Conservation and Science at the National Zoo
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| Acorns of a white oak. (Paul Wray/Iowa State University) |
Consider the acorn. Compared with other seeds found in the forest, it's big, tasty, and nutritious. Because it has a hard cover, it can survive on the forest floor or in a hidden cache for months without rotting. It is up to 25 percent fat and, with the exception of the cap and the outer covering, easily digested by most animals. In a good year, hundreds of thousands of acorns can rain down on an acre of oak forest in the eastern United States.
Is it any wonder that acorns are the most important food source for animals in the eastern deciduous forest?
Oaks (Quercus spp.) are the only abundant and widespread trees in North America that produce such useful seeds. American chestnuts (Castanea dentata) were once as plentiful as oaks, and produced a nut that was valuable to humans and wildlife. Unfortunately, virtually every mature chestnut in North America was killed between 1920 and 1950 by a fungus (Cryphonectria parasitica) introduced from Asia. Beech trees (Fagus grandifolia) produce nutritious nuts, but are diminishing due to an introduced beetle and fungus. The nuts of hickory trees (Carya spp.) are too tough to be useful to any animals save sharp-toothed rodents and bears. North American wildlife that evolved to eat acorns, chestnuts, and beechnuts now rely primarily on acorns.
Fossil pollen records indicate that oak forests have flourished in North America for about 10,000 years. Today oaks are common everywhere except some areas in the northern Rocky Mountain states. More than 50 species live in the United States, and diversity increases southward; Mexico is home to more than 100 species of oak.
Unfortunately, our oak forests are in trouble. Although mature oaks still dominate many forests, the percentage of young oaks in the understory is declining rapidly. According to the Forest Inventory and Analysis program of the U.S. Department of Agriculture's Forest Service, the percentage of oaks in the understory declined from 32 percent to 21 percent from 1989 to 2000. Although acorns are plentiful in forests today, wildlife biologists and foresters are concerned they might not be so abundant in the future.
"If we want large wildlife populations, we need to have a hard seed crop," says Bill McShea, a research scientist in the Smithsonian National Zoo's Center for Species Conservation. "Acorns are an essential food for them in the winter."
Dozens of species of birds and mammals in eastern forests depend on acorns, including black bear, white-tailed deer, white-footed deermouse, squirrel, turkey, blue jay, red-headed woodpecker, and ruffed grouse. The health of these and other animal populations is tied to the number of acorns produced each year.
Oaks are "masting" trees, meaning that in some years they produce prodigious amounts of seed and in others, they produce virtually none. The abundant years are called "mast" years, or "good mast" years. Oaks of one species in a localized area are often synchronized, producing many or few acorns in lockstep. Scientists haven't worked out exactly how this happens, but in many cases, weather plays a role. For example, a late frost might kill the flowers on all the oaks of a particular species in a particular area. (Chestnuts produced seeds more regularly, partly because they flowered in June, after the threat of frost had passed.)
In forests with more than one oak species, the number of acorns varies from year to year; some species take one year to go from flower to acorn while others take two, and each species produces a different amount of acorns. The effect of weather on acorn production also varies with each species. But every so often, most or all of the oak species in an area produce either a bumper crop or a paltry crop at the same time. The effect of all this variation on wildlife is significant.
Take black bears, for example. In Great Smoky Mountains National Park in North Carolina and Tennessee, and in Shenandoah National Park in Virginia, acorns are the bears' most important fall food. Before spending the winter sleeping and fasting in dens, which are often in big old oak trees, the bears fatten up on acorns. Females give birth in these winter dens, and if the acorns are abundant in the fall, they gain more weight and are more likely to produce cubs that survive. In years of few acorns, females either fail to give birth, or their cubs die soon after birth.
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| In the autumn in some parts of North America, black bears rely on acorn crops. (Steve Pfiffer/Coldwell Banker, U.S.) |
In mountainous areas that are not protected within national parks, bears have a more varied diet that includes berries and fruit trees, which are less common in deep forest habitats, and food from human sources such as garbage and agricultural crops. However, these bears' reproductive success is still tied to acorns, according to Gordon Warburton, a supervising wildlife biologist with the North Carolina Wildlife Resources Commission who studies bears in the mountains of western North Carolina. "We do see an increased failure rate in reproduction even outside the park in poor mast years," he says.
Because black bears are omnivorous, not all populations depend as heavily on acorns. Those living in the coastal plain of North Carolina, for example, fatten up in the summer and fall on crops such as corn, while those that live in coastal Virginia primarily eat inkberries (Ilex glabra). But for those living in oak forests, acorns are so crucial that they can cause synchrony in bear cub production. "During a bad mast year, nobody reproduces. In a good year, everybody reproduces," says Warburton.
Acorn production also strongly influences movement—and mortality—of adult bears. When an abundant acorn crop falls on the forest floor, bears don't cover large territories, because they don't have to. "It's like a big buffet," Warburton says. Because they aren't traveling, bears aren't as likely to get hit by cars, wander into people's backyards, or be spotted by hunters. They are eating more and exercising less than usual, so they go into their dens with plenty of fat and come out in spring with fat reserves to spare.
"The complete opposite occurs during a bad year," Warburton says. "Bears are traveling more and burning more energy. They are running into cars and getting hurt; causing more nuisance complaints; and getting into the den in poor shape."
Acorns influence movements of white-tailed deer as well, but in a different way. During the autumn of a good mast year deer spend more time in oak forests, according to McShea. During the autumn of a bad mast year, deer are more likely to inhabit other types of forests—maple forests, for example—where they may eat small plants and seeds.
Many rodents eat acorns, and because they have short life cycles, they are particularly responsive to variations in acorn abundance. "The number of mice and chipmunks and squirrels you have one year is a function of how many acorns you had the previous fall," says McShea. Long-term studies in Virginia, Maine, New Zealand, and Europe have repeatedly shown this relationship, with various species of seed-producing trees and rodents. In eastern forests in the United States, it occurs between the white-footed deermouse (Peromyscus leucopus) and oak trees. In years with many acorns, white-footed deermice begin breeding about a month earlier than in other years, which allows them to produce more offspring.
Rodents play important roles as both prey and predators in the lives of many other forest animals, so acorn abundance has wide-ranging ramifications in the forest ecosystem. For example, a good mast year is bad for ground-nesting birds such as the ovenbird (Seiurus aurocapillus), wood thrush (Hylocichla mustelina), and veery (Catharus fuscescens). Populations of white-footed deermice and eastern chipmunks (Tamias striatus) boom when acorns are plentiful, but by late spring, the acorns are long gone and the multitudes of rodents are hungry. "That's when they attack the eggs," says Rick Ostfeld, senior scientist at the Institute of Ecosystem Studies in Millbrook, New York. "When there are a lot of rodents, the nesting success of these ground-nesting birds is very low, so the following year bird abundance is low." Ostfeld and his colleagues have demonstrated this relationship in New York, and McShea and his colleagues have demonstrated it in Virginia.
Intriguingly, a poor mast year is also bad for ground-nesting birds. "When there are very few rodents in the system, we find the birds' nesting success is high, but there are about equally few birds the following year as when there are lots of mice," Ostfeld says. While he and his colleagues don't have direct evidence of the mechanism, he says, they believe that raptors such as hawks and owls eat more birds—both fledglings and adults—when they have a hard time finding rodents. "It's sort of the Goldilocks effect," he says. "The number of acorns or mice has to be just right."
White-footed deermice are players in another interesting ecological web, this one involving gypsy moths (Lymantria dispar) and acorns. Gypsy moths, which are native to Europe, arrived in Boston in 1868. Ever since, they have been spreading southward (to the Carolinas) and westward (to Wisconsin), but slowly, because female gypsy moths cannot fly. Their populations fluctuate quite a bit: Years of low to moderate populations are occasionally punctuated by a dramatic population increase, or "outbreak," lasting two or three years. During outbreaks, the moths can defoliate large tracts of forest, drastically decreasing acorn production in the area.
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| White-footed deermouse. (Charles H. Warren) |
While variations in gypsy moth populations are most likely caused by the interplay between several factors, white-footed deermice do play a role. Ostfeld and other ecologists have found that white-footed deermice are the main predators of gypsy moth pupae, which can be found in the summer on the forest floor or low to the ground on tree trunks. In natural areas where white-footed deermice were experimentally removed and kept out with barriers, many more pupae survived, and there were 35 times as many gypsy moth egg masses the following year as compared with similar control plots that contained mice. (Ostfeld had to hire extra field assistants to help destroy the egg masses so as not to cause a localized gypsy moth outbreak.) Ostfeld believes that in years of average to high mouse density, the mice keep the gypsy moths in check. In years with few acorns and few mice, gypsy moths experience rapid population growth. In theory, a positive cycle may exist in which more gypsy moths leads to more defoliated oaks, which leads to fewer acorns, which in turn leads to fewer mice and more gypsy moths, and so on. But this loop has not been quantitatively observed and recorded.
Ostfeld also studies an ecological system involving acorns, mice, deer, ticks, and Lyme disease. People contract Lyme disease when they are bitten by a tick infected with the bacterium Borrelia burgdorferi. In the northeastern United States, and as far south as the Carolinas, that tick is likely to be the black-legged tick (Ixodes scapularis), which has four stages in its two-year life cycle: egg, larva, nymph, and adult. When eggs hatch in the spring or summer, the larvae emerge without the bacterium, but if they acquire it as larvae, they can pass it on as nymphs or adults.
Tick larvae feed on a variety of vertebrates including lizards, birds, and mammals. A larva needs only one blood meal, which takes two to three days to consume. Afterward, it falls to the ground and molts into a nymph. The following spring or summer, the nymph becomes active and takes one blood meal from any of a wide variety of hosts, then molts into an adult tick. In the autumn of the same year, the adult takes one three- to four-day meal. The host is usually a white-tailed deer, but may also be another mammal such as a raccoon, coyote, or human. After feeding, the tick falls to the ground. Female ticks lay eggs the following spring before dying.
People usually get Lyme disease from nymphs rather than from larvae or adults, because larvae rarely carry the bacterium and adults prefer to feed on deer. Furthermore, adult ticks are larger (2 to 3 mm) than nymphs (1 mm) and therefore more likely to be spotted and removed by humans before the infection is transmitted.
Ostfeld and his colleagues tested various animals in forested areas of New York to determine which ones were carrying the Lyme disease bacterium and passing it on to tick larvae. They found that 92 percent of tick larvae that fed on wild-caught white-footed deermice became infected with the bacterium. That means the white-footed deermouse is a good reservoir species—it carries the bacterium without showing negative symptoms—of Lyme disease. In comparison, slightly more than half of larvae that fed on wild-caught chipmunks became infected, 15 percent of larvae that fed on squirrels became infected, and only about three percent of larvae that fed on opossums became infected.
White-footed deermice are not only extremely likely to carry the Lyme disease bacterium; they are also "permissive" hosts because they typically allow tick larvae to get a full blood meal without grooming them off. There are so many mice in a typical forest that collectively, they host as many tick larvae, if not more, than other species, even those that host more larvae per individual. Ostfeld found that the average white-footed deermouse carries 29 tick larvae, the average chipmunk 36, the average squirrel 142, and the average opossum 254. Based on the field work of Ostfeld and his colleagues, then, the most common way people get Lyme disease (at least in the forests of New York) is by serving as a host for a nymph that fed on an infected white-footed deermouse as a larva the previous summer.
Contrary to popular belief, deer very rarely transmit the Lyme disease bacterium to ticks. When it was first discovered in the 1970s, scientists studied the ecology of Lyme disease on islands in Long Island Sound. They found what they thought was a new species of tick, and because they observed it feeding mostly on deer, they called it the deer tick. Later that tick was identified as a northern population of the black-legged tick, but the colloquial term "deer tick" had already caught on. Furthermore, the ecological relationships on the islands were not typical elsewhere. "When studied in mainland sites," Ostfeld says, "things were a lot more complicated and adult ticks were not associated exclusively with deer. It shouldn't be called a deer tick."
Deer do, however, play a role in transporting black-legged ticks. Ticks can't cover much ground on their own; they go where their hosts go. And where deer go is strongly influenced by acorns.
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| This black-legged tick nymph can transmit the Lyme disease bacterium to humans. (Scott Bauer/ARS) |
In an autumn of acorn abundance, deer spend more time in oak forests, bringing adult ticks with them. Tick eggs hatch the following spring into a forest filled with large populations of white-footed deermice produced by the previous fall's acorns. A high proportion of the larvae feed on the mice and become infected with the Lyme disease bacterium; the following summer, as nymphs, they infect more mice, people, and other vertebrates. In other words, the risk of contracting Lyme disease in and around oak forests in the northeastern United States is higher two summers after a bumper crop of acorns.
Ostfeld and his colleagues confirmed this in a study published in the journal Ecological Applications in 2005. "What we found for Dutchess County [New York]—and this extended over to Connecticut as well—was an association between the actual per capita incidence of Lyme disease and acorn production two falls previous or the mouse population one summer previous," Ostfeld says.
There is also a connection between the presence of white-tailed deer and the occurrence of Lyme disease, but it's not a one-to-one relationship. "There's some low threshold of deer abundance that you need to get a burgeoning tick population, but once you've exceeded that any increase in deer abundance has little effect on ticks," Ostfeld says. "A single deer can feed hundreds to thousands of ticks in a given season." An intriguing study published in the journal Ecology in 2006 indicates that excluding deer from areas smaller than five acres does not decrease the number of ticks in that area, and may in fact increase it.
What's probably more important than the size of the deer population in controlling Lyme disease is the overall biodiversity of the area. Mice are generalists that can flourish in all kinds of habitats, whether degraded or relatively undisturbed. In degraded habitats without much biodiversity, there are many mice and few other species. "In a high-diversity community you have fewer mice and more other hosts," Ostfeld says, "and every one of the other hosts is a less competent reservoir for the bacterium." Other hosts are more likely to remove and kill tick larvae that try to feed on them. "Diversity plays a protective role in enticing ticks away from mice," Ostfeld says. There is also some evidence that West Nile virus and hanta virus are more easily transferred to humans in areas of low biodiversity.
Biodiversity among animals in eastern deciduous forests depends on oak trees and acorns. The Forest Inventory Analysis shows that maples (Acer spp.) are increasing in abundance while oaks are declining. "Maples are not producing seeds that are useful for very many things at all," McShea says.
For the past 10,000 years or so, the forests of America have for the most part been made up of the same species that we see today, but in varying combinations and proportions depending on changes in climate and other ecological factors. For various reasons, conditions today favor maple over oak.
Oaks don't require full sun, but neither do they prosper in full shade. Bill Healy, a retired research wildlife biologist with the U.S. Forest Service in Amherst, Massachusetts, says, "Because acorns are such big seeds, they can germinate in low light levels and live for a year or two, but to make a positive growth they need 50 to 75 percent light, so they need a stand that is pretty open and that has been disturbed." That "disturbance" can be caused by fire, tornadoes, hurricanes, ice storms, insect infestation, or even selective and targeted logging.
Native Americans used fire purposefully and regularly to open up forests for hunting, agriculture, travel, and to encourage growth of plants they found useful. Forest fires give oaks several advantages over other trees. Big old oak trees, for example, have thick bark that can often ward off a small fire's damage. Even if an oak dies back, it sends up new sprouts. Unlike maple seedlings, oak seedlings can resprout after a fire because they have a more developed root system. And repeated fires make for a sunnier, drier, more open forest, which suits oaks better than other trees.
Due to years of fire suppression, most of today's forests contain relatively dense understories of brush and young trees. Maples and yellow poplars (Liriodendron tulipifera) can sprout and grow in these conditions, but oaks are not as successful in the deep shade.
Fire suppression is not the only challenge facing oaks. They are also being hurt by browsing of seedlings by deer, defoliation by gypsy moths, logging for timber, acid precipitation, urban and suburban development, oak wilt (caused by the fungus Ceratocystis fagacearum), and sudden oak death (caused by the pathogen Phytophthora ramorum).
McShea says, "Maybe we're talking about oaks declining a few percentage [points] a year. We're not at a critical stage, but because things take so long…in forestry, and because oaks take 30 to 40 years to produce their first acorn, if you want to turn things around you've got to start now." McShea, Healy, and many other foresters and wildlife biologists are recommending that oak forests be managed to encourage new oak growth, which might involve prescribed burns and selected cutting. "The idea of putting a fence around this and letting nature take its course isn't going to work for oaks," McShea says. "It's going to take active management."
Before Native Americans began burning forests, natural fires and periods of warmer, drier weather allowed oaks to survive. But paleoecological evidence suggests that oak forest really began to prosper and spread between 5,000 and 7,000 years ago, when Native Americans began burning regularly.
"This world here in the East is manmade," McShea says. "We made the forests where the forests are. We made the fields where the fields are. We've gotten rid of so many species already—the passenger pigeon, American chestnut, wolves, bison. What's left is something that is a managed world."
Healy says that although the composition of America's forests has fluctuated naturally for thousands of years, what's going on now is different because the chestnut is gone and the beech is fading. "Why worry about the change?" he asks. "If the oak forest goes to maple and birch and yellow poplar, why does it matter? It matters because the wildlife that we enjoy in eastern forests, that whole vast community, is dependent on hard tree seeds, and that's been true for 10,000 years."
—Mary-Russell Roberson is a contributing editor to ZooGoer.
ZooGoer 36(4) 2007. Copyright 2007 Friends of the National Zoo.
All rights reserved.
