Lessons From a Project to Improve Prairie Quality – Part 1: Patch-Burn Grazing, Plant Diversity, and Butterflies

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.

Plant diversity and buttterfly habitat were the objectives of our 5-year project.
We’ve worked hard to get plant diversity in our restored prairies, including this one.  We wanted to know whether or not our management was maintaining that diversity, and also how it was affecting butterflies.  The prairie shown here was being grazed at the time of the photo – July 2009.  The Nature Conservancy’s Platte River Prairies, Nebraska.

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.

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Regal Fritillary Butterflies in Burned and Grazed Prairie

We’ve been conducting field surveys of regal fritillary butterflies for the last three years.  During that time, we’ve learned a lot about how those butterflies are responding our prairie management and restoration work.  So far, there are two overwhelming lessons we’ve learned from our work.

1. The number of regal fritillaries produced in our Platte River Prairies is primarily tied to two factors: violets and thatch.  During the spring, when adults are first emerging from their chrysalises, butterfly abundance is highest in degraded remnant (unplowed) prairies that have few showy native wildflower species, but lots of common blue violets (Viola sororia).  While they don’t have much to excite a prairie botanist, these prairies sure produce a lot of regal fritillaries.  We don’t find many regals in recently burned portions of these prairies – only in portions that have built up some thatch.

A regal fritillary feeds on a thistle – The Nature Conservancy’s Platte River Prairies, Nebraska.

2. After regals emerge and mate in those thatchy violet-rich prairies, they spread out into more flowery sites to feed.  In our Platte River Prairies, those feeding sites tend to be restored (reconstructed) prairies located around and between those degraded remnants.  Those restored prairies have significantly fewer violets than remnant prairies, but lots of the favorite nectar flowers for regals, including hoary vervain (Verbena stricta), milkweeds (Asclepias spp.), wild bergamot (Monarda fistulosa), and thistles (Cirsium and Carduus spp.).  Interestingly, while we don’t see regals emerging from recently burned prairie, some of the most-used summer nectaring sites are our most recently burned sites.

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