Evaluating effects of an expanding mountain goat population on native bighorn sheep: a simulation model of competition and disease

Abstract

Biological invasions create serious conservation problems at local to global scales, and decisions about their management require evaluation of the likely environmental consequences of an expanding population. An expanding population of exotic mountain goats ( Oreamnos americanus) occupy an area near Rocky Mountain National Park, Colorado, USA (RMNP), an area managed for preservation of natural communities and processes. If mountain goats colonized RMNP, they could have detrimental impacts on native bighorn sheep ( Ovis canadensis) in the Park. A simulation model was used to evaluate population-level interactions of these species. Simulated mountain goats and bighorn sheep each attained total population sizes of about 950 in the absence of disease or competition. Disease and competition resulted in bighorn sheep populations that were an average of 31% (moderate competition) and 47% (strong competition) smaller than populations without these influences, while competition reduced mountain goat populations by an average of 9%. Competition was responsible for most (>75%) of the reduction in average size of bighorn sheep populations, but disease increased variation in population size by a factor of 5. The combined population-level effects of competition (reduced average size) and disease (increased variance) represent a substantial risk to native bighorn sheep populations in RMNP.