Distribution, population structure and habitat use of the endangered Saint Francis Satyr butterfly, Neonympha mitchellii francisci

American Midland Naturalist, The, April, 2008 by Daniel Kuefler, Nick M. Haddad, Stephen Hall, Brian Hudgens, Becky Bartel, Erich Hoffman

Several species of Carex are common to abundant in these sites and are suspected to be larval food plants for Neonympha mitchellii francisci. Wetland grasses, particularly cane Arundinaria tecta, rice-cut grass Leersia oryzoides and witch-grasses Dichanthelium spp., are also common at these sites. While grasses are not suspected to be food resources, they do provide a large amount of herbaceous structure and can dominate the herbaceous community, particularly in drier sites.

Virtually all N.m. francisci sites are the result of past episodes of impoundment, which both opened up the canopy and produced deep deposits of organic sediment. Nearly all of the impoundments harboring subpopulations are located on the sites of abandoned beaver ponds. Abandoned beaver dams and beaver-felled tree trunks are present in the majority of sites where N.m. francisci occur. Outside of restricted areas, evidence of past beaver activity is missing only at the original two colony sites known to Parshall and Kral (1989). Within restricted areas, several sites lacked explicit evidence of beaver activity, (i.e., beaver-chewed stumps), although almost all showed signs of historical impoundment. Not all abandoned beaver ponds are occupied, however, even when located in close proximity to active colonies of the butterfly.

Another important factor related to habitat suitability appears to be the presence and abundance of Carex species, with C. mitchelliana, C. glaucescens, C. atlantica, C. lurida and C. lonchocarpa all regularly occurring as dominant members of herbaceous openings within occupied sites. Interestingly, C. stricta, a larval host plant used by northern populations of Neonympha mitchellii mitchellii, is absent from most of the sites occupied by the butterfly.

The sedge with the highest correlation to butterfly abundance is Carex mitchelliana, a relatively rare species that appears to have its largest populations in the state within Fort Bragg. From sampling across a variety of habitats at 33 sites (n = 184 plots), we observed that occupied sites have between 6-21% cover of C. mitchelliana, and that there appeared to be a positive relationship between its abundance of C. mitchellina and butterfly abundance. From further sampling across our four primary study sites (n = 75 plots) we found a perfect relationship ([R.sup.2] = 1.0) between the average percent cover of C. mitchelliana and butterfly population size. Additionally, we confirmed a positive relationship between C. mitchelliana abundance and habitat area ([R.sup.2] = 0.43). Because of the small number of occupied sites, we could not separate the effects of site area and C. mitchelliana percent cover on Neonympha mitchellii francisci abundance. Virtually all other sedge species found in conjunction with N.m. francisci have much wider distributions on Ft. Bragg and across the Southeastern U.S. We note that C. mitchelliana appears to be essentially absent at one colony site and very scarce at another (the two sole active colonies in one watershed). These findings indicate that multiple host plants are likely used by N. m. francisci.