Diet of Southern Toads (Bufo Terrestris) in Loblolly Pine (Pinus Taeda) Stands Subject to Coarse Woody Debris Manipulations

Article excerpt

ABSTRACT.-

In the southeastern United States, coarse woody debris (CWD) typically harbors high densities of invertebrates. However, its importance as a foraging substrate for southeastern amphibians is relatively unknown. We examined effects of CWD manipulations on diet composition of southern toads (Bufo terrestris) in upland loblolly pine (Pinus taeda) stands in the Coastal Plain of South Carolina. Twelve 9.3-ha plots were assigned one of the following treatments: removal- all CWD ≥10 cm in diameter and ≥60 cm long removed; downed- five-fold increase in volume of down CWD; and unmanipulated control stands. We collected southern toads ≥4 cm snout-vent length (SVL) during 14 d sampling periods in June and October 2002, June 2003 and during a 28 d sampling period in April 2003. We collected 80, 36 and 35 southern toads in control, downed and removal treatments, respectively. We found no difference in relative abundance or frequency of invertebrate groups consumed among treatments (P > 0.05). Average body weight (g), SVL (cm) and stomach content weight (g wet) of individuals also were similar among treatments (P > 0.05). The role of CWD as a foraging substrate for southern toads in loblolly pine stands of the southeastern Coastal Plain may be negligible, at least in the early stages of decay.

INTRODUCTION

Coarse woody debris (CWD) often harbors high densities of invertebrates in terrestrial habitats (Graham, 1925; Savely, 1939; Harmon et al., 1986; Hanula, 1996; Lockaby et al., 2002). Coarse woody debris also offers added protection from predation and dry ambient conditions during foraging periods by terrestrial insectivorous vertebrate species (Loeb, 1996; Whiles and Grubaugh, 1996). These characteristics are believed to make CWD ideal foraging sites for forest dwelling amphibians (Maser et al., 1979). Although use of CWD by southeastern amphibians has been documented (Whiles and Grubaugh, 1996), its importance as a foraging substrate in the region remains unknown.

The southern toad is an abundant species throughout the Coastal Plain of the southeastern United States. Southern toads, and other species of Bufo, feed predominately on ants (Formicidae) and beetles (Coleoptera; Bush and Menhinick, 1962; Brown, 1974; Clarke, 1974a; Punzo, 1992; Bellocq et al., 2000). For example, stomach contents of southern toads collected in Florida were composed primarily of beetles (19-31%) and ants (12-18%), with spiders (Araneae; 8-11%) and unidentified insects (22-35%) comprising the remainder (Punzo, 1992). Stomach contents of Fowler's toads (B. woodhousei fowlm) collected in agricultural fields and nearby hardwood forests in the Coastal Plain of Arkansas were limited almost entirely to ground-dwelling insects such as ants and beetles (Brown, 1974). Diet of American toads (B. americanus), Great Plains toads (B. cognatus), plateau toads (B. compactais) and Woodhouse's toads (B. woodhouseii woodhousii) in Oklahoma consisted primarily of different types of beetles and ants (Smith and Bragg, 1949). Bellocq et al. (2000) speculated that microclimate variation associated with different forest stand structural characteristics influenced insect assemblages and, therefore, affected food resource availability for insectivorous vertebrate species.

In a broad ecological sense, CWD is an important component of forest stand structure (Hunter, 1990; McComb and Lindenmayer, 1999). Many invertebrate groups associated with decomposing woody material are negatively affected by CWD volume reductions (Harmon et al., 1986; Lattin, 1993; Freedman et al., 1996; McCay et al., 2002). Abundance of ground beetles (Carabidae) and silken fungus beetles (Cryptophagidae) declined 1 y after removal of downed CWD in upland loblolly pine stands in the Coastal Plain of South Carolina (McCay et al., 2002). As predators dependent on invertebrates in forested ecosystems, species of Bufo may be heavily affected by fluctuations in invertebrate abundance caused by reductions in CWD volume. …