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Ecological Chain Reactions Involving Acorns

Richard S. Ostfeld
Institute of Ecosystem Studies

PI: R. Ostfeld; coPIs: C. Jones (IES), E. Schauber (IES and University of Connecticut); J. Wolff (University of Memphis); and P. Turchin (University of Connecticut).

Oak dominated forests are characterized by periodic or episodic production of large acorn crops, punctuated by several consecutive years of poor acorn production. This phenomenon is called masting. Because acorns constitute a high quality food for many vertebrate consumers, masting behavior results in a flush of resources available to wildlife every 2-5 years. We have used both long-term monitoring and large-scale experimentation to determine the consequences of masting behavior to some aspects of the functioning of forest ecosystems. Our studies reveal that acorn production sets off an ecological chain reaction that affects the population dynamics of both gypsy moths (Lymantria dispar), an introduced forest pest that defoliates forests, and black-legged ticks (Ixodes scapularis), the vector of Lyme disease.

In the autumn of good acorn years, white-tailed deer (Odocoileus virginianus) specialize on acorns when they are available, and are attracted to oak-dominated forest patches. Autumn is also the peak activity period for the adult stage of the black-legged tick. Because deer are the preferred host for adult ticks, oak forests are the sites where these ticks take their terminal blood meal, drop off the deer, and lay eggs, which hatch the following summer. We have found, via both monitoring natural variation in acorn production, and by experimentally adding acorns to large forest plots, that the density of larval ticks hatching from eggs in summer is highly predictable on the basis of acorn availability the prior autumn. However, these ticks hatch from eggs free of the Lyme-disease spirochete, Borrelia burgdorferi; therefore, they cannot transmit Lyme disease to humans or other hosts. Larval ticks that feed on white-footed mice (Peromyscus leucopus) are much more likely to acquire the Lyme-disease spirochete than are ticks that feed on a variety of other vertebrate hosts. Therefore, the white-footed mouse is considered the principal natural reservoir for Lyme disease.

Heavy acorn production also increases the over-winter survival and reproductive rates of the white-footed mouse, another species that consumes acorns. Results of our monitoring and experimental studies demonstrate that population density of mice in summer is highly predictable on the basis of acorn production the prior autumn. Consequently, acorn production causes the coincidence in space (oak patches) and time (summer following masting) of high densities of larval ticks and the tick host most capable of infecting ticks with the disease agent. The Ostfeld lab is currently testing whether this results in higher density of infected nymphal ticks two summers following heavy acorn production.


White-footed mice also are known to be key predators on the pupal stage of the gypsy moth. Each summer we test the effectiveness of white-footed mice in depredating experimentally introduced populations of gypsy moth pupae. Abundant populations of mice are capable of destroying entire cohorts of gypsy moth pupae, resulting in the regulation of low-density moth populations. When mouse populations are either naturally scarce (due to low acorn production the prior year) or manipulated experimentally at low density, a considerable proportion of the moth population escapes from predation by mice, resulting in the potentially rapid growth of the moth population to outbreak levels. Therefore, acorns indirectly influence population dynamics of gypsy moths by affecting population density of predatory mice. Because defoliation of oaks by gypsy moths may prevent or delay mast production, the potential exists for a feedback loop from oak trees to acorn production to mice to gypsy moths to oak trees and acorn production.

Supported by NSF, NIH, and The General Reinsurance Corporation

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