Response of small mammals to alternative stand structures in the mixed-conifer forest of northeastern CaliforniaThis article is one of a selection of papers from the Special Forum on Ecological Studies in Interior Ponderosa Pine — First Findings from Blacks Mountain Interdisciplinary Research.

A common, but largely untested, strategy for maintaining forest biodiversity is to enhance stand structural complexity. A silvicultural experiment was implemented from 1996 to 1998 at Blacks Mountain Experimental Forest, California, to test the efficacy of two levels of structural diversity (high versus low) and two levels of prescribed underburning (burn versus no burn) for maintaining or restoring biodiversity. Small mammals were trapped and tagged in experimental units for 2 noncontiguous weeks in fall 2003 and 2004. Total number of captures and number of captured individuals varied by year (P < 0.002). No treatment effects were detected for all species lumped together or for the three most frequent species analyzed separately ( Tamias amoenus J.A. Allen, 1890, Peromyscus maniculatus (Wagner, 1845), and Spermophilus lateralis (Say, 1823)), with the exception that T. amoenus was captured more often in burned units in 2004 (P = 0.004 for year × burn interaction). Mixed-effects regression models indicated that the number of captures and captured individuals of T. amoenus and P. maniculatus decreased with increasing residual basal area of overstory trees, but opposite results were obtained for S. lateralis. After accounting for residual stand density differences, T. amoenus was captured more frequently in units of low structural diversity and S. lateralis in units of high structural diversity.

Seasonal glucocorticoid responses to capture in wild free-living mammals

We determined baseline and capture-induced glucocorticoid concentrations during two different seasons in three species of wild free-living rodents: brown lemmings ( Lemmus trimucronatus), golden-mantled ground squirrels ( Spermophilus saturatus), and yellow-pine chipmunks ( Tamias amoenus). Initial blood samples were obtained within 3 min of capture, so that initial glucocorticoid levels reflect baseline titers of undisturbed animals. Animals were held for an additional 30 min, when a second blood sample was taken to measure stress-induced glucocorticoid titers. The primary glucocorticoid differed in each species. Lemmings secreted extremely large amounts of corticosterone (as high as 8,000 ng/ml). These high concentrations were accompanied by high corticosterone-binding globulin capacity and resistance to negative feedback. Squirrels and chipmunks secreted a mixture of cortisol and corticosterone (10–400 ng/ml). In males of all three species and female squirrels and chipmunks, glucocorticoid levels were significantly elevated 30 min after capture. Baseline and 30-min glucocorticoid levels differed seasonally in each species. Levels were higher during summer (with no snow cover) than in spring (with ∼60% snow cover) in female lemmings, higher during breeding than before hibernation in squirrels, and higher postreproductively than during breeding in chipmunks. Together, these data indicate that glucocorticoid responses to stress in these free-living species are similar to those in laboratory species, but the magnitude of the response appears to depend on life-history features specific to each species.

Population Dynamics of Deer Mice, Peromyscus maniculatus, and Yellow-pine Chipmunks, Tamias amoenus, in Old Field and Orchard Habitats

There are often several rodent species included in the small mammal communities in orchard agro-ecosystems. This study was designed to test the hypothesis that the population levels of Deer Mice (Peromyscus maniculatus) and Yellow-pine Chipmunks (Tamias amoenus) would be enhanced in old field compared with orchard habitats. Rodent populations were intensively livetrapped in replicate old field and orchard sites over a four-year period at Summerland, British Columbia, Canada. Deer Mouse populations were, on average, significantly higher (2.5 – 3.4 times) in the old field than orchard sites in summer and winter periods. Mean numbers/ha of Deer Mice ranged from 12.1 to 60.4 in old field sites and from 3.3 to 19.9 in orchard sites. Breeding seasons in orchards were significantly longer than those in old field sites, in terms of proportion of reproductive male Deer Mice. Recruitment of new animals and early juvenile survival of Deer Mice were similar in orchard and old field sites. Populations of Yellow-pine Chipmunks ranged in mean abundance/ha from 5.6 – 19.0 in old field sites and from 1.9 – 17.5 on one orchard site, with no difference in mean abundance in 2 of 4 years of the study. Recruitment and mean survival of Yellow-pine Chipmunks also followed this pattern. This study is the first detailed comparison of the population dynamics of these rodent species in old field and orchard habitats. These species should be able to maintain their population levels and help contribute to a diversity of small mammals in this agrarian landscape.

Age determination in yellow-pine chipmunks (Tamias amoenus): a comparison of eye lens masses and bone sections

Virtually all biological characteristics of organisms change with age, and thus, to assess the impact of these changes, accurate aging techniques are essential. However, many current methods are unable to accurately distinguish among adults of different ages. We determined the age of yellow-pine chipmunks (Tamias amoenus) from the Rocky Mountains of Alberta using eye lens masses, annuli from mandible sections, and annuli from femurs. Each of these methods was assessed against nine known-age animals and seven animals that had not been caught previously and were presumed to be juveniles. Eye lens masses could distinguish juveniles from adults but not adults of different ages. Mandibular sections were not practical in this species because of excessive tearing during sectioning. Femoral sections precisely predicted age. We found that the number of adhesion lines, minus one, accurately represented the ages of adults ranging from 1 to 5 years old. Femoral annuli have not previously been used to age mammals and our results suggest that they may be useful in aging other mammals, especially rodents.