Should We Manage for Rare Species or Species Diversity?

Land managers constantly make difficult decisions without really knowing the long-term consequences of their choices.

Balancing the sometimes conflicting needs of rare plants like Canada milkvetch (Astragalus canadensis), pollinators, and many other components of prairie communities can be a major challenge. 

For those of you who aren’t ecologists, here are some important vocabulary terms you’ll need to know for this post. 

 1. Conservative species – plants or animals primarily restricted to “intact” or “high-quality” natural areas, as opposed to species that commonly occur in degraded habitats.

2. Species richness – the number of species found in a certain area. High species richness means there are lots of different kinds of plants and/or animals present

3. Species diversity – a kind of modified species richness that also takes into account the evenness, or relative abundance, of each species. When one site has a few dominant species and lots of uncommon ones, it is less diverse than another site with the same total number of species but with more evenly distributed numbers of individuals.

Imagine this situation:  You’re put in charge of managing a tallgrass prairie with thriving populations of several rare plant species.  The prairie is located in a highly fragmented landscape dominated by rowcrop agriculture.  The prairie has been managed with frequent spring burning for many years, and the populations of those rare plants has been pretty stable for at least the last couple of decades.  As you take over, the previous manager tells you she’d recently been considering management changes that might increase overall plant and animal diversity but would likely reduce the population sizes of some rare plant species.  You have to decide whether to stick with the existing management regime or try something different.  What would you do?

Culver’s root (Veronicastrum virginicum) is a conservative plant species found in a small subset of today’s tallgrass prairies.

It would be perfectly rational and defensible to stick with the strategy that has sustained healthy populations of rare plants for a long time.  Because those plants aren’t found at many other sites, prioritizing them in this prairie makes good sense.  However, before you lock in that choice, let’s consider some other information.

First, there is often an assumption that an abundance of rare plants is an indication that the rest of the prairie community is also intact and healthy. While that assumption seems logical, it’s not always the case.  A good example of this comes from an Illinois study by Ron Panzer and Mark Schwartz.  Their research in the Chicago region showed that neither the number of conservative plant species or rare plant species predicted the number of conservative or rare insect species at a site.  Instead, Panzer and Schwartz concluded that overall plant species richness was more important for insect conservation.

Plant diversity also helps support healthy populations of pollinators and herbivores (invertebrate and vertebrate) by ensuring a consistent supply of food throughout the year.  A wide variety of plant species allows pollinators and herbivores to find high quality food at all times, even though each plant provides those resources at different times of the season.  For this and other reasons, increasing plant species richness can increase both the abundance and diversity of animals, especially invertebrates.  In addition, managing for a variety of vegetation structure types (including a wide range of both plant stature and density) can also help support more animal diversity, including birds, mammals, reptiles, and insects.

Grazing can decrease the size of rare plant populations, especially in comparison to sites under repetitive haying or burning management. However, carefully planned grazing can also increase plant diversity and provide more varied habitat structure for wildlife and invertebrates.

Every species of plant and animal plays a certain role within the prairie community.  High species richness provides redundancy of function and helps ensure that if one species disappears or can’t fill its role, others can cover for it.  That contributes to ecological resilience – the ability of an ecological community to respond to stress without losing its integrity.  Ecological resilience may be the most important attribute for any natural system, especially in the face of rapid climate change, continuing loss and degradation of habitat, encroaching invasive species and other threats.

Aside from the benefits of managing for species richness, a strict management focus on the needs of a few species can put others at risk.  The use of prescribed fire, for example, provides a competitive edge to some plant species, but has negative impacts on other plants, as well as on some animals.  There have been vigorous arguments between advocates for frequent burning and people concerned about rare butterflies and other insects, as well as reptiles and other animals that can be extremely vulnerable to prairie fires.  Repeated intensive grazing by cattle or bison is another management strategy that favors some plant and animal species, but can negatively impact many others, especially without adequate rest periods between grazing bouts. Management that consistently provides favorable conditions for a few species at the expense of others may eventually eliminate some species from a prairie altogether, or at least reduce their ability to effectively contribute to ecosystem functioning.  If those losses lead to decreased ecological resilience, the resulting impacts may end up negatively affecting the same species a site manager is trying to promote.

Regal fritillary butterflies are very sensitive to fire, and can be eliminated from isolated prairies if the entire site is burned at an inopportune time. However, populations can also thrive in large prairies managed with a combination of fire and grazing, as long as sufficient unburned areas are available, and many of their favorite nectar plants (like this Verbena stricta) are common, or even weedy.

So, what’s the right path?  Should we prioritize management for rare or conservative species, assuming that other species don’t need as much help?  Or should we focus on species diversity and ecological resilience because we need the strongest possible natural communities in today’s challenging environment?  How should scale (size of prairie) influence decisions?

There are plenty of potential benefits and risks associated with each path, and I’m not here to tell anyone which they should choose.  In most cases, my own tendency is to focus on diversity and resilience, but I completely understand why managers would go the other way, and I think every situation needs to be evaluated independently.  For example, if a species is teetering on the brink of extinction and we need to keep it alive while we create more habitat elsewhere, I’m perfectly fine with prioritizing management to favor that species.

In other cases, I worry that we’re too sometimes unwilling to manage prairies in ways that promote changes in plant composition.  Years of repetitive management (especially frequent haying or burning) create conditions under which plant communities seem very stable.  However, that stability may be a response to consistent management rather than an intrinsic quality.  Allowing plant populations, even of rare species, to fluctuate in size, or even persist at a lower abundance than we’re used to is not the same as driving those species to extinction.  If rare species survive in smaller populations but the surrounding community is more resilient, that may be a win.  Having said that, reducing the size of rare species’ populations can make them more vulnerable to local extinction, and I don’t take that kind of risk lightly.  These are challenging issues.

This bottle gentian plant (Gentiana andrewsii) is an extremely conservative plant, and was growing in a hayed meadow in the Nebraska Sandhills where management conditions are very stable from year to year.

The hard truth is that we don’t yet understand enough about ecological systems to make these kinds of decisions confidently.  I understand the impulse to manage conservatively, sticking with what seems to have been working for a long time – especially in small and isolated prairies.  At the same time, I also think we need to build as much diversity and resilience in our prairies as we can – focusing on both plants and animals – especially in landscapes where we don’t have many left.  I’m glad managers are experimenting lots of different strategies, but we should all take responsibility for collecting data that help us evaluate our management, and keep open minds as we share what we learn with each other.  None of this is easy, but it is certainly important.

32 thoughts on “Should We Manage for Rare Species or Species Diversity?

  1. Thanks, Chris, for the great synopsis of the values, alternatives, and uncertainty inherent to rare species management. I especially liked the insight about rare plants not being an indicator rare insects (or other groups of interest). From a resilience perspective, frequent burning as a management technique is interesting to me – when does a disturbance become so frequent that it’s no longer a disturbance. That is, when have you burned so much that the loss of fire is the “real” disturbance?

    • It is really interesting, isn’t it? I have a future blog post swirling in my head that addresses the question of whether it’s possible to call prairie a climax state. There are many in the tallgrass region who commonly refer to high quality prairie as “Climax communities”. Squaring that concept with disturbances and succession theory is an interesting mental journey.

  2. Hi Chris,

    I don’t see it as an either or decision. Increasing both species’ diversity and rare-species populations can be done simultaneously. We get a lot of practice in our little Eastern tallgrass prairie remnants. However, I recognize it can easily go the other way. Given that so few people are actively engaged as restoration practitioners, I try not to judge their particular management biases, rationalizing that in most cases, management is better than no management.

    Thanks for a thought provocative post.


  3. Interesting timing. Just this day I read an article “Rare species contribute disproportionately to the functional structure of species assemblages,” by Leitao et al. They conclude that “Disproportionately supporting the whole breadth of functional abilities within assemblages, rare species potentially play critical roles in maintaining ecological processes in space and time particularly under the ongoing rapid environmental transitions in the tropics. This justifies the application of the precautionary principle for conservation strategies and undermines arguments that many species are functionally redundant in highly diverse systems. ”

    • Thanks Linda – intriguing article! And to be clear, I certainly don’t mean to imply that rare prairie plants or animals are functionally redundant or unimportant to the community. I do wonder how abundant a species has to be to be a relevant contributor, and I’m sure that depends highly species by species and by the kinds of roles each plays. There’s just so much to learn!

      • I think about this a lot too, Chris, and I think you’re right that the importance or contribution of a particular species to any function can be quite species-specific. I’m thinking of some insects where body size is a strong predictor of function, with larger species making more contributions even if their abundance is low. Unfortunately, the responses of species to disturbances and management often are also species-specific, which makes it really hard to predict how a given action will change a community. And the biggest problems arise when those functionally important species are also the ones most likely to be extirpated by human actions, like species with high trophic position.

        • Thanks for those additions, Nick. It’s all very complex. Let me know if you ever get tired of working at Nachusa – we have housing available here in Nebraska if you or students want to come help us answer insect questions out here!

  4. I understand the benefits of burning, but if it’s an attempt to imitate nature, would frequent burning produce that result? Is the practice based on data indicating that a particular parcel of prairie would burn x times in a given period? How often has lightening started a burn on the prairie you manage? And is the frequent burning more directed toward invasive grasses and such?
    Actually, if you could just recommend a book geared toward the nonprofessional, it would save you having to write one to answer all these questions I’ve asked you. Thanks.

  5. Frequent burning has some of the same impacts as frequent haying, and it is not necessarily meant as something that mimics history but rather as a maintenance strategy. Frequent burning is most common in eastern tallgrass prairies. Further west, prairies are burned less frequently (often every 3-5 years). Of course there is variation in both regions.

    Lightning strikes start fires periodically on our Niobrara Valley Preserve (52,000 acres). Actually, most historic fires in prairies were likely started by humans (Native Americans) rather than lightning – or at least most of the larger fires were.

    If you haven’t seen it, check out my book “Ecology and Management of Prairies in the Central U.S.” It’s meant to be accessible to non-professionals.

  6. Some really great reflections Chris–thank you. As land managers, I think one of the most important things we can do is embrace diversity in management approaches rather than convincing ourselves that there is one right way to do things in all situations. I think of some of our sites in Minnesota where for so long the right approach was to plant brome/alfalfa strips for pheasants and broadcast spray 2,4-D everywhere else in order to control “weeds.” That was followed by burning entire units with no refuges at an unnatural fire return interval, effectively wiping out many of the invertebrates. I’m glad some landowners only hayed their land, or only grazed, or quite simply did nothing. Otherwise we would have very little native prairie left. So to your question–all of the above! And as you suggest, constantly monitor, evaluate, and share that information with others. And never do the same thing you’ve always done just because that’s what you’ve always done.

    • Thanks for those comments, Nate. I agree, we’ve come a long way in our understanding. I’m sure we’ll look back 20 years from now and shake our heads about some of today’s practices too. It’s one of the reasons I really worry about small isolated prairies. They are the most vulnerable to our mistakes. In larger, more connected landscapes, we have a little more leeway to screw up some places because those areas are connected to others. And yet, we can’t afford to be paralyzed by the fear of doing something wrong and just do nothing. Fun, huh?

    • I’ll echo what Nate said, with the exception of doing nothing. There are a lot of formerly good prairies on private land that are now solid stands of cedar, buckthorn, or both. The management need not be often, and shouldn’t take on the whole remnant at once, but without it prairies are doomed. A drive through the Loess Hills in Iowa or hill prairies in southeast Minnesota brings that into focus.

  7. I’m not an ecologist or land manager, but I enjoy posts like this one that pose a question and invite us to follow along as you explore possible solutions. I love the opportunity to peek into someone else’s world! Thank you!

  8. Chris, I’m curious…do you think your views on prairie management are influenced by whether the prairie is a native remnant versus a reconstruction, independent of the conservativeness of the species in question?

    • Good question. I do feel like it’s easier to justify crazier experiments on restored prairies than on remnants. I was just talking with a friend about this the other day. In some ways, restorations give us a chance to see how intensive grazing, for example, impacts conservative species without worrying about really impacting long-lived colonies. At the same time, we have to be careful extrapolating between remnants with their very mature and complex interrelationships (between plants, animals, microbes, etc.) and young restorations that are still developing those relationships. However, I think my overall philosophies of prairie management come less from working with restored prairies (as well as remnants) and more from working in landscapes with a decent amount of remaining prairie land and where grazing and fire are both culturally accepted practices. I didn’t start out with a bias against either fire or grazing, which has allowed me to judge them by how I see prairies respond to them.

      • Thanks for your response… I think I would naturally be more conservative with remnants too, only because there is still so much more we don’t know or don’t study regarding the many long-standing and complex relationships in these systems.

  9. There is a program in the Chicago Region that your readers might be interested in modeling in their own areas. This program has given land managers the understanding of needs for and impacts of management activities allowing them to better make the types of decisions you discussed.

    • This seems like a good program that should be emulated. Do you have any idea what kind of operating budget this group operates with?

      • I cannot answer that question. The Plants of Concern website under the option “about” has another option labelled “contacts” which lists three staff members. If you e-mail the program manager or her assistant then I am sure they could answer your question.

    • I should also mention all the other great monitoring that is occurring in the Chicago Region. I do not participate in monitoring because habitat management work keeps me very busy. However, I enjoy reading the monitors reports in local group’s newsletters.

      The link regarding the calling frog survey does not work because this program has been moved. The link to its new home is below.

  10. I do have a background in range ecology but have been out of the field for some time, but couldn’t you manage the area in a mosaic pattern. I am in a way agreeing with David Cordray’s comment, treat most areas for biodiversity and the sites with high endangered species concentration for that specific species?

    • Jan, good question. In some cases, yes, of course. In other cases, “protecting” an area from disturbances that might negatively impact a rare species can make the same area vulnerable to other impacts. An easy example is fire exclusion to protect fire-sensitive butterflies that ends up encouraging woody invasion that wrecks the habitat anyway. Each site is different and requires tough choices.

  11. Chris I really enjoyed your post this time – I have never not enjoyed a post. Having spent years trying to develop a program for the Platte River specifically designed under the ESA to focus almost entirely on just a few Endangered Species, it is really refreshing to see lots of serious discussion about for what species we really should be managing. I expect this is a fairly common debate in the managers’ literature, but for me it is new and very exciting.

  12. I think soil type is important in this discussion. I expect most of the prairies you manage can recover rather quickly from intensive grazing because they are located on sandy soils. In contrast, compaction of clay soils can take many decades to reverse. Secondarily, uncompacted soil can store more water and have better drainage which is more important in eastern prairies than the more arid conditions on the western edge.

    This is the reason I had previously suggested that feeding the soil was important when performing ecological restoration. The soil invertebrates both cycle organic matter into the soil and aerate helping alleviate soil compaction legacies from intensive grazing or agriculture.

  13. I read through the Panzer and Schwartz Study. I don’t think it matches with the discussion in your post. Although the preserves studied are not listed in the paper, I am sure I am intimately familiar with many of them. I can tell you from living in the area that grazing by cattle or bison is not occurring in areas with any natural area value. The only exception would be Midewin. Midewin is not considered to be virgin tall grass prairie because of the land use history that occurred. Also, the reintroduction of bison to Midewin did not occur until many years after the aforementioned study was published. I do know some preserves had a history of grazing, but they have not been used for this purpose in many decades.

    I think the result that should be taken away from Mr. Panzer and Mr. Schwartz study is that rare insects often need a diversity of plant communities. An example would be a dragonfly that needs a specific type of wetland to breed and an adjacent prairie for a hunting ground. Preserving assemblages of diverse communities will likewise preserve areas of high species richness. It is well known that edges between two types of habitats are often hot spots of diversity. I think this should be the take away from the study. Although, future studies at Nachusa may demonstrate the points you were making in your post.

  14. Pingback: Best of 2017 – Stories and Photos from The Year | The Prairie Ecologist

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