Tag Archives: habitat

Regal Fritillary Butterflies in the Platte River Prairies – 2011

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We completed our second season of regal fritillary butterfly data collection this summer.  Funded by State Wildlife Grant funds from the Nebraska Game and Parks Commission, our surveys are designed to help us better understand how this butterfly species is responding to our prairie restoration and management work.  The species is doing very well in Nebraska, and the central Platte River valley has some of the highest concentrations of regals in the country, but the species is of conservation concern because of it’s extreme rarity elsewhere in its historic range.  In addition to informing our work, we hope that studying the species where it’s common can help inform conservation efforts in those places where it’s rare.

Some people may be surprised to hear that regal fritillary butterfly populations are thriving in the central Platte River valley – a landscape dominated mainly by rowcrop agriculture and degraded prairie plant communities.  The neighborhood around our Platte River Prairies is about 30% grassland – a higher percentage of prairie than many eastern tallgrass landscapes, but certainly not a large unbroken prairie landscape.  Moreover, Platte River grasslands tend to have long histories of chronic overgrazing and broadcast herbicide use which has left them with limited plant diversity.  Even most of our annually-hayed prairies are missing many wildflower species that would be expected to be dominant in those prairie types.  The fact that regal fritillaries are thriving in this landscape seems counter to some of what is published about the species’ reliance on “high quality prairie”, but they’re here nonetheless! 

Regal fritillaries rely on violets as the sole food source for their larvae.  In our Platte River Prairies, the only perennial violet species we have is the common blue violet (Viola sororia).  In much of the eastern tallgrass prairie, prairie violets (V. pedatifida) or bird’s foot violets (V. pedata) are thought to be the preferred species, but we don’t get those species in our area.  Fortunately, the common blue violet thrives in pastures with long histories of annual intensive grazing, so the supply of larval food sources is not limited in our landscape.  It may be that the abundance of violets and the amount of grassland left in the valley are the two primary reasons for the regal fritillary’s continued success here.

Common blue violets (Viola sororia) are abundant in our degraded prairies - along with lots of Kentucky bluegrass. The species appears to support strong regal fritillary populations.

A brief report of our findings from 2011 can be found in this PDF, but I’ll lay out a few of the highlights here.  First, the seasonal patterns of butterfly abundance we’re seeing seems to match what we know of the life history of the species.  Males emerge first (late June), with highest numbers occurring in areas with relatively high amounts of thatch and lots of violets.  Those males tend to spend the next couple weeks flying around the same place they emerged from until the females also emerge and mating takes place.  That mating period is when we see the most fritillaries along our transects.  Following that, the number sightings drop as males begin to die off and females enter a period of “reproductive diapause”, during which they spend much of their time sitting around in the shade.  Finally, in late August, we see a little bump in our numbers again as females begin flying around and laying eggs.  (Those eggs then hatch, and the larvae – if they find enough violets to eat – will overwinter in the thatch.)

The effects of prescribed fire on regal fritillaries and other butterflies is a frequent topic of discussion among ecologists.  At our sites, we definitely see the highest numbers of regals emerging in June and July from prairies with high numbers of violets and with a build up of thatch.  Prairies that were burned in the spring have very few fritillaries during that peak period of emergence and mating – the larvae almost surely perish in spring fires.  However, during the summer of 2011, two of our top three sites for nectaring activity had been burned in the spring of that year.  It may be that in landscapes of relatively abundant habitat, a mixture of burned and unburned areas can complement each other by providing high quality larval habitat (unburned) and nectaring habitat (burned). 

In addition to prescribed fire, our other major management tool is cattle grazing – and the two are often combined within our various patch-burn grazing systems.  The burned areas mentioned above where nectaring activity was high were also being grazed.  Under our light stocking rates, cattle focus primarily on grasses, leaving most wildflowers to bloom – making them available to pollinators, including regal fritillaries.  Under higher stocking rates, fewer wildflowers escape grazing and regal fritillary are lower.  However, even areas that are burned and intensively grazed recover quickly, and within a year or two (as cattle focus on more recently burned areas) become hot spots for regal fritillaries again.  As stated earlier, it appears that a mixture of management treatments across a landscape with fairly abundant habitat is compatible with successful regal fritillary populations.

After our 2010 butterfly surveys, I posted a report on the nectar use by regal fritillaries and other butterfly species.  Those 2010 surveys included a slightly different mixture of prairies from our 2011 surveys, and included some transects where we counted all butterfly species.  Within that context, the clear favorite nectar plant was hoary vervain (Verbena stricta).  In 2011, vervain was still among the top nectar species for regal fritillaries – particularly in degraded remnants where it can be very abundant – but it was not quite as dominant as in last year’s surveys.  I think this was mainly because we included more restored prairies in our surveys this year, and the diversity of flowers was higher in those sites.  Even so, hoary vervain tied for first place in nectar species use with the combination of common and showy milkweeds (Asclepias syriaca and A. speciosa), followed closely by bee balm (Monarda fistulosa) and various thistle species (Carduus nutans and Cirsium spp).  It’s important to point out, however, that we only had 27 sightings of nectaring regal fritillary butterflies in 2011 (out of 352 total sightings), so our sample size was fairly limited.

Regal fritillaries in the central Platte River valley really like hoary vervain (Verbena stricta) as a nectar plant. Vervain survives intensive grazing very well, so it can be abundant in some of our more degraded pastures, as well as in our restored prairies.

Among those 27 sightings of regal fritillaries nectaring, all but four of them were in restored prairies (former crop fields).  At least based on the data we’ve collected so far, it appears that our regal fritillaries are tied to degraded remnants (lots of violets) for larval production but move into our restored prairies (lots of nectar plants) later in the season.  A quick and dirty correlation analysis showed that after the mating season was over, butterfly sightings were most likely to occur in portions of prairie with high abundances of their favorite nectar plants – but there was no such correlation prior to or during the mating season.  Again, a mixture of habitat types (this time, restored and remnant) appears to provide complementary resources for the butterflies in the Platte River Prairies. 

Clearly, regal fritillaries managed to survive just fine along the Platte River before our restored prairies were available, so it’s not that we’ve saved the species through our restoration work.  Intensively grazed pastures tend to have high numbers of hoary vervain flowers for nectaring, and some hayed prairies in the valley contain good numbers of milkweeds and other nectar species as well.  However, watching the behavior of fritillaries when areas of high nectar plant abundance (e.g., our restored prairies) become available may tell us about what their preferred habitat conditions are. 

Thistles, including this musk thistle, are among the favorite nectar plants for regal fritillaries - along with hoary vervain, bee balm, and common milkweed. At least for now, musk thistle is surviving just fine in the Platte Valley. (hooray?)

I feel pretty confident that regal fritillaries are managing to survive within the context of our current prairie restoration and management work.  However, we’re still not able to figure out where they’re laying their eggs.  According to the literature, regal fritillary females don’t lay their eggs on violets (why??) and no one is even sure if they select locations near violets.  I sure hope our regal fritillary females return to remnant prairies (where violets are abundant) to lay eggs, rather than laying them in our restored prairies.  If our restored prairies – with their abundant nectar plants – are luring those females away from prime egg-laying habitat, that’s not a good thing.  To date, we just can’t get enough sightings during that late summer egg-laying period to figure out what’s going on.  We even tried following females around to see if they were laying eggs but that turned out to be pretty difficult.  I guess since the population seems to be strong I’m not going to worry about it too much, but I’d still like to know what they’re doing. 

Because of the limited time we can devote to this project, it’s necessarily more of a pilot project than a full-blown research effort.  If there are scientists out there who would be interested in following up on our initial work, I’d be more than happy to work with you. 

In the meantime, you can see read the report from our 2011 surveys here, including lots of fun graphs and tables.

Report from the 2011 Grassland Restoration Network – Part 2: Grassland Birds

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The Grassland Restoration Network’s 2011 annual meeting was at The Nature Conservancy’s Kankakee Sands prairie/wetland restoration site in Indiana.  This year, we focused more than usual on creating habitat for various animal taxa, and I previously reported on the herpetology portion of the meeting.  Here is a summary of the discussions we had regarding grassland birds.

While the diversity of insects in a prairie is strongly tied to plant diversity, grassland birds have no such relationship.  The species richness, abundance, and breeding success of grassland birds are mainly related to variables such as vegetation structure, habitat patch size, and the amount of grassland in the neighborhood around their nesting sites.  At Kankakee Sands, the 6,000 acres of restored prairie/wetland habitat are obviously increasing both the patch size and total amount of grassland in the neighborhood, so our discussions focused on vegetation structure.  The ideal situation, of course, is to have a diversity of vegetation structure scattered across the site so that all grassland bird species can find the habitat they require for nesting.

This kind of tall vegetation is typical of most of the restored prairie at Kankakee Sands (and most other tallgrass prairie restoration sites). While valuable for many species, it's not much good for grasshopper sparrows and other animals that require short or patchy habitat structure.

We talked mainly about two different approaches to creating habitat structure in order to accomodate a rich variety of bird species.  The first approach is to design seed mixes for each desired habitat structure type, and the second is to design seed mixes that promote overall biological diversity, and then manage for structure with fire, grazing, and other tools.  Kankakee Sands is employing both strategies, so we were able to look up close at each.

We first looked at a restored prairie that had been seeded with about 60 species of short and medium-height plant species, and that included bunchgrasses like dropseeds and little bluestem instead of tall and strongly rhizomatous species like big bluestem.  The resulting structure was very favorable for grasshopper sparrows and other species that prefer that short to mid-height structure.  The seeding was nearly a decade old and seemed to be maintaining its structure and species composition with only a couple of prescribed fires as management.  The short patchy vegetation is definitely a contrast to the majority of other seedings at Kankakee Sands (and most other tallgrass prairie restoration sites) which are mainly tall and rank.  Those taller sites provide excellent habitat for Henslow’s sparrows and species with similar habitat preferences, but don’t do much for grasshopper sparrows.

Chip O'Leary (left) describes grassland bird habitat and research results with participants of the 2011 Grassland Restoration Network workshop. This restored prairie was seeded with short and medium-height plant species (including bunchgrasses instead of tall and strongly rhizomatous species).

The second example we visited was one of the first seedings done at Kankakee Sands (in the late 1990’s).  From the beginning, the several hundred acre prairie has been dominated by grasses and has been low on forb species diversity.  For the last several years, the Conservancy has been experimenting with patch-burn grazing as a way to create more heterogenous vegetation structure and to increase forb diversity in this prairie.  To date, forb diversity has neither increased nor decreased, but habitat structure has certainly changed.  The recently burned (and thus currently grazed) portions of the prairie provided excellent grasshopper sparrow habitat, while other portions were more tall and rank.  Though no change has been detected in forb abundance or diversity, the staff has noticed that a few forb species seem to bloom less abundantly than in the past – including compass plant, prairie dock, and Canada milkvetch.  Because of that, we discussed the value of fencing out a significant portion of the prairie each year to ensure that those species were given a complete break from grazing pressure periodically.

The two methods of creating bird habitat (seeding design vs. active management) both seem to be working well so far.  Both the prairie seeded with short/bunchy vegetation and the grazed prairie had significantly different vegetation structure than did the majority of the tall rank prairies around them – and birds are responding to that structure.  However, there are still plenty of questions about the long-term future of both approaches to creating bird habitat. 

In terms of the seeding design approach, one potential downfall is that the site was seeded with considerably fewer plant species than most other tallgrass prairie seedings at Kankakee Sands.  The potential effect of this lower diversity on insects and other species is unknown.  In addition, planting short and medium height plant species in soil/climate conditions that typically favor tall species could result in a relatively unstable prairie community.  In the long term, the fact that those shorter plant species aren’t using all of the available light/soil/moisture resources could lead to encroachment by either tall grasses (defeating the purpose of the design) or invasive species (which create obvious problems).  The planting we looked at was located on dry sandy soils with very low organic matter, so it probably is less at risk for that kind of instability than if it had been located in wetter or heavier soils. 

A final potential disadvantage of the seeding design approach is that the location of the short/medium habitat structure is static.  Grazing and other management tools for manipulating structure can be moved around a site from year to year, creating a shifting mosaic of habitat conditions.  That kind of mobility could help keep predator or pathogen populations from building up under consistently favorable conditions at any one site (this is speculation).  In addition, the staff will have to be careful to avoid repetitive management treatments aimed at maintaining the same structure year after year – that management could consistently favor some plant species over others, further reducing the plant (and insect?) diversity of the prairie.

There are plenty of concerns about the patch-burn grazing strategy as well.  To date, the plant diversity in the grazed prairie we looked at has not gone down, but neither has it increased – though increasing plant diversity in a grass-dominated prairie is very difficult with any strategy.  Because the prairie started with few forbs, it’s hard to know what the impact of patch-burn grazing would be on a more diverse plant community, but that needs investigating.  We discussed the possibility that a higher stocking rate and the addition of a large exclosure that changes location each year could help with both habitat and plant diversity over time. 

The burned patch of the patch-burn grazed prairie at Kankakee Sands. While the compass plant in this photo is blooming under grazing, other individuals (of compass plant, prairie dock, Canada milkvetch, and others) appear to be blooming less frequently than they did prior to the introduction of cattle. While this doesn't kill the plants over the short-term, it is a concern down the road, and the Kankakee Sands staff is considering strategies to mitigate those potential impacts.

A higher stocking rate would lead to more intense grazing of the dominant grasses such as big bluestem that are likely preventing existing forbs from becoming more abundant.  Currently many of the grass plants inside the most-recently burned patch are only being moderately grazed, and that can actually induce those plants to divert extra resources into rhizome production – leading them to expand their footprint (not the objective here).  More intense grazing on those grasses could create better opportunities for seed germination and seedling establishment around those plants, and would create even shorter vegetation structure, which might help attract upland sandpipers and as well as grasshopper sparrows. 

Regardless of stocking rate, the use of a grazing exclosure would help ensure a periodic break from grazing for plant species that otherwise be vulnerable to annual grazing – even in the unburned (and lightly grazed) portions of the prairie.  With a higher stocking rate, the exclosure would become even more important.  In addition to protecting plants from grazing, it would also protect fuel for the next year’s burn.  An exclosure roughly 1/4 or 1/3 of the size of the prairie would probably sufficient for protecting both plants and fuel, and would still leave cattle access to both burned and unburned portions of the prairie – something is important when promoting selective grazing. 

Even with those potential alterations to the current patch-burn grazing system, there are still plenty of unknowns about the long-term impacts of cattle grazing at Kankakee Sands.  It seems clear that grazing can create a shifting mosaic of habitat structure, but whether or not it can maintain the kind of plant diversity (and other diversity) desired by the Conservancy at this site is still an open question.

Discussing those kinds of questions while standing on the ground, however, is the best part of our Grassland Restoration Network workshops.  We don’t all agree on the best strategies, because we are all still experimenting with our own ideas on our own sites – and none of us feel like we have all the answers.  Being able to see for ourselves what various restoration and management treatment results look like helps us better compare those results to what we see on our own sites.  While we don’t have all the answers yet, we’re certainly moving much closer to them as a group than we would be individually.