Population Modeling of Peninsular Bighorn Sheep

Project purpose

Peninsular bighorn sheep are a distinct population segment of the desert bighorn sheep (Ovis canadensis nelsoni), listed as a California state threatened species in 1971. The peninsular population was also listed as federally endangered in 1998 due to habitat degradation and loss, disease, predation, human disturbance, low lamb recruitment, invasive toxic plants, and prolonged drought. Successful recovery and management of bighorn sheep requires detailed and up-to-date information on the status and distribution of populations.

However, the last population analysis for the species was completed in 2002 and estimates from that analysis were only expected to guide management over a 10-year period. Despite recent population increases, peninsular bighorn remain vulnerable to stochastic environmental conditions, habitat degradation and loss, disease, predation, and human disturbance. Thus, there is a need for an updated assessment of population responses to these threats and an analysis of the impacts on long-term population persistence.

Working in partnership, IEMM and SDSU’s Conservation Ecology Lab will support the California Department of Fish and Wildlife (CDFW) to analyze long-term datasets to determine demographic parameters, such as survival and reproduction rates, that drive population growth. Population models will provide CDFW managers with an updated assessment of the status of the species, a current and improved understanding bighorn survivorship, and allow for targeted management in response to predicted sources of decline.

Project goals and objectives

The goal of the project is to develop an updated population status assessment and population models for peninsular bighorn sheep to inform ongoing management and recovery actions within the nine recovery regions located in Riverside, Imperial, and San Diego counties. The research will focus on the following objectives:

1) Analyze population survey data and ewe group monitoring data collected from VHF- and GPS-collared bighorn sheep to estimate population size and ewe group size as well as emigration rates, reproduction, lamb recruitment, and cause-specific mortality.

2) Develop an integrated population model (IPM) for peninsular bighorn to produce an updated assessment of population status, including abundance estimates and population trajectory. We will quantify the relative impact of disease, predation, and human disturbance on bighorn sheep mortality among subpopulations and predict population persistence under various environmental conditions such as drought.

3) Determine the distribution and behavior of interacting peninsular bighorn sheep, mule deer, and mountain lions by analyzing the movement data from co-occurring VHF- and GPS-collared individuals using existing tracking and monitoring data (see map below).