Environmental change created by humans is one of the major challenges to survival for global biodiversity. I am passionate about conservation, and I am inspired by the scientific advances that better inform efforts to combat environmental threats. My research addresses complex questions at the intersection of ecology, evolution, and environmental science. I combine new methods from field ecology, genetics, and remote sensing to document habitat change in multiple environments, and measure its effect on the viability of plant and animal species. I use this information to predict species responses to continued environmental change, and direct conservation efforts to mitigate biodiversity loss. How does climate and environmental change affect the reproduction, dispersal, and evolution of plant and animal species?

Reproduction – The potential for future population stability
What is the role of interspecific interactions on ecological and evolutionary processes and how are these relationships impacted by shifts in suitable habitat distribution? Plant reproduction, plant-insect interactions, and pollination biology provide the foundation for my research. For instance, through two years of field study in fragmented Iowa prairie habitat, I identified a strong divergence in pollinator community assemblies among the major flowering families that indicated a high degree of specialization and susceptibility to pollinator collapse in the face of future disturbance (Gaddis & Hendrix 2004a). In a separate study in the Mojave National Preserve, I identified an economy-of-scale in floral production in the common shrub Acacia greggii, whereby, larger displays increase pollinator attraction and, subsequently, fertilization success. The resulting rise in seed production leads to predator satiation and a greater proportion of seeds evading destruction. These results suggest that a rise aridity that lowers floral production will exponentially decrease the reproductive potential of this and similar species (Gaddis et al. in review). These studies have been especially effective to train undergraduates that wish to gain a foundation in basic field ecology. My students have worked on independent projects related to my research. Over two years, a group of six students at Texas A&M ran an experimental germination trial and identified resilience of A. greggii to significant damage by simulated seed predation. Another group analyzed data from a summer project to investigate the effect of artificial floral plantings in Texas roadsides on pollinator diversity to help guide planting strategies for conservation groups in this area.

Dispersal – Maintaining genetic diversity and population resilience
How does climate and landscape change influence movement in non-model species? Movement allows a species to escape an unfavorable environment, find mates, and share genetic information that may help adapt in the face of habitat change. Restricted mobility increases the likelihood of local or global extinction. Over the last ten years, I have developed a suite of skills combining genomic and geospatial tools to track historic and contemporary dispersal in plants and animals. In the Mojave, I used these tools to determine how urban growth may influence regional connectivity in plants. We identified that dry-washes are a major corridor of seed and pollen dispersal in A. greggii, and that rechanneling these pathways through road establishment will likely isolate populations and limit the amount of genetic diversity traversing the landscape (Gaddis et al. 2016). In a separate study within tropical Southeast Asia, I examined the influence of a parasitic lifecycle on reproduction and dispersal within Rafflesiaceae species. In spite of a reproductive cycle outside of the host (unlike many animal parasites), there appears to be limited genetic exchange among hosts and regions, raising conservation concerns. If a single host dies, a significant component of parasite genetic variation would be loss, and a precarious relationship between extinction and recolonization that could lead to regional population loss should host mortality rise, or distance increase, among potential hosts (Barkman et al. 2016). Through these projects, I have trained undergraduate assistants in molecular genetic laboratory and analysis techniques that has opened the door for individual projects, career advance in academia, medicine, and industry, and continued graduate study in genetics and genomics.

Evolution – Adapting to climatic and environmental change
I address questions that examine the effect of environmental change on evolution: 

1 - What impact have historic environmental changes had on current genetic patterns?
In California, I set out to determine if isolation in refugia during the last glacial maxima drove parasitic acorn weevil host specificity. We found a generalized infection ability among weevil species that indicates a contrasting evolutionary pathway after glacial retreat compared to a similar European system (Bonal et al. 2016). In a study in Alaska, we found that glacial retreat led to rapid expansion of mountain hemlock into the Kenai Peninsula which significantly lowered genetic diversity relative to southern populations (Johnson et al. 2017a,b). These projects have helped to inform predictive modelling of evolutionary change in these ecosystems under alternative scenarios of future climate or environment.

2 - How will contemporary environmental changes affect a species’ evolutionary potential?
In a separate study, I investigated the influence environment has on reproductive mode in the tropical forest dominant Pentaclethra macroloba. We identified that moisture leads to a rise in the frequency of asexual reproduction relative to sexual, which suggests increased rainfall could lower genetic diversity, limiting potentially beneficial variants (Gaddis et al. 2014). My recent work has incorporated habitat suitability models to predict how species will respond under future climatic scenarios. For instance, within a suite of hybridizing Southern California oak species, rising temperatures are likely to drive greater range overlap and introgression (Riordan et al. 2016). In Alaska, I am completing a project where we are using habitat suitability models to predict the consequences of treeline advance on genetic diversity patterns in white spruce. Our preliminary analysis has shown that rapid range expansion actually leads to greater sexual reproduction relative to asexual, and more homogeneous dispersal, which increases the genetic diversity relative to more stable treelines. This project supported one NSF REU student, and led to the production of eight undergraduate conference posters.