Implications for Ecological Interpretations

The collection vectors described earlier mean that the sample of Pleistocene bones represents animals that lived mostly within a 5- to 18-km radius of Porcupine Cave. Wood rats generally collect within 50 m of their nest; raptors usually collect their prey within about 5 km of the sites where they regurgitate most of their pellets; and mammalian carnivores such as coyotes and badgers typically hunt within 5 km of their dens (Hadly, 1999). Porder et al. (2003) found that in Yellowstone Park, the bones from two deposits (Lamar Cave and Waterfall Locality) that are taphonomically similar to Porcupine Cave came from within an 8- to 18-km radius of the fossil accumulations.

The derivation of most of the fossil bones from raptor pellets and mammalian carnivore scats means that the sample represents primarily what the predators hunted. Typically the diets of predators such as coyotes, hawks, and owls reflect those small mammals and birds that are abundant on the landscape; that is, they eat what is out there, rather than selectively looking for a certain species. This situation results in a correlation between rank order abundance of small mammal species identified in the pellets and scats and rank order abundance of species in the living community, especially if the predators included a range of both diurnal and nocturnal hunters (Hadly, 1999). The range of mammalian predators that ultimately collected most of the Porcupine Cave specimens potentially included fishers, weasels, ermines, black-footed ferrets, minks, wolverines, badgers, skunks, coyotes, wolves, foxes, bears, bobcats, and cheetahs. Raptors and other avian predators or scavengers potentially included golden eagles, hawks, ravens, falcons, kestrels, great horned owls, and snowy owls. Fidelity between fossil assemblages and the communities they sample has been demonstrated in situations taphonomically similar to Porcupine Cave (Hadly, 1999; Porder et al., 2003). Observations of the modern fauna around the cave confirm that there is gross correspondence in rank order abundance of kinds of species that characterize the region today and those represented in the fossil deposits. For example, the most commonly sighted small mammals are Spermophilus spp. (ground squirrels), and individuals of that genus are most common as fossils. Voles likewise occur in high abundance in the modern environment and in the fossil deposits.

Time averaging, or the degree to which a given locality lumps together animals that lived at widely different times (up to thousands of years, for example), is notoriously difficult to assess in cave deposits (Graham, 1993; Gillieson, 1996). In late Holocene deposits that are somewhat analogous to those of the Pit locality in Porcupine Cave, stratigraphic levels averaging 10-30 cm were found to represent time spans from about 200 to 1000 years (Hadly, 1999; Hadly and Maurer, 2001). This degree of time averaging is probably a best-case

FIGURE 2.3 Map of Porcupine Cave. Left side = west half; right side = east half. Left side and right side slightly overlap. (Cartography [including figures 2.4-2.7] by Hazel Barton, from a Silva/Sunto and tape survey done by Evan Anderson, Hazel Barton, Michael Barton, Beth Branson, Kirk Branson, Greg Glazner, Mike Grazi, Ted Lappin, Fred Luiszer, Emma Rainforth, Don Rasmussen, Vi Shweiker, and Ken Tiner. Collecting sites labeled by A. D. Barnosky and C. J. Bell.)

0 0

Post a comment