Tag Archives: prairie restoration

Ecological Resilience in Prairies: Part 1

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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?

Allen:
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.

 

Tallgrass prairie at Camp Cornhusker (Boy Scouts of America) near Humboldt, Nebraska. What role does ecological resilience play in the future of prairies?

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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.

Species Diversity

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.

Plant species like hairy goldenaster can respond during drought years when other plant species suffer. Maintaining species like that can help a prairie’s resilience to drought.

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.

When eastern red cedar invasion reaches a certain extent, extensive changes occur, not only to plant species composition, but to ecological processes.

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.

Grassland Birds in Prairie Restorations: Response to a Research Paper

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A “Restoration Note” published in the most recent issue of the journal Ecological Restoration (March/June 2011) caught my attention last week.  I was initially interested because some of our sites had been part of the study but I hadn’t seen or heard about any of the results.  However, after reading the note (a short research article) I found myself musing yet again about the kinds of expectations people have for prairie restorations.  In this case, the authors were comparing bird communities in restored prairie to those in relatively degraded remnants with the idea that identical bird communities would equate to a measure of success for the restored prairies – something I strongly disagree with.  I apologize for not being able to provide you with anything more than the citation below and some paraphrasing of the article.  I requested permission from Ecological Restoration to post a PDF of the Restoration Note itself, but was turned down, and there is no abstract because of the short length of the article itself.  I guess if you want to read the whole article and don’t have a subscription to the journal, you’ll have to either go to a library to find it or pay their fee ($17!!) to read it online.

Nevertheless, here’s the citation for the article:

Ramírez-Yáñez, L. E., Chávez-Ramírez, F., Kim, D., Heredia-Pineda, F.  2011.  Grassland Bird Nesting on Restored and Remnant Prairies in South Central Nebraska.  Ecological Restoration 29:1-2, pgs 8-10.

In the study, the authors measured grassland bird abundance and vegetation structure/composition in six restored prairies (former cropland) and six remnant prairies (relatively degraded) along the Central Platte River valley in Nebraska.  The restored prairies ranged from 5-15 years in age, but the authors didn’t specify how many were in the younger vs. older stages.  Of the restored prairies on our property that I think were used in the study, two were seeded in 2002 and the other in 2001 (the paper isn’t clear about how many years of data were used for the analysis, or what years they were).  The researchers looked mostly at three bird species (bobolinks, grasshopper sparrows, and dickcissels) and located nests for each.

Grasshopper sparrows are nearly ubiquitous - often in high densities - in grazed pastures along the Platte River in Nebraska

From their vegetation data collection, the researchers found that the plant species richness of the remnant and restored prairies was very similar.  However, remnant prairies had more cover of low-growing grass species such as Scribner’s panicum and tall and sand dropseeds.  Restored prairies had more tall forbs such as sunflowers and goldenrods.  The average height of the vegetation was quite a bit taller in the restored prairies than the remnant prairies (average of 96 cm in restorations and 59cm in remnants).

The researchers found 242 grassland bird nests in remnant prairies and 264 in restored prairies.  There were significant differences between restored and remnant prairies in the abundance of nests of the three species they focused on.  Bobolinks and grasshopper sparrows had more than twice as many nests in remnant prairies than in restored prairies.  Dickcissels, on the other hand, had three times as many nests in restored sites as in remnants.

DIckcissels prefer tall forby vegetation structure like that found in young restored prairies.

Based on the vegetation data collected, the results of the bird data fits well with what would generally be expected – and what I observe in our sites.  Dickcissels tend to like sites with taller vegetation, especially when abundant tall forbs are present.  In contrast, grasshopper sparrows and bobolinks are most abundant in pastures and hayed prairies, respectively, where vegetation is kept short, and regular cropping of grasses favors those species with shorter stature.  Up to this point, as I read the paper, everything the authors were reporting fit with what I see for bird use in our Platte River prairies.  What threw me for a loop was the final paragraph of the article, in which the authors presented their interpretation of their data and the conclusions they had drawn from it.  The following is that last paragraph:

“Our preliminary data suggest that these restorations, at this point in time, are not creating the nesting habitat required or preferred by birds in more natural grasslands.  Most restoration practices focus on vegetation reestablishment and cover (Martin et al. 2005), which represent the restoration of primary production but do not ensure the recuperation of essential habitat components needed by native fauna (Whisenant 2005). Our preliminary results have promoted a change in our philosophy to include wildlife habitat requirements at the planning stage of restorations.”

There are three points I’d like to make about that paragraph and the conclusions drawn by the authors of the paper.  The first is something I’ve dealt with in a previous post.  The authors are assuming that the point of prairie restoration is to replicate existing prairies – at least, in this case, in terms of the vegetation structure and relative composition of the plant and bird communities.  Even in cases where remnant prairies are very high quality, I feel strongly that trying to replicate them through prairie restoration is not only unwise, it’s a strategy that is doomed to fail.  The management history, soil conditions, and many other factors are very different in a remnant prairie than they will be in a restored prairie.  Moreover, along the Platte River, the remnant prairies are almost all degraded – many significantly so – by overgrazing, repetitive haying, and/or broadcast herbicide application.  As a result most are missing many important prairie plant species and are largely dominated by grasses (including invasive grasses such as smooth brome and Kentucky bluegrass).  I think that most botanists would cringe at the idea of trying to replicate those prairie plant communities because of their degraded state.  Our restoration work, in fact, has focused not on replicating remnant prairies, but on enlarging and re-connecting them to each other by restoring cropland in between them with restored prairies planted with seed mixtures that maximize the diversity of locally-native plant species.  Those seedings, as they mature, are managed to maintain their biological diversity (including birds) – not to make them resemble the remnant native prairies adjacent to them.  In fact, we consider it to be a measure of success that the restored prairies contain many plant species no longer found in most nearby remnants.

The second point relates to the first, but deals specifically with the bird communities in the restored and remnant prairies.  Grasshopper sparrows and bobolinks are among the most abundant bird species in the landscape surrounding our restored prairies, and dickcissels (and other species that prefer tall forby vegetation structure) are relatively rare.  To me, creating restored prairies that complement the bird communities in the surrounding landscapes by providing habitat for dickcissels seems like a savvy conservation strategy.  If restored prairies favored the same bird species as the remnants, very little would be gained in the landscape since the amount of restored prairie is very small compared to the amount of remnant prairie.  We would simply be adding a few more grasshopper sparrows and bobolinks to the landscape – and we’d still be low on dickcissels.

Bobolinks are frequently found in hayed prairies along the central Platte River, as well as in light-moderately grazed prairies. (This one is sitting in a restored prairie)

The third point is that the authors are comparing grasslands (restorations to remnants) that differ significantly in both current and historical management.  The remnant prairies have long histories of either season-long intensive grazing and/or annual haying that has strongly influenced their plant species compositions.  Even though the remnants in the study are now primarily managed with fire and grazing, the plant community composition in those prairies is much more a product of historic than recent management.  In contrast to the remnant prairies, most of the restored prairies in the study have had mostly periodic dormant-season fire management that has encouraged dominance by the tall grasses and forbs that make up a large part of their plant community.  Plant species (such as panic grasses, dropseeds, and exotic cool-season grasses) that thrive in repetitively grazed or hayed prairies are not dominant.  Some of the restored prairies in the study are sufficiently established that they are now being grazed somewhat similarly to the remnant prairies – though generally at lighter stocking rates – but that management has not been going on long enough to greatly influence plant species composition.  It is no surprise, therefore, that the species composition and vegetation structure of the remnant and restored prairies are different from each other.  That difference reflects not a failure of the restoration process, but is rather a product of the difference in the length and type of management the sites have received.

To build a little further on the last point, IF the goal of the prairie restoration process along the Platte was to create prairies with plant species composition and vegetation structure similar to that of remnants, success would depend much more on management strategy than restoration planning and design.  Yes, alterations in the initial seed mixture to favor panic grasses, dropseeds (and invasives?) could give those species a greater relative abundance more quickly during the establishment of the restored prairie.  However, management is going to drive that establishment process much more than seeding design.  With periodic dormant-season fire, those species will not thrive, and warm-season native grasses and large forbs will still dominate – even if they start with lower abundances.  In contrast, relatively frequent and intensive grazing (or annual haying) would push the plant community toward high abundances of those lower-growing grasses (and probably fewer late-season tall forbs).  If you started with two young restored prairies – one with lots of low-growing grasses and few tall grasses and forbs, and one with the opposite composition – those plant communities could be pushed in opposite directions through management until each became nearly identical to the other.  In other words, if the authors of this paper want to favor grasshopper sparrows and bobolinks in future prairie restoration efforts, they could alter their restoration design to increase the abundance of low-growing grasses, etc., but they would be much better served spending their time on plans for an appropriate management regime instead.

To be clear, I don’t question or disagree with the data collected by the researchers on this project, I simply disagree with the conclusions and implications they drew from their results.  First, I’m not sure why they are disappointed to see an abundance of dickcissels in restored prairie, given their relative scarcity in the surrounding landscape.  More importantly, I have a much different set of objectives for restored prairies than they do.  Mine are related to contributions of restored prairie to landscape function and population/species viability, rather than to an attempt to replicate existing remnant prairies.

However, this kind of discussion over what prairie restoration should aim to accomplish (and what it CAN accomplish) is very productive.  The publication of articles that evaluate restored prairies – even those with a different perspective than mine! – is extremely valuable, and stimulates conversations that should move us significantly forward in our attempt to conserve and restore grassland ecosystems.  I would appreciate hearing from you about your perspectives on this and other similar topics.