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Community Ecology INTRODUCTION Every species population is part of an assemblage of species-plants, animals, and microorganisms-that share space and interact. We speak of this group of interacting organisms as an-"ecological community." It is difficult to define that term precisely, for two reasons. First, whether communities are discrete entities with higher-order "emergent" properties (not readily derivable from an analysis of constituent species) or simply groups of species from the available pool is controversial (Krebs, 1985; Simberloff, in press). Second, whether sympatric species (occupying the same area) should be considered parts of a community if they do not interact with many of the other species present is debatable. The debate, in part, is over the strengths of interactions and how they contribute to community structure. Many environmental problems arise because the alteration of some of these interactions leads to new arrangements of species populations that, from a human perspective, are less desirable than the former ones. Because the population dynamics of species depend on the kind and intensity of their interactions with other species- species that prey on them and compete with them and on which they prey a knowledge of these interactions is often valuable in managing individual species. Chapter 2 discusses population interactions as though they are generally indepen- dent of each other. Yet we know that each species interacts with many others and that the interactions are often indirect. For example, grazers 47
48 KINDS OF ECOLOGIC KNOWLEDGE ED THEIR PLACATIONS depend for food on the productivity of grasses, whose growth depends on the activity of earthworms in the soil, which can be affected by the addition of toxic materials to the soil. Human manipulations of the environment can therefore affect individual species, not only directly, but also in in- direct ways that can be understood only in the context of the functional structure of the whole community (Davidson et al., 19841. Ecological communities have numerous properties that transcend those of their constituent species and that require study themselves trophic structure, rates of energy and nutrient flow, growth form and physical structure, number of species and their numerical distribution, stability characteristics, and ecotones (zones of transition between two habitat types), to name a few. The composition of a community changes in space and time as a result of physical and biological processes. This variability makes it difficult to detect changes caused by human intervention, even if long-term data are available. Communities are usually named for the commonest or "most impor- tant" kinds of organisms found in them. Thus, we speak of "chaparral communities" and "blue mussel communities." The organisms chosen to identify communities are usually the ones that provide physical structure for them, such as plants in terrestrial environments, or that are the bases of food chains in systems lacking fixed structural elements, such as plank ton in the ocean. In addition to its value in managing individual species, a knowledge of community ecology is essential when the object is to manage communities themselves. For example, we often wish to maintain a diversity of or- ganisms in an area, as mandated by law in the U.S. national forests (Chapter 15), or to maintain a particular set of species together, as in parks or agricultural areas. Restoring degraded habitats requires not only a knowledge of the physical conditions that favor the growth of individual species, but also an understanding of how the species interact under dif- ferent conditions (Chapter 18~. We might need to know how the intro- duction of a species can alter community makeup or how well a new species can substitute for another in maintaining community stability. Or we might wish to know how much disturbance a community can tolerate before undergoing important compositional change. SPECIES COMPOSITION The species composition of a community the number and kinds of species present is one of its most obvious features. A knowledgeable ecologist can deduce a great deal about environmental conditions in an area simply by looking at a site and inspecting a list of species present
COMMUNITY ECOLOGY 49 (Dayton, in press). Systems under the influence of strong perturbations typically show reductions in the number of species that are numerically dominant. For example, continued eutrophication of lakes usually leads to an excess of some nutrients, a deterioration in water clarity due to algal proliferation, and a reduction in available oxygen due to increased rates of decay. Species that depend on clear water for foraging or on high oxygen content for respiration disappear from the lakes and are replaced by species that can use the increase in nutrients under relatively anoxic conditions. In Lake Washington (Chapter 20), continued addition of sew- age led to the appearance of the blue-green alga, Oscillatoria rubescens, now known to be characteristic of severe eutrophication; when the sewage was reduced, Oscillatoria disappeared and later the zooplankton com- munity changed. The presence or absence of an indicator species (Chapter 7) is in itself, however, not always a reliable sign of conditions resulting from human- induced perturbations. Species "typical" of some communities are not invariably present. Local populations can die out for various reasons, including disease, high predation rates, and unusually harsh weather. A species might have failed to colonize an area because of its isolation. Consequently, reliable use of species as indicators of perturbations requires knowledge of their distribution under "normal" conditions and knowledge of past conditions at the site. In addition, knowledge of the composition of an entire community can substantially improve the usefulness of the indicator approach. Patrick et al. (1967), for example, related a variety of stream pollutants to changes in the relative abundance of groups of algal species that had different tolerances to the pollutants. The number of species in a community (species richness) often changes in response to disturbance, and species richness has been used as an indicator of disturbance. Some types of stream pollution simplify the stream environment and reduce the number of available niches; others kill off many species outright (Patrick et al., 19671. But moderate disturbance can produce less clear-cut effects, and some perturbations can alter the relative abundance of species without changing the number of species present (Dickman, 1968~. More sophisticated measures and indexes of community composition weight the number of species with the relative abundance or biomass of each (Krebs, 19851. A diversity index increases both as the number of species increases and as the numerical distribution of species becomes more even. A large environmental change often leads to local extinction of many sensitive species and to the predominance of a few "disturbance- tolerant" organisms or organisms capable of using the new conditions for increased growth, so diversity indexes have been used as measures of
50 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS disturbance in a community. Such uses of diversity indexes have been controversial, because they have been applied with little regard for the functional changes that occur in disturbed ecosystems. Many ecologists hoped that diversity measures would capture key changes in fundamental community processes and thus obviate more detailed anal- yses of species composition. These hopes have been largely unrealized, and there is increasing recognition that the most important information is often discarded in calculating these indexes (May, 19851. A diversity index should be used with caution, for several reasons: · Many factors other than the disturbance of concern can cause a change in the index. This problem is particularly acute when communities in different areas are compared only once. Site differences in physical and biological factors nutrient availability, presence or absence of key spe- cies, climatic differences, etc. can cause differences in diversity. · The species present in a community can change substantially without any significant change in diversity indexes. · Some disturbances can increase diversity if they increase habitat heterogeneity, reduce the influence of competitively dominant species, or create opportunities for new species to invade (discussed below). Most of the complexities of the processes that change diversity are not captured in diversity indexes, which are appropriately used only when we are confident that they reflect the behavior of the system being measured. However, because comparison of long species lists from several com- munities is cumbersome and trends can be difficult to communicate with such lists, changes in diversity indexes within a community can be used to capture the most salient features of that community. The complete lists of species are still needed, but they need not always be presented in full. FACTORS AFFECTING SPECIES DIVERSITY Several processes that contribute to change in the number of species in a community can be of concern when the goal of management is to preserve or increase diversity. A primary determinant of species diversity is en- vironmental heterogeneity. On a large scale, species diversity increases as habitat types are added to the environment (Chapter 5~. More niches are also added as structural complexity increases within a habitat. For example, vertical complexity in the form of an increase in the number of foliage layers is associated with an increase in bird species diversity, because birds often partition a habitat by occupying different horizontal strata (MacArthur, 19651. Plantings that change the number of foliage
COMMUNITY ECOLOGY 51 layers can change the number of bird species that a park can support (Gavereski, 19761. Bird species diversity is correlated much less with plant species diversity than with the structural heterogeneity of vegetation (Kerr and Roth, 197 1; MacArthur and MacArthur, 1961; Orians, 1969; Recher, 19691. Birds forage widely every day and visit many plants in their search for food, responding differently to plant species primarily when those species are structurally very different, as are coniferous and broad-leaved trees (MacArthur and MacArthur, 1961) or spiny and nonspiny desert shrubs (Orians and Solbrig, 19771. In contrast, many herbivorous insects spend their entire foraging lives on one plant and choose their host plants on the basis of chemical characteristics, which often vary substantially with spe- cies (Caswell et al., 1973; Ehrlich and Raven, 1965; Fox, 1981; Rhoades, 1979; Westoby, 19781. Ground-dwelling vertebrates often partition habitats horizontally, and species diversity increases with increase in horizontal heterogeneity (Pianka, 19661. Horizontal heterogeneity also contributes to bird species diversity: patchier habitats support more species (Roth, 19761. Structural hetero- geneity is positively associated with insect diversity, with many species supported on a single plant, each specialized for foraging or hiding on a different substrate, such as upper leaf surface, twig, and trunk (Heinrich, 1979; Ricklefs and O'Rourke, 1975; Schultz, 1983a). In addition, chem- ical variations within a plant cause insects to move more often (Schultz, 1983b). Manipulation of physical and chemical structures of crop plants and mixtures of plants can thus be an effective way of combating pests (Crawley, 1983; Hare, 1983; Whitham et al., 19841. Repeated perturbations in a community change its makeup as species undergo local extinction and reinvade. The number of species is usually relatively small in highly disturbed communities, because few populations are able to re-establish themselves before they are reduced by later dis- turbances. In contrast, a low rate of disturbance provides few opportunities for pioneering species and might allow competitively dominant species to usurp limiting resources, particularly in space-limited systems, such as rocky intertidal zones or some terrestrial plant communities. Therefore, the number of species in a community is often greater at intermediate rates of disturbance (Cornell, 1978; Huston, 1979) than at either low or high rates. Some climax plant communities seem to require periodic disturbance for long-term maintenance. For example, some California chaparral com- munities (Biswell, 1974) and some grasslands (Wells, 1965) appear to be maintained by periodic fires. When fire is controlled, these communities are replaced by others.
52 KINDS OF ECOLOGIC KNOWLEDGE ED THEIR PLACATIONS Predation Predation and periodic disturbance of other types influence species diversity in similar ways. By removing competitively dominant species, predators can increase species diversity. For example, experimental re- moval of starfish from a rocky intertidal zone allows competitively dom- inant mussels to usurp space from other species; when present, starfish open up space for other species by selectively removing mussels (Paine, 1966, 19741. However, although mussels in the absence of starfish pre- dation can take over space and reduce the diversity of macroinvertebrates in the rocky intertidal zone, the mussel beds provide vertical structure that actually increases total diversity when microinvertebrates are also consid- ered; this shows how the effects of spatial heterogeneity and predation can interact (Suchanek, 19791. Herbivores can exert similarly powerful effects on community structure in terrestrial grazing systems. The effects of grazing on the diversity of plants depends on whether herbivores selectively graze on the competi- tively dominant species, which increases diversity, or on poorer compet- itors, which decreases diversity (Harper, 19691. The selective grazing of herbivores can lead to the replacement of naturally dominant but palatable species with species that are spiny or toxic. The influence of predators on species diversity seems to be most powerful in space-limited systems. Freshwater predators often select their prey by size, and that can result in large changes in the makeup of plankton communities (Zaret, 19801. In some cases, the introduction of planktivorous fish into fish-free lakes can reduce the numbers of larger, competitively dominant zooplankton and lead to increases in smaller species (Brooks and Dodson, 1965), although the changes can be complex and difficult to predict (DeMott and Kerfoot, 19821. Competition Competition can influence community structure by causing the elimi- nation of some species from local regions or habitats and by reducing the abundances of species in the habitats in which they occur. There are reasons for expecting competition to be strongest among closely related species (Darwin, 1859; Lack, 1954), and many such cases of competition have been documented. However, competition has been especially looked for among closely related species, and the frequency of competition among more distantly related organisms could be much higher than currently believed. Competition among distantly related species is especially prev- alent when space is the limiting resource. Plants of all taxonomic groups
COMMUNITY ECOLOGY 53 compete strongly with one another and, in the rocky intertidal zone, animals of different phyla compete with one another and with algae (Con- nell, 1975; Paine, 1966; Underwood and Denley, 19841. Territorial exclusion, a form of competition for space, can also occur between species. Interspecific territoriality is most common among closely related organisms, but does occur among more distantly related ones as well, especially among fish (Ebersole, 1977; Myerberg and Thresher, 19741. Some cases of interspecific territoriality among birds also involve distantly related species (Cody, 1969; Moore, 1978; Orians and Willson, 19641. Competition occurs among distantly related grazing mammals, such as moose and hares (Belovsky, 19841. In desert ecosystems, ants and rodents compete strongly for seeds (Davidson et al., 1980, 1984; Kodric-Brown and Brown, 19791. Many more cases of competition among distantly related species are likely to be uncovered as ecologists devote more effort to the study of such competition. Competition seems to be rare among herbivorous arthropods (Strong, 1984~. This suggests that management practices that exert their effects precisely on the target insect species are unlikely to result in unintended side effects on many other species in the community. The use of toxic substances that adversely affect many species has led to greatly magnified influence on population dynamics of other species (Chapter 244. Careful targeting of management toward the focal species decreases the likelihood of side effects that undermine the goals of management. Productivity Field studies of more or less natural ecosystems have shown a positive relationship between the number of species in an ecosystem and its pro- ductivity (Cornell and Orias, 19641. The most common interpretation is that in productive ecosystems more resources are above the minimal abun- dance required to support users than in unproductive ecosystems (Cornell and Orias, 1964; MacArthur, 1972) and that animals in productive eco- systems can therefore specialize on resources that in less productive en- vironments can be used only by generalists. Also, if productivity affects the number of species of plants, then the richness of species of animals that depend on the plants automatically increases as a consequence. In contrast, it is commonly observed that eutrophication of lakes re- duces, rather than increases, species richness (Rosenzweig, 19721. There is no generally accepted explanation of this response. One model assumes that increasing productivity makes predators more effective in eliminating some of their prey or in inducing wide oscillations in their abundances,
54 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS which are likely to lead to extinction from a number of causes (Rosen- zweig, 19721. Another suggests that competitive exclusion is more im- portant in enriched environments; enriched environments favor "weedy" species that dominate typical members of less productive environments (Huston, 19791. Managers must be alert to the possibility that changes in ecosystem productivity, a common goal or by-product of human inter- vention, will lead to unexpected changes in abundances and distributions of many species in a system. Among the affected species are likely to be some of aesthetic or commercial value. Spatial Factors Habitat patch size and isolation, two factors that can have strong effects on diversity, are discussed in detail in Chapter 5. Species diversity in- creases as the area of an " island" of habitat increases, and species diversity in a given habitat patch generally decreases as the patch becomes more isolated either by distance or by the unsuitability of intervening habitat. How these factors affect design and management of ecological reserves and biological control programs is important, and there is much discussion over how to apply understanding of them to the long-term preservation of species and communities (Franker and Soule, 1981; Simberloff and Abele, 19761. For example, to conserve some communities of species, it is necessary to maintain a mosaic of various successional stages (Pickett and Thompson, 19801. The resulting habitat patchiness allows species adapted to each stage to find suitable areas. A mosaic of stages can also afford temporary refuge to prey or host species; they will eventually be eliminated in any particular patch by predators or parasites, but by then they will have colonized other patches (Dodd, 19591. The spatial configuration of habitat patches also determines the extent and nature of ecotones. Ecotones support many species that would not be present in pure communities. Some forest management plans attempt to maximize diversity by creating configurations of habitat patches with much ecotone while retaining patch sizes and proximities that can support species that rely on single community types (Thomas, 1979~. What constitutes the most appropriate configuration of patch size, shape, and spacing de- pends on the requirements of the species to be maintained. Summary The exact roles of the factors that influence biogeographic patterns of species are controversial (Brown, 1984), but the factors reviewed above are all known to affect diversity. An understanding of these factors can
COMMUNITY ECOLOGY 55 provide an environmental manager with a powerful set of tools for ma- nipulating the environment to bring about or limit changes in diversity. Because management is usually targeted to particular species or groups of species and not toward diversity itself, however, additional knowledge of the natural history and population dynamics of the species of interest . is required. Competition, predation, and mutualistic interactions combine with com- ponents of the environment to influence species richness in various ways. Plant species richness is often high in the presence of low soil fertility and periodic disturbance, both of which interact to slow down the takeover of a site by competitively dominant species (Huston, 1979; Tilman, 1982; Chapter 181. A similar phenomenon occurs in rocky intertidal habitats, where animals are the dominant competitors for space (Menge and Suth- erland, 1976; Paine, 1966~. COMMUNITY ORGANIZATION Species richness is only one property of a community that influences its structure and dynamics. Abiotic factors, such as moisture and tem- perature (Holdridge, 1967), and the biological processes of competition (Strong et al., 1984), predation (Paine, 1980), evolution (Orians, 1975), and trophic structure (May, 1983; Paine, 1980; Pimm, 1982, 1984) act to influence the structure of a community by determining its makeup and the constraints under which its constituent species live. Because the link- ages between species can be at once complex, indirect, and strong (Paine, 1984), investigating the effects of perturbations by studying single species can be misleading (Kimball and Levin, 19851. Nonetheless, a small number of factors often dominate the organization of a given community and determine its response to particular stresses. For example, soil nutrients in many tropical forests are primarily tied up in the vegetation and superficial soil layers. When all the vegetation of these forests is removed for cultivation and the soils are unprotected from nutrient leaching, the soil can lose its capacity for regeneration for a long period (Gomez-Pompa et al., 19721. Particular species often dominate the visual appearance and structure of communities, providing physical struc- ture for the existence of many other species. For example, coral reef communities depend critically on the reef-building activities of living corals. In terrestrial communities, vascular plants provide the dominant substrate on which most biological interactions are carried out. A single species can be critical to the maintenance of a community in its "normal" state, such as starfish in some rocky intertidal communities. Those "keystone predators" exert an influence on community makeup
56 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS out of proportion to their numbers or biomass. The idea of keystone species was first applied to predators that have sessile prey, especially when the preferred prey is a dominant competitor for space in the absence of pre- dation. For example, the elimination of the sea otter along the Pacific coast of North America contributed to a large increase in the number of sea urchins, a major food item of otters; and the proliferation of urchins resulted in the decline of kelp beds through excessive urchin grazing (Duggins, 1980; Estes et al., 19821. Lobsters can function similarly as keystone species by preying on urchins off the Atlantic coast (Mann and Breen, 19721. The African elephant exerts a large effect on the landscape by destroying shrubs and trees; that results in the proliferation of grasses, an increase in the frequency of fires, and the conversion of woodland to grassland (Krebs, 19851. STABILITY AND RESILIENCE OF ECOLOGICAL COMMUNITIES Ecologists hold diverse opinions about the relationships between the numbers of species in an ecosystem and the complexity of their interactions and about the system's responses to perturbations. Some have asserted that simple ecosystems are much less stable than complicated ecosystems (Elton, 1958; Hutchinson, 1959; MacArthur, 1955; Watt, 1964), and others have asserted the opposite (Gilpin, 1975; Goodman, 1975; Horn, 1974; May, 1973; Pimm, 19791. This diversity of opinion reflects inad- equacies in information and the use of different definitions of stability and different types of communities and perturbations. "Stability" has been used to refer to lack of fluctuations (constancy), resistance to being changed by external perturbations (inertia), speed of recovery from perturbations (resilience), and other ideas (Goodman, 1975; Holling, 1973; Orians, 19751. Frank (1968) has pointed out that a community of long-lived species can appear to have some aspects of stability merely because the component species live a long time. A general relationship between stability in any general sense and species richness is unlikely. Many natural ecosystems are species-poor, but none- theless stable by some definition mentioned above (e.g., Arctic tundra). And some species-rich systems are sensitive to disturbance, because of the intricacies of the connections among their component species (e.g., tropical rain forests). Moreover, human-induced perturbations not only change species richness, but also create new patterns of interactions (e.g., Cairns, 19801. Until the species have adjusted through evolution to those new patterns, the systems might behave in ways that reflect not simply
COMMUNITY ECOLOGY 57 their altered richness, but the evolutionary novelty of the interactions (May, 19731. For management purposes, it is important that the meaning of "stabil- ity" most appropriate for the problem at hand be clearly specified. In some cases, such as preservation of valued species, it could be most important to prevent the system from being changed very much by the planned actions (Chapter 161. In other cases, such as control of erosion, it could be more important to quicken the return to a former condition of the community, because the problem depends primarily on the duration of a disturbance. Ecological knowledge probably will never be able to provide answers that are general and yet precise enough to replace the need for understanding specific systems and perturbations. Such knowl- edge can be expected, however, to help in focusing research more narrowly on the most important interactions. INVADAB IL IT Y An early survey of invasion by plants and animals was carried out by Elton (1958~. He concluded that invaders were more likely to establish populations in cultivated and otherwise disturbed environments than in pristine environments, and he noted that islands were more susceptible to invasion than mainland areas. This general perspective has been supported by recent research, although the reasons for the relationships are not much clearer than they were 30 years ago. Determining whether a species might invade new areas requires knowledge about its life history, relationships with other species, and responses to various agents that perturb ecosystems. Herbaceous plants have been among the most successful invaders of new environments. The flora of California now contains nearly 1,000 exotic plants, and much of the intermountain west is dominated by Eu- ropean and Asian annuals (Mack, in press; Mack and Thompson, 19821. Communities of freshwater fish also appear to be unusually susceptible to invasion by exotic species (Courtenay and Stauffer, 19841. Birds do not invade new areas as easily. Only three natural invasions of North America by birds have occurred during the last century: those of the cattle egret and two gulls. All three exploit food resources that have greatly expanded in recent decades (Orians, in press). Deliberately introduced species, such as starlings and house sparrows, primarily exploit human- modified environments. Many of the escaped captive birds that have es- tablished feral populations in North America also exploit new food re- sources, particularly those provided by extensive plantings of ornamental trees and shrubs in southern cities.
58 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS invasions by herbivorous insects are complex, but most species feed on plants that are closely related to the species on which they feed in their native range (Furniss and Carolin, 19801. Many insects colonize intro- duced plants in all parts of the world, but their natural food plants are generally unknown (Strong et al., 19841. SUBSTITUTABILITY When a species is removed from an ecological community, its roles are sometimes taken up entirely or in part by other species. The degree to which this occurs is referred to as substitutability. Because all species are involved in many different interactions, substitutability probably varies with the particular role being considered. For example, the blight-caused loss of the American chestnut in Appalachian forests resulted in only a temporary reduction in rates of photosynthesis in those forests, because other trees replaced chestnuts in the canopy. However, species that are specialists on the tissues of chestnut trees (folivores, frugivores) must have suffered major losses that will continue as long as chestnuts are rare. The likely effects of species losses on community dynamics depend on the details of current interactions, so an important part of project planning is a survey of competitive, predator-prey, and mutualistic interactions of an obligate and specialized nature. Such information can help in predicting which species losses are most likely to affect other species in the system. The significance of the potential effects can be evaluated, and steps to reduce the likelihood of their occurrence can be included in the project plan. ECOLOGICAL SUCCESSION As long as physical conditions do not change greatly, more or less distinct communities tend to replace others after disturbance in a pre- dictable way. Although ecological succession was originally thought of as a community process, examination of particular successions has shown that abrupt, wholesale extinction of the constituent species of one com- munity with concurrent colonization by the species of another is rare (Drury and Nisbet, 19731. The fates of some pairs or groups of species are inextricably intertwined, as are the fates of some mutualists, but these linkages are in a minority. Typically, the times of appearance and dis- appearance of most species in a succession are generally independent of those of others, and some species that seem late are present early, but in . · ~ an inconspicuous form.
COMMUNITY ECOLOGY 59 The nature of the interactions among species that determine their turn- over during a succession and the relative stability of the climax stage are poorly understood for many successions, partly because no community is exactly like any other. Recent research has suggested that processes and patterns of succession differ among communities and depend on which species are present at the start and are available to colonize later and on their life histories (Horn, 19761. Some early species modify the environ- ment to facilitate growth and recruitment of other species, as colonizers of sand dunes stabilize the soil and so allow others to become established (Olson, 1958~. Many pioneering plant species are so intolerant of shade that the shade they create inhibits growth of their own seedlings. Some species inhibit others chemically (Rice, 19741. Some late successional species persist because they are more tolerant of potential sources of mortality, such as fire or grazing (Harper, 1969; Sousa, 19841. Although successions are highly variable in detail, most have some characteristics in common. Odum (1969) listed many patterns of change in energy flow, biomass, and physical structure that are predictable. Some of these patterns, such as the ratio of gross production to respiration (PG/R) for the community as a whole, can indicate the stage of a succession and how long a given stage is likely to persist without intervention. Early successional stages typically have relatively high PER, whereas later stages have ratios approaching 1:1. Part of the reason is that early species are usually herbaceous, with most of each plant's resources devoted to growth and reproduction; many later species, which persist longer, support more woody tissue and devote more resources to competition than to reproduction. Thus, early stages do not lose in respiration most of the matter produced by photosynthesis, as do later stages, and usable (net) production is relatively high. The high net production of early succession is harvestable for human use, and this is taken advantage of in agriculture and forestry. Human societies usually try to maintain early successional stages pre- cisely because they are more productive, but maintaining them in the face of the natural tendency for change requires large expenditures of energy, effort, and materials. Prolonging normally short-lived early successional stages by calculated disturbances (such as plowing and weeding) or the use of chemicals (such as herbicides and pesticides) entails environmental and health problems (e.g., see Chapters 14, 23, and 241. Simply harvesting in the same site for a long period can result in slow degradation of the soil by erosion and leaching of nutrients. As the soil loses its capacity for production, the economic and environmental costs of maintenance grow with the use of fertilizers. The challenge for ecologists is to help to identify ways of minimizing
60 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS the large expenditures and environmental costs of maintaining early successional communities. Integrated pest-management programs aim at reducing the role of pesticides by integrating the use of pesticides with other modes of control (e.g., crop rotation and biological control) on the basis of detailed studies of pest life history and ecology. Soil erosion can be reduced by such techniques as reducing tillage and selecting optional contours (Greenland and Lal, 19771. Herbicide applications to powerline rights of way can be reduced by planting shrubs that impede succession (Niering and Egler, 19551. CONCLUSIONS All human-induced environmental disturbances alter interactions among species in some way that leads to direct and indirect affects on the com- position of ecological communities and their dynamics. Generally, the direct effects on species of concern are more readily identified and antic- ipated than are the indirect effects, especially the effects that influence community properties that are the summation of activities of many species. The major question is sometimes how long a community will remain in an altered state. At other times, the main question is how seriously a community is changed. Major changes might be intolerable, even if the community eventually returns to its predisturbance state. The problems described in this chapter are among the most difficult to deal with and are accordingly those for which careful planning and monitoring of a project are especially important, if unexpected and undesired ecological changes are to be avoided or reduced.