The Impacts of Climate and Land Use Change on Mojave Desert Tortoise (Gopherus agassizii) Habitat Suitability and Landscape Genetic Connectivity
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Authors
Parandhaman, Anjana
Issue Date
2023
Type
Dissertation
Language
Keywords
Conservation , Habitat Suitability , Landscape Genetics , Mojave Desert Tortoise , Sampling Design
Alternative Title
Abstract
The Mojave desert in the southwestern United States faces a multitude of anthropogenic stressors including urbanization, population growth, solar energy development, expansion of transportation infrastructure and military training, as well as climate change that has impacted the region through habitat fragmentation and altered precipitation and temperature regimes. The Mojave desert tortoise is a species that persists on this landscape despite these impacts that could influence its long-term population densities, distributions, and connectivity.A fundamental goal of conservation prioritization is understanding the distribution of suitable habitat and maintaining connected landscapes between these habitats to ensure species can adapt to changes in their environments. Species distribution models identify regions of suitable habitat based on statistical modeling relating location data for the species to environmental variables that influence the distribution of the species. Examining how landscape features, specifically landscape composition and configuration, interact with microevolutionary processes such as gene flow helps identify which landscape features facilitate or limit gene flow and subsequently connectivity for a species. In this dissertation I sought to understand how the compounded effects of land use and climate change would impact habitat suitability and landscape connectivity at multiple temporal and spatial scales for the tortoise using various spatial and genetic tools.
In chapter 1 I created a sampling design to guide targeted field efforts to obtain location and genetic data focused on under-surveyed edge regions of the tortoise’s range. I used this sampling design to obtain new genetic samples and used the new dataset to determine the efficacy of my initial design as well as examine whether my new samples represented new climatic niches.
In chapter 2 I built a range-wide habitat suitability model for the tortoise and forecasted this model to climate and land use change scenarios. I also explored the influence of scale by building regional habitat suitability models and forecasting these models. I found that precipitation, temperature, and soil variables influence habitat suitability at range-wide scales and the direction of these relationships changed at the regional scale. Forecast models predicted widespread loss of tortoise habitat under all future scenarios, with the highest net change in habitat across critical habitat units for the species, and least net change across military bases. Habitat was shifted northward over time, with the southern edge losing the most amount of habitat by the worst-case climate and land use scenarios in 2098.
In chapter 3 I used a range-wide genomics dataset to estimate population structure and to build habitat-based connectivity models using isolation-by-resistance (IBR) approaches at various temporal scales. I found that as habitat availability declined in the southern edge of the range, there was a concomitant loss of connectivity, and that connectivity was also gained in the northern edge of the range. I used comparative modeling in a maximum likelihood population effects (MLPE) framework and found that IBR based metrics were the best predictors of range-wide connectivity, indicating that habitat features on the landscape drive genetic differentiation and gene flow for the tortoise.
Overall, this work creates a novel sampling design methodology as well as identifies the impacts of climate and land use on tortoise habitat and connectivity using multiple approaches. This research identifies specific tortoise habitat and connective corridors, in vulnerable regions such as the southern and central range, that should continue to be protected to ensure persistence of the species currently as well as into the future.