Barry R. Noon

A gentle introduction to quantile regression for ecologists

Quantile regression is a way to estimate the conditional quantiles of a response variable distribution in the linear model that provides a more complete view of possible causal relationships between variables in ecological processes. Typically, all the factors that affect ecological processes are not measured and included in the statistical models used to investigate relationships between variables associated with those processes. As a consequence, there may be a weak or no predictive relationship between the mean of the response variable (y) distribution and the measured predictive factors (X). Yet there may be stronger, useful predictive relationships with other parts of the response variable distribution. This primer relates quantile regression estimates to prediction intervals in parametric error distribution regression models (eg least squares), and discusses the ordering characteristics, interval nature, sampling variation, weighting, and interpretation of the estimates for homogeneous and heterogen...

Spatially explicit population models: Current forms and future uses

Spatially explicit population models are becoming increasingly useful tools for population ecologists, conservation biologists, and land managers. Models are spatially explicit when they combine a population simulator with a landscape map that describes the spatial distribution of landscape features. With this map, the locations of habitat patches, individuals, and other items of interest are explicitly incorporated into the model, and the effect of changing landscape features on population dynamics can be studied. In this paper we describe the structure of some spatially explicit models under development and provide examples of current and future research using these models. Spatially explicit models are important tools for investigating scale-related questions in population ecology, especially the response of organisms to habitat change occurring at a variety of spatial and temporal scales. Simulation models that incorporate real-world landscapes, as portrayed by landscape maps created with geographic information systems, are also proving to be crucial in the development of management strategies in response to regional land-use and other global change processes. Spatially explicit population models will increase our ability to accurately model complex landscapes, and therefore should improve both basic ecological knowledge of landscape phenomena and applications of landscape ecology to conservation and man- agement.

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-

Mapping of Species Richness for Conservation of Biological Diversity: Conceptual and Methodological Issues

Biodiversity mapping (e.g., the Gap Analysis Program [GAP]), in which vegetative features and categories of land use are mapped at coarse spatial scales, has been proposed as a reliable tool for land use decisions (e.g., reserve identification, selection, and design). This implicitly assumes that species richness data collected at coarse spatio-temporal scales provide a first-order approximation to community and ecosystem representation and persistence. This assumption may be false because (1) species abundance distributions and species richness are poor surrogates for community/ecosystem processes, and are scale dependent; (2) species abundance and richness data are unreliable because of unequal and unknown sampling probabilities and species-habitat models of doubtful reliability; (3) mapped species richness data may be inherently resistant to scaling up or scaling down: and (4) decision-making based on mapped species richness patterns may be sensitive to errors from unreliable data and models, resulting in suboptimal conservation decisions. We suggest an approach in which mapped data are linked to management via demographic models, multiscale sampling, and decision theory. We use a numerical representation of a system in which vegetation data are assumed to be known and mapped without error, a simple model relating habitat to predicted species persistence, and statistical decision theory to illustrate use of mapped data in conservation decision-making and the impacts of uncertainty in data or models on the decision outcome.

Top 40 Priorities for Science to Inform US Conservation and Management Policy

To maximize the utility of research to decisionmaking, especially given limited financial resources, scientists must set priorities for their efforts. We present a list of the top 40 high-priority, multidisciplinary research questions directed toward informing some of the most important current and future decisions about management of species, communities, and ecological processes in the United States. The questions were generated by an open, inclusive process that included personal interviews with decisionmakers, broad solicitation of research needs from scientists and policymakers, and an intensive workshop that included scientifically oriented individuals responsible for managing and developing policy related to natural resources. The process differed from previous efforts to set priorities for conservation research in its focus on the engagement of decisionmakers in addition to researchers. The research priorities emphasized the importance of addressing societal context and exploration of trade-offs among alternative policies and actions, as well as more traditional questions related to ecological processes and functions.

POPULATION MODELS FOR PASSERINE BIRDS: STRUCTURE, PARAMETERIZATION, AND ANALYSIS

Population models have great potential as management tools, as they use information about the life history of a species to summarize estimates of fecundity and survival into a description of population change. Models provide a framework for projecting future populations, determining the effects of management decisions on future population dynamics, evaluating extinction probabilities, and addressing a variety of questions of ecological and evolutionary interest. Even when insufficient information exists to allow complete identification of the model, the modelling procedure is useful because it forces the investigator to consider the life history of the species when determining what parameters should be estimated from field studies and provides a context for evaluating the relative importance of demographic parameters. Models have been little used in the study of the population dynamics of passerine birds because of: (1) widespread misunderstandings of the model structures and parameterizations, (2) a lack of knowledge of life histories of many species, (3) difficulties in obtaining statistically reliable estimates of demographic parameters for most passerine species, and (4) confusion about functional relationships among demographic parameters. As a result, studies of passerine demography are often designed inappropriately and fail to provide essential data. We review appropriate models for passerine bird populations and illustrate their possible uses in evaluating the effects of management or other environmental influences on population dynamics. We identify environmental influences on population dynamics. We identify parameters that must be estimated from field data, briefly review existing statistical methods for obtaining valid estimates, and evaluate the present status of knowledge of these parameters.

Using SiZer to detect thresholds in ecological data

Ecological systems can change substantially in response to small shifts in environmental conditions. Such changes are characterized by a non-linear relationship between the value of the response variable and one or more explanatory variables. Documenting the magnitude of change and the environmental conditions that give rise to these threshold responses is important for both the scientific community and the agencies charged with ecosystem management. A threshold is defined as a substantial change in a response variable, given a marginal change in environmental conditions. Here, we demonstrate the usefulness of a derivative-based method for detecting ecological thresholds along a single explanatory variable. The “significant zero crossings” (SiZer) approach uses a non-parametric method to approximate the response function and its derivatives and then examines how those functions change across the range of the explanatory variable. SiZer makes fewer assumptions than conventional threshold models and explore...

Effects of Radio Tags on Spotted Owls

Fifteen of 47 radio-tagged northern spotted owls (Strix occidentalis caurina) died during a 2-year study in northern California, and 11 of 33 radio-tagged California spotted owls (S. o. occidentalis) died in 2.5 years in the Sierra Nevada. Female owls with 19-g radio tags had significantly lower annual survival rates than color-banded females (P=0.006) after a forest fire in 1987 in the Klamath Mountains. There were no differences in annual survival rates between males with and without radio tags. In 1988, fewer radio-tagged pairs (17%) fledged young than color-banded pairs (88%) (P<0.001), and in 1989 radio-tagged pairs nested (7%) less ofthen than color-banded owls (64%) (P=0.0002). We suggest that spotted owl researchers avoid using backpack-mounted radio tags

Post‐Fledging Survival Of Northern Goshawks: The Importance Of Prey Abundance, Weather, And Dispersal

Effective wildlife conservation strategies require an understanding of how fluctuating environmental conditions affect sensitive life stages. As part of a long-term study, we examined post-fledging and post-independence survival of 89 radio-marked juvenile Northern Goshawks (Accipiter gentilis) produced from 48 nests in northern Arizona, USA, during 1998-2001. Information-theoretic methods were used to examine within- and among-year variation in survival relative to environmental (prey abundance, weather), territory (hatching date, brood size), and individual (gender, body mass) sources of variation. The results support age- and cohort-specific differences in survival that were best explained by behaviors occurring at distinct stages of juvenile development, annual changes in the density of primary bird and mammal prey species, and gender-related differences in body mass. Survival between fledging and independence increased linearly with age and varied among annual cohorts of radio-marked juveniles from 0.81 (95% CI = 0.60-0.93) to 1.00 (95% CI = 0.95-1.00) in association with annual differences in prey density; the slope coefficient for the additive effect of prey density on survival was 1.12 (95% CI = 0.06-2.19). Survival declined to 0.71 (95% CI = 0.60-0.93) shortly after juveniles initiated dispersal (weeks 8-12 post-fledging) and moved to more open habitats at lower elevations. Survival was not closely associated with weather or territory-level parameters. A comparison of the predictions of environmental-, territory-, and individual-based models of survival demonstrated that food availability was the primary factor limiting juvenile survival. This finding indicates that forest management prescriptions designed to support abundant prey populations while providing forest structural conditions that allow goshawks to access their prey within breeding areas should benefit juvenile survival.

Habitat selection by northern spotted owls : The consequences of prey selection and distribution

We tested three predictions of a hypothesis that states Northern Spotted Owls (Strix occidentalis caurina) select habitat according to the distribution of their primary prey. Our predictions were that Northern Spotted Owls should (1) select larger (> 100 g) species among the assemblage of available prey, (2) select habitats according to the distribution of large prey, and (3) the owls reproductive success should be influenced by the availability of large prey. We also evaluated the potential energetic value of several prey species. We found that Northern Spotted Owls (n = 11 pairs and 1 single male) in northwestern California differentially selected the dusky-footed woodrat (Neotoma fuscipes), a moderately large sigmodontine rodent (x mass = 232 g). When foraging, owls selected late seral forest edge sites where dusky-footed woodrats were more abundant. Although the relationship between site selection, prey abundance, and the owls breeding success was not statistically significant, the average abundance of dusky-footed woodrats at sites foraged by breeding owls (x = 11.4 woodrats 100-trap-nights -1 ) was greater than at sites foraged by nonbreeding owls (x = 4.7 woodrats 100-trap-nights -1 ). We estimated that a male Spotted Owl would require 150,015 to 336,232 kJ over a 153-day period while helping to produce one young, and concluded that the selection of woodrats provided a potential energetic benefit over the use of other prey. These findings provide a partial explanation for the owls affinity for late seral forests.