Population Dynamics of Spotted Owls in the Sierra Nevada, California

Abstract

The California spotted owl (Strix occidentalis occidentalis) is the only spotted owl subspecies not listed as threatened or endangered under the United States Endangered Species Act despite petitions to list it as threatened. We conducted a meta‐analysis of population data for 4 populations in the southem Cascades and Sierra Nevada, California, USA, from 1990 to 2005 to assist a listing evaluation by the United States Fish and Wildlife Service. Our study areas (from N to S) were on the Lassen National Forest (LAS), Eldorado National Forest (ELD), Sierra National Forest (SIE), and Sequoia and Kings Canyon National Parks (SKC). These study areas represented a broad spectrum of habitat and management conditions in these mountain ranges. We estimated apparent survival probability, reproductive output, and rate of population change for spotted owls on individual study areas and for all study areas combined (meta‐analysis) using model selection or model‐averaging based on maximum‐likelihood estimation. We followed a formal protocol to conduct this analysis that was similar to other spotted owl meta‐analyses. Consistency of field and analytical methods among our studies reduced confounding methodological effects when evaluating results. We used 991 marked spotted owls in the analysis of apparent survival. Apparent survival probability was higher for adult than for subadult owls. There was little difference in apparent survival between male and female owls. Model‐averaged mean estimates of apparent survival probability of adult owls varied from 0.811 ± 0.021 for females at LAS to 0.890 ± 0.016 for males at SKC. Apparent survival increased over time for owls of all age classes at LAS and SIE, for adults at ELD, and for second‐year subadults and adults at SKC. The meta‐analysis of apparent survival, which included only adult owls, confirmed an increasing trend in survival over time. Survival rates were higher for owls on SKC than on the other study areas. We analyzed data from 1,865 observations of reproductive outcomes for female spotted owls. The proportion of subadult females among all territorial females of known age ranged from 0.00 to 0.25 among study areas and years. The proportion of subadults among female spotted owls was negatively related to reproductive output (no. of young fledged/territorial F owl) for ELD and SIE. Eldorado study area and LAS showed an alternate‐year trend in reproductive output, with higher output in even‐numbered years. Mean annual reproductive output was 0.988 ± 0.154 for ELD, 0.624 ± 0.140 for LAS, 0.478 ± 0.106 for SIE, and 0.555 ± 0.110 for SKC. Eldorado Study Area exhibited a declining trend and the greatest variation in reproductive output over time, whereas SIE and SKC, which had the lowest reproductive output, had the lowest temporal variation. Meta‐analysis confirmed that reproductive output varied among study areas. Reproductive output was highest for adults, followed by second‐year subadults, and then by first‐year subadults. We used 842 marked subadult and adult owls to...

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“…We used a 20-year (1991 to 2010) dataset collected as part of a long-term demographic study of California Spotted Owls in the central Sierra Nevada (Seamans et al 2001, Seamans andGutiérrez 2007) to test these hypotheses. Previous studies have shown that our study population exhibits biennial cycles in reproductive output (Blakesley et al 2010), a finding that was supported by an analysis of the 20-year dataset we considered here (Appendix A, Appendix A Figure 4). …”

“…Moreover, many Spotted Owl populations exhibit regionally synchronized, annual cycles in reproductive output, in which good reproductive years tend to be followed by bad reproductive years (i.e. an ''even-odd'' pattern; Anthony et al 2006, Blakesley et al 2010, Forsman et al 2011) that may represent the emergent effect of reproductive costs (Forsman et al 2011). Following a ''good'' year in which most individuals in the population breed, impacts to body condition stemming from reproductive costs could reduce breeding propensity in a large segment of the population during the subsequent year, resulting in low population-level output.…”

Modern departures in fire severity and area vary by forest type, Sierra Nevada and southern Cascades, California, USA

“…In the study area, the California spotted owl (Strix occidentalis occidentalis), northern goshawk (Accipiter gentilis), and California populations of fisher (Martes pennanti ) are carnivores whose life histories include a close association with old forest and complex forest structure, primarily in mixed conifer forests. Current population sizes of these species are small and population growth rates are near zero or negative (Blakesley et al 2010. These conditions have led to controversy, litigation, and a resultant agency ''hands off'' policy towards these species that is focused on avoiding the short term putative threats of active forest management.…”

Dynamics of Breeding-Season Site Occupancy of the California Spotted Owl in Burned Forests

“…To assess demographic rates and explore ecological relationships, biologists have surveyed California Spotted Owl sites annually for more than 20 years in three areas of the Sierra Nevada (see Blakesley et al 2010). In other areas of the Sierra Nevada, biologists with the U.S. Forest Service (USFS) surveyed historically occupied Spotted Owl sites prior to forest management such as logging.…”