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

INTRODUCTION

St. Francis' satyr, Neonympha mitchellii francisci, is one of the most imperiled butterflies in North America. First discovered in 1983, its range is restricted to Ft. Bragg, North Carolina (NC), where several small subpopulations persist in glades along streams (Parshall and Kral, 1989; Hall, 1993; Hall and Hoffman, 1994).

Neonympha mitchellii francisci is currently considered a subspecies of Mitchell's satyr (N. mitchellii mitchellii), which is also endangered. Neonympha m. mitchellii populations in the Great Lakes region have been the subject of detailed investigations (McAlpine et al., 1960; Ledge and Rabe, 1996; Shuey, 1997; Darlow, 2000; Kost, 2000; Szymanski et al., 2004; Barton and Bach, 2005). Since 1988 southern populations of N.m. mitchellii have also been discovered in Virginia, Alabama and Mississippi (Roble et al., 2001; Hart, 2004). A phylogenetic analysis indicates that these southern populations are more closely related to the nominate N.m.mitchellii than to N.m.francisci (Goldstein et al., 9004). Neonympha mitchellii francisci exhibits several morphological and behavioral similarities with N.m. mitchellii (Parshall and Kral, 1989). These butterflies share similar wingspans, coloration and flight behavior, and both utilize wetland grasses or sedges as larval food plants. Like N.m. mitchellii, N.m. francisci appears to be imperiled due to habitat loss (Hall, 2003; Szymanski et al., 2004). However, they differ markedly in terms of their ecology and life history, reflecting adaptations to very different environments (Table 1). In particular, northern populations of N.m. mitchellii in the Great Lakes region occupy relatively long-lasting fen habitats, are relatively sedentary and often have large populations with little or no evidence of subdivision (MacAlpine et al., 1960; Szymanski et al., 2004). In contrast, N.m. francisci inhabits a shifting mosaic of early successional wetland habitats and occupies relatively small subpopulations across the landscape. More specifically, N. m. francisci persist in a multi-tier metapopulation structure that appears to be influenced by disturbance dynamics associated with beaver activity. This complicated population structure and association with beavers is novel in studies of Lepidoptera and provides a unique case study in butterfly conservation.

Understanding the population structure, population dynamics, habitat requirements, and basic life history of Neonympha mitchellii francisci is critical for its management and potential delisting. To accomplish this objective, we have taken two separate lines of investigation. Along one line, between 1993-2005, we conducted reconnaissance surveys to determine the species' distribution and botanical assays to obtain a quantitative description of the butterfly's habitats. Concurrently, between 2002 and 2005 we conducted detailed demographic studies of N.m. francisci populations. The main purpose of the demographic studies was to determine the size, spatial structure and trends of butterfly populations. A secondary purpose was to determine the variability of demographic parameters with respect to butterfly sex, relative age, predation threats and environmental characteristics. Additionally, we report here on key aspects of its habitat requirements, as well as larval food preferences based on the results of experiments with reared larvae. Combined, these studies fill critical gaps in our understanding of N.m. francisci life history and habitat requirements. This paper complements several unpublished studies (available in full from the authors) that report on N.m. francisci from the time of first discovery and provide additional detail on the butterfly's distribution (Hall, 1993; Hall and Hoffman, 1994), its habitat associations (Hall et al., 2001; Hall, 2003; Hall and Haddad, 2005) and its population size and trends (Haddad et al., 2003, 2004, 2005).

METHODS

STUDY SITE

The only known population of Neonympha mitchellii francisci exists on Department of Defense lands at Ft. Bragg, NC. Created in 1918, Ft. Bragg is an active army installation comprising roughly 65,000 ha. This army installation is located in the Fall-Line Sandhills physiographic region and is completely contained within the Cape Fear River Basin. The topography of the Sandhills is quite hilly compared to the rest of the Coastal Plain and the portion within Ft. Bragg includes the highest ridges and some of the most dissected terrain located within this region in North Carolina. Despite its hilly terrain, the installation supports a biota typical of the Southeastern Coastal Plain. Approximately 40% of the installation is covered with longleaf pine communities. The installation also supports bottomland hardwood forests along stream floodplains, which are of particular importance as habitat for N.m. francisci, especially where modified by beaver activity.

Approximately 13,000 ha of the, installation are included within artillery impact areas that are off-limits to most forms of human intrusion. These areas differ somewhat compared to the surrounding installation in that fire is more frequent (annual return interval in many places) on the impact areas than on the surrounding installation (3 y return interval). Most entry for biological work in these areas was curtailed after 1995. Consequently, despite the discovery of several populations within these areas prior to 1995 (described below), our detailed studies were all conducted outside of these impact areas.