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The genes must flow: using movement ecology to understand connectivity of Mojave desert tortoise (Gopherus agassizii) populations in altered landscapes
AuthorHromada, Steven John
AdvisorNussear, Kenneth E.
Ecology, Evolution and Conservation Biology
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Maintaining historic connectivity across animal populations is important to ensure a species can persist into the future. Human infrastructure and activities often fragment habitat, so understanding how connectivity functions is important in mitigation efforts. Connectivity arises from the movement of individuals within and between populations; understanding the movement ecology of a species can provide crucial information in how to best manage populations to maintain gene flow across a landscape. The Mojave desert tortoise (Gopherus agassizii) is a threatened species of the southwestern United States that historically had range-wide genetic connectivity. Human activity has and continues to alter and fragment tortoise habitat and maintaining/restoring connectivity across the range has been identified as an important conservation goal. In this work, I study the movement ecology of Mojave desert tortoises to understand how natural and anthropogenic features contribute to patterns of connectivity in the species. Corridors are important areas of a landscape that allow movement of animals between population centers through areas of unsuitable habitat. Due to assumed modest dispersal capabilities of the species, tortoises have been classified as corridor-dwellers that primarily rely on overlapping home ranges within an area for gene flow through a corridor. I studied tortoise movement selection and home ranges to understand what delineates both natural corridors through mountain passes and artificial corridors of suitable habitat left on the landscape after construction of utility-scale solar installations. Tortoises avoided areas of high slope and low perennial vegetation cover, avoided moving near low-density roads, and traveled along linear barriers. Results suggested that corridors through mountain passes can function differently in allowing tortoise movement, supporting prior findings using genetic differences. Artificial corridors created with fencing may not function the same way as natural corridors as a result of alteration of movement behavior. Although tortoises will avoid certain features such as roads, they will still interact with them. To better understand how anthropogenic and natural features alter tortoise movement behavior, I studied fine-scale tortoise movements using Hidden Markov movement models. My findings suggested that tortoises may respond to the same anthropogenic features (e.g. paved roads) differently depending on the context. Tortoises also alter movement in disturbed areas such as those with off-highway vehicle recreation or wildfire scars, suggesting that these disturbances degrade tortoise habitat. Using simulations of tortoise movement, I show that the behavioral responses to these disturbances may alter how tortoises are distributed on the landscape. Describing the long-term space use of individuals is key to understanding how genetic information flows across the landscape. Using historic and contemporary telemetry datasets (4,861 years of data from 950 tortoises), I related long-term site fidelity and dispersal in desert tortoises to intrinsic (size and sex) and extrinsic (seasonal precipitation) covariates. Tortoises display high site fidelity, though this fidelity is altered by seasonal precipitation and sex. Dispersal is more likely to occur in smaller tortoises and in years with high winter but low summer precipitation or years with low winter but high summer precipitation. I forecast future connectivity across the Ivanpah valley area with an agent-based model to estimate how future precipitation may influence connectivity by altering dispersal propensity. I found no differences in connectivity across emission scenarios, though other anthropogenic stressors will likely play a role in the future of connectivity in this species. This work provides insight into how tortoise movement at different spatial and temporal scales interact with habitat features and disturbances to alter connectivity of tortoise populations.