RECOGNITION AND EVALUATION OF PATTERNING AND BIAS IN PREDATOR-DERIVED SMALL-VERTEBRATE DEATH ASSEMBLAGES

Scatological prey assemblages are common, but their complete taphonomic history and resulting biases remain largely unexplored. Disagreement exists over the utility of skeletal content and fragmentation patterns for the identification of both field assemblages and causative predator taxa. To test for distinctive patterns, I analyzed a literature-based scatological dataset using clustering, ordination, and discriminant function techniques. Whereas individual predator taxa do not appear to leave unique signatures, assemblages produced by owls, diurnal raptors, and mammalian carnivores differ due to the relative preservation potential of fragile vs. robust bones during digestion. Diurnal raptors (regurgitate from high-acid stomachs) and mammalian carnivores (excrete from entire digestive tract) preserve fewer bones than the regurgitated pellets of owls and concentrate teeth. Raptor pellets and mammalian scat also contain a lower proportion of intact bones.

Experimental work on pellets/scat has focused on bone modification and patterning from digestion, rather than on subsequent weathering, disintegration, transport, and burial. From a 5-week study of a naturally occurring Great Horned Owl (Bubo virginianus) pellet assemblage in a temperate forest on San Juan Island, WA, I show that damage increases and proportions of skeletal elements change during pellet residence on the forest floor. As pellet integrity breaks down, bones become scattered (up to 4 m), fragmentation increases, (1.2% in intact pellets, 25.9% in dispersed pellets) and small fragile bones are preferentially lost, leaving assemblages enriched in robust bones (mandible, femur, humerus, pelvis).

High skeletal content, along with roost fidelity and opportunistic hunting, suggest that owl pellet assemblages may provide valuable ecological snapshots of the small-vertebrate fauna of an area, and information on ecological/environmental change through time. These two analyses indicate: (1) assemblages produced by owls are taphonomically distinct from those produced by diurnal raptors and mammalian carnivores, and (2) the post-predator taphonomic history of assemblages (including variation with climate) needs exploration to quantify the strengths and weaknesses of their use in environmental and ecological analysis.