1993 Proceedings of the Midwest Oak Savanna Conferences
KARNER BLUE HABITAT SUITABILITY: CONSIDERATIONS FOR OAK SAVANNA MANAGEMENT IN MINNESOTA
Habitat management and restoration will be the primary methods for recovering populations of the federally endangered Karner blue butterfly (Lycaeides melissa samuelis). In order to develop habitat management and restoration plans, however, suitable habitat for the butterfly will need to be identified. Many researchers have suggested that succession of oak savanna to oak forest represents degradation of Karner blue habitat. Studies conducted in 1992 substantiated this hypothesis, finding adult butterflies in areas with less than 10% canopy cover. However, determining suitable habitat requires consideration of all life stages. Interestingly, preliminary studies on the larval stage found survival to be equal to or higher in closed canopied areas than open areas. Larval survival on a larger scale, and possible mechanisms for differential survival in open to closed canopy cover, will be studied in 1993.
The Karner blue butterfly (Lycaeides melissa samuelis) Nabokov) was recently added to the Federal Endangered Species list. Habitat management and restoration will be the primary methods to increase populations, however, efforts to develop management and restoration plans present researchers with several unknowns. One large area of uncertainty is the identification of optimal habitat.
Most Karner blue habitat includes various successional stages from completely open to closed canopied savanna. In areas where disturbance is absent, oak savanna is being converted to oak forest. Researchers believe that succession to oak forest results in degraded habitat for the Karner blue and that management techniques should be used to open the tree canopy (Schweitzer 1989). To what degree canopy should be opened and what density and distribution of these openings will result in optimal Karner blue habitat is unknown.
Many researchers and managers have approached this question by examining historical data. Land survey data, aerial photograph interpretation, tree ring analysis and carbon isotope analysis, have all been used in attempts to estimate the structure of oak savanna prior to loss of disturbance; particularly fire (Faber-Langendoen 1991, Grimm 1984). While these methods provide useful management guidelines, the information is not sufficient to determine optimal Karner blue habitat, since the information derived from these sources is the average canopy cover for entire sites. It does not address the degree of canopy cover that will provide for optimal Karner blue survival on a smaller scale or microhabitat level. Karner blue butterfly habitat was unlikely to have had uniform tree distribution. More likely, it included a mosaic of various sized, patches of open to closed tree canopies. Two questions arise: 1) at smaller and microhabitat scales, what amount of tree canopy cover will provide for optimal Karner blue adult and larval survival and under what conditions, and 2) does the size and distribution of open to closed areas affect habitat quality. Answers to these questions are critical to judge the success of management and restoration projects.
This study is part of a larger effort, begun in 1991, to promote recovery of the Karner blue butterfly presence and larval survival. The goal of this study is to determine what amount of canopy cover will provide for optimal adult performance and larval survival.
The study sites are in the Whitewater Wildlife Management Area (Whitewater WMA) in southeastern Minnesota. This area has approximately 1500 acres of oak savanna/forest on Plainfield sand and contains the only known populations of the Karner blue butterfly in Minnesota. The Whitewater WMA is within the Paleozoic Plateau which is believed to have escaped overriding during the Wisconsin glaciation, (Ojakangas and Matsch 1982). Oak savanna is found on areas of Plainfield sand which was deposited as glacial outwash, and blown into its current position (Howard Hobbs, pers. comm.).
To determine adult habitat preference, random transects were established in two of five Karner blue sites in which the largest numbers of butterflies occur (Lane 1992). The transects passed through areas ranging from completely open to almost fully closed tree canopy cover. The transects were walked three times per site and Karner blue numbers and locations noted. These same sites were then subdivided into recognizable habitat “subtypes” for vegetation analysis and were named: oak savanna closed, oak savanna open, sand bank, valley floor, oak savanna cut, oak savanna cut-open, and burned. Vegetation plots were established in each subtype area (for a total of 53 plots). Each plant species was identified for five height classes and percent cover estimated for each species in each class (Lane 1992).
To assess larval survival in areas with differing amounts of canopy cover, thirty larvae were placed in each of two study plots on July 2, 1992. In plot I, Karner blue females had been observed ovipositing (Lane 1992). It had an average tree canopy cover of 5%, with lupine mostly clumped with 20-50stems/clump. Study plot II had an average tree canopy cover of 95%. There lupines were mostly single stemmed and widely distributed (approximately 1-3 meters apart).
The Karner blue adults were present in habitat with open canopy, and were not found in those with a closed canopy, (Table 1), supporting the hypothesis that open areas are an important component of Karner blue adult butterfly habitat. Factors which contributed to habitat quality of open areas may include nectar plant abundance, good mate finding ability, availability of preferred oviposition sites, or other factors.
Open areas without Karner blue butterflies, (Table 1), may lack cerain habitat factors or may not yet have been colonized. Four of the oak savanna open plots without Karner blue adults are located in the Lupine valley site which has the greatest density of lupine (Lane 1992), but low abundance of nectar plants (preliminary results). this site is also the farthest from the occupied sites (approximately 2.5-6 km.).
It is yet to be determined whether and to what extent the adults use the closed canopied areas. Eighteen eggs were found during the 1992 field season in areas with 80-90% canopy cover, suggesting occasional use of these areas by Karner blue females for oviposition.
Preliminary results suggest that Karner blue larvae may have different microhabitat requirements than the adults. In plot II (closed canopy) larvae survived longer, and therefore grew to a greater size, than those in plot I (open canopy), (Fig. 1). Half of the larvae in the open area had disappeared by the second day of the study and all were gone by the nineteenth day. None of the larvae in this plot appeared large enough when they disappeared to have pupated. Of the 30 larvae in the closed canopy area, 12 survived to a size large enough for pupation (Savignano 1990).
Greater survival in the area with higher tree canopy cover may result from a more favorable microclimate, less predation from natural enemies, or higher lupine quality.
Areas with open canopies may be critical habitats for adult Karner blue butterflies, especially as nectaring sites and mate locating sites. Oviposition has been observed in areas with open to completely closed canopy cover. It is still unknown if there is any preference for egg laying in open or closed areas. The possibility of greater larval survival in shade, warrants further research.
Larval habitat studies will be done in 1993 with larger numbers in each treatement and over a wider variety of canopy covers. Research will also examine possible variation in lupine quality, microclimate differences, and natural enemy presence in the same habitat types. Given this information, the next step will be to examine patch size, and inter-patch distance as determined by canopy cover.
To restore oak savanna, and also provide habitat for the Karner blue butterfly, it will be necessary to more fully understand what amount of tree canopy cover at small scales, and what size and distribution of these patches on larger scales, will provide optimal habitat for Karner blue adults and larvae.
Faber-Langendoen, D. 1991. Community structure at Allison savanna: the impact of settlement and fire management. Final report to The Nature Conservancy, Minnesota.
Grimm, Eric C. 1984. Fire and other factors controlling the big woods vegetation of Minnesota in the mid-nineteenth century. Ecological Monographs 54(3) 291-311.
Lane, C. 1992. The status of the Karner blue butterfly (Lycaeides melissa samuelis: Lycaenidea) and its Associated Plant Resources in Minnesota, 1991. Final report to U.S. Fish and Wildlife Service, Minnesota Department of Natural Resources, and the Minnesota Chapter of The Nature Conservancy.
Ojakangas, R. W. and C.L. Matsch 1982. Minnesota’s Geology, University of Minnesota Press, Minnesota.
Savignano, D.A. 1990. Field Investigations of a Facultative Mutualism between Lycaeides melissa samuelis Nabokov (Lycaenidae), the Karner Blue Butterfly, and Attendant Ants. Ph.D Dissertation, University of Texas, Austin, Texas.
Schweitzer, Dale 1989. Fact sheet for the Karner blue butterfly with special reference to New York. The Nature Conservancy, internal document.
Andow, D., R. Baker, and C. Lane (eds.), (in press) Karner Blue Butterfly: Symbol of a Vanishing Landscape, Agricultural Experiment Station, University of Minnesota, St. Paul, Minnesota.