher species diversity doesn’t mean that it’s more resilient than a shortgrass prairie that has fewer species. However, between two tallgrass prairies where latitude, soil type, and other factors are similar, the one with higher species diversity is likely more ecologically resilient.
Why? As Craig talked about in my interview with him, species have different ways of interacting with their environments – even species that we categorize together (e.g., annual legumes, perennial cool-season grasses, herbivorous ground beetles, etc). When something stresses a prairie – such as drought, intensive grazing, or a pest outbreak – some species will be better suited to respond to that stress than others. The prairie community’s “response” to the stress is that those species better suited to deal with the stress will become more abundant as other species become less abundant. In a resilient prairie community, the species that increase in abundance will fill at least somewhat similar roles to those that decreased in abundance so that the overall ecological processes in the prairie continue on as before. Moreover, when a stress in the other direction (e.g., a wet period that follows a drought) occurs in that same resilient prairie, the previously abundant species will regain their dominance under the conditions they are best suited for. In contrast, a less resilient prairie might experience a drastic shift in its ecological processes in reaction to the same stress, and might not recover its prior composition when conditions change back to what they were.
By the way, I’m presenting these changes as linear – species composition changing in one way and then straight back to the previous condition again – because it makes for easier illustration. In reality, it’s better to envision a ball rolling around in a constantly moving bowl. As long as the bowl (the ecological community) remains in the bowl (the stable state) it can theoretically return to a previous point, but it’s unlikely to happen very often. However, if the ball leaves the bowl because something drastic happens to push it out, it’s very difficult – or impossible – for it to return to the same bowl again.
Let’s look at a couple examples. When a prairie experiences drought conditions for a couple consecutive seasons, some plant species will become less abundant – or at least produce fewer flowers – while other species will respond in the opposite way. In order for pollinator insects to survive those drought years, the flowering species that respond positively to the drought conditions must provide the same kind of opportunities for those insects to get nectar and pollen as the plant species that are now much less abundant. If there are fewer overall blooms across the prairie, or the size/shape of flower that a particular pollinator needs is not available, pollinator species (and pollination services) will suffer. That loss of pollination can cascade through the prairie system, affecting seed production by plants, food availability for seed-eating insects and animals, and so on.
Herbivores have somewhat similar needs to pollinators. An insect that feeds on the leaves of warm-season grasses or the flowers of legumes needs suitable leaves or flowers to be available in dry years as well as wet. Prairies with higher species diversity are more likely to have the redundancy across those plant species categories and provide consistent food sources for those herbivores, regardless of climatic variations.
Prairie Size and Redundancy of Habitats
Besides species diversity, there are other important components of ecological resilience in prairies. Among those, two interrelated examples are the size of a prairie and the redundancy of its habitat types. Prairie size has become more of an issue, of course, as many prairies have become fragmented by human development. Smaller prairies can hold fewer individual plants and animals than larger prairies, making species more vulnerable to local extinction. Isolation from other prairies makes the situation even worse, because it further increases the likelihood that species will disappear from a particular prairie (butterflies in two prairies close to each other can exchange individuals between sites, helping to compensate when numbers drop in one of the prairies). Equally important, once a species disappears from an isolated prairie, it’s unlikely to ever recolonize from other prairies.
Small prairies not only have fewer individuals of each species, they tend to have fewer species overall. This phenomenon is laid out in MacArthur and Wilson’s Theory of Island Biogeography. Both the lower number of species and the vulnerability of those species to local extinction reduce the overall ecological resilience of small prairies for the reasons explained above.
In addition to the size of a prairie, the number of habitat types it has influences its species diversity. A prairie that has wet, mesic, and dry habitats is likely to have more species than a prairie that has only one habitat type. Besides the total number of habitat types, the redundancy of those habitats within a prairie is important as well. Many species of plants and animals are tied to relatively specific habitat conditions (e.g. sub-irrigated meadows or south-facing dry slopes, etc.) so the boundaries of those habitat types are also the boundaries of populations of those species. In that way, those habitat types and populations function and interact much like small prairies (island biogeography again). A prairie that has multiple examples of the same habitat type, especially if they’re close enough for species to interact between them, is more likely to sustain viable populations of the species that rely on those habitats than prairies that have fewer (or more disjunct) examples of each habitat type. Thus, prairies with multiple examples of habitat types are more resilient, in that they are less likely to suffer species extinctions. AND – since the number of habitat types is likely to be higher in large prairies than smaller prairies, we’re back to talking about prairie size again.
Examples of Stable States and Thresholds in Prairies
Assuming that ecological resilience is relevant to prairies, it’s extremely important that we learn more about how to recognize the boundaries between current and potential stable states – and how to predict when those thresholds might be crossed. One fairly obvious example of multiple stable states relative to prairies involves eastern redcedar invasion. A prairie with a few scattered cedar trees is still a prairie, but at some point, the trees become dense enough that the prairie becomes a woodland. When the trees are small and scattered, fire can still push the prairie back to a less wooded state (the ball is still in the bowl), but at some point the trees become large enough that they are almost invulnerable to fire, and when the size and density of trees reach a certain point, there is not enough grass beneath them to carry a fire anyway.
Once a prairie has converted to a cedar woodland, everything is different. The woodland obviously hosts a completely different set of species living in it now because most prairie plants, insects, and other animals can’t survive in the dense shade of the cedar trees. The soils begin to change too, because they are no longer being built and maintained by the annual growth and death of prairie plants – or by the countless species of tiny invertebrates, bacteria, and fungi that drove that process of decomposition. Both the plants and their decomposers are gone and are replaced by those species that can live under cedar trees. The seed bank in that soil changes too, because many prairie seeds have a relatively short life span in the soil.
As a result, even if a massive tree clearing operation takes place and removes all of the cedar trees, there are a tremendous number of obstacles that can prevent the site from becoming a prairie anything like the one that previously existed. The ball is in a different bowl.
Another change of stable states seems to occur when a prairie plant community loses plant diversity and becomes increasingly dominated by invasive grass species. I’m not sure whether the loss of plant diversity allows the grasses to gain dominance or the grasses push the other plants out (chicken and egg?) but the phenomenon often occurs as a result of chronic overgrazing and/or broadcast herbicide application. Once grasses such as smooth brome, Kentucky bluegrass, or tall fescue become dominant, it appears to be nearly impossible to regain plant diversity in those prairies – the ball is in a different bowl again. As opposed to the cedar example, above, we don’t yet fully understand what makes it so difficult to reverse the loss of plant diversity in this example. Simply altering management doesn’t seem to have much impact (it’s possible to switch dominance from cool-season invasive grasses to warm-season native grasses, but forb diversity remains low). Clearly, forbs can’t regain dominance if they and their seeds are no longer present, but even when seed or seedlings are reintroduced, most people have experienced only moderate success – at best – in regaining some degree of plant diversity. It’s likely that important insect-plant host and soil microbe-plant host relationships have been broken and that those play a large role in holding back recovery. It’s also possible that soil nutrient levels have altered (e.g., more nitrogen) in ways that favor continuing invasive grass dominance. Whatever the reasons, it’s clear that avoiding crossing the threshold into the low diversity/invasive grass dominance state should be a high priority for prairie managers.
This discussion is continued in Part 2 of this two-part blog post. That post focuses on applying ecological resilience as we restore and manage prairies.
I presented this argument to a Nebraska symposium on grassland birds in 2008 and managed to escape relatively unscathed. Now I’m testing my luck with a wider audience. At least no one can throw things at me through the computer…
Let me start by saying that I’m a big fan of birds. I really enjoyed working on my graduate research, which focused on grassland birds and their vulnerability to prairie fragmentation. I also think birds are generally pretty and interesting. However, the truth is that prairie birds make up only a tiny percentage of the species in prairies (most of which are invertebrates, followed by plants).
However, grassland birds are often held up as indicators of whether or not a prairie – or a prairie landscape – is “healthy” or “high quality.” A common refrain in prairie conservation goes something like this; “If we have our full complement of grassland birds in this prairie and/or landscape, it’s a good bet that all the other species are also doing well.” Unfortunately, while prairie birds are relatively easy to study and monitor, they may not do a good job of reflecting how the rest of the prairie is doing. Let’s look at some of the most important attributes of prairies and some of their major threats – and consider how well birds correlate with them.
Species Needs – Survival, Reproduction, and Dispersal.
First and foremost, species have to survive and reproduce in order to persist in a prairie. This applies to every species, from large vertebrates to tiny invertebrates and the entire suite of plants. It’s important for us to know that grassland birds are surviving and reproducing, but can they tell us whether other species are doing the same? You could argue that because they eat insects, grassland birds could have an impact on the survival of some insect species. That’s true to a point, but grassland birds are generalist feeders – they tend to eat whatever insects are easiest to catch at any particular time – so while the abundance of grassland birds might impact the overall abundance of insects, you can’t really tie the presence of a particular grassland bird species to the survival of a particular insect species (or vice versa). In other words, the plight of a rare leaf hopper or butterfly species is unlikely to be correlated with grassland birds. Nor are grassland birds good predictors of plant species survival – the presence of meadowlarks or Henslow’s sparrows tell us nothing about whether or not compass plant or leadplant is thriving. Grassland birds require certain habitat structure types (short vegetation, tall/dense vegetation, etc.) but they don’t much care whether that vegetation consists of smooth brome and sweet clover or a large diversity of native plants.
In addition to basic survival, animal and plant species need to be able to move around the landscape in order to recolonize places where they have disappeared, and to maintain genetic interaction between populations (important for genetic diversity). In landscapes where prairies exist as isolated remnants, moving between prairies becomes very difficult. Corridors of prairie vegetation between prairies become important in those landscapes, and prairies near each other provide better opportunities for interaction within species than do more isolated prairies. Because most grassland birds fly south at the end of each season and return the next year, (and the ones that don’t can still fly long distances between prairies) they don’t rely on those physical connections between prairies like most other animals and plants do. Since grassland birds are pretty unique in terms of long-distance flying ability, they are a poor indicator of conditions that affect less mobile species.
Ecological Services and Ecological Function – Pollination, Seed Dispersal, etc.
Apart from the needs of individual species, prairies rely on certain processes to keep everything humming along. Pollination and seed dispersal are two good examples. Both affect the viability of prairie plant species, and neither has much to do with grassland birds. Pollination primarily relies on plant diversity and bees – the most important pollinator group in prairies – and both plant diversity and bees are pretty disconnected from grassland birds. We don’t know much about the role of prairie birds as seed dispersers, but it’s a good bet that they do very little seed dispersal during the summer when they’re primarily eating insects. The role of migrating grassland birds as seed dispersers would be an interesting thing to study – but our use of grassland birds as indicators of prairie quality is always based on their presence during the breeding season. If you were going to measure whether or not pollination and seed dispersal were functioning adequately, you’d likely evaluate the diversity of plants, the abundance of bees, and some of the potential obstacles to seed dispersal (tree lines, isolation of prairies, etc.), but I don’t think measuring grassland birds would tell you much.
Resilience – Redundancy and the Ability to Withstand Stresses and Invasive Species.
One way to think about the resilience of a prairie is as a measure of how well the prairie can bounce back from stresses. For example, species diversity adds resilience to a prairie because when many species are present – especially when they overlap in the roles they play – the loss of an individual species can be a relatively minor blow. If there are dozens of bee species pollinating flowers in a prairie, a disease that wipes out one or two species will probably not have a huge impact on seed production. A diversity of plant species can also help to dampen the impacts of an event such as a severe drought or intensive grazing that temporarily weakens the vigor and growth of dominant plant species. When there are lots of plant species present, the weakening of some leads to increased growth and abundance of others. This helps maintain a stable supply of food for herbivores, and also helps prevent encroachment by invasive species that might otherwise take advantage of the weakened plant community.
Grassland birds may help bolster the resilience of a prairie in some ways. They might, for example, help suppress an outbreak of grasshoppers by focusing their feeding on that easy-to-find prey species, and thus limit its abundance. However, as discussed earlier, they don’t have much to do with pollination, nor would they help provide food for herbivores during a drought.
Threats to Prairies – Habitat Fragmentation, Invasive Species, Broadcast Herbicide Use, and Chronic Overgrazing.
One of the best arguments for grassland birds as an indicator of prairie health is that they are vulnerable to the loss and fragmentation of grassland habitat. This is true. A diverse and successfully reproducing community of grassland birds requires relatively large and unfragmented grassland. In addition, because some grassland breeding birds need short vegetation and others need tall/dense vegetation, a diversity of birds can indicate a diversity of available habitat structure – and that’s important to many other wildlife species as well. Greater Prairie Chickens are often promoted as particularly good indicators because they are a single species that needs both large grasslands and a diversity of habitat structure.
However, there are a couple of other things to consider. First, while some grassland bird species need large prairies, we don’t really know whether the minimum area required by grassland bird species is larger or smaller than that required by plant or insect species. We know that many prairie plant species have survived for a very long time in tiny isolated prairies. But because individual plants of many species can live almost indefinitely, due to their ability to generate new plants through rhizomes and other asexual means, the populations of plants in those tiny prairies could be in a death spiral due to the lack of genetic interaction with other populations. You could make the argument that plants need much larger prairies than birds (thousands of acres, perhaps?) in order to maintain genetic fitness. We simply don’t know. And we know even less about the prairie size needs of insects.
Second, while grassland birds do require fairly large grasslands, most don’t actually require PRAIRIES. Prairie chickens and many other grassland bird species have benefitted greatly from Conservation Reserve Program (CRP) fields that have added large numbers of acres of switchgrass, brome, and low-diversity grasslands to agricultural landscapes. However, those same CRP fields have done very little for native wildflower populations or pollinator insects (or other insects that rely on diverse communities of native plants) so the increase in grassland birds in landscapes with CRP doesn’t really tell us much about the health of most other prairie species.
Because grassland birds can live comfortably in grasslands made up of a few native grasses, or even non-native grasses, they are poor indicators of the impacts of most invasive species – a major threat to prairies. Similarly, broadcast herbicide use that greatly reduces the number of plant species in a prairie has little impact on the grassland birds nesting there. Finally, a prairie that is being overgrazed would certainly have different bird species than one that is not being grazed at all, but a prairie that was chronically overgrazed for decades – and then managed well again – would have a pretty low number of prairie wildflower species but a very nice-looking grassland bird community.
One threat that grassland birds can be an excellent indicator for is tree encroachment on prairies. Most grassland birds avoid nesting anywhere near even a solitary tree, let alone a grove of them, so a prairie with few birds could indicate a tree problem. On the other hand, it’s pretty hard to miss a bunch of trees growing in a prairie…
Here’s the real point. Grassland birds are an important component of prairies. A prairie without all of its appropriate prairie bird species, or in which those species are not successfully raising broods, is missing something valuable. Improving grassland bird success in prairie landscapes is an important and worthwhile objective. At the same time, however, a prairie that has a full complement of successful grassland bird species doesn’t necessarily have diverse plant and insect communities, functioning ecological processes, or a low risk of invasive species or other threats. In other words, grassland birds are an important component of high quality prairies, but their presence and/or success doesn’t necessarily mean a prairie is high quality.
Now that I’ve spent 1,500 words bashing prairie birds (I really do like birds…) the relevant question is, “What SHOULD we use as indicators of prairie conservation success?” I wish I had a simple answer. Part of the answer, of course, depends on how you visualize prairie quality (see my earlier post on this subject) because evaluation needs to reflect objectives. But if our vision of a high-quality prairie includes species diversity, habitat heterogeneity, and other complexities, our evaluation methods will have to be complex as well. Figuring out how to “take the pulse” of prairies may be the most important conservation challenge we face, because without that information we can’t design effective conservation strategies.
While we still have a lot to learn about how to take that pulse, it’s clear that we’ll have to do more than just count birds…