Doctor of Philosophy

dissertationChapter 1 introduces a classic question from optimal foraging theory regarding space-use strategies of a forager, and gives context for addressing similar questions in groups of foraging ants. Chapter 2 generalizes the marginal value theorem (MVT) model by describing a rate-maximizing forager searching for pointwise resources with a specific searching distribution around previous resource finds, and giving-up value (GUV) strategy at resources. The model shows that the optimal ARS breadth increases, and the optimal GUV decreases, with increased dispersion of the resource distribution. Chapter 3 builds an agent-based model (ABM) and corresponding PDE model derived from an isotropic diffusion limit. The model links individual movement biases in the presence of pheromone to the colony-wide searching distribution. Parameterized with movement data obtained from Tetramorium caespitum (the pavement ant), the model predicts bistability in pheromonal recruitment at resource distances of 3 - 6 m; the onset-distance of bistability increases with colony size. Data collected from the field are used to estimate parameters of the PDE model for T. caespitum in Chapter 4. The ability of T. caespitum to find autocorrelated resources during recruitment is analyzed using a Cox proportional hazards model, the results of which are compared to those predicted by the PDE model developed in Chapter 3. Finally, Chapter 5 develops a simulation to assess the effect of individual trail fidelity on the ability of a colony to capitalize on autocorrelated resources in different resource scenarios; the results suggest that T. caespitum is tuned to exploit large, nonautocorrelated resource distributions

Suppressed basal melting in the eastern Thwaites Glacier grounding zone

Thwaites Glacier is one of the fastest-changing ice–ocean systems in Antarctica1,2,3. Much of the ice sheet within the catchment of Thwaites Glacier is grounded below sea level on bedrock that deepens inland4, making it susceptible to rapid and irreversible ice loss that could raise the global sea level by more than half a metre2,3,5. The rate and extent of ice loss, and whether it proceeds irreversibly, are set by the ocean conditions and basal melting within the grounding-zone region where Thwaites Glacier first goes afloat3,6, both of which are largely unknown. Here we show—using observations from a hot-water-drilled access hole—that the grounding zone of Thwaites Eastern Ice Shelf (TEIS) is characterized by a warm and highly stable water column with temperatures substantially higher than the in situ freezing point. Despite these warm conditions, low current speeds and strong density stratification in the ice–ocean boundary layer actively restrict the vertical mixing of heat towards the ice base7,8, resulting in strongly suppressed basal melting. Our results demonstrate that the canonical model of ice-shelf basal melting used to generate sea-level projections cannot reproduce observed melt rates beneath this critically important glacier, and that rapid and possibly unstable grounding-line retreat may be associated with relatively modest basal melt rates

A little goes a long way: Weak vaccine transmission facilitates oral vaccination campaigns against zoonotic pathogens.

Zoonotic pathogens such as Ebola and rabies pose a major health risk to humans. One proven approach to minimizing the impact of a pathogen relies on reducing its prevalence within animal reservoir populations using mass vaccination. However, two major challenges remain for vaccination programs that target free-ranging animal populations. First, limited or challenging access to wild hosts, and second, expenses associated with purchasing and distributing the vaccine. Together, these challenges constrain a campaign's ability to maintain adequate levels of immunity in the host population for an extended period of time. Transmissible vaccines could lessen these constraints, improving our ability to both establish and maintain herd immunity in free-ranging animal populations. Because the extent to which vaccine transmission could augment current wildlife vaccination campaigns is unknown, we develop and parameterize a mathematical model that describes long-term mass vaccination campaigns in the US that target rabies in wildlife. The model is used to investigate the ability of a weakly transmissible vaccine to (1) increase vaccine coverage in campaigns that fail to immunize at levels required for herd immunity, and (2) decrease the expense of campaigns that achieve herd immunity. When parameterized to efforts that target rabies in raccoons using vaccine baits, our model indicates that, with current vaccination efforts, a vaccine that transmits to even one additional host per vaccinated individual could sufficiently augment US efforts to preempt the spread of the rabies virus. Higher levels of transmission are needed, however, when spatial heterogeneities associated with flight-line vaccination are incorporated into the model. In addition to augmenting deficient campaigns, our results show that weak vaccine transmission can reduce the costs of vaccination campaigns that are successful in attaining herd immunity

Bat case study example.

Specific example for D. rotundus on the level of pathogen reduction achieved across various times of vaccination with different vaccine R0 values indicated by the different colors. Solid lines represent the level of pathogen reduction achieved for a given date of vaccine introduction. The grey region outlined by the dashed lined represents the birthing season where day 1 corresponds to the first day of the birthing season. The remaining parameters used were: an average population size of 240 individuals (), s = 2.59, an average lifespan of 3.5 years (d = 1/(365 × 3.5), R0,P = 1.5, 24 vaccines are distributed each year (NV = 24), individuals can disseminate the transferable vaccine for 7 days on average (γg = 2−1), individuals remain infectious with the transmissible vaccine for their entire life (γV = 0), individuals remain infectious with the pathogen for 21 days on average (γP = 21−1), the transferable vaccine is groomed off individuals after 6 days on average (α = 1/15000, and the pathogen is virulent (ν = 0.005).</p

Fractional reduction in baits required for herd immunity in wildlife.

The y-axis shows the reduction in the vaccine deposition rate due to vaccine transmission in campaigns that maintain herd immunity in North American wildlife (ϕ = 0.5), and at higher levels (ϕ ≈ 0.7) that are achieved in coyote populations.</p

Description of state variables and model parameters.

Description of state variables and model parameters.</p

Cost reductions predicted by variations of the spatial model.

Cost reductions predicted by variations of the spatial model.</p