Spatial inheritance in a dynamic risk landscape: How magnitude, scale, and persistence of spatial heterogeneity in predator activity affects prey persistence

Background/Question/Methods

Spatial heterogeneity in predation risk can ameliorate impacts on prey populations, particularly for prey of generalists. Spatially heterogeneous risk implies the existence of refugia, and the spatial scale of those refugia and their persistence over time affect whether prey can avoid predation by aggregating therein. Our objective was to quantify and assess the impacts of the magnitude, spatial scale, and temporal persistence of heterogeneity in risk of predation by white-footed mice (Peromyscus leucopus), an abundant generalist predator, and short-distance dispersal on persistence of gypsy moths (Lymantria dispar). We used track plates to measure white-footed mouse activity at >170 trees in each of 3 forest plots in upstate New York during summers of 2003-2005. We quantified the mean and coefficient of variation of track activity among trees by fitting the beta-binomial distribution to data from each plot and study period, temporal persistence by disattenuated autocorrelation, and spatial scale by fitting exponential variograms. We also measured lifetime dispersal of gypsy moth caterpillars by curve-fitting to the decline in larvae under burlap bands on trees radiating out from isolated egg masses. Finally, we combined the spatiotemporal data on predation risk with the dispersal kernel for gypsy moths to estimate the spatial inheritability of risk between gypsy moth generations and potential for moth persistence.

Results/Conclusions

Mice were much less abundant in 2005 than the other 2 years, leading to lower overall track activity but higher coefficient of variation among trees. Mouse track activity at individual trees was positively autocorrelated between monthly study periods in 2003 and 2004, and even between the two years, whereas temporal autocorrelation in 2005 was much weaker. Track activity showed positive spatial autocorrelation over lag distances from ca. 30 to >1000 m. These findings indicate that mouse activity, and hence risk to their prey, varies substantially in space at spatial and temporal scales that appear responsive to mouse population dynamics. Gypsy moth larvae typically dispersed short distances, with median 30-50 m, within the scale of heterogeneity in mouse activity. Resulting estimates of spatial inheritability of risk imply that gypsy moths could persist despite abundant predators causing spatial mean fitness <1.