Earlier, in the context of biogeography, there was a reference to animals "colonizing". This may sound like the behavior of humans only, but other animals are also capable of colonizing. Nor are humans the only creatures that crossed the Atlantic Ocean from the Old World to colonize parts of the New World. During the nineteenth century, an Old World bird species known as the cattle egret managed to cross the Atlantic, perhaps driven by a storm, and founded a breeding colony in Brazil. Since then, it has expanded the range of its habitats, so that cattle egret colonies can be found as far north and east as Ontario and as far south and west as southern Chile.
Colonization is one example of the phenomena studied within the realm of biogeography. Other examples involve islands, and, indeed, island biogeography is a significant subdiscipline. The central idea of island biogeography, a discipline developed in 1967 by American biologists R. H. MacArthur (1930-1972) and Edward O. Wilson (1929-), is that for any landmass a certain number of species can coexist in a state of equilibrium. The larger the size of the landmass, the larger the number of species. Thus, reasonably enough, a large island should have a great number of species, whereas a small one should support only a few species.
These principles have helped in the study of other "island" ecosystems that are not necessarily on islands but rather in or on isolated lakes, mountain ranges surrounded by deserts, and patches of forest left behind by clear-cut logging. As a result of such investigations, loggers in the forests of the Pacific Northwest or the Amazon valley have been encouraged to leave behind larger stands of trees in closer proximity to one another.
This makes possible the survival of species at higher trophic levels (positions on the food web) and of those with very specialized requirements as to food or habitat. Examples of the latter species include Amazonian monkeys and the northern spotted owl, which we discuss later. Because studies in island biogeography have made land-use planners more aware of the barriers posed by clear-cut foresting, it has become common practice to establish "forest corridors." These long, thin lines of trees connecting sections of forest ensure that one section will not be isolated completely from another.
In discussing succession earlier, it was noted that a disturbance usually sets the succession process in motion. Examples of such disturbances can include seismic events (earthquakes, tidal waves, or volcanic eruptions) and weather events (hurricanes or tornadoes). Across larger geologic timescales, the movement of glaciers or even of plates in Earth's crust (see Paleontology for more about plate tectonics and its effect on environments) can set succession processes in motion.
There are also causes directly within the biosphere, or the realm of all life, that can bring about disturbances. Among them are wildfires as well as sudden infestations of insects that act to defoliate, or remove the leaf cover from, a mature forest. Quite a few disturbances can result from activities on the part of the biosphere's most complex species: Homo sapiens. Humans can cause ecological disturbances by plowing up ground, by harvesting trees from forests, by bulldozing land for construction purposes—even by causing explosions on a military reservation or battlefield.
Disturbances can take place on a grand scale or a small scale. It is theoretically possible for disturbances—even man-made ones—to wipe out forests as large as the Ardennes in northwestern Europe or the Amazon rain forest in South America. Fortunately, neither shelling in the Ardennes during the world wars nor logging in the Amazon valley in the late twentieth century managed to destroy those biomes, but it is quite conceivable that they could have. On the other hand, a disturbance can affect an individual life-form, as when lightning strikes and kills a mature tree in a forest, creating a gap that will be filled through the growth of another tree—an example of microsuccession.
Once succession begins, it can take one of several courses. It may lead to the restoration of the ecosystem in a form similar to that which it took before the disturbance. Or, depending on environmental circumstances, a very different ecosystem may develop. For example, suppose that a forest fire has wiped out a biological community and secondary succession has begun. It is conceivable that this succession process will restore the forest to something approaching its former state. On the other hand, the wildfire itself may well have been a signal of a climate change, in this case, to a drier, warmer environment. In this instance, succession may bring about a community quite different from that which preceded the disturbance.
The "disturbance" itself actually may be the alleviation of a long-term environmental stress that has plagued the community. Suppose that a biological community has suffered from a local source of pollution, for instance, from a factory dumping toxins into the water supply. Suppose, too, that pressure from state or federal authorities finally forces a cleanup. How does this affect the biotic environment? In all likelihood, species that are sensitive to pollution (i.e., ones that normally could not survive in polluted conditions) would invade the area.
Removal of an environmental stress may not always be a matter of pollution and cleanup. For instance, a herd of cattle may be overgrazing a pasture, thus holding back the growth of plant species in the area. Imagine, then, that the cattle are moved elsewhere; as a result, new plant species will proliferate in the area, and, in all likelihood, the biological diversity of that particular ecosystem will increase.
As we noted earlier, primary succession occurs in an environment where there has never been a significant biological community or in the wake of
These statements should be qualified in two ways, however. When it is said that an area has not maintained a significant biological community, this refers only to the recent or relatively recent past. In the case of the parking lot area, there probably have been countless biological communities in that spot over the ages, each replaced by the other in a process of primary succession. Also, by significant biological community we mean a biological community that exists above ground; even in the instance of the parking lot, there would be an extensive biological community underground. (See The Biosphere for more about life in the soil.)
Glacier Bay, in southern Alaska, is an example of an ecosystem that experienced primary succession in the wake of deglaciation, or the melting of a glacier. The glaciers there have been melting for at least the past few hundred years, and as this melting began to occur, plants moved in. The first were mosses and lichens, flowering plants such as the river-beauty ( Epilobium latifolium ), and the mountain avens ( Dryas octopetala ), noted for their ability to "fix" or transform nitrogen into forms usable by the soil. (See The Biosphere for more about nitrogen fixing and biogeochemical cycles.)
These were the pioneer species, and over time they were replaced by larger plants, such as a short version of the willow. Later, taller shrubs, such as the alder (also a nitrogen-fixing species), dominated the area for about half a century. In time, Sitka spruce ( Picea sitchensis ), western hemlock ( Tsuga heterophylla ), and mountain hemlock ( T. mertensiana ) each had its turn as dominant plant species. With the last group, Glacier Bay reached climax, meaning that the dominant species are not those most tolerant of stresses associated with competition. The habitat thus has reached maturity, and access to resources is allocated as fully as it can be among the dominant species. Accompanying these changes have been changes in nonliving parts of the ecosystem as well, including the soil and its acidity.
When a disturbance has not been so intense or sweeping as to destroy all life within an ecosystem, regeneration may occur, bringing about secondary succession. But regeneration of existing species is not the only mechanism that makes secondary succession possible; invasions by new plant species typically augment the succession process. While much else changes in the environment of a secondary succession, the quality of the soil itself remains constant, as do other characteristics, such as climate.
Because it is rare for a disturbance to be powerful enough to obliterate all preexisting life-forms, secondary succession is much more common than primary succession. Examples of the type of disturbance that may serve as a precursor to secondary succession are windstorms, wild-fires, and defoliation brought about by insects—provided, of course, that the destruction caused by these phenomena is less than total. The same is true of most disturbances associated with human activities, such as the abandonment of agricultural lands and the harvesting of forests by cutting down trees for lumber or pulp.
In a forest of mixed species in the eastern United States, the dominant trees are a mixture of angiosperms and coniferous species (respectively, plants that reproduce by producing flowers and those that reproduce by producing cones bearing seeds), and there are plant species capable or surviving under the canopy, or "roof," provided by these trees. Suppose that the forest has been clear-cut. This means that most or all of the large trees have been removed, but the entire biological community has not been wiped out, since loggers typically would not bother to cut down smaller plants that are not in their way.
As soon as the clear-cutting is over, regeneration begins. One form that this takes is the formation of new sprouts from the stumps of the old angiosperms. These sprouts are likely to grow rapidly and then experience a process of self-thinning, in which only the hardiest shoots survive. Within half a century, a given tree will have only one to three mature stems growing from its stump.
At the same time, other species regenerate seemingly from nowhere, though actually they are growing from a "seed bank" buried in the forest floor, where trees have dropped countless seeds over the generations. Species such as the pin cherry ( Prunus pennsylvanica ) and red raspberry ( Rubus strigosus ) are particularly adept at regeneration in this form. Therefore, these species are likely to feature prominently in the forest during the first several decades of secondary succession.
On the other hand, some tree species simply do not survive clear-cutting, or at least not in large numbers; if they are to obtain a stake in the secondary succession, they must do so by a process of re-invasion. Such often happens in the case of coniferous trees. Other species may also invade when they have not previously been a part of the habitat, yet they enter now because the temporary conditions of resource availability and limited competition make the prospect for invasion attractive. A great number of species, from alders and white birch to various species of grasses, fit into this last category.
Plants are not the only organisms involved in secondary succession. In a mature forest of the type described, the dominant bird forms probably include species of warblers, vireos, thrushes, woodpeckers, and flycatchers. When clear-cutting occurs, however, these birds are likely to be replaced by an entirely new avian community—one composed of birds more suited to the immature habitat that follows a disturbance. As time passes, however, and the forest regenerates fully, the bird species of the mature forest re-invade and resume dominance, a process that may well take three to four decades.
Old-growth forests represent a climax ecosystem—one that has come to the end of its stages of succession. They are dominated by trees of advanced age (hence the name old-growth ), and the physical structure of these ecosystems is extraordinarily complex. In some places, the forest canopy is dense and layered, whereas in others it has gaps. Tree sizes vary enormously, and the forest is littered with the remains of dead trees.
An old-growth forest, by definition, takes a long time to develop. Not only must it have been free from human disturbance, but it also must have been spared various natural disturbances of the kind that we have mentioned, disturbances that bring about the conditions for succession. Typically, then, most old-growth forests are rain forests in tropical and temperate environments, where they are unlikely to suffer such stresses as drought and wildfire. Among North American old-growth forests are those of the United States Pacific Northwest as well as in adjoining regions of southwestern Canada.
These old-growth forests of North America are home to a bird that became well known in the 1980s and 1990s to environmentalists and their critics: the northern spotted owl, or Strix occidentalis caurina. A nonmigratory bird, the spotted owl has a breeding pattern such that it requires large tracts of old-growth, moist-to-wet conifer forest as its habitat. These are the spotted owl's environmental requirements, but given the potential economic value of old-growth forests in the region, the situation was bound to generate heated controversy as the needs of the spotted owl clashed with those of local humans.
On the one hand, environmentalists insisted that the spotted owl's existence would be threatened by logging, and, on the other, representatives of the logging industry and the local community maintained that prevention of logging in the old-growth forests would cost jobs and livelihoods. The question was not an easy one, pitting the interests of the environment against those of ordinary human beings. By the early 1990s the federal government had stepped in on the side of the environmentalists, having recognized the spotted owl as a threatened species under the terms of the U.S. Endangered Species Act of 1973. Even so, controversy over the spotted owl—and over the proper role of environmental, economic, and political concerns in such situations—continues.
Another concern raised by the logging of old-growth forests has been the need to preserve dead trees, which provide a habitat for woodpeckers and other varieties of species. This concern, too, has brought about conflict with loggers, who find that dead wood gets in the way of their work. Dead wood, after all, is an expression for something or someone that is not performing a useful function (as in, "We're removing all the dead wood from the team"), and to loggers this literal dead wood is nothing more than a nuisance.
Unfortunately, the United States logging industry typically has not pursued a strategy of attempting to manage old-growth forests as a renewable resource, which these forests could be, given enough time. Instead, logging companies—interested in immediate profits and not much else—have tended to treat old-growth forests as though they were more like coal mines, home of a nonrenewable resource. In this "mining" model of tree harvesting, the forest is allowed to experience a process of succession such that a younger, second-growth forest emerges. Over time, this might become an old-growth forest, but the need to turn a quick profit means that the forest likely will be cut down before that time comes.
The average citizen, who typically has no vested interest in the side of either the loggers or the environmentalists, might well find good and bad on both sides of the issue. Certainly, the image of radical environmentalists chaining themselves to trees is as distasteful as the idea of loggers removing valuable natural resources. There is also a class dimension to the struggle, since a person deeply concerned about environmental issues is probably someone from an economic level above mere survival. This results in another distasteful image: of upper-middle-class and upper-class environmentalists inhibiting the livelihood of working-class loggers.
On the other hand, as we have already suggested, the logging companies themselves are big business and hardly representative of the working class. Largely as a result of pressure from environmentalists, these companies have attempted to develop more environmentally responsible logging schemes under the framework of what is called new forestry. These practices involve leaving a forest largely intact and removing only certain trees. Many environmentalists contend, however, that even the new forestry disturbs the essential character of old-growth forests.
Browne, E. J. The Secular Ark: Studies in the History of Biogeography. New Haven: Yale University Press, 1983.
Cox, C. Barry, and Peter D. Moore. Biogeography: An Ecological and Evolutionary Approach. Malden, MA: Blackwell Science, 2000.
The Eastern Old Growth Clearinghouse (Web site). <http://www.old-growth.org/> .
Environmental Biology—Grasslands (Web site). <http://www.marietta.edu/~biol/102/grasslnd.html> .
Forestry: Ecosystems: Forest Succession. Saskatchewan Interactive (Web site). <http://interactive.usask.ca/skinteractive/modules/forestry/ecosystems/forest_succession.html> .
Introduction to Biogeography and Ecology: Plant Succession. Fundamentals of Physical Geography <http://www.geog.ouc.bc.ca/physgeog/contents/9i.html> .
Old-Growth Forests in the United States Pacific Northwest (Web site). <http://www.wri.org/biodiv/b011-btl.html> .
Reed, Willow. Succession: From Field to Forest. Hillside, NJ: Enslow Publishers, 1991.
Succession (Web site). <http://www.cpluhna.nau.edu/Biota/succession.htm> .