Host-Parasitoid Spatial/Landscape Ecology (continued)

Behaviorally Based Landscape-Level Modeling

Dr. John Reeve and I are collaborating on the development of behavior-based landscape-level models to address one of the most significant questions in ecology today – how does spatial heterogeneity affect the long-term temporal population dynamics and persistence of interacting species? The main objective is to develop a spatially realistic model that will reveal the mechanisms underlying the effects of habitat fragmentation and loss, and the invasion of exotic plant species, on the dynamics and persistence of a predator and its prey.

Hypothetical landscape consisting of five host patches surrounded by a matrix of grass (smooth brome) or mudflat. The landscape is divided into six domains (matrix = 1, patches = 2-6).

We are currently developing such models for the planthopper (Prokelisia crocea) and its egg parasitoid (Anagrus columbi) residing in a landscape consisting of host-plant patches embedded in a heterogeneous matrix (Cronin & Reeve 2005, Reeve & Cronin in preparation), in which movement rates and boundary behavior vary with the composition of the matrix (Haynes 2004, Haynes & Cronin 2006). We briefly illustrate this approach on a hypothetical landscape using the software package FEMLAB 3.1 (COMSOL AB 2005).

The solution process consists of drawing the landscape and then specifying diffusion, oviposition, mortality, and parasitoid attack rates within each domain, as well as boundary behavior on the patch-matrix edge. The video below shows the movement of planthoppers across the landscape. Numerical solutions were obtained for two matrix types, mudflat and brome (a grass), using parameter values estimated from observations of P. crocea movement (Reeve, Haynes & Cronin in preparation). Observations have yet to be made for A. columbi, so for simplicity we assume its dispersal behavior is similar to P. crocea. The solutions illustrate the importance of matrix type, patch size, and edge behavior on host and parasitoid abundance <insert graphs>. Densities were consistently higher for a cordgrass-mudflat (left) vs. cordgrass-brome landscape (right), because boundary behavior on the cordgrass-mudflat landscape retains dispersing insects within cordgrass patches. Large cordgrass patches also had higher densities than small ones, as would be expected, and patches closer to the large central patch also had higher densities.

Abundance of host eggs (blue) and juvenile parasitoids (parasitized eggs; green) for 50 generations. Numbers correspond to the patches in the hypothetical landscape.

Testing and refinement of the model will be made possible by fitting the model to independently derived experimental data. The final model will be used to address a number of important ecological questions including how 1) habitat fragmentation and loss affects predator and prey local and regional persistence and long-term population dynamics, 2) matrix composition influences the efficacy of corridors and stepping stones, and 3) the invasion and spread of exotic plants affect patch connectivity and predator-prey dynamics.

It is our goal to develop a modeling framework that is flexible and can be applied to a wide range of study systems that exist in a spatially heterogeneous landscape. By employing FEMLAB, the use of partial differential equations is simplified to drawing a landscape and then specifying diffusion rates and boundary behavior using drop-down menus. It is our hope that by making such models accessible to a wider ecological audience, we will stimulate more theoretical and empirical work in the area of landscape ecology. The suite of software templates and programs that are derived from this research will be made available, free of charge, at this web address.