9th International Deer Biology Congress

AUTHORS: Eric M. Schauber, Cooperative Wildlife Research Laboratory, and Department of Zoology, Southern Illinois University; Marie I. Tosa, Department of Fisheries and Wildlife, Oregon State University; Lene J.Kjær, Technical University of Denmark, National Veterinary Institute, Copenhagen, Denmark; Clayton K. Nielsen, Cooperative Wildlife Research Laboratory and Department of Forestry, Southern Illinois University

ABSTRACT: Wildlife managers are often concerned whether disease transmission is density-dependent (DD, force of infection proportional to the density of infective animals [I]) or frequency-dependent (FD, force of infection proportional to the fraction of hosts that are infective [I/N]).  DD transmission ensures that disease will not deterministically drive its host to extinction, and allows the possibility of eliminating disease by holding host density below a threshold level.  In contrast, diseases with FD transmission could eradicate their hosts and are much harder to control by culling or other population reductions.  Empirical studies comparing measured values of force of infection to I and I/N in wildlife populations often point to transmission dynamics intermediate between DD and FD.  However, I and I/N change in parallel early in an epizootic, which greatly hampers the ability of researchers to reliably distinguish DD and FD dynamics using this "top-down" approach because inconclusive results are likely even if disease dynamics are strictly DD or FD.  Our research has focused on an alternative "bottom-up" approach that combines behavioral data with modeling to formulate mechanistic hypotheses for how specific behavioral patterns could alter disease dynamics, and then testing those hypotheses with focused empirical studies.  Using data from long-term studies of white-tailed deer (Odocoileus virginianus) social behavior and movement, we built a spatially explicit individual-based simulation and observed that the disease dynamics that emerged in the model were primarily density dependent.  However, that modeling exercise pointed to an information gap regarding how deer respond to loss of social group (either due to disease or management activities).  Another simulation model confirmed that that a strong "need" to be social after group loss (i.e., remnant animals always join other groups) could generate FD transmission.  A controlled experimental test showed that adult does responded very little to group removal, but young deer (ca. 9 months) expanded and shifted their home ranges, which could enhance opportunities for between-group disease transmission.  These findings encourage caution when interpreting tests of DD vs. FD early in an epizootic, and point to the benefits of focused behavioral study aimed at testing specific mechanistic hypotheses.