Population Viability of the Southern Idaho Ground Squirrel (Spermophilus Brunneus Endemicus): Effects of an Altered Landscape

Publication Date

12-2005

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Biology

Department

Biology

Supervisory Committee Chair

James C. Munger

Supervisory Committee Member

Eric Yensen

Abstract

Surveys of the southern Idaho ground squirrel have detected a long-term downward trend in population size, and habitats within its range have been fragmented by agricultural development and invaded by exotic plants. Is habitat degradation having a negative effect on southern Idaho ground squirrel (Spermophilus brunneus endemicus) populations? To answer this question, I monitored seven local populations of southern Idaho ground squirrels in a variety of habitats and quantified differences in the available vegetation among these local populations. By trapping resident ground squirrels, I estimated sizes of age/sex classes, estimated reproductive and survival rates, and monitored body condition changes through measurements of body mass and estimates of body fat. Using estimated demographic rates and sizes of age/sex classes I estimated the long-term persistence of the local populations.

The reproductive rates estimated for the southern Idaho ground squirrel were very similar to other Spermophilus species. However, the survival rates of southern Idaho ground squirrel juveniles averaged substantially lower than those of other species during normal years, similar to congeneric rates during drought periods and population declines. Overall, the demographic rates of this species were not sufficient to maintain the long-term viability of isolated local populations. The survival rate of juvenile females was found to be the parameter with the greatest influence on population viability, and the survival rate of both juvenile males and females was found to be positively correlated with pre-hibernation body mass.

In the local populations studied, the body condition of juveniles was poorer, the emergence body condition of all age/sex classes was more variable between years, and the predicted duration of population persistence was less when there were more invasive grasses. In contrast, forbs appeared to provide beneficial qualities. Forbs were positively correlated with a decrease in the year-to-year variation in emergence body condition of all age/sex classes, an increase in reproductive rates, and an increase in the survival rate of juvenile males. Enhancing the quality of the habitat and, more specifically, limiting the encroachment by invasive annual grasses, should be a high priority in any attempt to reverse the long-term decline in the southern Idaho ground squirrel.

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