Seminars & Events

Environmental Science Division Seminar
"Point Pattern Modeling for Degraded Presence-Only Data over Large Regions"

DATE: April 19, 2010
TIME: 10:30 AM - 11:30 AM
SPEAKER: Avishek Chakraborty, Duke University
LOCATION: Bldg. 203 Room E-142, Argonne National Laboratory

Description:
Explaining species distribution using local environmental features is a long standing ecological problem. Often, available data is collected as a set of presence locations only thus precluding the possibility of a presence-absence analysis. We propose that it is natural to view presence-only data for a region as a point pattern over that region and to use local environmental features to explain the intensity driving this point pattern. This suggests hierarchical modeling, treating the presence data as a realization of a spatial point process whose intensity is governed by environmental covariates. Spatial dependence in the intensity surface is modeled with random effects involving a zero mean Gaussian process. Highly variable and typically sparse sampling effort as well as land transformation degrades the point pattern so we augment the model to capture these effects. The Cape Floristic Region (CFR) in South Africa provides a rich class with such species data. The potential, i.e., nondegraded presence surfaces over the entire area are of interest from a conservation and policy perspective. Our model assumes grid cell homogeneity of the intensity process where the region is divided into _ 37,000 grid cells. To work with a Gaussian process over a very large number of cells we use predictive process approximation. Bias correction by adding a heteroscedastic error component is implemented. The model was run for a number of different species. Comparison is made with the now popular Maxent approach, though the latter is much more limited with regard to inference. Additional inference such as investigation of species richness immediately follows from our modeling framework.

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