The Effects of the Hemlock Woolly Adelgid on Abundance and Nymphal Infection Prevalence of Black-Legged Ticks in Maine

The black-legged tick (Ixodes scapularis) has recently made a tremendous impact in Maine due to its role as a vector for the bacterial pathogen Borrelia burgdorferi, the causative agent of Lyme disease. A lesser known, but equally concerning, invasive insect is the hemlock woolly adelgid (HWA; Adelges tsugae), a sap-sucking scale that is primarily responsible for the ongoing widespread decline of eastern hemlock in the northeast. Maine is currently experiencing a co-invasion of these species, and this study tests the hypothesis that the phenomenon of hemlock loss may facilitate the invasion of the black-legged tick by a combination of indirect effects. By killing eastern hemlock trees, the HWA alters forest structure (e.g., letting more light through the canopy) and changes the species composition of plant and wildlife communities, including important hosts of the black-legged tick. My study simulates the consequences of HWA infestation by comparing tick abundance and nymphal infection prevalence (NIP) in hemlock and hardwood stands in southern Maine and the Bangor area. I hypothesized that the HWA is indirectly increasing both tick abundance and Lyme disease risk in Maine by creating ecological conditions that alter abundance of deer and provide a more suitable microhabitat for the tick. I also predicted that NIP would differ between the two treatments, with ticks collected in deciduous stands having higher infection prevalence. My results showed no significant difference in either tick abundance or NIP between the two treatments. Additionally, I tested one mechanism that could explain these patterns by conducting deer scat surveys using standardized transects. There was no significant difference in deer scat counts between the treatments. Conclusions from this work could inform park managers and Maine citizens about the likelihood of Lyme infection or tick bites in certain areas of forests or parks

Temporal Dynamics and Seed Dispersal in Plant-Frugivore Communities of the Dominican Republic

Plant-animal mutualisms are a foundational component of biodiversity in terrestrial ecosystems. Most tropical forest plants have adapted to produce fleshy fruits to attract frugivorous animals to disperse seeds. Interaction patterns among plant taxa and their seed dispersers are driven by a complex suite of factors involving their evolutionary history and environmental context, and the structure of these mutualistic networks are theoretically tied to their ecological function. I carried out a series of field studies to investigate the temporal dynamics of mutualistic interactions of plant and avian frugivore communities in the central Dominican Republic and how their characteristics affect seed dispersal in agricultural landscapes. I first investigated the effects of reproductive phenology of a tropical tree (Guarea guidonia) on the temporal variation of avian foraging behavior and seed dispersal patterns. I found that temporal variation in seed dispersal was driven most by landscape-level dynamics in the availability of alternative resources rather than tree– or neighborhood–level fruit production. I proceeded to expand my focus on the processes of frugivory and seed dispersal by monitoring the phenology of six local communities and characterizing the temporal dynamics of plant-frugivore networks across a full annual period. By applying multilayer network analyses, I identified a tendency of birds to shift between temporally defined modules in nonrandom patterns that suggest a prevailing influence of resource partitioning on consumer preferences across seasonal time periods. By systematically sampling seed dispersal at a subset of these monitoring sites, I demonstrated how frugivory measures from network data predict their dispersal potential and ability to colonize new patches in heterogenous landscapes. Finally, I applied network data from frugivorous bird species to design an experiment to test the effect sounds of frugivore taxa with varying degrees of fruit consumption on the movement behavior and use of artificial perches in abandoned pastures by potential seed dispersers, finding that frugivorous bird sounds stimulate an increase in the frequency of avian visitors to degraded habitat. Collectively, my investigations provide insight into the processes of frugivory and seed dispersal in a previously undocumented region and reveal how interaction patterns can translate to ecological outcomes

A novel method for sensitivity analysis of time-averaged chaotic system solutions

The direct and adjoint methods are to linearize the time-averaged solution of bounded dynamical systems about one or more design parameters. Hence, such methods are one way to obtain the gradient necessary in locally optimizing a dynamical system’s time-averaged behavior over those design parameters. However, when analyzing nonlinear systems whose solutions exhibit chaos, standard direct and adjoint sensitivity methods yield meaningless results due to time-local instability of the system. The present work proposes a new method of solving the direct and adjoint linear systems in time, then tests that method’s ability to solve instances of the Lorenz system that exhibit chaotic behavior. Promising results emerge and are presented in the form of a regression analysis across a parametric study of the Lorenz system

Random subgraphs of finite graphs: I. The scaling window under the triangle condition

We study random subgraphs of an arbitrary finite connected transitive graph $\mathbb G$ obtained by independently deleting edges with probability $1-p$. Let $V$ be the number of vertices in $\mathbb G$, and let $\Omega$ be their degree. We define the critical threshold $p_c=p_c(\mathbb G,\lambda)$ to be the value of $p$ for which the expected cluster size of a fixed vertex attains the value $\lambda V^{1/3}$, where $\lambda$ is fixed and positive. We show that for any such model, there is a phase transition at $p_c$ analogous to the phase transition for the random graph, provided that a quantity called the triangle diagram is sufficiently small at the threshold $p_c$. In particular, we show that the largest cluster inside a scaling window of size |p-p_c|=\Theta(\cn^{-1}V^{-1/3}) is of size $\Theta(V^{2/3})$, while below this scaling window, it is much smaller, of order $O(\epsilon^{-2}\log(V\epsilon^3))$, with \epsilon=\cn(p_c-p). We also obtain an upper bound O(\cn(p-p_c)V) for the expected size of the largest cluster above the window. In addition, we define and analyze the percolation probability above the window and show that it is of order \Theta(\cn(p-p_c)). Among the models for which the triangle diagram is small enough to allow us to draw these conclusions are the random graph, the $n$-cube and certain Hamming cubes, as well as the spread-out $n$-dimensional torus for $n>6$