Composition, if you remember, is the act of combining a variety of parts or elements to form a whole. As most people think of forest—which is strictly above ground—composition refers to the aboveground kinds of plants that compose its basic living parts. Structure is the configuration of those parts or elements, be it simple or complex. Structure can be thought of as the organization or arrangement of the plants that form the living foundation of a forest. Function, on the other hand, is what a particular structure either can do or allows to be done within the forest. So, the composition of a forest creates its structure, and the structure determines how the forest functions. Conversely, how the forest functions dictates its necessary structure, that in turn dictates the necessary composition.
To maintain ecological function means that the characteristics of the ecosystem must be maintained in such a way that its processes are sustainable. In other words, if you want large woodpeckers to live in your forest, you must have species of trees that not only grow large enough to accommodate them but also are allowed to grow large enough to accommodate them.
Pileated woodpeckers (among the largest in the forests of North America) require nest trees that are at least 22 inches (59 centimeters) in diameter (USDA Forest Service photograph) 5 feet (1.5 meters) above the ground. As well, their main diet is carpenter ants, which often live in the dead wood of live trees that must be large enough to accommodate their colonies, as attested by the holes a pileated woodpecker has drilled into the mainstream of ant life.
But, Nature's disturbance regimes, such as fires, floods, and windstorms, often act as environmental constraints because they control how succession proceeds and thereby alter habitat. True, we humans can tinker with disturbance regimes, such as the suppression of fire, but in the end our tinkering catches up with us and we pay the price—witness the "fire storms" that burned millions of acres of our national forests throughout the western United States in 2002 and 2003.1
In addition to tinkering with a disturbance regime, we can change the trajectory of an ecosystem, such as a forest, by altering the kinds and arrangement of plants within it through "management" practices because that composition is malleable to human desire and, being malleable, is negotiable within the context of cause and effect. At this juncture, it must be understood that composition is the determiner of the structure and function in that composition is the cause of the structure and function, rather than its effect.
By negotiating the composition, we simultaneously negotiate both the structure and function. Once the composition is ensconced, structure and function are set on a predetermined trajectory—unless, of course, the composition is drastically altered, at which time both the structure and function are altered accordingly. So, it is clearly the composition, structure, and function of a plant community that determines what kinds of animals can live there, how many, and for how long.
Both the red-legged frog and the long-toed salamander require aquatic habitats in order for their offspring to develop from eggs, through metamorphosis, into adults. Any event, natural or human-caused, that dries up their aquatic habitat—thereby shifting it to a terrestrial habitat—can spell the demise of the affected population.
If we change the plant composition of a forest (such as clear-cutting all the large old trees), we change the structure, hence the function, and that affects the animals. Under Nature's scenario, the animals are ultimately constrained by the composition because, once the composition is in place, the structure and its attendant functions operate as a unit in terms of the habitat opportunities and requirements for the various species of animals.
For a baby little brown bat to be born and raised within a forest, there must be such habitat components as old trees or snags with large, hollow areas in their upper portions; large, abandon woodpecker cavities; or large, loose slabs of bark that are sufficiently separated from the tree's trunk to allow the mother access, as well as room in which to give birth.
But then, people and Nature are continually manipulating the composition of plants, thereby changing the composition of the animals that are dependent on the structure and function of the resultant habitat. These manipulations in turn determine what uses humans can make out of the ecosystem.2 Therefore, in order to maintain or repair the biological health of a forest so it can produce the things we valued it for in the first place, we must figure out how such a forest functions and then work backward through the required structure to the necessary composition in order to achieve that outcome. To see how this scenario might work in practice, let's consider those forest insects that are deemed "pests" because they can disrupt the economic predictability of the expected timber harvest.
A fire-maintained "grassland" on a mountaintop in the Coast Range of Oregon, with clear-cuts and patches of forest in different stages of growth in the distance.
Before discussing "pests" insects in the context of forests, it needs to be understood that, since biblical times, most insects that in one way or another feed on plants we humans value for our own uses have been considered to have only negative effects on the resource, and so are thought of as "pests." On the other hand, insects are not considered pests if they feed on plants for which we find no social or economic value.
The term pest reflects this traditional bias and the perceived necessity of always having to battle them for control of the resources. Only within the past couple of decades or so has evidence emerged to show that many of the so-called "insect pests"—like all other species—enrich the world, and in the process provide largely unrecognized benefits to the forest, even during apparently destructive epidemics.
Patterns of vegetation across a forested landscape, especially those created over the centuries and millennia by fire or other major disturbance regimes, influence populations of insects in the following ways: (1) by establishing the spatial arrangement and degree of diversity exhibited in their sources of food, (2) controlling the quality, quantity, and distribution of the habitat for their predators, (3) influencing how insects move across a forested landscape, and (4) determine how insects alter their habitat. Not surprisingly, insects multiply and disperse much more effectively when suitable food plants are uniformly distributed; "suitable" in this sense means a given species of plant or a group of species of a certain age or size. In lush, rapidly growing, tropical rainforests, on the other hand, insect herbivory helps to created and maintain the number of available habitats.
The implications of "homogenizing" forested landscapes (simplifying a forest's composition, structure, and function) as related to insect activity are interesting and instructive. Taking a landscape of diverse, indigenous forest and homogenizing it through clear-cutting and the planting of single-species monocultural plantations has the effect of eliminating predators and physical barriers to insect dispersal, such as habitat diversity that is maintained through Nature's various interactive disturbance regimes. Loss of such habitat diversity increases both the survival of forest-damaging insects, such as bark beetles, and the likelihood of region-wide outbreaks.
The diversity of Nature's forest acts as a "habitat confusion factor," much like a buffet, which forces you to choose certain foods, not only from a multitude of options but also from a number of cultural cuisines. A monocultural, even-aged tree farm, on the other hand, offers one choice, which greatly simplifies—and thus increases—the opportunity for a given insect to find an abundance of food, reproduce, and begin to stress its food supply.
To illustrate, in the Cascade Mountains of Oregon, old forests (such as the one depicted above on the left) support far more predatory invertebrates (mainly spiders) because of their diversity of habitats than do "plantation forests" or even-aged stands (such as the one shown on the right). This difference raises the question of how much help foresters can expect from invertebrate predators that originate in diverse, old stands when it comes to controlling insects in homogeneous, even-aged stands. Although the consequences of reduced numbers and kinds of predators are impossible to predict with any certainty, it seems likely that severe epidemics of plantation-damaging insects will become increasingly frequent as diverse, indigenous forests are replaced by genetically controlled, monocultural, economically arranged "fiber farms."
How might this work? The success of plantation-damaging insects increases when the landscape is intersected with roads and managed for young, single-species monocultures of small trees. This modification of the forest makes it easier for the insects to find a suitable abundance of host trees by removing the confusion factors inherent in a diverse habitat and thereby reducing the time it takes the insects to locate food.
Simplification of the forest also reduces the diversity of habitats and the variety of species of prey necessary to maintain the populations of opportunistic predators, such as spiders and birds. These opportunistic predators are more important in preventing outbreaks of insects than are host-specific predators, such as parasitic wasps, that are dependent on finding a particular species of prey to exploit.3
Vegetation patterns within a forested landscape also influence the habitats available to birds and mammals that prey on defoliating insects. To clarify this point, let's examine the bird communities in coniferous forests of the Pacific Northwest because they are dominated by species that feed on insects in the foliage. Roughly 80 percent of the food consumed by northwestern birds is invertebrate prey, mostly foliage-feeding insects. It would cost about $1,800 per 1.5 square miles (2.4 kilometers) per year in insecticides (not counting the cost of aerial application) to kill the same number of spruce budworms (a larval moth) that are eaten by birds in the forests of north-central Washington, and this does not even count the predaceous ants that complement the birds. These insect-eating birds (such as warblers) and mammals (such as bats) depend on forests in mature to old-growth age classes for nesting and roosting. Where landscapes no longer contain the required habitat components, the numbers of these species are declining.
Hence, indigenous, mature to old-growth forests in the Pacific Northwest, with their complex array of species of both trees and predators, large size of stands, and high diversity of age classes are less vulnerable to epidemics of forest-damaging insects than are the simplified, exploitive plantations and even-aged stands. For all that, circumstances can be somewhat different in forests outside of the Pacific Northwest, such as occasional old-growth forests in the Rocky Mountains, where epicenters of insects build to epidemic proportions, yet the forest survives.
Can economic plantations—planted forests—and landscapes be designed in such a way that problems of forest-damaging insects are minimized? Yes, at least temporarily, but it will be neither simple nor easy because different insects respond differently to a given landscape pattern. A pattern that reduces problems with one insect may well create problems with another. Further, air pollution and the changing global climate will stress some forests and further stress some plantations. Such stress often translates into increased problems with forest-damaging insects and could, in time, change the plant-species composition, which presumably would alter the populations of insects and their predators.
In addition, global influences, beyond those mediated by climate, extend outward in unforeseen ways from a given forest. The most serious threat to insectivorous birds in the Pacific Northwest, for instance, is the loss of their winter habitat in Central and South America due to logging, as well as slash and burn agriculture—rather than the loss of summer habitat once they reach the Northwest. Roughly one half of the species of insectivorous songbirds in the Pacific Northwest spend the winters in tropical forests. The large-scale destruction of these forests lowers the numbers of insectivorous birds returning to temperate forests, where they breed, rear their young, and thereby help to control tree-damaging insects.4
In caretaking a forest, it must be recognized and accepted that insects, including those causing damage to trees, are natural components of the forest even though they can reach epidemic proportions. As such, they are necessary to its long-term health as part of a complex of organisms that forms the forest's interactive "immune system." The immune system includes diseases, parasites, and a variety of predators, such as spiders, bugs, beetles, flies, wasps, ants, birds, and bats, to name a few.
If these complex, interdependent feedback loops among plants and animals are gradually simplified, so too will be the cultural aspects of humanity, such as the forest-related jobs that once depended on the feedback loops. The species that composed the feedback loops will be lost—and the feedback loops with them, both ecological and cultural, which is how the evolutionary process works.
Ecologically, it is neither good nor bad, right nor wrong for these changes to occur, although they will most assuredly make the ecosystem less attractive and less usable by the humans who used to rely on it for their livelihoods and for products. If we and the generations to come are to have sustainable ecosystems over the next millennium, we have to think about the interrelationships of animals with plants, both in the context of one with the other and concurrently within the patterns we humans are creating across landscapes.
©Chris Maser 2009. All rights reserved.