Population Estimation

Science Center Objects

Determining the size of animal populations is fundamental to wildlife management and how such populations change over time and space can provide insights into ecosystem function and associated processes.   Patuxent scientists’ utilize a number of robust statistical techniques to generate these estimates and to improve our understanding of population dynamics.

USGS-PWRC population modeling software web-page

grams that were developed at PWRC and elsewhere for the analysis of animal populations for wildlife biologists/managers. (Public domain.)


Development of Computer Software for the Analysis of Animal Population Parameters

Biologists at USGS Patuxent, as well as cooperating agencies are constantly looking for new ways of answering questions about the status of animal populations and how animal populations change over time. To address these questions, data are collected on captures and or sightings of animals which can be used to estimate parameters which affect the population using legacy software. Over time, new questions and methods for addressing these questions arise which require new computer software.






West Indian manatees, Trichechus manatus

(Public domain.)



Modeling, Estimation, and Adaptive Management of Florida Manatees

Florida manatees are threatened by watercraft-related mortality, the potential loss of warmwater habitat, red tide events, and other anthropogenic factors. The USFWS and the Florida Fish and Wildlife Conservation Commission have regulatory authorities under the Endangered Species Act (ESA), the Marine Mammal Protection Act (MMPA), and state statutes to recover manatees. To support management decision-making, these agencies need quantitative assessments of population status.


Spatial Capture-Recapture by Royle,Chandler, Sollmann, & Gardner

 Royle, J. A., Chandler, R. B., Sollmann, R., and Gardner, B., 2014, Spatial capture-recapture: Amsterdam,  Elsevier.    xxix, 577  p. (Public domain.)



Spatial Capture-Recapture Models to Estimate Abundance and Density of Animal Populations

For decades, capture-recapture methods have been the cornerstone of ecological statistics as applied to population biology.  While capture-recapture has become the standard sampling and analytical framework for the study of population processes (Williams, Nichols & Conroy 2002) it has advanced independent of and remained unconnected to the spatial structure of the population or the landscape within which populations exist. Furthermore, capture-recapture does not invoke any spatially explicit biological processes and thus is distinctly non-spatial, accounting neither for the inherent spatial nature of the sampling nor of the spatial distribution of individual encounters.  Linking observed encounter histories of individuals to mechanisms of spatial population ecology will enable ecologists to study these processes using new technologies such as noninvasive genetics, remote cameras and bioacoustic sampling.





Survival Probability

Survival Probability (Public domain.)



Hierarchical Models of Animal Abundance and Occurrence

Research goals of this project are to develop models, statistical methods, sampling strategies and tools for inference about animal population status from survey data. Survey data are always subject to a number of observation processes that induce bias and error. In particular, inferences are based on spatial sampling – we can only ever sample a subset of locations where species occur --and imperfect detection – species or individuals might go undetected in the sample. Principles of hierarchical modeling can be applied directly to accommodate both features of ecological data. Prior to the development of hierarchical models at PWRC, studies of unmarked populations focused on simplistic descriptions of distribution patterns and temporal trends. Hierarchical models have advanced the field of population ecology by enabling the estimation of demographic and movement parameters that previously could only be obtained using costly field methods. Ecologists can now make inferences about population dynamics at broad spatial and temporal scales using models designed specifically for this task.


Population Change and the BBS

Log-linear hierarchical model of population change for BBS data (Public domain.)


Design and Analysis of Surveys for Estimation of Temporal and Spatial Change in Animal Populations

Population status information is required for management of migratory bird populations, and structured decision making and adaptive anagement place additional emphasis on the need for rigorous survey designs and robust estimation methods. The North American Breeding Bird Survey (BBS) and Christmas Bird Count (CBC) provide continent-scale information on breeding and wintering populations of >450 species of North American birds, and for many species these two surveys are our only data source for population status and trend information. Appropriate analyses of these important surveys require sophisticated methods to accommodate variation in survey efficiency over the large areas covered by the surveys and to control for factors that influence detection of birds. Factors such as observer quality and effort, if not appropriately controlled for in the analysis, can lead to biased estimates of population change.


Endangered Karner blue butterfly, Lycaeides melissa samuelis

Endangered Karner blue butterfly, Lycaeides melissa samuelis (Public domain.)


Estimation of Density and Abundance of Biological Populations on National Parks and Wildlife Refuges through Distance Sampling

Assessing the status and trends of populations of biological organisms is an important management goal and a recurrent theme in USGS research. Often, the most basic question of “how many are there?” remains elusive, thus making management decisions more difficult. This study continues a long-term commitment of technical support for the use of distance sampling for wildlife population abundance estimation in our National Parks and Wildlife Refuges.




Black Duck, Anas rubripes, Breeding Range Map

Black Duck, Anas rubripes, Breeding Range Map (Public domain.)

Population Change and Abundance of Black Ducks and Mallards in Eastern North America

Over the last half of the 20th century, the breeding range of American black duck (Anas rubripes) has contracted from central Canada and the Northeastern United States toward eastern Canada. This reduction in size of the breeding range has been reflected in a steady decline of black ducks counted during winter surveys, both the midwinter Waterfowl Survey conducted by the U. S. Fish and Wildlife Service and the Christmas Bird Count. The causes of the declines are unclear. Hunting regulations have been restricted to decrease the harvest. To improve our understanding of black duck population dynamics and of the role of harvest in population change, a variety of surveys, banding studies, and research activities have been conducted for this economically important species. Patuxent staff have been active participants in these activities, and have served on the Technical Committee and Management Board of the Black Duck Joint Venture of the North American Waterfowl Management Plan. Here, we describe Patuxent’s role in assisting with development and analysis of surveys for black ducks in eastern Canada.


Ducks eating brine flies

(Credit: Ian Thomas, US Fish and Wildlife Service. Public domain.)


Survival and Reintroduction of the Laysan Teal

The Laysan Teal is an endangered, endemic, Hawaiian dabbling duck that has been pushed to the brink of extinction numerous times. The previous range includes the Main and Northwestern Hawaiian Islands, and its current range is less than 10 sq. km within the National Wildlife Refuges of Papahānaumokuākea Marine National Monument. This non-migratory waterfowl was eliminated from all the Hawaiian Islands except for Laysan by the 1860’s through anthropogenic effects (i.e., introduced rats, shipwrecked mariners, etc.). The Laysan Island population was threatened when guano miners inhabited the island, hunted the duck, and introduced rabbits, devastating the native habitat until they were removed in 1923. Presently, extreme events (e.g., tsunamis, hurricanes, drought, or flooding), disease (e.g., Avian Botulism), sea-level rise, accidental predator or competitor introductions, are ongoing threats to this duck’s survival.