Forest Structure and Squirrel Populations in Relation to Fire History; Habitat Use of Endemic Tree Squirrels in the Sierra de San Juan Pedro Martir National Park and Saguaro National Park

Koprowski, John

Management of forests in national parks is a challenge that highlights the potential conflict between passive preservation and active management strategies. Fire suppression in many forests of the United States lasted for >50 years during the 20th century. A recent shift in strategy has occurred in many of these forests in an effort to restore natural fire regimes. Tree squirrels have often served as indicators of forest health. The Mearns’s squirrel (Tamiasciurus mearnsi) is an endemic species located in the Sierra de San Pedro Mártir (SSPM) Mountains in Baja California, Mexico. Although the species is listed as threatened in Mexico and as endangered by the IUCN, nothing is known about the biology and ecology of this southernmost Tamiasciurus. From November 2004 to August 2008, we studied the ecology of Mearns’s squirrels and their habitat in the Sierra de San Pedro Mártir National Park using live trapping, observation, and radiotelemetric monitoring. These forests have experienced little fire suppression relative to forests such as those in the Rincon Mountains of Arizona in the Saguaro National Park (SAGU), the sister park of SSPM. We radiocollared 38 adult Mearns’s squirrels and marked 41 juveniles. The home ranges of adult males were larger than those of adult females. Similarly, juvenile males traveled longer distances than juvenile females before settling in a new area of their habitat. Unlike other Tamiasciurus, Mearns’s squirrels do not defend territories, nor do they build middens, but do feed heavily on pine (Pinus) and fir (Abies) seeds among other food items like fungi, pollen, bones, and branch tips. Mating season begins in March with litters emerging from May through October. Adult survival was greater for males than for females during both years of study. Mearns’s squirrels do not construct leaf nests, relying only on tree cavities for nesting. We detected 115 tree cavities, of which 21 were used for reproduction (maternity nests). Most tree cavities were located in Jeffrey pines (Pinus jeffreyi); however, Mearns’s squirrels used more white firs (Abies concolor) than available. Mearns’s squirrels, like tree squirrels in the fire-suppressed forests of Saguaro National Park, use specific characteristics of their habitat for nesting. Tree size and canopy cover influence nesting behavior: however, scarcity of dead trees and tree cavities may be an important factor in the reproduction and survival of Mearns’s squirrels. We did not detect the presence of eastern gray squirrels (Sciurus carolinensis) previously introduced within the SSPM National Park. The high elevation conifer forests of Baja California that have received frequent low intensity ground fires due to lack of fire suppression are significantly more open than the fire-suppressed forests of Arizona. Tree squirrels show promise in serving as potential indicators of forest condition and forest change in the national parks of the Mexico-United States borderlands.