Population Viability and Vital Rate Sensitivity of an Endangered Avian Cooperative Breeder, the White-Breasted Thrasher (Ramphocinclus brachyurus).

Jennifer L Mortensen,J Michael Reed,Antoni Margalida

doi:10.1371/journal.pone.0148928

Abstract

Social behaviors can significantly affect population viability, and some behaviors might reduce extinction risk. We used population viability analysis to evaluate effects of past and proposed habitat loss on the White-breasted Thrasher (Ramphocinclus brachyurus), a cooperatively breeding songbird with a global population size of <2000 individuals. We used an individual-based approach to build the first demographic population projection model for this endangered species, parameterizing the model with data from eight years of field study before and after habitat loss within the stronghold of the species’ distribution. The recent habitat loss resulted in an approximately 18% predicted decline in population size; this estimate was mirrored by a separate assessment using occupancy data. When mortality rates remained close to the pre-habitat loss estimate, quasi-extinction probability was low under extant habitat area, but increased with habitat loss expected after current plans for resort construction are completed. Post-habitat loss mortality rate estimates were too high for projected populations to persist. Vital rate sensitivity analyses indicated that population growth rate and population persistence were most sensitive to juvenile mortality. However, observed values for adult mortality were closest to the threshold value above which populations would crash. Adult mortality, already relatively low, may have the least capacity to change compared to other vital rates, whereas juvenile mortality may have the most capacity for improvement. Results suggest that improving mortality estimates and determining the cause(s) of juvenile mortality should be research priorities. Despite predictions that aspects of cooperative systems may result in variation in reproduction or juvenile mortality being the most sensitive vital rates, adult mortality was the most sensitive in half of the demographic models of other avian cooperative breeders. Interestingly, vital rate sensitivity differed by model type. However, studies that explicitly modeled the species’ cooperative breeding system found reproduction to be the most sensitive rate.

Highlights

Understanding the drivers of population decline and the dynamics of the extinction process are interesting research problems (e.g., [1,2,3]), with important applications to species management [4, 5]
We examined the sensitivity of White-breasted Thrasher population dynamics to shifts in vital rates in three ways: (1) perturbation analysis, (2) relative sensitivity, and (3) a logistic regression approach
From the analyses described above, we found that White-breasted Thrasher population dynamics are sensitive to adult mortality rates

Summary

Introduction

Understanding the drivers of population decline and the dynamics of the extinction process are interesting research problems (e.g., [1,2,3]), with important applications to species management [4, 5]. A common tool used to investigate the effects of these drivers is demographic modeling [6]; the most common approaches have been matrix models (e.g., [7]) and individual-based models (e.g., [8]), but new approaches are being explored, such as multi-agent systems modeling (e.g., P systems models [9]). When these models are used to understand, predict, and manage extinction risk of small or declining populations, they are often called population viability analysis (PVA) [10]. As opposed to obligate systems, helpers are not necessary for a breeding pair to raise young to independence

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