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(1) Dobson and Foufopoulos (2001) define EIDs as infectious diseases that are increasing in prevalence, spatial range, or number of host types. Most are not newly evolved but occur historically in only a few populations and are exotic to recently invaded populations.

(2) There is no recovered population in SI models, implying that vaccination is not an option. This is the case for many EIDs because vaccines: (a) must be developed for particular disease strains and so may be ineffective against new outbreaks: (b) often only protect against clinical signs of the disease and not the disease itself, making it harder to detect an actual outbreak; and (c) can cause inoculated animals to test positive for the disease, risking sanctions by trading partners (e.g., USDA-APHIS 2002).

(3) The [R.sub.0] = 1 criterion is often discussed for unmanaged populations. In this case the result that invasion cannot occur when host density combinations result in [R.sub.0] < 1 should not be interpreted as a policy prescription because harvest mortality is not explicit in the model.

(4) The pathogen is likely to be introduced first into a single host population, but disease transmission in our model is such that the pathogen would be introduced into the second host population at the next instant. Because of this, we simplify matters and assume both hosts are initially infected, so as to not worry about which is infected first.

(5) Infected animals are unobservable, so detection and response likely occur after T. For simplicity we ignore this delay, but note that delay in switching from prevention to control: (a) will depend on monitoring effort, which will also be endogenously determined, and (b) will likely increase the incentives to invest in prevention.

Richard D. Horan is Associate Professor, Department of Agricultural Economics, and Eli P. Fenichel is Research Assistant, Department of Fisheries and Wildlife, both at Michigan State University.

Funding was provided by the Economic Research Service-USDA cooperative agreement number 58-7000-6-0084 through ERS's Program of Research on the Economics of Invasive Species Management (PREISM), and by NRI, USDA, CSREES, grant #2006-55204-17459. The views expressed here are the authors'.

This article was presented in a principal paper session at the AAEA annual meeting (Portland, OR, July 2007). The articles in these sessions are not subjected to the journal's standard refereeing process.