Daniel K. Rosenberg

Biological Corridors: Form, Function, and Efficacy

H abitat loss and fragmentation are among the most pervasive threats to the conservation of biological diversity (Wilcove et al. 1986, Wilcox and Murphy 1985). Habitat fragmentation often leads to the isolation of small populations, which have higher extinctionrates (e.g., Pimm et al. 1988). Ultimately, the processes of isolation and population extinction lead to a reduction in biological diversity. Concern for this loss has motivated conservation biologists to discuss the actions that are needed to increase the effective size of local populations. Predominant among these possible str,ltegies has been the recommendation that corridors be induded in conservation plans (Figure 1) to increase the connectivity of otherwise isolated patches (Meffe and Carroll1994). The indusion of corridors in reserve designs has become an importa nt conservation tactic for protecting biological diversity. This strategy was motivated by theoretical and empirical observations demonstrating that increased interchange of in-

Estimation of habitat selection for central-place foraging animals

Analyses of habitat use for individuals occupying discrete home ranges are typically based on comparison with null models that implicitly assume no spatial context for habitat use within the home range. For species that regularly return to a central place, a more appropriate null model for estimation of habitat selection may be that of a declining expectation of resource use with distance from the central place, such as a nest site. When this null expectation is ignored and a uniform-use expectation is used for central-place foragers, we predicted (1) positive bias of selection for habitat types near the central place, and (2) bias will increase with the degree to which habitat types are spatially correlated to the central place. We explored these predictions with simulated data, using a range of selection intensities and spatial correlations. Results from the simulations confirmed our predictions: biases were large and positive for those habitat types proximal to the central place. To correct for these biases, we included distance from the central place as an explanatory variable in habitat selection models of simulated central-place foraging, and we found that including distance as a linear factor successfully reduced these biases. We then applied these models to field data from northern spotted owls (Strix occidentalis caurina) and red-cockaded woodpeckers (Picoides borealis). For both species, distance-based models performed better than the nonspatial (uniform) model: the models were both statistically superior and produced results more in concordance with our biological understandings. Estimates of selection for habitat types that were disproportionately located near the central place were lower in the distance-based models than in the uniform model, corroborating the results from the simulations. The simple distance-based models we used provide a reasonable means to estimate foraging habitat selection for animals for which a central place can be identified.

CARRYOVER AQUATIC EFFECTS ON SURVIVAL OF METAMORPHIC FROGS DURING POND EMIGRATION

In organisms with complex life cycles, physiological stressors during early life stages may have fitness-level impacts that are delayed into later stages or habitats. We tested the hypothesis that body size and date of metamorphosis, which are highly responsive to aquatic stressors, influence post-metamorphic survival and movement patterns in the terrestrial phase of an ephemeral pond-breeding frog by examining these traits in two populations of northern red-legged frogs (Rana aurora aurora). To increase variation of body size at metamorphosis, we manipulated food availability for 314 of 1045 uniquely marked tadpoles and estimated the probability that frogs survived and emigrated using concentric rings of drift fencing surrounding ponds and Bayesian capture-recapture modeling. The odds of surviving and emigrating from the ponds to the innermost drift fences, approximately 12 m, increased by factors of 2.20 (95% credibility intervals 1.39-4.23) and 2.54 (0.94-4.91) with each millimeter increase in snout-vent length and decreased by factors of 0.91 (0.85-0.96) and 0.89 (0.80-1.00) with each days delay in metamorphosis for the two ponds. The odds of surviving and moving to the next ring of fencing, 12 m to approximately 40 m from the ponds, increased by a factor of 1.20 (0.45-4.06) with each millimeter increase in size. Our results demonstrated that body size and timing of metamorphosis relate strongly to the performance of newly metamorphosed frogs during their initial transition into terrestrial habitat. Carryover effects of aquatic stressors that reduce size and delay metamorphosis may have population-level impacts that are not expressed until terrestrial stages. Since changes in both aquatic and terrestrial systems are implicated in many amphibian declines, quantifying both immediate and delayed effects of stressors on demographic rates is critical to sound management.

Estimation of animal abundance when capture probabilities are low and heterogeneous

Obtaining reliable estimates of abundance or relative abundance under conditions of low numbers of captures and recaptures is crucial to properly assess population status of species that are of management concern; however, these characteristics make estimation difficult. We applied the commonly used jackknife (Burnham and Overton 1978, 1979) and moment (Chao 1988) estimators of abundance to capture-recapture data from northern flying squirrel (Glaucomys sabrinus) populations that had low (p ≅ 0.10), heterogeneous, capture probabilities and low densities (approx 2 squirrels/ha). The jackknife estimator selection procedure, higher-order jackknife estimators, and moment estimator were sensitive to the number of trapping occasions. These estimators tended to have low precision. Comparisons of estimators suggested specific, lower-order jackknife estimators performed well. Monte Carlo simulations corroborated results from field data. The moment estimator tended to have low bias, but the high root mean square error made the estimator less reliable than lower-order jackknife estimators. First- and second-order jackknife estimators tended to be the most reliable (low bias and precise) estimators when the number of trapping occasions (t) was ≥ 12. However, confidence interval coverage (% replications in which the constructed confidence interval included true N) was low with the first-order jackknife estimator, reflecting the negative bias of the variance estimator. We improved confidence interval coverage by an ad hoc adjustment to the variance estimator; coverage with the adjusted estimator approached the nominal 90% level at t ≥ 12. Reliable estimates of abundance can be achieved under conditions often encountered in field studies (small N and low, heterogeneous, capture probabilities) with lower-order jackknife estimators, a modification of the variance estimator for the first-order jackknife estimator, and ≥ 12 trapping occasions

Space use and pesticide exposure risk of male burrowing owls in an agricultural landscape

We estimated home-range size and habitat selection in a population of burrowing owls (Athene cunicularia) living within an agricultural landscape in the Central Valley of California, USA, in 1998 and 1999. We modeled home-range size and habitat selection of breeding male owls (n = 33) as a function of biological and physical factors. Biological factors included number of young fledged and diet, and physical factors included cover-type composition around the nest. We also examined patterns of space use in conjunction with agricultural pesticide application records for evidence of secondary poisoning risk to the owls. Owl home ranges varied in size within (but not between) years, and not in conjunction with any of the biological factors we measured. Foraging versus random locations were differentiated most strongly by distance from the nest, with 80% of nocturnal foraging observations falling within 600 m of the nest burrow. No single cover type was selected when distance to nest was also included in the model. Owls did use agricultural fields recently treated with pesticides, although we did not find evidence of owls selectively foraging in these fields.

Differences in Townsend's Chipmunk Populations between Second- and Old-Growth Forests in Western Oregon

Because Townsends chipmunks (Tomias townsendii) may be important in maintaining natural ecosystem processes in forests in the central Oregon Cascade Range, we compared their population characteristics in young second-growth and old-growth forests. We live-trapped Townsends chipmunks in 5 young (30-60 yr old) second-growth and 5 old-growth (>400 yr old) Douglas-fir (Pseudotsuga menziesii) stands during spring and autumn 1987-90 in western Oregon. We tested the null hypothesis of no difference in characteristics of chipmunk populations in these 2 stand age-classes. Densities ranged from 0.4 to 10.3 chipmunks/ha and were greater (P < 0.05) in old-growth (x ± SE, 5.1 ± 0.4) than in second-growth (2.8 ± 0.3) stands. Chipmunk densities were related to large (≥50 cm diam at breast height [dbh]) snags in old-growth (P = 0.002) but not in second-growth (P = 0.6) stands

Monitoring survival rates of Swainson's Thrush Catharus ustulatus at multiple spatial scales

We estimated survival rates of Swainsons Thrush, a common, neotropical, migratory landbird, at multiple spatial scales, using data collected in the western USA from the Monitoring Avian Productivity and Survivorship Programme. We evaluated statistical power to detect spatially heterogeneous survival rates and exponentially declining survival rates among spatial scales with simulated populations parameterized from results of the Swainsons Thrush analyses. Models describing survival rates as constant across large spatial scales did not fit the data. The model we chose as most appropriate to describe survival rates of Swainsons Thrush allowed survival rates to vary among Physiographic Provinces, included a separate parameter for the probability that a newly captured bird is a resident individual in the study population, and constrained capture probability to be constant across all stations. Estimated annual survival rates under this model varied from 0.42 to 0.75 among Provinces. The coefficient of variat...

Breeding Dispersal and Nesting Behavior of Burrowing Owls Following Experimental Nest Predation

ABSTRACT Nest predation is considered a primary factor affecting the life-history characteristics and particularly dispersal of many avian species. We tested the hypothesis that nest predation would increase dispersal probability, dispersal distance and the frequency of renesting. We removed eggs from burrowing owl (Athene cunicularia) nests to simulate nest predation in southeastern California. Owls responded to egg removal with increased dispersal probability, nesting attempts and egg production. We found that nest predation tended to increase dispersal probability (50% depredated nests vs. 14% control nests), which occurred fairly soon after nest predation (0–25 d). Dispersal distance was highly variable among owls (range: 148–13,012 m). Following experimental nest predation, burrowing owls increased the number of nesting attempts and thus the total number of eggs produced in a season, regardless of dispersal. Clutch size, however, decreased as the number of breeding attempts increased. Despite large initial clutch size, burrowing owls in the Imperial Valley may have adapted to nest predation by both dispersal and the ability to renest frequently.

Post-Breeding Dispersal of Burrowing Owls in an Extensive California Grassland

Abstract Understanding patterns of dispersal is key to developing effective conservation plans, yet dispersal is poorly known for most species. We radio-tracked 15 adult burrowing owls (Athene cunicularia) from 13 nests within the Carrizo Plain National Monument in southern California. Our goal was to describe post-breeding movements in this extensive grassland system. Of nests that failed (n = 9 nests), 8 radio-tagged individuals from 7 nests dispersed, whereas none of the owls from successful nests (n = 4 nests) dispersed. Dispersal distances ranged from 0.2 km to 53 km (median = 3.1 km). The large dispersal distances we observed within the breeding season were greater than previously published estimates of between-year breeding dispersal based on mark-recapture methods and provide insight into the lack of genetic differentiation observed among burrowing owl populations.

Radiotransmitter Mount Type Affects Burrowing Owl Survival

JENNIFER A. GERVAIS, Department of Forest, Range, and Wildlife Sciences, Utah State University, Logan, UT 84322, USA DANIEL H. CATLIN, Department of Forest, Range, and Wildlife Sciences, Utah State University, Logan, UT 84322, USA NATHAN D. CHELGREN, Department of Forest, Range, and Wildlife Sciences, Utah State University, Logan, UT 84322, USA DANIEL K. ROSENBERG, Department of Forest, Range, and Wildlife Sciences, Utah State University, Logan, UT 84322, USA