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Population Interactions Every species in an ecological community is connected to many others. Each is both a predator (if plants can be regarded as predators on photons) and a prey, each can compete with other species that use the same re- sources, and each can engage in mutualistic interactions with other species. In a particular environmental problem, usually one of the roles that a species can play is of prime concern, but neglect of other interactions might cause management efforts to fail or produce unwanted side effects. The processes in which species are involved are measured in different units. Rates of photosynthesis are measured in units of mass change; feeding rates are measured in energetic units. Competitive interactions might decrease reproductive rates or cause species to be absent from some areas. Not all these effects can be expressed in energetic terms. If dis- ruption of mutualisms lowers seed set or dispersal, the energy content of the lost seeds is only a small component of the significance of those changes. The overall unit for expressing these effects is fitness (relative reproductive success), but total fitness rarely can be measured in the field, and even partial measures can be difficult to obtain. As a result, various units are used by environmental problem-solvers, the most appropriate one depending on the problem and the objectives of manipulation. Human uses of ecological systems commonly involve altering the nature and extent of interactions among populations, whether or not this is the prime objective of the management programs. We commonly eliminate large carnivores, because they are dangerous and because they prey on wild or domesticated species that we wish to exploit. Herbivorous insects 38
POPULATION INTERACTIONS 39 compete with us for the tissues of crop plants, and weeds compete with those plants for light, water, and nutrients. Our efforts to remove these species or reduce their abundances are based on the assumption that doing so will increase the yields of valued products front the species we wish to protect. The assumption is probably true in most cases, but there are remarkably few quantitative data on the effects of weeds on yields of crop plants (Mortimer, 19844. There are many more estimates of losses of crops to herbivores. Damage is often severe enough not only to justify control, but to cause economic hardship to agriculturalists (Barrons, 1981; May, 1977; Pimentel et al., 19801. It is clear, however, that control measures are often initiated when populations of pests are so small that economically significant damage is unlikely to occur (Pimentel et al., 19801. Restricting the use of control measures to times and places when they are cost-effective is important, because many of the materials used are toxic and exert other, usually detrimental, effects on ecological communities (Barrons, 1981; Bull, 1982; Carson, 1962; Dunlap, 1981) and because resistance to toxic chemicals evolves as a function of the frequency with which they are used (Chapter 11. Green plants account for about 97% of all the carbon fixed in terrestrial and aquatic ecosystems, and their photosynthetic activity supports almost all other components of those systems. Plants carry out photosynthesis by only three different mechanisms, which differ in a variety of ways, in- cluding optimal temperature and amount of carbon that can be fixed per unit of water lost (Berry, 1975; Bjorkman and Berry, 19731. Nearly all woody plants at all latitudes and herbaceous plants at high and middle latitudes use the same pathway. Because of the relative uniformity of mechanisms of photosynthesis, plants are generally replaceable in their role as photosynthesizers. That is the basis of single-crop agriculture and the reason why forest productivity is largely independent of the number of species of trees growing on the site. Plants differ greatly, however, in physical structure, chemistry, depth of soil from which they extract water and minerals, and kinds and numbers of mutualistic interactions. Those differences make it possible for particular species of plants to play different roles. All animals require high-energy organic compounds as food, so much of their diversity is related to what they eat and how they find it. Animals are involved in many kinds of competitive interactions, and they participate in many convolved mutualisms with plants (e.g., pollination, fruit dis- persal, and plant protection), other animals (e.g., mimicry and protection), and microorganisms (e.g., digestion and bioluminescence). Animals exert most of their effects through consumption of prey. Because energy-rich molecules are usually found at low to moderate densities in ecological
40 KINDS OF ECOLOGIC KNOWLEDGE ED THEIR PLACATIONS communities, most animals are motile and move around to find food. In special environments, however, such as coral reefs and the rocky intertidal zone, each wave brings a new supply of food that cannot be depleted by the foraging activities of the attached animals, and animals compete for space instead of food and actually form the dominant structural elements of the ecological communities. Like animals, microorganisms are diverse in their modes of obtaining energy. They use various substrates for their synthetic abilities, use various photosynthetic pigments, and derive energy by oxidizing many simple substrates, such as gaseous hydrogen, inorganic nitrogen, sulfur, and iron. Microorganisms live in the bodies of all species of larger organisms. They are able to synthesize compounds required by plants and animals (e.g., vitamins and nitrates), and they are the only organisms that can break down many biologically important molecules (e.g., cellulose, waxes, and lignins) PREDATOR-PREY INTERACTIONS It is useful to think about predator-prey interactions from the perspective of the relative sizes of predators and prey. "Typical" predators are larger than their prey, kill and consume all or most of each prey item they capture, and must find many different prey items daily or over their life spans; in the extreme, predators like baleen whales feeding on krill are so much larger than their prey that the prey are handled en masse. How- ever, predators can be smaller than their prey and consume only parts of them. If they live externally on their prey, these predators are called herbivores or external parasites. If they live internally, they are usually called parasites or parasitoids (Price, 1980; Thompson, 19821. These differences are important from a management perspective. Pre- dators that eat most types of prey captured are unlikely to be sensitive to the loss of any particular prey species. Predators that are specialists, eating only a single species or a few closely related species of prey, are desirable for many biological control purposes (Chapter 14), because they are un- likely to feed on nontarget prey species (which would cause unexpected and unwanted side effects). Knowledge of size relations can be helpful in directing research efforts, because specialist predators are often smaller than their prey. Predator species being considered for biological control must, of course, be screened individually to determine their diets. Narrow diets can evolve in the absence of size differences between predators and prey. For example, dietary specialists should be more prev- alent in stable habitats, where the availability of specific resources fluc- tuates over rather narrow limits and particular prey types are therefore
POPULATION INTERACTIONS 41 reliably available throughout the year (Charnov and Orians, 1973; Cowie, 1977; Pyke et al., 1977; Schooner, 1971; Tinbergen, 1981; Werner and Hall, 19741. Monophagy is prevalent among predators with special ad- aptations for capturing and eating prey with unusual defenses, such as armor, spines, and toxic chemicals. Predators can be dietary specialists because their simple nervous systems leave them unable to use other than simple criteria for recognizing prey (Levins and MacArthur, 19691. Mon- ophagy can be so narrow that predators eat tissue of only some types found in their prey; herbivores of large, woody plants are predators of this kind (Crawley, 1983; Strong et al., 19844. The consequences for an ecological system of adding or removing predators depend on whether the system is predator-controlled or "donor- controlled" (Pimm, 1979, 19801. In a donor-controlled system, the supply of prey is determined mainly by factors other than predation; therefore, the predators have little influence on their food supplies. Familiar examples of donor-controlled systems are communities of sessile, planktivorous animals of rocky intertidal shores, where a fresh supply of food is delivered with each successive wave; consumers of dead plants and animals; and consumers of fruits and seeds. In predator-controlled systems, the pre- dators, by their feeding, reduce the supply of prey and their reproductive ability (i.e., the future supply). The distinction is important, because removal of predators in donor-controlled-systems has little effect on pop- ulation dynamics and interactions among their prey species, whereas re- moval of predators in predator-controlled systems often results in outbreaks of the prey and large changes in the relative abundances of the prey species (Estes et al., 1982; Simenstad et al., 19781. The ability to predict such changes is obviously important for the design of management schemes. An important and rapidly growing application of predator-prey rela- tionships is the use of the natural defenses of plants against their predators as a substitute for synthetic toxic chemicals. A major advantage of using natural defenses is that effects on nontarget organisms are minimized and the plant itself synthesizes and distributes the defenses. Breeding of pest- resistant crop plants has a long tradition, but improved knowledge of the chemical bases of resistance now enables managers to search for specific kinds of chemical and physical defenses in plants to deal with particular herbivores, rather than relying simply on randomized field trials to see what happens to work (Maxwell and Jennings, 19801. Defensive chemicals are in two major classes acute toxins and di- gestibility-reducing substances. Acute toxins are present in plant tissues in small amounts and exert their effects in the bodies of the herbivores by interfering with a basic metabolic process, such as nerve transmission, protein synthesis, or hormone balance. They are effective because animals
42 KINDS OF ECOLOGIC KNOWLEDGE ED THEIR APPLICATIONS have many tissue types not present in plants tissue types that are targets for chemicals not toxic to the plants producing them. Digestibility-reducing substances act in the guts of the herbivores (technically outside their bodies, inasmuch as no cell membrane has to be crossed), by combining with proteins in the food to reduce their availability to the herbivore. Tannins and resins are common substances of this type. Acute toxins are most effective against generalized herbivores, because specialized her- bivores rapidly evolve the ability to detoxify chemicals in their host plants. It is more difficult to evolve counteradaptations to digestibility-reducing substances, so they are often effective against specialized herbivores (Feeny, 1976; Rhoades, 1979; Rhoades and Cates, 19761. Herbivores can, how- ever, evolve resistance to hydrolyzable tannins (Fox and Macauley, 19771. The use of such information in screening for strains of crop plants to use in different situations is still largely a task of the future, but the necessary ecological knowledge is being accumulated. An important recent discovery is that plants can respond to herbivore attacks by turning on inducible defenses. These induced defenses can cause marked reductions in growth rates, survival, and pupal weights among insects feeding on foliage of previously attacked branches or trees (Haukioja and Hakala, 1975; Haukioja and Neimela, 1979; Ryan and Green, 1974; Schultz and Baldwin, 19821; Rhoades (1979) has reviewed the evidence. The full significance of this discovery is not yet clear, but once the mechanisms underlying such responses are identified, they might be manipulated to induce defenses in advance of any attacks. Moreover, damaged plants might release volatile materials that induce neighboring plants to increase their defenses (Baldwin and Schultz, 1983; Rhoades, 19821; if so, that opens up still other possibilities to manipulate plant defenses against herbivores to reduce reliance on toxic substances. COMPETITIVE INTERACTIONS Competition occurs when a number of individuals use common re- sources for which the demand exceeds the supply or when resources are not scarce and organisms harm one another in the process of seeking those resources (Birch, 19571. Competition can occur among individuals of a single species (intraspecific competition) or among individuals of different species (interspecific competition). Interactions can be especially strong between two species or spread out over a larger number of species, each of which contributes a small part of the total (diffuse competition). Un- derstanding competition can be helpful in solving environmental problems. The most direct method of assessing the importance of competition is
POPULATION INTERACTIONS 43 to remove individuals of one species and measure the responses of the remaining species. Often such a manipulation cannot be made, however, and even if it can, it reveals only the short-term responses to the reduction of competition. Longer-term responses, which might involve genetic changes in the component species, cannot be measured this way, except in the case of very short-lived species, such as bacteria, protists, algae, and some insects. Competition is much more difficult to study than predation. Unlike predation, a more or less continuous process (all predators must eat reg- ularly), competition can be intermittent, with periods of competition al- ternating with long periods in which competition is weak or absent. Nonetheless, patterns due in part to competition might persist through periods when competition is absent. Indeed, if species evolve traits that are adaptations to environments with particular sets of competitors, their behavior might reflect competition that occurred long ago a phenomenon referred to as the "ghost of competition past" (Cornell, 19801. Most studies of competition have not involved measuring competition directly, but have attempted to predict and find the patterns that should be found in nature if competition were occurring. Necessary though this procedure might be in many cases, it increases the probability of mistak- enly assuming either the presence or absence of competition because predictions about resulting patterns have been incorrect. The literature is biased, owing to underrepresentation of negative results, and, not sur- prisingly, there is much controversy among ecologists about the impor- tance of competition in nature and about the extent to which patterns observed in ecological communities can be attributed to it (Cody, 1974; Diamond and Gilpin, 1982; Gilpin and Diamond, 1982; Strong et al., 1979, 1984; Tilman, 19821. All observers do agree, however, that com- petition occurs and that it must be understood better if we are to understand ecological communities. Except for competition for space in marine communities, where animals and plants compete vigorously (Paine, 1969, 1980), competition generally occurs among organisms at the same trophic level in a community. Some authors have suggested that competition is stronger at some trophic levels than at others. For example, Hairston, Smith, and Slobodkin (1960), observing that the world is green, suggested that competition was intense among plants, rare among herbivores, common among carnivores, and common among detritivores (coal and oil do not appear to be forming at appreciable rates today). They stressed that their conclusions were trophic- level generalizations and did not imply that all carnivores or no herbivores compete. Menge and Sutherland (1976), working in rocky intertidal en- vironments, suggested that the importance of competition should rise with
44 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS trophic level and that competition should be especially noticeable in troph- ically simple communities. They differed from Hairston et al. in predicting low competition among plants. These and other predictions about competition have recently been ex- amined by Connell (1983) and Schoener ~ 19831. Connell's criteria for inclusion of studies in his sample were more stringent than Schoener's, so the two reached different conclusions. Schoener found strong support for the Hairston et al. hypothesis in both terrestrial and freshwater com- munities, but only weak support in marine communities. An important exception was that some groups of terrestrial carnivores, such as spiders and predatory insects, often do not compete strongly. The reason might lie in the fact that these small vulnerable carnivores are preyed on by many potential competitors for their prey (Schoener, 19831. Connell found little support for the Hairston et al. hypothesis. Thus, the general importance of competition in nature is not settled; but there is general agreement about some patterns in competitive inter- actions. First, small animals are more vulnerable than large ones to va- garies in the physical environment and predators and, as a result, are less likely to compete. Second, most competitive interactions are highly asy-m- metrical in their effects (Cornell, 1983; Lawton and Hassell, 1981; Schoe- ner, 1983), i.e., the removal of one species has a strong effect on the other, but the reverse is not true. Indeed, many cases are so asymmetrical that the effects on one of the species are almost undetectable. Such a situation was predicted by theoretical considerations: a slight difference in competitive abilities between two species can lead to the elimination of one of them in simple environments (Gause, 1934; Park, 19481; there- fore, in nature, even a slightly subordinate species might be much less common and found in many fewer habitat types than if the dominant species were not present. Indeed, the experimental removal of the larger of two competitors usually has a much greater effect than the removal of the smaller this was true in 27 of 32 cases in Schoener's sample. Knowledge that removal of the larger of two competing species is likely to result in substantial population increases among the smaller species and that the larger, despite its competitive dominance, is more likely to be eliminated by human-caused perturbations might suggest likely conse- quences to which responses should be prepared, especially in cases of biological control where two or more predators are introduced and they differ in size and hence in behavioral dominance. Human activities often introduce into ecological communities new species that, through their competitive and predatory activities, strongly affect indigenous species. Indeed, some of the most important biological problems have been caused by the introduction of species, whether inadvertent or deliberate. The
POPULATION INTERACTIONS 45 general response to such problems has been the enactment of strict leg- islation governing importation of species. However, because importation is certain to continue, better understanding of its likely consequences will be of great value to the environmental problem-solver. MUTUALISTIC INTERACTIONS Mutualism occurs when two species benefit one another. As in com- petition, the reciprocal effects are rarely of equal strength. Mutualisms, which are found among all major groups of living organisms, range from obligate (as in the association of some algae and fungi to form lichens) to facultative. Most mutualisms are based on the transfer of energy and materials between the partners, and many mutualisms are believed to have evolved from predator-prey interactions (Thompson, 19821. Mutualistic interactions are of particular importance for the environmental problem- solver, because loss or severe reduction of a mutualist can have a major impact on a target species that would be unexpected if the nature of the mutualistic interaction were not known. Most frugivores eat many different kinds of fruits over the course of the year, and many include animals in their diets (Wheelwright, 19833. However, many plants have brief fruiting seasons and are visited by only a few species of frugivores. Therefore, plants might be more seriously affected by the loss of one species of frugivore than would a frugivore by the loss of one of the species of plants whose fruits it eats. In contrast, most plants are visited by many species of potential pollinators, whereas many species of pollinators, especially bees, restrict their foraging to one or a few species of plants. (Agriculture, however, depends heavily on the honey bee, Apis mellifera, which has a catholic diet and ready access to the flowers of most cultivated plants. The loss of bees, as sometimes occurs when pesticides are used extensively in a region, can cause serious losses of fruits and seeds, even when plant growth remains normal and the harvesting activity of the bees is a rather minor component of the total energy flux in the plant community.) Interactions among plants, their pollinators, and disseminators of their fruits and seeds are generally rather obvious, and managers are usually sensitive to the need to maintain these mutualistic relationships. Rela- tionships between plants or animals and microorganisms, although less apparent, are equally important. Fixation of nitrogen in terrestrial eco- systems depends on the presence of a few species of bacteria and cyano- bacteria associated with the roots of some species of plants, especially legumes (Alexander, 1971; Bond, 19671. Digestion of cellulose by most animals depends on the presence of specific microorganisms in their guts.
46 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS Generally, however, environmental problem-solvers need not be seriously concerned about the preservation of these mutualisms, because the mi- croorganisms appear to be nearly universally distributed and the probability that a plant or animal will fail to establish its required mutualistic rela- tionships is very low, even in the presence of severe environmental per- turbations. INDIRECT EFFECTS We have considered direct effects of interactions, but species often influence other species with which they have no direct contact. For ex- ample, adult Heliconius butterflies in the American tropics depend for food primarily on the flowers of vines in the genera Anguria and Gurania, both in the cucumber family (Cucurbitaceae). The ability of the butterflies to lay eggs and hence their rate of attack on the larval food plant, pas- sionflower (Passiflora spp.) vines, can depend on the abundance of cu- curbits, even though the two groups of vines do not interact directly. Starfish (Pilaster), by preying on competitively dominant mussels in rocky intertidal environments, make possible the presence of many species of animals that they do not eat or otherwise directly affect (Paine, 19801. Such indirect effects are less well documented than are direct effects of interactions among species, because more careful experimentation would be needed; and such effects are likely to be missed, simply because in- vestigators do not think to monitor the relevant species in the system likely to be affected by indirect interaction. CONCLUSIONS A component of many environmental problems is alteration in the in- teractions among species. Such a perturbation is a likely candidate for unexpected side effects of a project, most of which are undesirable from a human perspective. Assessing which species are likely to be lost from systems as a result of planned perturbations and which species are likely to be affected (whether beneficially or adversely) by those losses is an important part of preproject analysis and one in which ecological knowl- edge is central. The knowledge needed to avoid serious problems is often easy to obtain, but no pertinent data are likely to be gathered unless the effects of interactions are considered.