Are American Black Bears (Ursus Americanus) Legitimate Seed Dispersers for Fleshy-Fruited Shrubs?

By Auger, Janene; Meyer, Susan E. et al. | The American Midland Naturalist, April 2002 | Go to article overview

Are American Black Bears (Ursus Americanus) Legitimate Seed Dispersers for Fleshy-Fruited Shrubs?


Auger, Janene, Meyer, Susan E., Black, Hal L., The American Midland Naturalist


ABSTRACT.-Seeds of seven fleshy-fruited shrubs used by American black bears (Ursus americanus) were extracted from fresh fruit (controls) and from scats of free-ranging and captive bears (ingestion treatments). Effects of the digestive process on viability, germinability and germination rate were measured against controls. Results were species specific. Filled control seeds of all species were highly viable (>74%). Digestion by bears had no significant effect on initial viability, except for serviceberry (Amelanchier alnifolia) for which viability of seeds passed through captive bears decreased 14%. Percent germination of control seeds summed over five chilling durations at 1 C was significantly different from that of at least one bear ingestion treatment for five of the seven species. For chokecherry (Prunus virginiana), Oregon grape (Mahonia repens) and skunkbush sumac (Rhus trilobata), seeds ingested by wild black bears germinated to a significantly higher percentage than controls, but for snowberry (Symphoricarpos oreophilus) control seeds germinated significantly better. For serviceberry all viable seeds germinated after sufficient chilling, but germination of controls was higher than ingested seeds after shorter chilling durations. When seeds were prewarmed for 5 wk before chilling, percent germination of Oregon grape, skunkbush sumac and snowberry significantly increased. For serviceberry, prewarming lengthened the minimum chilling requirement from 7 to 14 wk. Results from ingestion treatments suggest that the digestive process of black bears may simulate prewarming of seeds. Future work clarifying the role of bears as seed dispensers should involve characterization of (1) scat deposition sites, (2) activities of seed predators and secondary dispensers at the scats and (3) the relative importance of bears in communities containing other dispersal agents.

INTRODUCTION

Throughout their range, American black bears (Ursus americanus) rely heavily on both hard and soft mast in late summer and fall to increase fat reserves before hibernation. In those seasons fleshy fruits have importance values in the diet of 38 to 91% (Grenfell and Brody, 1983; Raine and Kansas, 1990; Richardson, 1991). Hard mast such as pine nuts or acorns is chewed, but fleshy fruits are typically bolted whole, and unbroken seeds-the objects of this study-are often the dominant component of scats. This suggests that bears could have a significant role as seed dispersers for fleshy-fruited species. The number of seeds which can be transported by bears is certainly noteworthy. For example, one male in our study area passed approximately 60,000 Oregon grape (Mahonia repens) seeds within 24 h. Those seeds represented about 10,000 individual fruits. Seeds numbering in the hundreds are typically found in scats collected in the summer and fall (Richardson, 1991).

Given these observations, it is surprising that the role of bears in seed dispersal is so poorly described. Chavez-Ramirez and Slack (1993) suggest that the neglect of carnivores stems from an emphasis on closely coevolved systems in the seed dispersal literature (McKey, 1975; Howe, 1977; Herrera and Jordano, 1981; Tomback, 1982). Diffuse coevolution (Janzen, 1983; Herrera, 1985), however, is more likely the case in communities containing black bears and fleshy fruits because: (1) other dispersal agents (i.e., birds) co-occur with bears, (2) modern plant communities have existed for only a few thousand years at most (Herrera, 1989; Harper and Collins, 1993) and (3) black bear diets vary widely throughout North America (Willson, 1993). Bears eat fruits diverse in size, color, odor and nutritional value and would seem to exert only weak selective pressures on fruit characters; there is no evidence that those pressures contributed significantly to the character syndrome for fruits dispersed by mammals (Debussche and Isenmann, 1989; Herrera, 1989; Willson et al., 1989).

In any case, defining the role of black bears in seed dispersal first requires an understanding of the effects of black bear digestion on seed viability and germination. …

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