Tag Archives: plant diversity

When is a Prairie Restoration (Reconstruction) Project Successful?

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This is a follow-up to last week’s post on using prairie restoration to enlarge and reconnect remnant prairies.  In this week’s post, I present a case study of a remnant sand prairie and an adjacent prairie restoration, and give thoughts about how to measure the effectiveness of that restoration project.  We’re (all of us) just getting started figuring out how to measure this kind of thing, so I’m hoping my thoughts will stimulate others to come up with their own ideas to improve upon – or contradict – mine.

Last week, I wrote about how we can improve our chances of conservation success in small isolated prairies by using prairie restoration (reconstruction) to enlarge and reconnect prairie fragments.  I even made a goofy analogy about catching falling popcorn.  At the end, I mentioned that when measuring the success of a prairie restoration – as a tool for enlarging or reconnecting remnants – we need to take a different approach than simply comparing the remnant and restored prairies to see how similar they are.  If the point of the restored prairie is to reduce the level of threat to species and natural communities inside the remnant prairie, that’s what we need to measure.

To explain what I mean, let me use a restored/remnant prairie complex along Nebraska’s Platte River as an example.  In 2000, The Nature Conservancy added several hundred acres to our Platte River Prairies through a land acquisition.  Most of the new land was cropland, but it also included 60 acres of remnant mixed-grass sand prairie with good plant diversity.  Two years later, using seed harvested from the remnant prairie and other nearby sites, we seeded 110 acres of cropland directly adjacent to the sand prairie.  The restored cropland has the same kind of hilly topography as the remnant, but also includes some low areas more appropriate for mesic tallgrass prairie.  Thus, the 162 species in our seed mixture included plant species from both mixed-grass sand prairie and mesic tallgrass prairie.

Remnant sand prairie at The Nature Conservancy's Platte River Prairies.

In June of 2010 I collected plant data from both the remnant and restored prairie (in its ninth growing season).  The data were collected by counting the plant species inside a 1m2 plot frame from 100 locations across each prairie.  Those data allowed me see the frequency of occurrence of each species (the % of plots in which each species was found).  To make the results easier for you to visualize, I’ve used a color-coding system to create what I call a plant composition signature for each prairie.  The complete comparison of the two prairies, with additional interpretation, can be seen here if you’re interested, but for this example, I’m just going to show some representative excerpts.

After the latin and common name for each species, you’ll see a column labeled “C”, which is the C-value (or coefficient of conservatism – defined by Swink and Wilhelm 1994).  If you’re not familiar with this categorization of species, a quick explanation is that lower C-value species are more opportunistic plants that can generally thrive in very disturbed environments and higher C-value species are more tied to intact native communities.  Another way to look at it is that higher C-value species are more vulnerable to habitat degradation.  All species are ranked on a scale from zero to ten (the values I’m using are specifically for Nebraska) and all exotic species get an automatic zero.

In general, the restored prairie has the same grass species as the remnant, although many are less abundant. Most of those less abundant species will spread over time as the restored prairie continues to mature. A few sedges, including sun sedge, do not establish well from seed, and we're attempting to bring them in as transplants and let them spread from there.

The main difference in "weedy" forbs between the remnant and restoration is the abundance of goldenrods in the restoration. Canada and late goldenrod were both from the seedbank, but stiff goldenrod was planted by us. At this point, I'm not concerned about the goldenrods (they don't appear to be as aggressive here as in some places) because they haven't been decreasing species diversity over time.

As with other species, I expect many of the more conservative forbs species will increase over time in the restoration.

Based on experience, I'm sure Kentucky bluegrass and smooth brome will increase over time in the restoration, but so far we've been able to manage those species to keep them from overwhelming the plant diversity in other older restorations. Apart from those two species there are no serious invaders that in the restoration that might threaten the remnant, which is good to see.

It’s easy to find differences between the remnant and restored plant communities in this example – some plant species are much more abundant in one than the other.  On the other hand, very few plant species from the remnant are missing completely from the restored prairie, and those that are less abundant are likely to increase over time.  As a prairie ecologist, I can see some obvious visual differences between the restored and remnant prairies, but most visitors to our site see the two as one large prairie.  But… Does any of this matter?  How do I decide?

First, remember that the objective of this restoration project was NOT to replicate the remnant sand prairie, but to increase the viability of the species and communities living in it.  Given that, the real questions I need to answer include the following:  Does the restored prairie increase the population size of species formerly constrained by the small remnant prairie?  Does the combination of the restored and remnant prairies provide suitable habitat for species that don’t occur in prairies the size of the remnant alone?  Does the restored prairie add to the overall resilience or ecological function of the remnant prairie?  Any questions about similarities or differences in the abundance of individual plant species need to be framed within the context of these kinds of broader questions – and tied to the specific objectives for the restoration project.  Comparisons outside of that context are relatively meaningless.

To begin evaluating the impact of the restored prairie, one first step could be to look at a few at-risk species in the remnant prairie to see if the restoration appears to benefit them.  If the remnant prairie has been harboring a small population of Franklin’s ground squirrels, for example, it’d be good to find squirrels (and their burrows) in the restored prairie as well.  If there was a rare penstemon species in the remnant (bumblebee pollinated) it’d be interesting to follow bumblebees from the plants in the remnant to see if they also visit penstemon plants in the restored prairie  – indicating that the restored prairie has facilitated growth of a genetically-interactive penstemon population.

Besides at-risk species, it would be worthwhile to search the restored prairie for the presence and/or abundance of species from other categories as well.  These categories might include:

–          Species that are representative of various types of relationships (e.g. predators and their prey, parasites/parasitoids and their hosts, insects and their larval host plants, etc.).

–          Species that have a cascading effect on other species and ecological processes (e.g. allelopathic or parasitic plants, burrowing insects/animals, etc.).

–          Species that are particularly important as food sources for a range of other species (e.g. springtails – aka Collembola, grasshoppers, “soft-bodied insects” like caterpillars and other similar larvae, etc.).

–          Area-sensitive species that may not have been able to survive in the small remnant alone but that might have a chance in the combined restored/remnant prairie (e.g. prairie chickens, badgers, and other vertebrates).

It’s also important to evaluate impacts of the restoration project on groups of species that influence ecological processes – such as pollinators and seed dispersers.  Pollinators are relatively easy to observe, and both the pollinators themselves and the resources they depend upon can be evaluated.  Ideally, of course, it’d be great to have several years of data on the species richness and abundance of pollinating insects in a small remnant prior to initiating a restoration project, followed by similar data collection after the restoration has established.    However, simply looking at whether or not purple prairie clover plants (for example) in the restored prairie are getting pollinated by the same species and numbers of pollinators as the prairie clover plants in the remnant could be very informative.  From the resource perspective, if the remnant prairie tends to lack an abundance of flowering plants at a particular time of year (late spring, for example, or early fall), measuring whether or not the restored prairie provides appropriate blooming plant species to fill that gap is very important.

Purple prairie clover being pollinated by a native bee.

There are numerous other things that could be measured, including taxonomic groups we really don’t know much about at this point.  For example, soil fauna, fungi, and obscure groups of invertebrates may very well have strong roles to play in ecological functioning of prairies, but we don’t know much about what those roles might be or how to evaluate them.  While it’s certainly important to learn more about those other taxonomic groups, our lack of knowledge shouldn’t stop us from measuring what we do know in the meantime.

The last thing to consider is whether or not a restored prairie could be actually be negatively impacting the adjacent remnant prairie or its species.  One example of this could be an invasive species that becomes established in the restored prairie – thus threatening the remnant.  A second possibility is that the restoration could function as an “ecological sink” for some species from the remnant, in which a species is drawn out of suitable habitat into attractive-looking but perilous habitat instead.  We’ve actually been testing for one possible example of this in our Platte River Prairies.  Regal fritillary larvae feed only on violets, but adults don’t lay their eggs directly on violet plants.  Our lowland remnant prairies have lots of violets, but our restored prairies have very few (so far) because we are unable to harvest large numbers of seeds.  We’re trying to make sure fritillaries aren’t laying eggs in the restorations where the larvae would be doomed to starve because of the near absence of violets.  (So far it looks like it’s not a big problem.)

As I mentioned at the beginning, we’re just starting think about how to measure the effectiveness of restored prairies as conservation tools.  Since the initial practical work of a prairie restoration project involves the establishment of a new plant community, it’s natural to assess the success of the various species we included in the seed mixture.  Unfortunately, it’s also easy to overemphasize the importance of floristic differences between a restored prairie plant community and nearby remnant prairies.  For many reasons, it’s not practical to recreate a historic prairie or replicate an existing remnant prairie.  However, it is possible to use prairie restoration to increase the viability of our remaining remnant prairies.  It is imperative to set clear objectives for this kind of restoration work, including the specific ways we want the restored prairie to help abate threats to species and communities.  Clear objectives will lead to easier decisions about how to measure success.

Many of the suggestions here are just first steps, and they and subsequent steps will require considerable resources, as well as collaboration with academic researchers.  Yes, there’s a lot to measure, but as we start to establish consistent patterns of success with some kinds of species or ecological processes, we can start focusing attention more narrowly on others.  We don’t have to test everything at once, and the most important measures at each site are those that evaluate whether or not specific objectives for that restoration project are being met.  However, it will be critical that we all share what we learn – successes and failures alike – to build up our cumulative knowledge as quickly as possible.

There are a number of examples of restoration projects where remnants have been enlarged or reconnected by restoring adjacent lands.  We should look closely at those existing sites to see if we can find evidence of success or failure (based on some of the suggested strategies above – and others).  That knowledge can guide us as we plan and implement new projects in the coming years.  It’s unlikely that we’ll be able to design restoration projects to benefit every prairie species and function, but we can certainly do a lot of good.  There’s a lot of work to be done, but I’m very optimistic about our ability to make a real difference.

Measuring Success in Prairie Conservation – Species Composition vs. Structure and Process

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Stick with me – this isn’t as complicated as the title might lead you to believe…

I was involved in an interesting discussion a couple weeks ago among some fellow prairie ecologists about what makes a “good prairie”.  The discussion brought into sharper focus something I had thought a lot about but only in general terms.  Here is the discussion – with some over-simplification of the respective positions:

Position A – Species Composition:

A high quality prairie (tallgrass prairie, in this case) can be judged largely by its plant species composition.  A “good” prairie might have 20 or 30 plant species per square meter, for example, and more than 100 species per acre – including a mixture of both common and rare species.   Besides the value for the direct conservation of species, that kind of plant diversity provides multiple ecological benefits.  More plant species means more choices for animal species that rely on them as food and/or habitat resources, as well as a more consistent supply of those resources through the season – because some plants will always be emerging/blooming as others go dormant.  Because of that, prairies with lots of plant species tend to have lots of insect and other small animal species as well.  In addition, prairies gain resilience from plant species diversity, because if multiple species fill similar ecological roles the prairie community can better withstand the temporary decline of some species due to drought or pest outbreak.   A prairie that is missing many of its species, or that is dominated by a few species with only scattered small populations of others, can’t be considered to be of high quality or to be “conserved.”

A diverse prairie at the Madison Arboretum – Madison, Wisconsin.

Position B – Structure and Process:

What really defines successful grassland conservation is the presence of large-scale and intense disturbances (e.g. fire and grazing).  The combination of fire and grazing shaped historic prairies and that combination is needed today to maintain them.  Without fire, prairies lose integrity in several ways – the most obvious being the encroachment of trees that fire otherwise suppresses.   Fire also helps drive the cycling of nutrients and regulates the amount of standing dead vegetation and thatch in prairies.  Furthermore, the high-quality of the fresh vegetative regrowth following a fire attracts intense grazing by herbivores large and small.  Historically, there would have been few cases where a prairie would burn and not be intensively grazed right afterwards.  That intense grazing suppresses dominant prairie grasses, opening up space for the abundant growth of weedy vegetation once the grazers move on.  As the prairie recovers, the dominant grasses reassert themselves and the vegetation becomes tall and dense enough to carry fire once more.  A landscape consisting of a heterogeneous mixture of recently burned patches and patches that haven’t burned for several years provides the full range of habitat structure – from very short to very tall – and thus supports the full range of prairie wildlife species.  Perhaps most valuable in that range of habitat structure is the post-fire/grazing recovery phase that provides simultaneously provides a wealth of stemmy vegetation cover and abundant seeds for wildlife food.

An expansive prairie landscape in the Nebraska sandhills.

So is Position A or Position  B correct?  Well, yes.  It’s like owning a sports car.  You need to keep all the sparkplugs, tires, and other parts – and keep them in good condition.  On the other hand, a sports car is no good if you can’t drive a standard transmission and/or don’t have good roads to drive on.  Aldo Leopold said, “To keep every cog and wheel is the first rule of intelligent tinkering” – but those important components include both species and the processes that maintain them.

Interestingly, the two positions seem to be strongly correlated with geography.  People who work with the fragmented tallgrass prairies in Midwestern States like Illinois, Wisconsin, and Indiana tend to fall more heavily in the first camp – emphasizing species composition.  In contrast, people who work in the large expanses of grassland in Oklahoma, Kansas, western Nebraska, and the Dakotas, tend to hold Position B – emphasizing process and structure.  In both cases, the positions are reasonable based on the local situation.  In the eastern tallgrass prairie, most of the prairie is gone, so any remaining grassland is precious and there is great concern about the loss of prairie plant and insect species.  Further west, where grasslands still dominate the landscape, there is much less concern about the loss of plant and insect species and more focus on larger wildlife like prairie chickens that require a range of habitat conditions not always found in those ranching-dominated landscapes.

As usually happens in a discussion among reasonable people, a partial consensus emerged in ours, and there was general agreement that neither Position A or B was sufficient by itself.  Proponents of species composition as the measure of prairie conservation success would surely not be fully satisfied with a flowery prairie that was missing species like prairie chickens, upland sandpipers, and bison.  They just don’t often have the opportunity to work with grasslands large enough to support all of those species.  Likewise, proponents of structure and process wouldn’t be happy with 20,000 acres of switchgrass just because it had a heterogeneous mix of fire and bison grazing and lots of prairie chickens.

The point here is not that we need to subscribe fully to either Position A or Position B, but that we can’t afford to ignore either one.  Those working in the fragmented eastern tallgrass prairie need to be sure to emphasize strategies like prairie restoration that can strategically convert crop fields back to prairie vegetation and enlarge remnant prairies to the point where they have a chance of supporting prairie chickens and upland sandpipers, if not bison.  And even at smaller scales, finding creative ways to reintroduce the combination of fire and grazing, where possible, may help provide better wildlife habitat – and might even pay dividends for plant species conservation (more discussion on this in posts to come.)

Meanwhile, ecologists with the luxury of large unplowed expanses of native grassland need not to forget the importance of restoring and/or maintaining both large-scale and small-scale plant diversity.  While adequate habitat structure for species like prairie chickens can be created in a landscape dominated by grasses and weedy forbs, pollinators and many other insects may have a much more difficult time surviving there.  In addition, whether the intervening landscape is dominated by grass or corn, small isolated populations of prairie forbs (and the insects that rely on them) aren’t likely to survive forever if they’re not able to cross pollinate or otherwise interact with each other.  Native bees that have to find consistent sources of nectar within a small radius from their nest rely on small-scale plant diversity to provide abundant blooms every day of the growing season – and pollination by those bees is critically important for the survival of many plant species.  Finally, proponents of process should recognize and appreciate the potential (but understudied) values associated with a diverse plant community – including a diverse and vigorous soil fauna, and the overall resilience offered by a mix of species that provides redundancy of ecological function.

The danger for all of us is that we tend to look at prairie conservation through a cultural lens – and we sometimes don’t see what our prairies, and our strategies, are missing.  It would be great if we could facilitate some cultural exchanges, in which we sent Texas cowboys to the Illinois black soil prairies and Wisconsin prairie restoration experts to the flint hills of Kansas.  Imagine the discussions that would ensue – not to mention the neighborhood coffee shop gossip.

Participants in the Grassland Restoration Network discussing prairie conservation at The Nature Conservancy’s Nachusa Grasslands, Illinois.

I feel fortunate to be involved in two groups that do a great job of stimulating interaction and discussion – the Grassland Restoration Network and the Patch-Burn Grazing Working Group.  The former is a loose affiliation of ecologists working to use prairie restoration as a tool for grassland conservation in fragmented landscapes.  The latter is a network of scientists, land managers, and ranchers trying to find better ways to combine fire and grazing and create heterogenous prairie landscapes.  (I’ll provide more information on both of those groups in future blog posts.)  Now if I can just figure out how to convince the two groups to hold a joint meeting…