Where Do All the Nuts Go?
The relationship between oak trees and populations of acorn consuming animals

INTRODUCTION
Some tree species, such as oaks and hickories, produce large seed crops at irregular intervals of 3 to 12 years. Seed production in these species is often synchronous over large regions (Koenig et al. 1996). This type of reproductive strategy is called masting, and though efforts have been made to determine the evolutionary significance of masting, no one theory has yet been able to account for this phenomenon (Janzen 1969, Silvertown 1980, Sork 1993).
Oak-hickory forests dominate much of eastern North America and their nuts provide an abundant, but unpredictable food source for seed-eating animals. Nut crops may provide an inexhaustible food supply one year, but no food the following year. Many birds and mammals utilize mast, but the degree to which their population biology is affected by mast availability varies greatly (Christisen and Korschgen 1955, Bock and Lepthien 1976, Ostfeld et al. 1996, Smith and Scarlett 1987). For some, a poor mast crop simply means that the animal must use other food items more than they would if there were a large mast crop (Harlow et al. 1975, Williams and Batzli 1979a). Others rely heavily on mast as a winter food source, and a poor mast crop may lead to reduced survival and perhaps poor reproductive performance (Koenig and Mumme 1987, Wolff 1996).
In turn, birds and mammals play a large role in the reproductive success of mast producing trees. Birds, mammals and insects eat a large proportion of the mast produced by a tree. Although most birds and mammals are seed predators, some cache seeds and are effective at dispersing seeds to locations favorable for germination. Thus, the relationship between mast producing trees and the animals that utilize mast is reciprocal and is an important force driving the evolution of species within the oak-hickory forest ecosystem.


INFLUENCE OF MASTING ON BIRD AND MAMMAL POPULATIONS

Specialists vs. Generalists
Population fluctuations are generally greatest for species that are most specialized on mast (Watts 1969, Smith 1986, Smith and Scarlett 1987). Species that depend almost completely on mast are rare. Many birds and mammals show higher survivorship and reproduction in mast years, but are not completely dependent on mast.  Mice (Peromyscus sp.), chipmunks (Tamias striatus), red squirrels (Tamiasciurus hudsonicus), and white-tailed deer (Odocoileus virginianus) show population increases the year after a large seed crop, but may maintain populations at moderate densities in non-mast years (Harlow et al. 1975, Rusch and Reeder 1978, Ostfeld et al. 1996, Wolff 1996).
Although many animals utilize mast extensively when it is available, some are able to include higher proportions of other food items in their diet when mast crops fail. Crows (Corvus brachyrhynchos) and raccoons (Procyon lotor) are true generalists and utilize a wide variety of food items (Christisen and Korschgen 1955). Although these animals use acorns significantly when available, they are not likely to suffer greatly in years of low acorn abundance due to the diversity of their diet.
Animal mobility
Some species get around the difficulties of poor mast years by tracking mast crops. In general, birds are more mobile than mammals and can move long distances when faced with a bad mast year. Mammals are less mobile, and their populations often fluctuate with the number of seeds produced by masting trees. Mammals such as deer, squirrels, and deer mice will move from one habitat to another to find more abundant food (Gashwiler 1979, Harlow et al. 1975, Ostfeld et al. 1996), but cannot track mast crops over vast areas like birds (Smith and Balda 1979).
Research Question
My hypothesis is that the response of any particular mast-eating species to fluctuations in mast abundance will be determined by the species’ degree of specialization on mast and the relative mobility of the species. To test this hypothesis I will answer the following questions:
1. Do mobile species have better survivorship and reproduction in years of low mast abundance than less mobile species?
2. Are species that include a high proportion of mast in their winter diet more likely to show poor survival and reproduction after a poor mast?
3. Do mast-eating species switch to soft mast (fruit) in years of poor mast abundance?

 Acorn Production and Winter Reproduction in White-footed Mice (Peromyscus leucopus)
From 1999 to 2003 I conducted a study of white-footed mice  to determine how population size and demography fluctuate in relation to acorn abundance. Previous work in this area has been done in the northern US, Canada or in the Appalachian Mountains of Virginia where it was found that mouse populations increase in years following large acorn crops. One question was whether or not mice in the milder climate of the Carolina Piedmont would show the same pattern. My study did not show a dramatic effect of mast abundance on population size, though there was some decline in years of low acorn abundance. The most notable result of the study was that Peromyscus bred during the winter, even in low-acorn years. In northern and Appalachian populations, Peromyscus only bred in winter after a large acorn crop resulting in a large population surge in spring and early summer, often followed by a population decline or crash. Low acorn abundance did not have the same affect on Piedmont populations of mice.

Scarlett, T. L. 2004. Acorn Production and Winter Reproduction in White-footed Mice (Peromyscus leucopus) in a Southern Piedmont Forest. Southeastern Naturalist 3:483-494 


 

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Last updated August 1,  2009