We recently completed a large multi-year restoration and management project at our Platte River Prairies. Our specific objectives were to improve habitat quality for various at-risk prairie species and evaluate the impacts of our management on at-risk butterflies – particularly regal fritillaries. The project was supported by the Nebraska Game and Parks Commission, who funded our work with two State Wildlife Grants (U.S. Fish and Wildlife Service money). Over five years, we conducted fire/grazing management in our prairies and enhanced plant diversity through overseeding and seedling plugs. We measured the results of that work by measuring changes in prairie plant communities and by looking at the use of our prairies by regal fritillaries and other butterflies.
The following is a brief summary of the major lessons we’ve gleaned from the fire/grazing component of the project, including implications for future management and restoration work. I will summarize the overseeding/seedling work in a separate upcoming post. If you want more details, you can see our entire final report to the funding agencies here. As a warning, the report is 14 pages long, with an additional 21 pages of Appendices, full of tables and graphs.
What We Did
Between 2008 and 2012, we treated over 1,500 acres of prairie with varying applications of patch-burn grazing management. During that time, we altered the timing of burning and the intensity of grazing from year to year, and included years of complete rest from grazing in some prairies. For the purposes of this project, we evaluated the results of our work in two main ways:
- We measured changes in plant diversity and mean floristic quality.
- We conducted three years of butterfly surveys to evaluate how regal fritillaries and other butterfly species responded to our restoration and management work.
What We Learned
Here are the seven major lessons we learned from this project. Some of this information has been covered in previous blog posts, so in addition to providing you the link to our full final report for this project, I am also providing links to blog posts in which I covered these topics more completely.
1. During our use of prescribed fire and grazing, plant diversity and mean floristic quality have been either stable or increasing in most of our prairies. This holds up when looking at both short-term (3-4 years) and longer term (up to 10 years) data sets. In addition to collecting data on the overall plant community, we also tracked individual plant species. Even conservative forbs (those most vulnerable to prairie degradation) are maintaining stable populations.
With a few exceptions, we haven’t been able to compare patch-burn grazing against other management techniques (fire only, other grazing systems, etc.) in our prairies, so we can’t say our patch-burn grazing is better at promoting plant diversity than those other alternatives. However, we have been able to increase or maintain plant diversity while creating a mosaic of habitat patches that we think benefits a wide range of plant and animal species.
2. Periodic complete exclusion of grazing appears to be important to prevent annual grazing of a few plant species. Within our patch-burn grazing systems, cattle mostly constrain their grazing within recently burned patches to the exclusion of unburned areas. However there are a few plant species that cattle appear to seek out and graze regardless of whether they are in burned or unburned patches. These include common and showy milkweed (Asclepias syriaca and A. speciosa), Canada milkvetch (Astragalus canadensis), and entire-leaved rosinweed (Silphium integrifolium).
All three plant species are still surviving in prairies that have been grazed annually for 10 years or more, but the vast majority of individuals within each species are being grazed each year before they can flower. If that continues, they will probably disappear over time, as existing plants die off without replacing themselves. To prevent this, we are building periodic exclusion of grazing into each of our pastures, either by pulling cattle out of the pasture for a whole year now and then, or by using electric fence exclosures that shift in location from year to year.
3. We can increase plant diversity in prairies dominated by invasive grasses, but it is less important to measure the extent of invasive grasses than it is to measure plant diversity. I dealt with this topic extensively in a recent post on Kentucky bluegrass, so I’ll skip over most of it here. Basically, we’ve found that we can increase plant diversity without necessarily decreasing the frequency or abundance of invasive grasses. Because of that, measuring the invasive grasses might indicate a failure in our management, even though we’re achieving our ultimate objective.
4. Butterflies are nectaring primarily on ”weedy” wildflower species in our prairies. Again, I’ve dealt with this in a previous post. Essentially, regal fritillaries and most other butterfly species in our prairies are primarily nectaring on hoary vervain (Verbena stricta), thistles (Carduus nutans and various Cirsium species), and milkweeds (Asclepias species) – which are considered to be weeds by many people. Those “weeds” appear to be awfully important to butterflies and other pollinators.
5. Regal fritillary populations appear to survive well under patch-burn grazing management despite some mortality from fire. This was one of our most important findings from this project, and it backs up similar results from Ray Moranz from Oklahoma, Missouri, Kansas, and Iowa prairies. In fact, we worked with Ray to mimic his survey techniques to allow us to compare results more easily. In our prairies, we are certainly killing regal fritillary caterpillars with our spring burns. However, regals are doing very well in the unburned portions of our sites, which make up 2/3 to ¾ of the total area in most years. Could we have higher numbers of regals if we weren’t burning? Maybe, but we’d certainly see decreases in other species we also think are important, and it’d be much more challenging to keep invasive and aggressive plants at bay and create heterogeneous habitat structure.
6. Our restored (formerly cropped) prairies appear to be complementing our degraded remnant (unplowed) prairies in terms of nectar resources. Most regal fritillary production – egg laying, larval feeding, and emergence as adults – seems to occur in thatchy portions of our remnant prairies, where we have high populations of violets (the sole food plant for larvae). However, once the adults emerge from their chrysalises and mate, they tend to spend much more time in our restored prairies, where the abundance of nectar plants is much higher. Since the main objective of our restoration work is to increase the effective size and function of our fragmented remnant prairies, this result is very encouraging.
7. While we looked only at butterflies for this project, we did some work with bees over the same period, and as we synthesize all of our pollinator work there are some apparent lessons that emerge. We know that to maintain plant diversity, it’s important to allow all desired plant species to bloom and complete their life cycles periodically. Because plant species respond differently to management treatments, that means varying those treatments from year to year. However, from a pollinator standpoint, it’s also important to stagger those management treatments across the landscape to maximize the availability and diversity of blooming plants for pollinators at any one time.
Because most bees, for example, have a very limited range of travel from their nest, haying or burning an entire 200 acre prairie at once would mean that the only plant species available for bees are those that do well under that management treatment. Since many bees are specialists on certain flower species, that can have important consequences for those bees, as well as for bees that need a variety of flower species in order to maintain steady food supplies. For bees and other pollinators (and likely many other species) it’s probably important to scatter a range of management treatments across space so that wherever a bee nests, there is a large variety of flowers available to it. However, I don’t know how to decide how many management units to use within a prairie, or how big each should be. I’ll explore this topic more in a future blog post.
As always, we still have a lot to learn about how to manage prairies for the diversity of species that live in them, as well as for overall ecological resilience. However, this project helped us better understand the impacts of our fire and grazing management on plant diversity and floristic quality. More importantly, it pushed us to look at the impacts of our management through the eyes of species other than plants – particularly butterflies. Overall, I think we’re doing well for both plants and butterflies, but we also learned some lessons that will help us tweak our management to benefit both.