Research Article: Survey design for broad-scale, territory-based occupancy monitoring of a raptor: Ferruginous hawk (Buteo regalis) as a case study

Date Published: March 22, 2019

Publisher: Public Library of Science

Author(s): Tracey N. Johnson, Kristen Nasman, Zachary P. Wallace, Lucretia E. Olson, John R. Squires, Ryan M. Nielson, Patricia L. Kennedy, Mark S. Boyce.


Given the uncertain population status of low-density, widely-occurring raptors, monitoring changes in abundance and distribution is critical to conserving populations. Nest-based monitoring is a common, useful approach, but the difficulty and expense of monitoring raptor nests and importance of reliable trend data to conservation requires that limited resources are allocated efficiently. Power analyses offer a helpful tool to ensure that monitoring programs have the ability to detect trends and to optimize financial resources devoted to monitoring. We evaluated alternative monitoring designs for raptors to identify appropriate survey effort to detect population trends. We used data collected from a territory-occupancy study of ferruginous hawks throughout Wyoming to guide simulations and evaluate the ability to detect trends in occupancy rates. Results suggest that greater gains in precision of trend estimation may be achieved through the addition of more sites and not more visits; statistical power was ≥80% when monitoring lasted 20 years and population declines were 20%; and probability of detection affected statistical power less than rates of population decline. Monitoring at least 150 sites for 20 years would provide reasonable estimates of trend in occupancy given certain rates of detection and occupancy, but only for population declines of 20%. Removal sampling did not result in substantial changes of any metrics used to evaluate simulations, providing little justification for employing the standard design if territory occupancy is the variable of interest. Initial rates of territory occupancy may be biased high, a problem inherent to many studies that monitor territory occupancy. We explored the effects of lower rates of initial occupancy on the ability to detect trends. Although we present data from a study of ferruginous hawks, our simulations can be applied to other raptor species with similar life history and population dynamics to provide guidance for future trend estimation of territory occupancy.

Partial Text

Anthropogenic transformation of natural systems is currently a primary driver of species abundance and distribution patterns, leading to population declines for many species [1, 2]. Consequences are of particular concern for species that are already rare, especially those that occupy a high trophic position, restricted geographic ranges, and narrow environmental gradients [3]. Apex predators, such as raptors, often meet criteria for rarity and consequently, many species are of conservation concern. Presently, the International Union for Conservation of Nature (IUCN) reports 86 species within Order: Accipitriformes (34%) as either Critically Endangered, Endangered, Vulnerable, or Near Threatened [4]. In the United States, ten raptor species (45%) within the same order have been designated as species of conservation concern in the last decade [5]. Raptors fill unique ecosystem roles as indicators of environmental pollutants [6, 7], flagship species [8], and apex predators responsible for trophic cascades [9, 10]. Given the implications of their loss from ecological communities to ecosystem functioning, it follows that there is substantial interest in their conservation and management from regional to global scales (e.g., Hawks Aloft, Inc., Hawk Mountain Sanctuary, Hawkwatch International, The Peregrine Fund, and many others).

Briefly, we used data presented in [27] to guide simulations allowing us to evaluate the ability to detect trends in occupancy rates for ferruginous hawks. We evaluated trends in occupancy over a time period of 10 or 20 years given various characteristics of the ferruginous hawk population and survey effort. Wallace et al. [27] reported data collected from territory-based detection-non detection surveys in Wyoming during the ferruginous hawk breeding season from 2011–2013. The authors present rates of re-occupancy of known territories estimated from single-season models and related to environmental covariates including oil and gas infrastructure, sagebrush cover, nest substrate, and prey populations. Using the same data presented in Wallace et al. [27], we simulated trends in territory occupancy for ferruginous hawks using a multi-season site-occupancy approach [33]. We identified distributions for initial probability of occupancy, probability of detection, and probability of colonization and extinction, and evaluated the ability to detect trends using multiple statistical metrics described below.

The plausibility of a removal design was of primary interest given the logistical and financial difficulties of monitoring raptors at broad spatial scales. Thus, we present results from the removal design first, followed by results from a standard sampling design for comparison. Furthermore, we concentrate on results from 20 years of monitoring, assuming the long-term data would provide the most robust estimates of occupancy trend, followed by results from 10 years of monitoring to address whether trends could be detected in a shorter time frame.

We demonstrated through simulation that a site-occupancy framework for monitoring ferruginous hawks or other territorial, low-density raptors has the power to detect population declines as small as 10% after a 20-year period under certain conditions of site occupancy and detection rates, many of which have empirical justification. Our results further the science of species monitoring by demonstrating the impact of decisions regarding trade-offs between number of sites and number of visits, and how these trade-offs are influenced by demographic parameters. Similar to Barata et al. [54], our simulations suggest that even when a survey design provides reasonable precision and statistical power, only moderate to large declines in occupancy will be detected, and smaller declines may require up to twice as many years of data to detect.




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