A MANAGEMENT MODEL TO SIMULATE FORAGE UTILIZATION BY ELK, MULE
DEER, AND CATTLE ON SUMMER RANGE
Alan A. Ager
Operations Research Analyst, USDA Forest Service Umatilla National Forest
2517 Hailey Ave., Pendleton, OR 97850 phone, (541-278-3740) fax (541-278-37370),
email aager@fs.fed.us,
Bruce K. Johnson
Project Leader, Oregon Department of Fish and Wildlife
1401 Gekeler Lane, La Grande, OR 97850, phone (541-962-6556) fax (541-962-6504),
email johnsobd@eou.edu,
Michael J. Wisdom
Research Wildlife Biologist, USDA Forest Service, Pacific Northwest Research Station
La Grande, Oregon 97850, phone (541-962-6532), Fax (541-962-6504),
email mwisdom@fs.fed.us,
Priscilla K. Coe
Wildlife Biologist, Oregon Department of Fish and Wildlife
1401 Gekeler Lane, La Grande, OR 97850, Phone (541-962-6550) Fax (541-962-6504),
email pcoe@eou.edu
There is a keen interest in modeling forage consumption by herds of free-ranging ungulates to
better understand interactions among domestic and wild species, and to simulate how changes in
herd management might affect forage availability in space and time. Predicting forage removal
by multiple species of ungulates across large landscapes is a complex problem owing to the
temporal and spatial variability in species distributions and forage production. A number of past
efforts to build forage allocation models progressed only to experimental phases due to
insufficient data and modeling frameworks that were difficult to adapt to complexities of the
forage allocation problem (e.g., linear programming). We developed model to simulate how
Rocky Mountain elk (Cervus elaphus), mule deer (Odocoileus hemionus), and cattle would
respond to altered population levels and forage availability on a typical summer range conditions
found in the Blue Mountains. Our model has 4 primary components that simulate animal
distributions, vegetation growth, diet selection, and animal energetics. Animal distributions were
simulated using resource selection functions during peak foraging periods developed from
Starkey telemetry data. The vegetation submodel considered daily increments in forage biomass
as a function of habitat type with adjustments for tree canopy closure and summer precipitation.
Forage quality was adjusted monthly to account for seasonal changes in digestibility. Diet
selection and interspecific diet preferences were modeled within a two step process that involved
the selection of feeding patches and subsequent selection of forage within the feeding patch. The
form of this model was motivated by literature and concepts in optimal foraging theory and
ecology of ungulates. The energetics component incorporated forage quality, intake, and
nutritional demands of gestation and lactation to predict monthly weight gains or losses of
ungulates. We used this model to simulate the interactions among stocking levels, diet selection,
and seasonal changes in animal distributions to predict spatial occurrence and intensity of forage
removal and competition among the 3 ungulate species. This model may be useful to managers
in allocating forage between cattle and wild ungulates, particularly in mixed forest-range
ecosystems on public lands. The modeling framework may also be useful in future
investigations of how free-ranging herbivores might affect plant succession and species
distributions.