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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
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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
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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
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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~.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.
Representative terms from entire chapter:
single populations
