Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
1 Individuals and Single Populations Environmental concern commonly focuses on populations of organisms, whether the goal is protection of valued species, harvest of economically profitable species, or control of economically destructive species. We are interested in predicting and controlling changes in size and structure of populations that occur in response to environmental change, whether an- thropogenic or not. The ecology of individuals and populations is of particular relevance to this interest. Population biology, the subject of this chapter, encompasses many kinds of research, from the study of the details of life history and behavior to the construction of mathematical models of the dynamics and genetics of multiple populations of a species over a large area. A life history encompasses an individual's interactions with its physical and biological environment throughout its lifetime. Research into life histories has taken several paths, all of them valuable in the management of populations: detailed studies of the ecology of individual species (e.g., Chapters 12-17), comparative studies of groups of species (e.g., Chapters 13 and 15), and theoretical treatments of the evolution of life-history patterns (e.g., Stearns, 1976; Chapters 15 and 17~. Studies of particular species yield the detailed information needed for management with respect to nutrient and habitat requirements, important interactions with other species, reproductive requirements, and significant behavioral idiosyn- crasies. Moreover, research on organisms with complex life cycles has shown the importance of choosing the appropriate stages in the life cycle 23
24 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS for management intervention. Comparative studies reveal general patterns that help focus individual studies and provide managers with guidance in the absence of detailed information. Theoretical research focuses attention on the elements of life history important in solving long-term management problems (e.g., Beddington, 1974; Lewontin, 1965; May, 19801. Thor- ough knowledge of species' life histories has a broad range of applicability to problems of population management, including captive propagation programs for endangered species (Franker and Soule, 1981), pest and disease control (Chapters 13-15), species protection (Chapter 17), har- vesting (Chapter 12), predicting environmental impacts (Chapter 16), and restoring plant communities (Chapter 181. As valuable as life-history information is for the prediction and control of population behavior, it provides only partial insight into the causes and consequences of changes in population numbers and composition. To determine how a population will respond to an increase in mortality due to harvesting or stress or how effective a given procedure might be for improving reproductive output, we need some understanding of population dynamics. Research in population dynamics has ranged from field studies designed to determine what factors affect population sizes in an area to theoretical studies of how such factors can act together to "regulate" population size over long periods. Some simple models use an "account- ing" formulation to calculate future population size on the basis of current size and rates of growth, death, and birth. More complex models deal with such phenomena as dispersal, breeding structure, interchange be- tween populations, environmental variability, and the effects of intra- specific interactions on population behavior. Traditional models of population dynamics are based on the assumption that organisms do not change genetically during a period of management. However, genetic changes do occur when populations are exposed to repeated manipulations, and evolution can take place with startling rapid- ity e.g., the evolution of insect resistance to pesticides and of bacterial resistance to antibiotics. Both insects and bacteria have the short life spans and high reproductive rates that speed evolution (May and Dobson, in press), but long-lived organisms can also evolve quickly if management results in large differences in mortality or reproduction among individuals. Size-selective harvesting of fish can lead to a reduction in the average age and size at maturity (Ricker, 1981), possibly as a result of changes in competition among age classes and in the frequencies of particular genes. When populations are small, harvesting practices can lead also to loss of genetic variability and can increase the deleterious effects of inbreeding. Preserving a species that is distributed into many small and isolated pop- ulations as many endangered species are involves an understanding of
INDIVIDUALS AND SINGLE POPULATIONS 25 both population dynamics and population genetics (Franker and Soule, 1981). Dynamic and evolutionary effects of manipulations are not always sep- arable. Changing such population characteristics as total numbers, distri- butions of ages and sizes, and sex ratio not only changes the dynamics of a population, but also establishes a potential for evolutionary change in traits that exhibit genetic variation. Population management that is based only on dynamic considerations can in the long run produce results op- posite to those intended. DDT works miracles on untreated populations of mosquitoes that transmit malaria, but within 5-50 generations the evo- lution of resistance might largely negate the effectiveness of the chemical; at the same time, human resistance to malaria can decrease in the absence of the disease (Chapter 151. Using large-mesh nets to ensure the harvest of male but not female migrating salmon can lead to an increase in the proportion of early-maturing small male salmon (jacks) (Gross, 1984, 19851. An increase in the proportion of jacks that escape the nets can lead to an increase in the proportion that breed and thus in the proportion that are hatched in the next generation. IDENTIFYING KEY FACTORS Organisms are influenced by many components, or factors, of their physical and biological environments. Those factors are not equally im- portant; often, a few dominate the dynamics of populations and need careful study for successful management. Of particular interest are factors that exert especially powerful effects on population size (usually by in- fluencing birth and death rates). Key-factor analysis is a method for identifying and understanding the stages of an organism's life history in which critical controlling processes occur. The method has been used for designing pest-control strategies (Clark et al., 19671. The basic data used in key-factor analysis are the number of individuals in each developmental stage or age group, survival from one stage to the next, and fecundity rates i.e., the standard elements of life tables (Ricklefs, 1979~. When the variance in these elements is partitioned among environmental causes, the key factors are the ones that cause high mortality (Harcourt and Leroux, 19671. Key-factor analysis is less applicable in heterogeneous than in uniform environments (Hassell, 19854. In organisms with complex life histories (e.g., salmon), different stages use resources differently and sometimes live in different habitats. The behavior of the different stages might therefore be controlled by largely independent events. Too few individuals of such a species might survive
26 KINDS OF ECOLOGIC KNOWLEDGE ED THEIR PLACATIONS one stage to saturate the habitat of the next. In addition, the more inde- pendent the habitats are, the less likely it is that their carrying capacities will vary concurrently. Therefore, populations of species with complex life histories often vary more widely in density than they would if all resources were gathered in one habitat (Istock, 1967), and study of a population in only a single stage is unlikely to reveal the causes of fluc- tuations. BEHAVIOR Biological processes of animals are tied to the behavior of individuals as they acquire nutrients, select habitats and mates, avoid predators, and interact socially. Some behavior can be predicted from knowledge about general environmental conditions and the types of species under consid- eration, but in many cases understanding behavior requires detailed knowl- edge about particular species and how they make behavioral "decisions." We discuss below decisions made by individual organisms that are im- portant in dealing with environmental problems. Habitat Selection' By natural selection, species evolve to prefer environments in which survival and reproduction are greatest. Responses to habitats are influenced not only by characteristics of the environment, but also by the presence or absence of individuals of the same species. Other individuals provide information about choices made by previous settlers, and they modify the environment forlater settlers (Orians, 1980; Partridge, 19781. Individuals might not settle in an otherwise suitable area if no other individuals are there. Young individuals might need to learn the locations of suitable sites from older ones, and slight changes in a habitat can lead to rejection, even if it is otherwise suitable for the species. For example, a management plan for the spotted owl (Chapter 17) avoids such rejection by recognizing that the owls require undisturbed old-growth forests for hunting and are reluctant to cross open areas. The collapse of the whitefish fishery in Southern Indian Lake was due not to a decline in the fish population, but to changes in selection of habitats as a result of raising the level of the lake (Chapter 21~. The control of malaria was less suc- cessful than expected, because of a behavioral polymorphism in habitat selection (Chapter 151. Attempts to find suitable natural predators to control agricultural and forest pests usually involve searches in areas with climates similar to those of the areas into which the control agent is to be introduced (Chapter 14~.
INDIVIDUALS AND SINGLE POPULATIONS 27 Particular attention is paid to climate, rather than biological interactions, because the required food source-the pest to be controlled is known to exist in the areas of introduction and because experience has shown that predators and parasites of a single host species typically change over the range of the host. It is rare for a single predator to be an important control agent over the entire range of its host. It is difficult, in practice, to find an agent that can control over even part of a pest's range, because many introduced predators fail to establish viable populations or fail to become common enough to achieve effective control (e.g., Chapter 141. Attempts at biological control therefore often involve importing a variety of potential control agents, in the hope that some will prove effective (Chapter 141. Mating Systems Mating systems vary widely. In the simplest, sex cells are shed into the surrounding medium, usually water, where fertilization takes place. In more complex animal systems, reproductively mature adults gather, choose mates from the available pool, and associate with one another for various periods after fertilization to care for each other or their offspring. In many species, mate selection is combined with habitat selection for breeding, especially in species in which one adult holds space that contains resources for the reproductive cycle. Much research into the evolution of mate choice is directed at determining the relative roles of mate quality and habitat quality and the criteria used for selection of each (O' Donald, 1967; Searcy, 19821. A successful biological control program that has capitalized on the use of laboratory-reared sterile males is the control of screwworm populations in the southern United States (Perkins, 1982; Scruggs, 19751. The program depends both on the ability to produce enough sterile males to ensure that most males in the field are sterile and on the mating behavior of the flies. No long-term pair bonds are formed, but the first male to copulate with a female blocks her reproductive tract with a plug that prevents other males from copulating with her. If multiple matings were the rule, a much higher ratio of sterile to fertile males would be required to achieve the same degree of population control. A similar mating system is the basis of strategies to control Mediterranean fruit flies (Dacus). Social luteractions Social grooming is a component of social behavior that sometimes accompanies mate selection, but also occurs in other contexts. The ob- servation that vampire bats groom each other extensively suggested a
28 KINDS OF ECOLOGICAL KNOWLEDGE ^D THEIR APPLICATIONS control strategy that facilitated transfer of a control agent applied to the fur of only a small number of bats (Chapter 131. The control agent was highly specific and had no effects on other species living in the same caves. The vampire bat case study is an excellent example of the value of detailed natural-history observation for population management. POPULATION DYNAMICS Population Regulation Environmental changes can influence a population in two basic ways, each having important management implications. Fires, bad weather, and other events can reduce a population by removing a fraction of it that is independent of its density. If a population or a part of it, such as an age group" is limited primarily by such density-independent factors, it can be reduced with little effect on the remaining individuals. Many environmental changes, however, influence individual growth, birth, and death rates in a manner that depends on population density. Even though populations grow rapidly at low densities, growth often slows as density-dependent effects appear through individual interactions (e.g., competition for food or nest sites) or through the actions of predators, disease, and other factors that increase deaths or reduce births. When individuals are removed from a population, density-dependent compen- sation can occur-for example, an increase in the birth rate as population size decreases. The operation of density-dependent factors can stabilize population densities and thus cause displaced populations to tend to return to equilibrium, in which the rate of loss of individuals of a species equals the replacement rate. The greatest number of individuals of a species that the environment can support is called the carrying capacity (K); it too can vary. May (1973) has discussed mathematical studies that showed how density dependence can lead to cyclic or even chaotic population change. Species with a very high reproductive potential (r) in the absence of competition are potentially able to recover quickly from population re- duction, but they are more likely than species with a low r to show extreme population fluctuations. Many pest species, such as weeds, are poor com- petitors, but have good dispersal abilities and high r (Baker, 1974~. Species with low r, such as whales and spotted owls (Chapter 17), often have stable populations consisting of long-lived individuals. Overharvesting of lower species can easily lead to species extinction. Conversely, lower species can more easily be controlled with measures used only periodically (e.g., vampire bats, as discussed in Chapter 131. Commercial or recreational harvesting reduces populations to some
INDIVIDUALS AND SINGLE POPULATIONS 29 value below the carrying capacity. Managers often try to maintain pop- ulations near the size that permits the greatest yield of individuals or biomass. Because yield is a result of both the reproductive rate and the number of individuals reproducing, yield is highest when the population is between low density (few individuals but high individual reproductive rate) and the high density of natural equilibrium (strong density-dependent reduction of average individual reproductive output or survival). In theory, if environments are constant, every population has a "max- imal sustainable yield" (MSY); but in practice, the MSY is very difficult to identify, let alone achieve. Accurate estimates of abundances are re- quired, especially when an unstable equilibrium exists near the MSY (see below). Both yields and population sizes fluctuate more as the MSY is approached, the effect being most pronounced in large mammals, such as whales, in which density-dependent effects are strongest near K and r is low (May, 1980~. Reducing the risk of errors resulting from population management re- quires at least estimation of relative changes in population size and com- position; good estimates of absolute population size are more desirable. But accurate estimates of the age distributions and sizes of populations are often difficult to obtain (Eberhardt, 1976; Seber, 19821. Although density and age of forest stands can be estimated accurately, errors in density and age estimates in bird, mammal, and fish populations are often large or unknown. Most marine fish populations are managed without knowledge of population sizes (e.g., Pacific halibut, as discussed in Chap- ter 121. Shepherd (1984) discusses the advantages and shortcomings of techniques for managing fish populations in the face of limited information; some difficulties in obtaining the information have been set forth clearly by Larkin ~ 19781. Populations of many species are often managed indirectly by managing the habitat. Habitat quality is commonly judged by population density, on the assumption that high density indicates high quality of habitat. This assumption is often valid, but data on survival and reproductive success are generally needed to understand differences in population densities. High densities can occur in suboptimal habitats composed mostly of sub- dominant individuals that have been forced out of other habitats (Van Home, 19831. Population Stability A population with a zero growth rate is in equilibrium. The equilibrium is stable if a population displaced above or below the equilibrium point tends to return to it. However, a population can have more than one stable
30 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS equilibrium point, and some equilibria are unstable (Berryman, 19811. Unstable equilibria often occur at low population densities of large, long- lived, slowly maturing species with low r (Southwood et al., 1974), as well as in a variety of heavily harvested fish populations (Beverton, 19841. Increased difficulty in finding mates, reduced effectiveness of cooperative defense against predators, and other phenomena can result in a lower threshold of density below which the population collapses unless there is immigration. Density-dependent compensatory mechanisms might also break down at very low densities (Beverton, 1984~. That possibility is of particular concern for rare and endangered species with few, isolated populations that are already small. Environmental fluctuations, disease, and other factors can easily reduce such a population to below its stability threshold. Some fish stocks have collapsed to extremely small numbers after overexploitation, and a few have not recovered even after the release of fishing pressure (e.g., Beverton, 1984; Peterman, 19784. In other spe- cies, the maximal yield could be very close to the harvest magnitude that would cause the population to collapse (Ricker, 19631. In any case, en- vironmental variation appears to interact with fishing pressure to compli- cate the fluctuations in stock size (Chapter 81. It is difficult to estimate minimal safe population sizes. M. L. Shaffer (unpublished manuscript) has modeled minimal population sizes and areas for grizzly bears in Yellowstone National Park by using successive com- puter simulations based on estimated population parameters with random variation. Using several scenarios, he estimated that 35-70 bears were needed to have a 95% probability of population survival for 100 years in that ecosystem. Studies of this type can be helpful in focusing attention on the need to choose the time scales at which to judge the acceptability of a given probability of extinction, but the underlying assumptions are not subject to rigorous testing. Long-term management plans can accommodate the possible presence of low-density thresholds by providing a substantial safety margin, par- ticularly for isolated populations with little immigration. It might be dif- ficult to predict these thresholds with the available data, and guesses based on experience with similar species could be necessary (Chapter 17; Soule and Wilcox, 19801. Many defoliating insects appear to have multiple stable equilibria. Such species appear to be controlled at low densities in a density-dependent fashion by predators, parasites, or pathogens. These agents cannot "con- trol" the pest populations above particular sizes, because other factors prevent the populations of the controlling agent from increasing enough (e.g., Campbell and Sloan, 1977; McNamee et al., 19811. Traditional
INDIVIDUALS AND SINGLE POPULATIONS 31 approaches to controlling these species have relied on the massive appli- cation of insecticides during outbreaks. However, control might be more effectively applied to subpopulations threatening to escape from the low- density control (Campbell and Sloan, 1978; MacLeod, 19774. Environ- mentally induced reductions in the vigor of trees can trigger outbreaks of some forest pests, and control techniques can be applied to areas where such conditions appear imminent (Berryman, 1981~. Such-techniques use a system of "risk classification" of the habitats of the pests in determining when and where to apply controls. Another successful approach is the use of a biological control agent that has a shorter generation time than the pest and that can respond to outbreaks by increasing its own population. An example is the partial control o gypsy moths in the northeastern United States with bacteria and viruses (Leonard, 1974; Massachussetts Department of Environmental Manage- ment, 19811. McNamee et al. (1981) have provided a useful framework for classifying defoliating forest insects so that appropriate control strat- egies can be chosen. Dispersion and Population Movements Species vary in their patterns of dispersion in space (clumped, random, or even), density, dispersal, and migratory behavior. Most terrestrial spe- cies occupy habitat patches of various sizes and degrees of isolation. These variations have implications for management. Management of a migratory species must take into account the relationships between populations in breeding and nonbreeding habitats. For example, if the population of salmon at sea is near K, then increasing hatching success in the rivers might have little benefit (Peterman, 1978, 19841. Fretwell (1972) has shown theoretically and by examples that popula- tions in strongly seasonal environments are ultimately controlled in one "bottleneck" season. For example, winter resources might limit the num- ber of individuals that survive to breed, so an increase in the preceding summer reproduction could cause little change in the number of individuals breeding in the following summer. This idea is particularly important in managing bird species that winter in tropical habitats undergoing extensive deforestation. Recognizing that the protection and enhancement of pop- ulations might not be sufficient, in itself, to prevent extinction, legislators of many countries have cooperated in formulating treaties to protect mi- gratory bird species. Migratory patterns can also make species susceptible to local disturbances (e.g., Chapter 16~. Salmon can be affected by a single dam in a migration of thousands of miles, and some migratory birds
32 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS (such as geese, cranes, and swans) depend on a very small number of resting and feeding areas along their migration routes. Animals that organize into dense aggregations can easily be harvested to very low numbers, because the profitable unit for harvest is the aggre- gation. Fish stocks that form aggregations (e.g., many clupeoids) appar- ently are subject to catastrophic collapse when fishing effort is continuously increased (May, 1980, 1984; Murphy, 19771. The situation with some very large whales is analogous, in that it can be profitable to hunt and harvest a single animal. May (1980) has discussed various problems en- countered in managing the harvest of fish that form schools. Isolated populations have less potential for recovering from temporary reductions when immigration rates are low, as happens when an occupied habitat becomes an island surrounded by unsuitable habitat. Maintaining travel corridors of appropriate habitat to connect populations, particularly of species at low density and with strict habitat requirements and poor dispersal ability, might be a key component of management (MacClintock et al., 1977; Willis, 19741. The spotted owl (Chapter 17), a strict obligate of old-growth coniferous forest, is an example of such a species (Foreman et al., 19841. However, if control is the objective, disruption of corridors can be effective in preventing dispersal. The size of the habitat patch necessary to support even a single breeding female or pair depends on the size, diet, and behavior of the species (Gall) et al ., 1 976; Schonewald-Cox et al ., 1 9831. Carnivores generally require more area than herbivores, and area requirements increase with body size (McNab, 1963; Schoener, 19681. Required areas for managing populations of large social carnivores, such as wolves and bears, can be huge (M. L. Shaffer, unpublished manuscript; Soule, 19801. The extinction of bird species on Barro Colorado Island constitutes a particularly revealing example of spatial isolation. The island became isolated from the Central American mainland during the construction of the Panama Canal. One of the best predictors of local extinction was susceptibility to ground predators, such as snakes (Kerr, 19821; the pop- ulations of these predators were increasing, probably because large pre- dators were decreasing as a result of human activities. Growth Rates, Age, and Size The operation of forestry, fisheries, and agriculture depends on an understanding of how growth rates are influenced by interactions with the physical environment and with other organisms. Size is often used as an indicator of age in a continuously growing species, because it is easy to measure and is usually reliable. When age and size are poorly correlated
INDIVIDUALS AND SINGLE POPULATIONS 33 (e.g., Caswell and Werner, 1978; Policansky, 1983), the use of size as an indicator of age can lead to inappropriate management decisions, as it did in a population of roe deer (Capreolus capreolus) in Scotland. The ages of culled deer in that population were estimated on the basis of body size and weight, as is common for deer (Ratcliffe, 1984), but the calves were growing so fast that the technique underestimated their number and overestimated the number of fertile females. The result was overharvest- ing. If resources, such as food and light, are limiting to animals or plants, removing large individuals usually increases the growth rate of those remaining. These interactions are well known to wildlife and fishery man- agers, foresters, and agriculturalists. In the New Brunswick forest-man- agement case (Chapter 19), the inverse relationship between planting density and growth rates of individual trees was an important part of the man- agement plan. An understanding of the factors affecting growth rates among competing species allowed the successful regeneration of land in the derelict-lands case (Chapter 181. Environmental manipulations affecting fecundity or survivorship have the greatest effect on population dynamics if the perturbations are intro- duced at early reproductive stages (Beddington, 1974; Emlen, 1970; Le- wontin, 19651. Expected reproductive output is smaller in late reproductive stages than in early stages, so late stages have a smaller per capita effect on population production. Similarly, many individuals in prereproductive stages will not survive to breed, so their removal affects population re- production less than the removal of early breeders. This is the basis for fishing and hunting regulations that specify minimal sizes or ages. Age Structure Variations in life expectancy or age structure in a population can in- fluence its responses to management. Length of life and age structure are associated with many variables such as spatial distribution, reproductive potential, and feeding habits that are hard to estimate, so generalizations as to the implications of age structure for management can be unreliable. Nonetheless, a few points should be mentioned. Beverton (1984) classified the expected response of fish species to fishing pressure on the basis of their ecological characteristics (including life history). Fish populations comprising long-lived individuals in many reproductive age classes are generally more stable and easier to manage than those having only few age classes comprising short-lived individuals. There is also a tendency for species that are densely aggregated when young to be more stable in the face of fishing pressure. Beverton classified
34 KINDS OF ECOLOGICAL KNOWLEDGE kD THEIR APPLICATIONS halibut as "reliable, steady, and robust," and these life-history charac- teristics probably have been partly responsible for success in halibut man- agement; (Chapter 12~. It is important to think about population stability in terms of the life spans of individuals. A population of long-lived organisms might appear to be maintaining itself when in fact it is in danger of serious decline. If habitat degradation by pollution or other factors prevents young organisms from surviving, the long life span of adults can mask the loss of those age classes. The June sucker (Chamistes liorus) in Utah has maintained populations without recruitment for 15 years (U.S. Fish and Wildlife Service, 1984) and saguaro cactuses (Carnegiea gigantea) in the Sonoran desert remain common in areas that have seen no recruitment for even longer periods (Turner et al., 19691. The other side of this coin is that such populations are able to survive long periods of adverse conditions, even if conditions favorable for reproduction occur only infrequently. Sex Ratios and Sex Biases Either intentionally or unintentionally, human activities often produce mortality differences between the sexes and thus biased sex ratios. The effects on population reproduction and dynamics depend on the form of the mating system. The degree to which differences in mortality can be controlled depends on the difficulty of distinguishing the sexes in the field. In many animal species with polygynous mating systems, one male can fertilize the eggs of many females, so a reduction in the number of males has a relatively small effect on reproductive rates. Management of poly- gynous species in which the sexes are easily distinguished (e.g., deer, crabs, and pheasants) often involves permitting the harvest only of males. In the sockeye salmon (Oncorhynchus nerka), males are larger than fe- males, so gillnets, which select for larger fish, take disproportionately many males. In spite of this skewing of sex ratio, Mathisen (1962) has shown that a 15:1 ratio of females to males led to egg hatching only 5% lower than that with a 1:1 ratio. In contrast, differences in mortality can have large effects in species whose reproduction is limited by whichever sex is the rarer, such as species in which both parents (e.g., geese) or only the males (e.g., rheas and seahorses) care for the young. Sex ratios can be manipulated to increase productivity, including food production, as in the case of dairy cattle. Only the female structures of many commercially important crop plants are eaten (seeds and fruits). In plants with separate sexes, fruit yields can be increased by manipulating the sex ratio. In cosexual species those with both sexual functions in
INDIVIDUALS AND SINGLE POPULATIONS 35 the same individual manipulations are directed toward altering the al- location of investment in female versus male function. Such schemes work when allocation patterns can be genetically selected for or when they are influenced by environmental conditions that can be manipulated. In the Cucurbitaceae (cucumbers, melons, squash, etc.), some genes shift the allocation of resources from male to female function (Kubicki, 1969a,b,c; Robinson et al., 1976), and cultivars that favor female function have been selected (Velich and Satyko, 19741. GENETIC AND EVOLUTIONARY CONCERNS Artificial selection has been successful in producing desirable traits in animals and plants, and the genetic resources of domestic species can be maintained by intentional cross-breeding. Manipulations of the population dynamics of nondomesticated organisms, however, can produce the evo- lution of undesirable genetic consequences, often within a few generations. Conspicuous examples include the evolution of resistance to pesticides and antibiotics (e.g., Anderson and May, 1982; Peters, 1984, in press) and perhaps increased pathogen virulence (Ewald, 1983), decrease in size (Ricker, 1981), and increased inbreeding depression (see below). The Evolution of Resistance to Pesticides One of the best-documented cases of undesired evolutionary change is the development of resistance to DDT (Chapter 241. In retrospect, it should have been obvious that it would happen; in fact, the use of poisons to produce a class of resistant organisms for experimental purposes has been a major technique of microbiology for more than 50 years. The short generation times, high reproductive rates, and large populations of most pests favor rapid evolution (May and Dobson, 19851. The race between chemists and the evolutionary capacities of organisms parallels the race between the evolution of new antiherbivore devices by plants and the evolution of resistance to them by insects (Rhoades, 19831. A number of approaches can be taken to reduce the rate of evolution of resistance. First, agents that interfere with fundamental and invariant processes, such as eating and oxygen uptake, can be used. Changes in fundamental processes are unlikely to evolve over a short period. However, such agents can affect nontarget species, because the processes in question are shared by many of them. The use of anticoagulants against vampire bats (Chapter 13) depended on the spread of the materials by intraspecific social grooming, a process that kept the materials within the target species. Second, two or more chemicals can be used at the same time or in
36 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS close alternation (e.g., Peters, 1984, in press; Smith, 19821. This reduces the number of survivors, because survival would require the evolution of two or more types of resistance simultaneously, which is unlikely in view of the small probability that several mutations will occur at once. Third, agents can be used intermittently. This allows the frequency of evolved resistant types to decrease through competition with nonresistant types (e.g., Levin and Lenski, 19831. However, Plasmodium resistance to chloroquinone actually seems to be associated with a general biological advantage over nonresistant strains (Doberstyn, 1984), so this approach might not be universally applicable. Genetic Consequences of Differential Harvesting by Sex and Size Harvesting according to age or sex is a feature of many management schemes. Animals of one sex might be singled out for harvest (e.g., in deer and crabs) or the capture of animals might be sex-biased for other reasons, such as differences in size or activity (consider, for example, the evidence of size and sex selectivity of fishing gear). Nonrandom harvests could have produced such changes perhaps genetic as early maturity at smaller size in fish populations, e.g., salmon (Mathisen, 1962; Ricker, 1981), Atlantic cod (Borisov, 1978), an African cichlid (Silliman, 1975), and lake whitefish (Handford et al., 1977~; Moav et al. (1978) have suggested that this is a general phenomenon. Many large male salmon are harvested with size-selective nets before they have a chance to spawn; this harvesting favors the survival of early- maturing, small males ~ jacks). Increased proportions of jacks, which have low commercial and sport value, are a matter of growing concern to the managers of exploited salmon populations (Ricker, 19811. Theoretical studies (Gross, 1984, 1985) have suggested that jacks are not abnormal, as previously thought, and that they and larger males exist in an equilibrium that depends on both differences in mortality and conditions in spawning areas. The theory suggests ways of reducing the proportion of jacks despite heavy harvesting of larger males. For example, spawning sites could be manipulated to reduce the cover in which the jacks seek protection from larger males, thus lowering their reproductive success and the proportion of jacks in the next generation. The jack story shows how management can lead to undesirable evo- lution. For example, trophy hunting, which removes the finest specimens from the breeding population, leads to the evolution of deer genotypes with smaller antlers. Hunting in general is known to make animals furtive, and genetic changes in dispersals foraging, and social behavior are pos- sible. In short, the evolutionary effects of management must be considered
INDIVIDUALS AND SINGLE POPULATIONS 37 if it can lead to differences in reproductive success of different genotypes (Law, 19791. Genetic Consequences of Small Population Size Genetic deterioration has often led to extinctions in captive (including laboratory) populations, and small natural populations might also be in such danger, because of inbreeding depression and loss of genetic vari- ability (Franker and Soule, 1981; Schonewald-Cox et al., 1983; Soule and Wilcox, 1980) the former reduces fitness, and the latter reduces adaptive potential. Efforts to protect species threatened by the genetic effects of small population size involve estimating a minimal "effective" population size (minimal Ne) that is based on estimates of tolerable degrees of in- breeding, which in turn are based on experience with domestic and lab- oratory animals. (Ne is the number of potential breeding individuals in an "ideal" population one with random mating, a sex ratio of 1: 1, discrete generations, constant size, and a Poisson distribution of family size that retains the same amount of selectively neutral genetic variability as the population under consideration. If the population departs from "ideal" conditions, as occurs commonly in nature, Ne is lower than actual N.) Selecting tolerable rates of loss of genetic variability is much more difficult than choosing tolerable degrees of inbreeding, because the long- term consequences of reduced variability are not known and could depend on the species involved. Soule (1980) crudely estimated a minimal Ne of 50 on the basis of inbreeding alone, according to an expectation of sur- viving extinction for 1.5 Ne' or 75, generations. Franklin (1980) suggested that, for long-term adaptive potential, Ne should be 10 times as high, i.e., about 500. The consequences of inbreeding might be less serious in species that normally engage in much inbreeding or that are already strongly homozygous (e.g., many self-fertilizing plants). Sex-biased harvesting can also lower genetic variability, perhaps enough to have serious consequences. Ryman et al. (1981) have shown how various moose- and deer-hunting policies can lead to loss of genetic var- iability, with Ne perhaps as low as 5% of N.