OVERVIEW

HISTORY

ACTORS

MAPS

DIMENSIONS:

ecosystem
wildlife
economic
policy
recreation/aesthetic
social

STUDY TEAM

REFERENCES

Competition and Resource Segregation (Houston 1982):

I. Resource Segregation:

Elk are resource generalists, and relatively versatile.  For this reason, in order for other less abundant ungulates to remain extant, they must be better equipt than elk at exploit some part of the resource base.  Where species differ in their ability to exploit a resource base, resource segregation results.  This can occur due to interspecific differences in: 1) Ungulate distribution, and 2) Food preferences.

1)       The distribution of ungulate species can be evaluated by describing the ecological, or spatial, segregation of ungulate species found in Yellowstone.

Elk vs. Moose and Bison: Elk, moose, and bison show pronounced separation.

Moose Exploit:  Widely scattered pockets of browse in very deep snow, which effectively segregates them from elk and bison.

Bison Exploit:  Wet meadows, when snow conditions force elk into other habitats, which effectively segregate bison from moose and elk.  Bison are more restricted to high productivity forage in order to meet food required by their unselective grazing strategy.          

 Elk vs. Mule Deer and Bighorn Sheep:  Relationships between elk, mule deer and bighorn sheep are poorly understood.  Some information does exist to delineate separation of these species.

Sheep exploit:  Cliffs and steep slopes more efficiently than elk, which results in spatial segregation. 

Mule deer exploit:  A larger range of habitat during the winter, and are less bound by traditional winter range than sheep.  Mule deer have great mobility, which increases when threatened by elk competition, or winter severity. 

*Ungulate distribution can also be shaped by allometric, or body size, relationships.   To view this idea in more detain, visit allometry concepts.

2)      Interspecific differences in food preferences also play a role in determining resource segregation.  Food preferences are evident at the level of     forage class.  The most prominent differences exist between mule deer and elk, and between pronghorn and elk.

                        *Food selection, driven by body size relationships, or allometry, presents additional sources of species separation.

II. Interspecific Competition as a means of species separation:

Competition is harder to document than resource segregation.  It is difficult to determine whether resources are being partitioned, or if competition is driving species separation. 

• With large changes in elk, minimal effects were observed in the late 1960’s and 1970’s.  Bison recruitment was negatively associated with increasing elk numbers.

• Theoretically, moose and mule deer would be able to respond to declines in elk numbers, or reduction in competition for resources with elk, due to their high reproductive potential.

• The effects of prior human predation on pronghorn and bison obscure their relationship with elk to date.  We need more study of this subject.

On the other hand…  Facilitation may be, or have been, important to sympatric species of ungulates in Yellowstone.  Historically, large bison herds in the Yellowstone region heavily modified plant communities.  These modifications may have been beneficial to pronghorn (Barmore 1980).   Currently, these relationships are not apparent.  Rangelands have been drastically altered and the pronghorn and bison are separated in space. 

Current Facilitation Possibilities:

·        Smaller ungulate species (deer) following snow trails created by elk and or bison. 

·        Elk may slow or hamper forest invasion by periodic heavy browsing of conifers.  This allows more resource availability overall.