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Product Details
| ISBN-13: | 9780520270084 |
|---|---|
| Publisher: | University of California Press |
| Publication date: | 07/21/2011 |
| Series: | Studies in Avian Biology , #40 |
| Edition description: | First Edition, Published for the Cooper Ornithological Society |
| Pages: | 120 |
| Product dimensions: | 7.10(w) x 10.10(h) x 0.50(d) |
About the Author
Read an Excerpt
Population Demography of Northern Spotted Owls
By Eric D. Forsman
UNIVERSITY OF CALIFORNIA PRESS
Copyright © 2011 Cooper Ornithological SocietyAll rights reserved.
ISBN: 978-0-520-27008-4
CHAPTER 1
Population Demography of Northern Spotted Owls
Abstract. We used data from 11 long-term studies to assess temporal and spatial patterns in fecundity, apparent survival, recruitment, and annual finite rate of population change of Northern Spotted Owls (Strix occidentalis caurina) from 1985 to 2008. Our objectives were to evaluate the status and trends of the subspecies throughout its range and to investigate associations between population parameters and covariates that might be influencing any observed trends. We examined associations between population parameters and temporal, spatial, and ecological covariates by developing a set of a priori hypotheses and models for each analysis. We used information-theoretic methods and QAICc model selection to choose the best model(s) and rank the rest. Variables included in models were gender, age, and effects of time. Covariates included in some analyses were reproductive success, presence of Barred Owls (Strix varia), percent cover of suitable owl habitat, several weather and climate variables including seasonal and annual variation in precipitation and temperature, and three long-term climate indices. Estimates of fecundity, apparent survival, recruitment, and annual rate of population change were computed from the best models or with model averaging for each study area. The average number of years of reproductive data from each study area was 19 (range = 17 to 24), and the average number of captures/resightings per study area was 2,219 (range = 583 to 3,777), excluding multiple resightings of the same individuals in the same year. The total sample of 5,224 marked owls included 796 1-yr-old subadults, 903 2-yr-old subadults, and 3,545 adults (≥ 3 yrs old). The total number of annual captures/recaptures/ resightings was 24,408, and the total number of cases in which we determined the number of young produced was 11,450.
Age had an important effect on fecundity, with adult females generally having higher fecundity than 1- or 2-yr-old females. Nine of the 11 study areas had an even–odd year effect on fecundity in the best model or a competitive model, with higher fecundity in even years. Based on the best model that included a time trend in fecundity, we concluded that fecundity was declining on five areas, stable on three areas, and increasing on three areas. Evidence for an effect of Barred Owl presence on fecundity on individual study areas was somewhat mixed. The Barred Owl covariate was included in the best model or a competitive model for five study areas, but the relationship was negative for four areas and positive for one area. At the other six study areas, the association between fecundity and the proportion of Spotted Owl territories in which Barred Owls were detected was weak or absent. The percent cover of suitable owl habitat was in the top fecundity model for all study areas in Oregon, and in competitive models for two of the three study areas in Washington. In Oregon, all 95% confidence intervals on beta coefficients for the habitat covariate excluded zero, and on four of the five areas the relationship between the percent cover of suitable owl habitat and fecundity was positive, as predicted. However, contrary to our predictions, fecundity on one of the Oregon study areas (KLA) declined with increases in suitable habitat. On all three study areas in Washington, the beta estimates for the effects of habitat on fecundity had 95% confidence intervals that broadly overlapped zero, suggesting there was less evidence of a habitat effect on fecundity on those study areas. Habitat effects were not included in models for study areas in California, because we did not have a comparable habitat map for those areas. Weather covariates explained some of the variability in fecundity for five study areas, but the best weather covariate and the direction of the effect varied among areas. For example, there was evidence that fecundity was negatively associated with low temperatures and high amounts of precipitation during the early nesting season on three study areas but not on the other eight study areas.
The meta-analysis of fecundity for all study areas (no habitat covariates included) suggested that fecundity varied by time and was parallel across ecoregions or latitudinal gradients, with some weak evidence for a negative Barred Owl (BO) effect. However, the 95% confidence interval for the beta coefficient for the BO effect overlapped zero ([??] = -0.12, SE = 0.11, 95% CI = -0.31 to 0.07). The best models from the meta-analysis of fecundity for Washington and Oregon (habitat covariates included) included the effects of ecoregion and annual time plus weak effects of habitat and Barred Owls. However, the 95% confidence intervals for beta coefficients for the effects of Barred Owls and habitat overlapped zero ([??]BO = -0.104, 95% CI = -0.369 to 0.151; [??]HAB1 = -0.469, 95% CI = -1.363 to 0.426). In both meta-analyses of fecundity, linear trends (T) in fecundity were not supported, nor were effects of land ownership, weather, or climate covariates. Average fecundity over all years was similar among ecoregions except for the Washington–Mixed-Conifer ecoregion, where mean fecundity was 1.7 to 2.0 times higher than in the other ecoregions.
In the analysis of apparent survival on individual study areas, recapture probabilities typically ranged from 0.70 to 0.90. Survival differed among age groups, with subadults, especially 1-yr-olds, having lower apparent survival than adults. There was strong support for declining adult survival on 10 of 11 study areas, and declines were most evident in Washington and northwest Oregon. There was also evidence that apparent survival was negatively associated with the presence of Barred Owls on six of the study areas. In the analyses of individual study areas, we found little evidence for differences in apparent survival between males and females, or for negative effects of reproduction on survival in the following year.
In the meta-analysis of apparent survival, the best model was a random effects model in which survival varied among study areas (g) and years (t), and recapture rates varied among study areas, sexes (s), and years. This model also included the random effects of study area and reproduction (R). The effect of reproduction was negative ([??] = -0.024), with a 95% confidence interval that barely overlapped zero (-0.049 to 0.001). Several random effects models were competitive, including a second-best model that included the Barred Owl (BO) covariate. The estimated regression coefficient for the BO covariate was negative ([??] = -0.086), with a 95% confidence interval that did not overlap zero (-0.158 to -0.014). One competitive random effects model included a negative linear time trend on survival ([??] = -0.0016) with a 95% confidence interval (-0.0035 to 0.0003) that barely overlapped zero. Other random effects models that were competitive with the best model included climate effects (Pacific Decadal Oscillation, Southern Oscillation Index) or weather effects (early nesting season precipitation, early nesting season temperature). Ownership category, percent cover of suitable owl habitat, and latitude had little to no effect on apparent survival. Apparent survival differed among ecoregions, but the ecoregion covariate explained little of the variation among study areas and years.
Estimates of the annual finite rate of population change (λ) were below 1.0 for all study areas, and there was strong evidence that populations on 7 of the 11 study areas declined during the study. For four study areas, the 95% confi dence intervals for λ?overlapped 1.0, so we could not conclude that those populations were declining. The weighted mean estimate of λ for all study areas was 0.971 (SE = 0.007, 95% CI = 0.960 to 0.983), indicating that the average rate of population decline in all study areas combined was 2.9% per year. Annual rates of decline were most precipitous on study areas in Washington and northern Oregon. Based on estimates of realized population change, populations on four study areas declined 40 to 60% during the study, and populations on three study areas declined 20 to 30%. Declines on the other four areas were less dramatic (5 to 15%), with 95% confidence intervals that broadly overlapped 1.0.
Based on the top-ranked a priori model in the meta-analysis of λ, there was evidence that ecoregions and the proportion of Spotted Owl territories with Barred Owl detections were important sources of variation for apparent survival (φt) and recruitment (ft). There was some evidence that recruitment was higher on study areas dominated by federal lands compared to study areas that were on private lands or lands that included approximately equal amounts of federal and private lands. There also was evidence that recruitment was positively related to the proportion of the study area that was covered by suitable owl habitat.
We concluded that fecundity, apparent survival, and/or populations were declining on most study areas, and that increasing numbers of Barred Owls and loss of habitat were partly responsible for these declines. However, fecundity and survival showed considerable annual variation at all study areas, little of which was explained by the covariates that we used. Although our study areas were not randomly selected, we believe our results reflected conditions on federal lands and areas of mixed federal and private lands within the range of the Northern Spotted Owl because the study areas were (1) large, covering [approximately equals to] 9% of the range of the subspecies; (2) distributed across a broad geographic region and within most of the geographic provinces occupied by the owl; and (3) the percent cover of owl habitat was similar between our study areas and the surrounding landscapes.
Key Words: Barred Owl, fecundity, Northern Spotted Owl, Northwest Forest Plan, population change, recruitment, Strix occidentalis caurina, Strix varia, survival
During the last 40 years, the management philosophy on federal forest lands in the United States has undergone profound changes as government agencies have become increasingly aware of the importance of federal lands in species conservation. Nowhere has this change been more controversial than in the Pacifi c Northwest (Washington, Oregon, and northern California), where attempts to maintain viable populations of Northern Spotted Owls (Strix occidentalis caurina), Marbled Murrelets (Brachyramphus marmoratus), red tree voles (Arborimus longicaudus), and other plants and animals that thrive in old forests have resulted in large reductions in harvest of old forests on federal lands (Ervin 1989, Durbin 1996). Because of the controversial nature of these changes and the need to know whether management policies were achieving desired objectives, the U.S. Forest Service and U.S. Bureau of Land Management initiated eight long-term mark-recapture studies of Northern Spotted Owls during 1985 to 1991 (Lint et al. 1999). The primary objective of these field studies was to provide federal agencies and the public with data on the status and trends of Spotted Owl populations and to determine if the management plans adopted by the agencies were resulting in recovery of the owl, which was listed as a threatened subspecies in 1990 (USDI Fish and Wildlife Service 1990). In addition, the recent invasion of Barred Owls (Strix varia) into the range of the Spotted Owl represents a competitive threat that many research groups are trying to assess. The information generated in these studies has been featured in many publications (Franklin 1992, Burnham et al. 1994, 1996, Forsman et al. 1996a, Franklin et al. 2000, Kelly et al. 2003, Hamer et al. 2007, Olson et al. 2004, 2005, Anthony et al. 2006, Bailey et al. 2009, Singleton 2010) and has played a key role in several court cases and in the development of the Northwest Forest Plan (NWFP). The NWFP is an interagency plan that was designed to protect all native plants and animals on federal lands within the range of the Northern Spotted Owl, while at the same time providing jobs and wood products (USDA Forest Service and USDI Bureau of Land Management 1994). The data from the long-term demography studies were also considered by the team that prepared the 2008 recovery plan for the Northern Spotted Owl (USDI Fish and Wildlife Service 2008) and by a committee of The Wildlife Society (2008) who commented on the plan. Research on the long-term demography of the Spotted Owl has focused attention on forest management and conservation of forest wildlife in the western United States. This research, and the controversy it has created, have changed forest management in the region and helped to bring about a general reassessment of the role of forest management in species conservation, forest ecosystem management, and human health (Thomas et al. 1993, USDA Forest Service and USDI Bureau of Land Management 1994, Dietrich 2003).
With any large-scale, long-term monitoring program, important criteria are consistency in methods and funding, and a consistent protocol for analyzing the data and reporting the results. Standard protocols are especially important in cases like the Spotted Owl, where (1) the economic stakes are high, (2) there is occasional disagreement regarding the potential for bias in the estimates of demographic parameters (Loehle et al. 2005, Franklin et al. 2006), and (3) where many different agencies and stakeholders are responsible for collecting the data. For the Northern Spotted Owl, the methods for collecting, analyzing, and reporting demographic data have been described by Franklin et al. (1996), Lint et al. (1999), Anderson et al. (1999), and Anthony et al. (2006). Because of considerable scientific and public interest in these studies, one of the key features in the monitoring program has been regularly scheduled workshops in which all of the researchers who are conducting demographic studies of Northern Spotted Owls, meet and conduct a meta-analysis of all of the demographic data (Lint et al. 1999). Since 1993, there have been four cooperative workshops, the results of which have been described in three published articles (Burnham et al. 1994, 1996, Anthony et al. 2006) and one unpublished report (Franklin et al. 1999). The most recent of these workshops was conducted in January 2009, where we completed an updated meta-analysis in which we analyzed all of the demographic data currently available on the Northern Spotted Owl, including an additional five years of data from 2004 to 2008, and modeled the demographic parameters as a function of a new set of environmental covariates. Our demographic analyses, which represent the most complete and up-to-date summary of the population status of the subspecies, are the focus of this volume of Studies in Avian Biology.
Estimates of vital rates and population trends are more interesting when there is some understanding of the environmental factors that may influence those estimates. Anthony et al. (2006) included covariates for the cost of reproduction and presence of Barred Owls in their analyses of survival and population trends of Spotted Owls, but they were not able to include habitat or weather covariates in their analysis. In our analysis, we included the same covariates examined by Anthony et al. (2006) but add several new range-wide weather covariates and habitat covariates in Washington and Oregon. Thus, our analysis is the most comprehensive to date in terms of the number of covariates examined. Our analysis also differed from earlier analyses of Spotted Owl populations (Burnham et al. 1994, 1996) in that we use the f-parameterization of Pradel's (1996) temporal symmetry model to estimate the annual rate of population change (λ), and examine trends in the components of population change, including survival and recruitment rates. Our analyses have led to some valuable insights regarding our ability to discern the possible influence of environmental covariates (e.g., habitat, Barred Owls, weather) on a species that has high temporal variation in survival and reproduction. Our general approach will be of interest to other research groups investigating population dynamics of other long-lived vertebrates with similar life histories.
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Excerpted from Population Demography of Northern Spotted Owls by Eric D. Forsman. Copyright © 2011 Cooper Ornithological Society. Excerpted by permission of UNIVERSITY OF CALIFORNIA PRESS.
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Table of Contents
INTRODUCTIONSTUDY AREASFIELD METHODSANALYTICAL METHODS DEVELOPMENT OF COVARIATES Barred Owl covariate Habitat covariates Weather and climate covariates Land ownership, ecoregion, and latitude covariates Reproduction covariate FECUNDITY Individual study areas Meta-analysis of fecundity APPARENT SURVIVAL Individual study areas Meta-analysis of apparent survival ANNUAL RATE OF POPULATION CHANGE Individual study areas Estimates of realized population change Meta-analysis of annual rate of population change STATISTICAL CONVENTIONS WORKSHOP PROTOCOLSRESULTS FECUNDITY Individual study areas Meta-analysis of fecundity APPARENT SURVIVAL Individual study areas Meta-analysis of apparent survival on all areas Meta-analysis of apparent survival on the eight NWFP monitoring areas Potential cost of reproduction on survival Effects of Barred Owls on recapture and survival ANNUAL RATE OF POPULATION CHANGE Individual study areas Estimates of realized population change Meta-analysis of annual rate of population change DISCUSSION FECUNDITY APPARENT SURVIVAL ANNUAL RATE OF POPULATION CHANGE AND REALIZED RATES OF POPULATION CHANGE Individual study areas Meta-analysis of annual rate of population change STATUS OF OWL POPULATIONS IN THE EIGHT NWFP MONITORING AREAS ASSOCIATIONS BETWEEN DEMOGRAPHIC PARAMETERS AND COVARIATES Cost of reproduction on survival Weather and climate Barred Owls Habitat POTENTIAL BIASES IN ESTIMATES OF DEMOGRAPHIC PARAMETERS Fecundity Apparent survival Annual rate of population change Estimating goodness-of-fit and overdispersion SUMMARY, CONCLUSIONS, and RECOMMENDATIONSACKNOWLEDGMENTSLITERATURE CITEDAPPENDICESTABLESFIGURESWhat People are Saying About This
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