The effects of American chestnut mast on consumer population dynamics

Background/Question/Methods   American chestnut (Castanea dentata) was a foundation species of eastern deciduous forests, comprising between 25 and 50% of canopy trees throughout its historic range. The loss of the chestnut due to an introduced pathogen in the early 20th century likely reduced the production and stability of hard mast as food for wildlife. Using historical data to parameterize a logistic population growth model with stochastic carrying capacity, we compared population dynamics of mast consumers both pre-and post-blight. We explored the effect of three general characteristics in determining the response of populations to the loss of chestnut mast: maximum intrinsic population growth rate (λ_max), degree of density dependence, and reliance upon mast as a food source. We used published data to compare the effects of American chestnut on five mast consumers: Peromyscus leucopus (white-footed mouse), Tamias striatus (eastern chipmunk), Sciurus carolinensis (grey squirrel), Odocoileus virginianus (white-tailed deer), and Ursus americanus (black bear).

Results/Conclusions   Annual mast production decreased by an estimated 35%, and the coefficient of variation (CV) increased 60% post-blight. Simulated populations with greater intrinsic growth rate, density dependence and reliance on mast responded most to chestnut loss. For example, populations with a λ_max of 1.5 and high reliancee on mast experienced population declines of 67% post-blight while the CV nearly doubled, from 0.36 ± 0.02 to 0.64 ± 0.04. Increased variability also significantly increased the frequency with which the population dropped below 10% of the pre-blight mean: 0.02% to 9%. White-footed mice exhibited the greatest changes post-blight (CVpre: 0.41 ± 0.02; CVpost: 0.60 ± 0.03) followed by eastern chipmunks (CVpre: 0.29 ± 0.01; CVpost 0.38 ± 0.02). White-tailed deer and black bear were the least responsive to the loss of chestnut with no difference in CV pre- and post-blight. The loss of chestnut resulted in smaller, more variable mast crops, which translated to increased variability in small mammal consumer populations. Through its effect on small mammal consumers, the reintroduction of blight-resistant American chestnut hybrids may alter community and trophic-level interactions such as gypsy moth outbreaks and Lyme disease hot-spots.