Mast producing plants play critical roles in forested ecosystems. These plants regularly produce large crops - often in synchrony with other species - followed by years of reduced crop size, and sporadically produce an enormous crop followed by one or more years with an almost absent crop. The 2007 crop of acorns was huge, while in 2008 many regions of the United States have seen a total lack of acorns.
Seed Consumers Fluctuate With Food Supply
Seed consumer populations fluctuate much as the mast crop does, but the fluctuation is delayed because the consumers cannot increase until they reproduce. After years of low seed production, seed predators' population sizes drop to such a low level that when a heavy mast year arrives, the superabundance of seeds saturates the consumers' abilities to eat the seeds. In these years, the trees produce enough seedlings to replace the few mature trees that die each year.
- Weevils (Curculonids) usually lay many eggs on an acorn and the larvae eat the whole seed and embryo. With a large acorn crop, fewer eggs are laid on each acorn and the larvae only partially consume the cotyledons of acorns. The competition between larvae is reduced and more larvae survive.
- Rodents (mice, squirrels, and chipmunks) and
- birds (jays, turkeys, grouse, quail, and some woodpeckers) that rely on seeds as a food source, are more fit to reproduce and produce larger numbers of offspring in the next summer.
In years of low seed output after a heavy mast year, the abundance of seed eaters is reduced through two processes:
- not being able to find enough to eat, and
- increased reproduction of predators: insectivores, foxes, hawks, and owls.
Other Animals are Affected by Mast Cropping.
After a heavy mast crop stimulates increased numbers of seed consumers, the populations of insectivores, foxes, owls, and hawks increases – usually in the second year following the heavy crop. As these carnivores gorge on the abundance of seed eating prey, their ability to produce offspring increases, and after a summer of feasting, their reproductive output is boosted. The abundance of prey is only short-lived as the increased pressure placed on the prey by their predators and the reduction in seeds from the trees squeezes the seed consumers and rapidly reduces their abundance.
For similar reasons, the abundance of parasites of seed predators also cycles. Some of these parasites are the ticks that carry Lyme and associated diseases. Where 2007 was a boom year for acorns, 2008 will be a boom year for the mice that feed on acorns, and 2009 and 2010 are projected to be boom years for the ticks that feed on these mice. It would be prudent to consume more garlic to repel ticks (it works better for most people than over-the-counter insect repellents), dress to reduce the number of ticks, and conscientiously check yourself for these insidious parasites after each out of doors session.
Acorn densities also affect the predators of oak trees. Gypsy moths feed on the foliage of many tree species including oaks. When acorns are plentiful, mice increase dramatically in numbers and feed in the late winter and spring on gypsy moth pupae, reducing their numbers. As the mice populations are reduced by increased predation and reduced seed output, their effect on gypsy moth pupae is drastically reduced and the moths can undergo a population explosion that can defoliate the trees. This explosion of gypsy moths subsequently enhances the reproduction of mice and moth populations again decline. Chart 1 (Picture 2) shows that increases in mouse populations are followed by gypsy moth declines and mouse declines are followed by larger gypsy moth populations.
Thus, mast cropping affects organisms that do not feed on plants as well as the ability of trees to reproduce and the animals that eat the seeds. This cascading of effects on a wide variety of organisms shows how important a single species is in the web of life and should remind us humans to disturb the system as little as possible when going about our lives.
For more information about acorns, click here.
