Factors affecting gray wolf (Canis lupus) encounter rate with elk (Cervus elaphus) in Yellowstone National Park

Despite encounter rates being a key component of kill rate, few studies of large carnivore predation have quantified encounter rates with prey, the factors that influence them, and the relationship between encounter rate and kill rate. The study’s primary motivation was to determine the relationship between prey density and encounter rate in understanding the mechanism behind the functional response. Elk (Cervus elaphus Linnaeus, 1758) population decline and variable weather in northern Yellowstone National Park provided an opportunity to examine how these factors influenced wolf (Canis lupus Linnaeus, 1758) encounter rates with elk. We explored how factors associated with wolf kill rate and encounter rate in other systems (season, elk density, elk group density, average elk group size, snow depth, wolf pack size, and territory size) influenced wolf–elk encounter rate in Yellowstone National Park. Elk density was the only factor significantly correlated with wolf–elk encounter rate, and we found a nonlinear density-dependent relationship that may be a mechanism for a functional response in this system. Encounter rate was correlated with number of elk killed during early winter but not late winter. Weak effects of snow depth and elk group size on encounter rate suggest that these factors influence kill rate via hunting success because kill rate is the product of hunting success and encounter rate.

The role of variability and uncertainty in testing hypotheses involving parameters in stochastic demographic models

Hierarchical / random effect models provide a statistical framework for estimating variance parameters that describe temporal and spatial variability of vital rates in population dynamic models. In practice, estimates of variance parameters (e.g., process error) from these models are often confused with estimates of uncertainty about model parameter estimates (e.g., standard errors). These two sources of “error” have different implications for predictions from stochastic models. Estimates of process error (or variability) are useful for describing the magnitude of variation in vital rates over time and are a feature of the modeled process itself, whereas estimates of parameter standard errors (or uncertainty) are necessary for interpreting how well we are able to estimate model parameters and whether they differ among groups. The goal of this comment is to illustrate these concepts in the context of a recent paper by A.W. Reed and N.A. Slade (Can. J. Zool. 84: 635–642 (2006)) . In particular, we will show that their “hypothesis tests” involving mean parameters are actually comparisons of the estimated distributions of vital rates among groups of individuals.

Injuries among children in home and out-of-home care

As the number of children receiving care in out-of-home settings increases in the United States, the risk of injury in such settings has become the subject of intense research.

OBJECTIVES

This study examined the relative safety of out-of-home care compared with care in a child's own home.

METHODS

This community based prospective cohort study of 656 families in three adjacent counties in the Piedmont region of North Carolina characterizes the patterns and rates of injuries among children less than 5 years of age in three child care settings, home care (HC), center based care (CBC), and other out-of-home care (OOHC). Information about minor and severe injuries was obtained from parents using monthly telephone interviews over a one year period. Statistical modeling designed to handle unbalanced data with correlated observations was used as the primary tool for analysis.

RESULTS

Rate of minor injuries was highest in CBC, followed by HC, and then OOHC. However, these differences for OOHC may have been due to reporting biases and errors in rate estimates. There were no significant differences in severe injury rates among the three settings.

CONCLUSIONS

The risk of serious injury among children under 5 in CBC is not different from that of children in HC or OOHC despite the fact that the risk of minor injury is higher.