Critical Habitat Breadth for Gopherus Tortoises: A New Paradigm for Managing Threatened and Endangered Species in a Non-Stationary World
Kevin Shoemaker | University of Nevada, Reno
Conserving threatened and endangered species (TES) on Department of Defense (DoD) installations, without sacrificing vital military objectives, is necessarily a complex balancing act. A changing climate only intensifies these challenges, as habitat protection and restoration initiatives must rigorously account for new risks and uncertainties. Reliance on standard habitat models for performing climate vulnerability assessments may overestimate the risk from climate change because most TES assessments place more focus on the nature and magnitude of exposure to change than species’ adaptive capacity to change. To overcome these shortfalls, this project advances the concept of critical habitat breadth as the foundation for a new generation of rigorous TES conservation planning and vulnerability assessment tools. This concept, defined as the full range of environmental conditions capable of harboring viable populations, represents a novel operationalization of the niche concept in ecology. Gopherus tortoises provide an excellent model system for developing and applying the concept of critical habitat breadth because their populations have been extensively studied, many occupied habitats exhibit steep environmental gradients, and many prior translocations have been conducted (“common garden” experiments which enable investigation into the inherent ability of individuals to acclimate to novel environments). This project hypothesizes that Gopherus tortoise populations (G. agassizii and G. polyphemus) are resilient to a wider range of environmental changes than previous research suggests. The research objectives are to (1) understand the complex pathways through which environmental conditions influence population vital rates for two Gopherus species, (2) use this information to derive population growth rates, assess viability, and quantify critical habitat breadth for these species, and (3) develop a conservation planning tool that applies the critical habitat breadth concept to aid DoD installation managers in protecting Gopherus metapopulations and planning for an uncertain future.
Gathering the data for this project requires (1) measuring population growth and survival rates using existing, long-term survey datasets; (2) conducting detailed field studies at prior translocation sites to investigate the relative importance of local adaptation and phenotypic plasticity; and (3) investigating within-population changes in key vital rates across steep environmental gradients (e.g., elevation, temperature). Field data-collection efforts will focus on measuring vital rates for which linkages with environmental drivers are poorly understood – including offspring production, age-at-maturity, hatchling sex ratio, and nest success. Field data collection will occur both on and off DoD installations to sample across the broadest possible range of environmental conditions. These data will be used, along with a suite of field-validated climate and habitat datasets (Geographic Information Systems [GIS] layers), to model critical habitat breadth for both Gopherus species, specifically through hierarchical population models to estimate population growth rate as a function of pathways involving inter-correlated vital rates, body condition and spatiotemporally varying environmental conditions. Finally, fitted population models will be used to produce spatiotemporal projections of population growth rates and critical habitat, and to assess metapopulation viability under multiple plausible future scenarios.
The project framework, which focuses on Gopherus tortoises but is generalizable to a wide array of TES and other species of concern, represents a major departure from previous approaches for TES conservation, and has great potential to transform the management of TES metapopulations in uncertain and changing environmental conditions on DoD lands and beyond. Maps of critical present and future habitat for both Gopherus species will be created to guide conservation planning efforts. This project will also develop a web-based tool that DoD biologists and natural resource managers can use to rank alternative conservation scenarios, produce rigorous and cost-effective TES impact assessments, and guide recommendations for efficiently fulfilling conservation mandates in an uncertain and non-stationary world, via on-site solutions (e.g., protection of existing habitat) and/or establishment of connected, multi-jurisdictional habitat reserve networks.