Bruce C. Lubow

Structured decision making as a conceptual framework to identify thresholds for conservation and management

Thresholds and their relevance to conservation have become a major topic of discussion in the ecological literature. Unfortunately, in many cases the lack of a clear conceptual framework for thinking about thresholds may have led to confusion in attempts to apply the concept of thresholds to conservation decisions. Here, we advocate a framework for thinking about thresholds in terms of a structured decision making process. The purpose of this framework is to promote a logical and transparent process for making informed decisions for conservation. Specification of such a framework leads naturally to consideration of definitions and roles of different kinds of thresholds in the process. We distinguish among three categories of thresholds. Ecological thresholds are values of system state variables at which small changes bring about substantial changes in system dynamics. Utility thresholds are components of management objectives (determined by human values) and are values of state or performance variables at which small changes yield substantial changes in the value of the management outcome. Decision thresholds are values of system state variables at which small changes prompt changes in management actions in order to reach specified management objectives. The approach that we present focuses directly on the objectives of management, with an aim to providing decisions that are optimal with respect to those objectives. This approach clearly distinguishes the components of the decision process that are inherently subjective (management objectives, potential management actions) from those that are more objective (system models, estimates of system state). Optimization based on these components then leads to decision matrices specifying optimal actions to be taken at various values of system state variables. Values of state variables separating different actions in such matrices are viewed as decision thresholds. Utility thresholds are included in the objectives component, and ecological thresholds may be embedded in models projecting consequences of management actions. Decision thresholds are determined by the above-listed components of a structured decision process. These components may themselves vary over time, inducing variation in the decision thresholds inherited from them. These dynamic decision thresholds can then be determined using adaptive management. We provide numerical examples (that are based on patch occupancy models) of structured decision processes that include all three kinds of thresholds.

SMALL‐MAMMAL DENSITY ESTIMATION: A FIELD COMPARISON OF GRID‐BASED VS. WEB‐BASED DENSITY ESTIMATORS

Statistical models for estimating absolute densities of field populations of animals have been widely used over the last century in both scientific studies and wildlife management programs. To date, two general classes of density estimation models have been developed: models that use data sets from capture–recapture or removal sampling techniques (often derived from trapping grids) from which separate estimates of population size (N) and effective sampling area (Â) are used to calculate density (D = N/Â); and models applicable to sampling regimes using distance-sampling theory (typically transect lines or trapping webs) to estimate detection functions and densities directly from the distance data. However, few studies have evaluated these respective models for accuracy, precision, and bias on known field populations, and no studies have been conducted that compare the two approaches under controlled field conditions. In this study, we evaluated both classes of density estimators on known densities of e...

Fitting population models to multiple sources of observed data

The use of population models based on several sources of data to set harvest levels is a standard procedure most western states use for management of mule deer (Odocoileus hemionus), elk (Cervus elaphus), and other game populations. We present a model-fitting procedure to estimate model parameters from multiple sources of observed data using weighted least squares and model selection based on Akaikes Information Criterion. The procedure is relatively simple to implement with modern spreadsheet software. We illustrate such an implementation using an example mule deer population. Typical data required include age and sex ratios, antlered and antlerless harvest, and population size. Estimates of young and adult survival are highly desirable. Although annual estimates are desirable, the procedure also can be applied-with less precision-to data sets with missing values in any of the data series. The model-fitting procedure adjusts input estimates and provides estimates of unobserved parameters to achieve the best overall fit of the model to observed data. Rigorous, objective procedures such as those described here are required as a basis for wildlife management decisions because diverse stakeholder groups are increasing the intensity with which they scrutinize such management decisions.

Field trials of line transect methods applied to estimation of desert tortoise abundance

We examine the degree to which field observers can meet the assumptions underlying line transect sampling to monitor populations of desert tortoises (Gopherus agassizii). We present the results of 2 field trials using artificial tortoise models in 3 size classes. The trials were conducted on 2 occasions on an area south of Las Vegas, Nevada, where the density of the test population was known. In the first trials, conducted largely by experienced biologists who had been involved in tortoise surveys for many years, the density of adult tortoise models was well estimated (-3.9% bias), while the bias was higher (-20%) for subadult tortoise models. The bias for combined data was -12.0%. The bias was largely attributed to the failure to detect all tortoise models on or near the transect centerline. The second trials were conducted with a group of largely inexperienced student volunteers and used somewhat different searching methods, and the results were similar to the first trials. Estimated combined density of subadult and adult tortoise models had a negative bias (-7.3%), again attributable to failure to detect some models on or near the centerline. Experience in desert tortoise biology, either comparing the first and second trials or in the second trial with 2 experienced biologists versus 16 novices, did not have an apparent effect on the quality of the data or the accuracy of the estimates. Observer training, specific to line transect sampling, and field testing are important components of a reliable survey. Line transect sampling represents a viable method for large-scale monitoring of populations of desert tortoise; however, field protocol must be improved to assure the key assumptions are met.

Dynamics of interacting Elk populations within and adjacent to Rocky Mountain National Park

We studied population subdivision and density-dependent and independent factors influencing population processes between 1965 and 2001 for elk (Cervus elaphus) inhabiting Rocky Mountain National Park (park) and the adjacent Estes Valley (town), Colorado, USA. Elk numbers within the park were held relatively constant by management controls until 1967, after which time they were allowed to increase without human interference. Radiotelemetry of 73 elk indicated limited exchange between the subpopulations; combined with clear distinctions in population dynamics, this suggests that these subpopulations are relatively independent despite the absence of physical barriers between them. The elk subpopulation within the park initially increased at 6.5%/year between 1968 and 1970, then growth gradually slowed-exhibiting density-dependent reductions both in calf survival and recruitment with increasing population size-and approached an estimated carrying capacity of 1,069 ± 55 (?±SE). Since 1991, this subpopulation has remained within ±5% of this equilibrium. The adjacent Estes Valley subpopulation grew at an estimated maximum 5-year average rate of 11.0% from 1979 to 1983 and is still increasing at 5.2%/year (1991-2001 average). Estimated town population currently is about 70% of our projected carrying capacity of 2,869 ±415 elk based on projection of observed calf recruitment decline with increasing population. Both carrying-capacity estimates are consistent with independent estimates based on forage biomass and energy considerations. Adult cow survival rate was not found to differ between park and town, and we estimated a constant rate of 0.913 [95% CI = 0.911, 0.915]. Bull survival rates increased in the park from 0.52 to 0.79 between 1965 and 2001, but remained constant at 0.42 [0.35, 0.47] in the Estes Valley. Colder winter temperatures were correlated with reduced calf recruitment (calves:cow at age 0.5 yr) and with reduced calf survival (between age 0.5 and 1.5 yr) in town. Recruitment of town elk also increased with warmer summer temperatures and greater summer precipitation. No weather covariates were significantly correlated with calf recruitment or survival in the park. Declining calf recruitment has been nearly linear and similar in both the park and town. In the park, calf survival responded little to density when the population was well below carrying capacity, but responded at an increasing rate as the population neared carrying capacity. This pattern may explain why calf survival response to density has not yet been detected in town. We estimated current combined population size of 3,049 [2759, 3369] elk in 2001. Elk in the town sector currently outnumber elk in the adjacent national park by almost 2:1 and are projected to increase by 46% before being nutritionally limited, suggesting that human-elk conflicts likely will increase in the absence of active management intervention.

Optimal Translocation Strategies for Enhancing Stochastic Metapopulation Viability

Numerous methods have been proposed for enhancing species viability. Much attention has been given to the minimum required number of individuals, size and number of nature reserves, and value of habitat corridors. Surprisingly, however, the po- tential value of active management of a population through a program of translocations has only rarely been suggested, and explicit formulations of a theoretical basis for such a program are nonexistent. By drawing on the mathematical optimization technique known as dynamic programming, I develop an optimal dynamic strategy for translocation in a model population. I demonstrate this approach using a simple model for a hypothetical remnant population with two available habitat reserves incorporating demographic, envi- ronmental, and catastrophic forms of stochasticity. I then generalize the results by examining the effect on the cost and effectiveness of optimal translocation management of reserve size, population growth rate, environmental stochasticity, catastrophe size and frequency, translocation mortality rate, spatial correlation of population dynamics, and reserve size asymmetry. Simulated application of the optimal strategies, under a wide range of condi- tions, demonstrates that managed translocations averaging between 1 and 6 individuals/yr might dramatically enhance the probability of species persistence and reduce the required size of nature reserves, potentially by >1 order of magnitude. Given prohibitive financial and political costs of land acquisition for nature reserves, this technique could provide an important alternative for saving many endangered species.

Population dynamics of the Jackson elk herd

Abstract We fit data on elk (Cervus elaphus) population size and composition, survival rates measured from their first week of life, reported harvest, and local weather to a series of alternative population models of the elk herd in Jackson, Wyoming, USA, for the period 1980–2002. Data were corrected for biases in aerial survey visibility, misclassification of juveniles in ground surveys, and harvest reporting. The models included explanatory variables for sex, age, population size, weather, and autocorrelation of survival rates in different periods. Using information–theoretic model selection, we identified the most strongly supported models and effects. Model complexity ranged from 12 to 70 fitted parameters, and the best-supported model contained 25 parameters. We estimated annual natural survival (excluding harvest) of mature (≥1 yr) elk of 96.8% (SE = 1.5%) for males and 97.2% (SE = 2.2%) for females. Natality was 60.4 juveniles/100 mature females (SE = 3.9 juveniles/100 mature females). Sex ratio at birth strongly favored females (45.8% males, SE = 1.6%, Akaike weight = 99.9%). The dynamics of this population were well explained by annual variation in survival of neonates (birth to 31 Jul), juvenile survival during late winter (20 Feb–19 May), and harvest. Survival of neonates was correlated with several weather covariates that apparently affected nutritional status of their mothers. Survival of juveniles during late winter was related to weather conditions during the preceding summer and early winter. We found a compensatory effect of juvenile harvest on subsequent juvenile survival in late winter; 89% of increased juvenile harvest was offset by reduced natural mortality. We also found evidence for a decline in survival of neonates with increasing population size (density dependence). However, the density effect was weak at current population size and recent supplemental feeding rates. Thus, only continued or increased female harvest can maintain this population at current or lower levels if current feeding policies are continued—unless disease prevalence, predator impacts, or other factors substantially alter the historical dynamics. Simulations suggested that harvest rates of mature females must be increased to 15.1% from recent levels of 11.9% to reduce the current population of 15,680 elk (SE = 407) to the target population size of 11,029 set by the Wyoming Game and Fish Department (WGFD). Sensitivity of equilibrium population size at the WGFD target level to harvest rate was very high, requiring regular monitoring and adjustment of harvest to maintain a stable population.

Impacts of Climate Changes on Elk Population Dynamics in Rocky Mountain National Park, Colorado, U.S.A.

Changing climate may impact wildlife populations in national parks and conservation areas. We used logistic and non-linear matrix population models and 35 years of historic weather and population data to investigate the effects of climate on the population dynamics of elk in Rocky Mountain National Park (RMNP), Colorado, U.S.A. We then used climate scenarios derived from Hadley and Canadian Climate Center (CCC) global climate models to project the potential impact of future climate on the elk population. All models revealed density-dependent effects of population size on growth rates. The best approximating logistic population model suggested that high levels of summer precipitation accelerated elk population growth, but higher summer minimum temperatures slowed growth. The best approximating non-linear matrix model indicated that high mean winter minimum temperatures enhanced recruitment of juveniles, while high summer precipitation enhanced the survival of calves. Warmer winters and wetter summers predicted by the Hadley Model could increase the equilibrium population size of elk by about 100%. Warmer winters and drier summers predicted by the CCC Model couldraise the equilibrium population size of elk by about 50%. Managers of national parks have relied on effects of weather, particularly severe winters, to regulate populations of native ungulates and prevent harmful effects of overabundance. Our results suggest that these regulating effects of severe winter weather may weaken if climate changes occur as those that are widely predicted in most climate change scenarios.

Validating Aerial Photographic Mark‐Recapture for Naturally Marked Feral Horses

Abstract Accurately estimating large mammal populations is a difficult challenge because species of interest often occupy vast areas and exhibit low and heterogeneous visibility. Population estimation techniques using aerial surveys and statistical design and analysis methods provide a means for meeting this challenge, yet they have only rarely been validated because wild populations of known size suitable for field tests are rare. Our study presents field validations of a photographic aerial mark–recapture technique that takes advantage of the recognizable natural markings on free-roaming feral horses (Equus caballus) to accurately identify individual animals and groups of animals sighted on multiple occasions. The 3 small populations of feral horses (<400 animals each) in the western United States used in the study were all closely monitored on a weekly basis by local researchers, thus providing test populations of known size. We were able to accurately estimate these population sizes with aerial surveys, despite rugged terrain and dense vegetation that created substantial heterogeneity of sighting probability among horse groups. Our best estimates at the 3 sites were within −6.7%, 2.6%, and −8.6% of known truth (−4.2% mean error, 6.0% mean absolute error). In contrast, we found undercount bias as large as 32% before any statistical corrections. The necessary corrections varied both temporally and spatially, in response to previous sighting history (behavioral response), and by the number of horses in a group. Despite modeling some of the differences in horse-group visibility with sighting covariates, we found substantial residual unmodeled heterogeneity that contributed to underestimation of the true population by as much as 22.7% when we used models that did not fully account for these unmeasured sources. We also found that the cost of the accurate and validated methods presented here is comparable to that of raw count (so called, census) methods commonly employed across feral horse ranges in 10 western states. We believe this technique can assist managers in accurately estimating many feral horse populations and could be applied to other species with sufficiently diverse and distinguishable visible markings.

Evaluation of a linked sex harvest strategy for cervid populations

We evaluated the ability of the linked sex harvest strategy (LSHS) proposed by Mccullough et al. (1990) to determine optimum harvest of cervid populations from harvest statistics alone. This strategy purports to optimize total harvest by adjusting female harvest in response to observed changes in male harvest, without knowing the populations size or vital parameters (age-specific survival and productivity), and without an explicit population model. To examine LSHS, we evaluated a series of population models spanning a range of assumptions and parameter values that encompass many cervid populations. Deterministic simulations and numerical optimization were used to examine the response of these models to LSHS. Both ready state and dynamic responses of harvest statistics proposed for detecting optimal yield were examined. Based on our analyses, we were unable to identify general conditions under which LSHS, as currently proposed, provides a sound basis for harvest management. Reliable information regarding a populations vital parameters and current size of each age-sex class remains the only sound basis for near-optimum big game management involving female harvests