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Linzey: Vertebrate Biology 10. Population Dynamics Text © The McGraw−Hill
Companies, 2003
CHAPTER 10
Population Dynamics
■
INTRODUCTION
Animal populations, which are dynamic and constantly
changing, depend on successful reproduction to maintain
their existence. Other important factors in maintaining viable
populations include an adequate food supply, sufficient home
sites, and the effects of dispersal, immigration, emigration,
climate, predation, disease, and parasites. The impact of
some of these factors is density-dependent—that is, the
effect varies according to the population density; for others,
the impact is density-independent—that is, unrelated to
population size.
■
POPULATION DENSITY
Population density is an important variable that can influence
the level of competition for scarce resources. Every habitat
has a theoretical maximum number of individuals of a given
species that it can support for an extended period of time.
This level is known as the carrying capacity (Fig. 10.1a) and
is determined by environmental resistance factors acting on
the reproductive (biotic) potential of a population. It is pri-
marily determined by the availability of food and shelter.
Vertebrates exhibit three basic types of population growth.
Once many species reach the environmental carrying capacity
of their range, they maintain relatively stable populations (Fig.
10.1b). This is especially true of species inhabiting some trop-
ical regions where temperature and rainfall show little vari-

ability. Some species that normally maintain relatively stable
populations experience sharp population increases at irregular
intervals. Such irruptions, which cause the population to
exceed its carrying capacity, may be the result of such transient
factors as a reduction in predators, an increase in food, a favor-
able change in the weather, or any combination of these. Still
other species experience sharp increases in their population
sizes at regular intervals, followed by crashes. Species exhibit-
ing regular cyclic population increases usually do so either every
3 to 4 years or approximately every 10 years.
Reproductive (Biotic) Potential
The maximum number of young that a population can
produce under ideal conditions during a particular time
period is referred to as the reproductive (biotic) poten-
tial of that population. In a healthy, natural population,
the birth rate will equal, if not exceed, the death rate, but
due to environmentally limiting factors, the reproductive
potential is rarely, if ever, reached. Dispersal, immigra-
tion, and emigration may affect the reproductive poten-
tial to a limited degree.
Most populations will level off after the population
reaches a certain size (the carrying capacity). The point at
which population growth levels off varies with the species,
the habitat, and the climate. A natural population will con-
tinue to show fluctuations (seasonal, annual), but they will
generally not be far removed from the average carrying
capacity (Fig. 10.1c).
Each individual can affect the reproductive potential of
its species in one or more of the following ways:
1. By producing more offspring at a time.

2. By having a longer reproductive life, so that it repro-
duces more often during its life span.
3. By reproducing earlier in life. The shorter the genera-
tion time of a species (that is, the younger its members
when they first reproduce), the higher its reproductive
potential (Fig. 10.2).
Reproductive rates vary widely among the vertebrates.
Some fishes such as sturgeon and cod may produce several
million eggs annually, whereas many mammals normally give
birth to only a single young. Factors such as climate and pre-
dation of eggs and/or young have undoubtedly been factors
in the evolution of egg production. Numerous hypotheses
have been proposed to explain clutch size in birds. These
were summarized by Lack (1954), who presented arguments
for and against each hypothesis. Among the principal
hypotheses are the following:
1. Females produce as many eggs as they are physiologi-
cally capable of producing.
Linzey: Vertebrate Biology 10. Population Dynamics Text © The McGraw−Hill
Companies, 2003
Population Dynamics 325
Environmental resistance
Biotic (reproductive) potential
Carrying capacity
(a)
Time
Number of individuals
Irruptive
Cyclic
Stable

(b)
Time
Number of individuals
(c)
Year
Number of breeding pairs
400
300
200
100
0
1940 1950 1960
(a) The theoretical number of individuals of a given species that can be
supported for an extended period of time in a habitat is known as the
carrying capacity. It is determined by environmental resistance factors (pri-
marily food and shelter) acting on the reproductive (biotic) potential of the
population. (b) Some populations remain relatively stable after reaching
the carrying capacity of their range; some experience regular cyclic pop-
ulation increases; and others experience sharp population increases
(irruptions) at irregular intervals. (c) A stable population is illustrated by the
number of breeding pairs of gray herons (Ardea cinerea) in northwestern
England. After recovering from the severe winter of 1947, this population
showed little fluctuation over a 15-year period.
Source: (c) Data from D. Lack, Population Studies in Birds, 1966, Claren-
don Press, New York.
FIGURE 10.1
Time (years)
Brood size (b) = 2
Age at first
reproduction = 1 year

P
opu
l
a
ti
on s
i
ze
10
8
6
4
2
0
12345
2 years
3 years
4 years
Also curve for:
Brood size = 4
Age at first
reproduction = 4
Population growth is dramatically affected by the age at which females
first reproduce. In each of these examples, females produce two off-
spring per year, but the age at which females first reproduce differs for
each curve (first reproduction at 1, 2, 3, or 4 years of age). Changing
the age of first reproduction from 4 to 3 years has the same effect as
doubling the brood size from two to four.
Source: Data from Cole, Quarterly Review of Biology, 29:103, 1954.
FIGURE 10.2

2. Females produce as many eggs as they can successfully
incubate.
3. Females produce approximately the number of eggs
and young that the parent(s) can satisfactorily feed and
care for.
Whereas each of these hypotheses holds true for many
species, many exceptions exist. For example, many birds will
lay additional eggs in their nests if one or more of the orig-
inal eggs is removed. This fact has been of extreme impor-
tance in the attempt to increase the population of endangered
whooping cranes (Grus americana). Females normally pro-
duce two eggs. When biologists remove one egg for artifical
incubation in order to increase the size of captive flocks, the
female usually will produce and incubate a third egg.
Whereas many birds apparently produce as many eggs
as they can satisfactorily incubate, there are other species that
seemingly could incubate more than the number of eggs they
produce in the average clutch. Critics of the third hypothe-
sis point out that precocial birds do not need to expend time
and energy feeding their offspring.
In studies where clutch size was adjusted experimentally
during incubation, larger clutches were associated with signif-
icantly lower percentage hatching success in 11 of 19 studies;
longer incubation periods in 8 of 10 studies; greater loss of
adult body condition in 2 of 5 studies; and higher adult energy
expenditures in 8 of 9 studies (Thomson et al., 1998). Since
incubation does involve metabolic costs and since the demands
of incubation increase sufficiently with clutch size to affect
Linzey: Vertebrate Biology 10. Population Dynamics Text © The McGraw−Hill
Companies, 2003

326 Chapter Ten
breeding performance, Thomson et al. (1998) proposed that
optimal clutch size in birds may in part be shaped by the
number of eggs that the parents can afford to incubate.
Among mammals, small prey species such as mice, voles,
rabbits, and ground squirrels usually produce several litters
annually, each of which consists of several young. Many of
these species make up the primary consumer level in a food
web or food pyramid; they are subsequently consumed by
secondary and tertiary consumers (Fig. 10.3). Bailey (1924)
recorded a meadow vole that produced 17 litters within 12
months. Larger species, such as most ungulates, breed only
once a year and produce a single offspring. Although smaller
species generally produce greater numbers of young annually
than larger species, longevity is also a factor. Many small
mammals have a life expectancy of approximately 1 year.
However, most bats, although small, are long-lived—up to
at least 34 years in Myotis lucifugus (Keen and Hitchcock,
1980; Tuttle, pers. comm., 1992). With the exception of lasi-
urine bats, most North American species produce a single
young annually. Many predators, such as mustelids, canids,
and felids, produce only one or two litters annually.
The ratio of adults to juveniles varies during the year.
Juveniles form a larger proportion of the population during
and immediately after the breeding season in temperate
regions. By fall and early winter, many juveniles have either
matured and become part of the adult population or have
been lost. Most temperate populations, therefore, reach their
largest size in late fall and early winter. Loss of individuals
during the harsher conditions of winter usually leads to low

population levels in late winter, just prior to the breeding
season of many species. In years when the climate is favor-
able and food is plentiful, many species may breed well into
the fall, resulting in larger populations the next year.
Environmental Resistance
Although populations have the biotic potential to increase,
a variety of factors act to limit the number of young actu-
ally produced or that survive. These factors represent the
environmental resistance. Climate (including rainfall, flood-
ing, drought, and temperature) is a primary controlling fac-
tor. Other controls are exerted by intraspecific aggression,
inadequate supply of den sites, predation, disease, and par-
Northern Harrier
Upland plover
Garter snake
Crow
Cutworm
Meadow frog
Grasshopper
Grassland
Badger
Coyote
Weasel
Prairie vole
Pocket gopher
sparrow
A food web for a prairie grassland in the midwestern United States. Arrows flow from the grassland (producer) to various levels of consumers.
FIGURE 10.3
Linzey: Vertebrate Biology 10. Population Dynamics Text © The McGraw−Hill
Companies, 2003

Population Dynamics 327
asites. Resistance factors can be grouped into two categories:
density-dependent and density-independent.
Density-Dependent Factors
Density-dependent factors are those whose effects vary directly
with the density of the population. For example, as population
density increases, suitable home sites and food may become
scarcer per individual. As the rate of individual contacts
increases, intraspecific aggression may increase; females may
stop breeding; the rate at which nestling young may be killed
and/or cannibalized by their parents may increase; and the rate
at which juveniles are forced to disperse may be greater than
what occurred at lower densities. Parasites can increase in
response to population size of the host, and diseases can spread
much more rapidly. For example, when waterfowl congregate
in dense flocks, the incidence of infection and the chances of
an epizootic are increased (Fig. 10.4).
Approximately 600 Mediterranean monk seals (Monachus
monachus) remain in the wild, mostly in groups of about 20.
From May to August, 1997, a catastrophic epizootic struck
the largest social group (Osterhaus, 1997; Harwood, 1998).
Of 270 seals living in a pair of caves on West Africa’s Mau-
ritanian coast, only about 70 survived the disease. Osterhaus
(1997) reported that most of the seals examined harbored a
dolphin morbillivirus, a virus similar to the one that causes
distemper in dogs. Hernandez et al. (1998), however, carried
out histopathological examination of lung and other tissues
from 14 fresh carcasses and found no indication of typical
morbillivirus lesions. There was no evidence of primary viral
damage or secondary opportunistic infections in lung tissue,

which are hallmarks of morbillivirus infections in other
aquatic mammal species. The terminally ill seals exhibited
clinical signs of lethargy, motor incoordination, and paraly-
sis in the water—symptoms consistent with drowning caused
by paralysis due to poisoning. Hernandez et al. (1998), who
identified three species of toxic dinoflagellates in eight water
samples collected from near the colony during the mortality
event, suggested that poisoning by paralytic algal toxins may
have been the cause of death.
Optimal densities may vary seasonally in temperate
areas. For example, the lower flow of many streams during
summer determines the annual carrying capacity for species
like trout (Salmo). The winter food supply may determine the
annual carrying capacity for many species, even though more
animals can be supported during the summer months. In
ungulates, food limitation as a cause of density-dependent
population regulation has been shown for roe deer (Capreo-
lus), wild reindeer (Rangifer), kangaroos (Macropus), wilde-
beests (Connochaetes), and white-eared kobs (Kobus) (reviewed
by Skogland, 1990).
One effect of high population density in herbivores is
overgrazing. Skogland (1990) reported increased tooth wear
and lowered body size and fat reserves in wild reindeer
(Rangifer tarandus). Skogland (1990) stated:
During late winter foraging, lichen mats in the Loiseleria-
Arctostaphylion plant alliance become the only available
vegetation type due to snow cover [Skogland, 1978]. As
the unrooted lichens are grazed off, the animals substi-
tute easily digestible lichens in their diet by the dead
parts of grasses, dwarf shrubs, and also mosses, with

insufficient nutrient content [Skogland, 1984a].
Increased use of crustaceous lichens with encrusted small
rock particles as well as soil particles and detritus in the
ingested diet accelerated molar wear. This lowers chew-
ing efficiency and increases the passage time of larger
plant particles into the digestive system whose ability to
process energy is slowed down [Skogland, 1988].
Although adult female survival rate was not affected, a sig-
nificant negative correlation existed between population den-
sity and juvenile winter survival rate. Calves normally were
not able to compete successfully with conspecifics of higher
rank. Neonatal survival was directly related to maternal con-
dition during the last part of gestation and the calving sea-
son (Skogland, 1984b).
Male common toads (Bufo bufo) tend to call at low den-
sities, but are more likely to remain silent at high densities
(Hoglund and Robertson, 1988). Male wood frog (Rana syl-
vatica) density has a significant effect on the behavior of
searching male wood frogs at breeding ponds (Woolbright
FIGURE 10.4
A concentration of snow and blue geese. When waterfowl congregate
in dense flocks, the incidence of infection and the chances of an epi-
zootic such as fowl cholera are increased.
Linzey: Vertebrate Biology 10. Population Dynamics Text © The McGraw−Hill
Companies, 2003
328 Chapter Ten
et al., 1990). When the male population density is low, males
are more likely to be stationary. As male density increases,
more males actively search for females.
High density reduces growth rates in amphibians, thus

lengthening exposure to predators and possible unfavorable
environmental conditions (e.g., Petranka and Sih, 1986;
Wilbur, 1987, 1988). Van Buskirk and Smith (1991) recorded
significantly reduced survival and growth rates and an increase
in the skewness of the size distributions of individuals with
increasing density in blue-spotted salamanders (Ambystoma
laterale) in Michigan. Individuals in high-density populations
showed an increased skewness in body size, with only a few
salamanders becoming large and most remaining small. This
tendency did not occur in populations with lower densities.
Some species seem to have an inherent self-regulating
population control mechanism. This is especially true in ter-
ritorial species in which individuals space themselves so that
they have an adequate supply of food and shelter. In other
species, when the population increases to a certain size, food
and home sites become scarcer, intraspecific aggression
increases, and breeding decreases or ceases. Many, but not all,
members of the population may emigrate from the area. The
best known example is the lemming (Lemmus) of Norway. As
many members leave the area, more home sites and food
become available for those left behind. Intraspecific aggres-
sion falls, individuals become better nourished, breeding
resumes, and the population begins growing again.
Some species exceed the carrying capacity of their range.
Because of all the environmental factors acting to control
population increase, this is rare for a natural population. It is
most common in those populations that are managed by
humans, such as herds of deer and elk that are confined to
military reservations, parks, and refuges. In addition, herds
of elephants whose ancient migration routes and feeding areas

are being encroached on by an expanding human population
are, through no fault of their own, exceeding the carrying
capacity of their dwindling range. In their attempts to locate
food, they frequently trample crops and break down fences.
In some areas, predator control measures are undertaken
in efforts to increase the numbers of another species, usually
a game species. Predators often cull sick, lame, injured, and
old individuals from a population. When the predator con-
trol measure is implemented, the protected species often
increases and may exceed the carrying capacity of its range.
To prevent overpopulation, the levels of many game species
are controlled by federal and state agencies. These agencies set
“limits” on the number of individuals of each sex of a given
species that can be killed during certain seasons of the year. For-
merly, many people were subsistance hunters and utilized most
parts of an animal that they killed. Today, some hunters still fall
into this category, but most hunters are looking for trophy ani-
mals (biggest rack of antlers, etc.). They attempt to kill the
largest, healthiest male specimens in order to mount their heads.
Only in recent years have regulatory agencies promoted efforts
to cull females from the population in order to balance the sex
ratio and manage reproductive rates. In deer and many other
species, one male will breed with multiple females; thus, pop-
ulations can be more efficiently managed by culling some
females rather than by focusing exclusively on males.
A coyote-proof enclosure was erected encompassing 391
hectares (ha) of pasture on the Welder Wildlife Foundation
Refuge in Texas in 1972 (Teer et al., 1991). The immediate
response was an increased size of the deer (Odocoileus virgini-
anus texanus) herd (Fig. 10.5). Fawn survival was 30 percent

Year
Inside
Deer per sq/km
2
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990
Outside
FIGURE 10.5
White-tailed deer (Odocoileus virginianus texanus) population estimates inside and outside the coyote-proof pasture from 1972 until 1990 at
the Welder Wildlife Foundation Refuge in Texas.
Source: Data from J. G. Teer, et al., “Deer and Coyotes: The Welder Experiments” in Transactions of the 56th North American Wildlife and Natural
Resources Conference 550–560, 1991.
Linzey: Vertebrate Biology 10. Population Dynamics Text © The McGraw−Hill

Companies, 2003
Population Dynamics 329
TABLE 10.1
Number of Predators Removed from Kaibab Deer Habitat
Dates Mountain Lions Coyotes Wolves Bobcats
1906–1923. U.S. Bur. of Biol. Survey 674 3,000 11 120
1929–1939. Private hunters and trappers 142 4,488 20 743
for fur, sport and lion bounty
1940–1947. USFWS 36 1,401 0 396
1948–1963. USFWS 18 733 0 282
1964–1976. Private hunters and trappers
for fur and sport. 13 No records 0 No records
Lion bounty ended 1970
From C. Y. McCulloch, in Southwestern Naturalist, 31(2):217, 1986. Copyright © The Southwestern
Naturalist. Reprinted by permission.
higher inside the enclosure than outside, where coyotes
(Canis latrans) were uncontrolled. After several years, the
high number of deer caused forage to become scarce, and deer
began to die. Studies showed that mortality was caused pri-
marily by lack of adequate food. Parasite loads also had
increased. The deer herd reached a low point in 1980, after
which it began to increase as food supplies returned to nor-
mal. Beginning in 1982, coyotes were once again present
within the enclosure, and predation prevented the herd from
increasing as it had during the years of predator control.
A classic example of a species exceeding the carrying
capacity of its range involved the Kaibab mule deer (Odocoileus
hemionus) in northern Arizona. In 1906, President Theodore
Roosevelt set aside approximately 750,000 acres as a game
refuge. At that time, an estimated 4,000 deer inhabited the

area. Not only was hunting prohibited, but a predator con-
trol program was begun. Within the next 10 years, 600 moun-
tain lions were killed. The small wolf population living on the
area was almost exterminated by 1926, and it was completely
eliminated by 1939. By 1939, more than 7,000 coyotes had
been killed (Table 10.1). The winter food supply was the lim-
iting factor that determined the annual carrying capacity,
which was estimated to be about 30,000 animals. By 1920,
however, an estimated 100,000 deer were present on the
refuge. During the next two winters, 60 percent of the pop-
ulation died of starvation. An estimated 75 percent of the
fawn crop was lost during the winter of 1924–25.
The numbers continued to decrease due to the depleted
range. By 1939, the population had declined to 10,000 animals.
However, publicly funded coyote control (trapping, shooting,
and poisoning) continued in the area from 1940 until May
1963, resulting in phenomenally high deer densities in the early
1950s (McCulloch, 1986). Presumably due to the high density
and inadequate food supply, deer were in poor physical condi-
tion and reproduction was low. McCulloch (1986) noted that
absolute comparisons of herd size estimates could not be made
for the different eras such as 1906–1940 vs. 1950–1961 vs.
1972–1979 because the deer inventory methods varied and
were not compatible. Private sport hunting and fur trapping
continued after 1963. Mountain lions were designated as game
animals in 1971, and now can be taken normally only during
designated hunting seasons by sport hunters. As of 1977, an
estimated 40 adult mountain lions inhabited the Kaibab
(McCulloch, 1986). During the period 1972–1979, the deer
population experienced a decline of 9 percent per year (Barlow

and McCulloch, 1984). Barlow and McCulloch (1984) stated:
“Reasons for the decrease in deer abundance from 1972 to 1979
are not yet known. Climatic factors and increased natural pre-
dation are both suspected. We know, however, that the decline
has not continued. Pellet counts indicate that the number of
deer in Kaibab has increased dramatically since 1979, and now
may have exceeded the 1972 population size.” Thus, high lev-
els of reproduction and deer in good physical condition have
been achieved without predator control programs. As of 1996,
an estimated 30,000 deer were living on the refuge.
In many areas of the world, increasing wildlife popula-
tions are creating problems. The spread of communicable
diseases, such as rabies, has been slowed by reducing the pop-
ulations of striped skunks, raccoons, and foxes (Bickle et al.,
1991). Various control methods involving shooting, trapping,
and poisoning have been used. More recently, fertility-
inhibiting implants and contraceptive vaccines are being
tested for birth control purposes (Moore et al., 1997). Nor-
plant implants containing levonorgestrel, a synthetic proges-
tin, have proved to be effective fertility inhibitors in several
species, including Norway rats (Rattus norvegicus), rabbits
(Oryctolagus cuniculus), striped skunks (Mephitis), and humans
(Homo sapiens) (Phillips et al., 1987; Brache et al., 1990; Bickle
et al., 1991). A single administration of immunocontraceptive
vaccine was effective for more than 3 years in gray seals (Hali-
choerus grypus) (Brown et al., 1996). A vaccine made from pig
ovaries has been used successfully as a contraceptive to control
wild horse populations on Assateague Island in Maryland and
white-tailed deer on Long Island (Kemp, 1988; Daley, 1997).
The vaccine is made of minced pig ovaries that are distilled

until only the membrane of the eggs (zona pellucida) is left.
This is then mixed with a substance that helps stimulate the
immune system. When the mixture is injected, it causes the
horse’s body to form antibodies that bind to the outside of the
egg when the female ovulates, blocking the sperm receptor
Linzey: Vertebrate Biology 10. Population Dynamics Text © The McGraw−Hill
Companies, 2003
330 Chapter Ten
sites there and preventing fertilization (Daley, 1997). In lab-
oratory tests, the vaccinations have proven to be 90 percent
effective and reversible. Fertility returns within a year.
BIO-NOTE 10.1
Elephant Birth Control Programs
Female African elephants usually are in heat just 2 days
of every 17 weeks. In an attempt to control the expand-
ing elephant population in South Africa’s Kruger
National Park, testing began on two forms of contracep-
tion in 1996. The first method involved injecting
specially-designed estrogen implants into 31 non-
pregnant females. The implant is designed to slowly
release hormones into the bloodstream in much the same
manner as the contraceptive pills used by women. In 6
months, no cow became pregnant. However, the contra-
ceptive caused the females to be permanently in heat,
which in turn caused the bulls to be in a perpetual state of
sexual excitement. These unintended side effects resulted
in a breakdown of the close-knit elephant societies and
social responsibilities, including the loss of several baby
elephants because their mothers were permanently dis-
tracted by as many as eight sexually excited males at one

time. Although unwanted pregnancies were prevented, the
social cost was too high, and this population control pro-
gram was discontinued in April 1997.
The second method is based on creating an
immunological response: a vaccine made from the outer
coating of egg cells taken from pigs produces antigens
that prevent elephant eggs from recognizing elephant
sperm. Initial data from 21 elephant cows has shown the
vaccine to be only about 60 percent effective, a problem
the researchers predict they can overcome by giving
booster inoculations every 10 months.
Daley, 1997
Old World rabbits (Oryctolagus cuniculus) were intro-
duced successfully to Australia by British settlers in 1859, and
to New Zealand a few years later (Grzimek, 1990). The rab-
bits reproduced until they numbered in the hundreds of mil-
lions, causing an ecological disaster in the southern half of
Australia (see Fig. 3.34). Unchecked, the burgeoning rabbit
population creates deserts by devouring plants, shrubs, and
seedlings. The widespread destruction of vegetation seriously
harmed the sheep-raising industry. A number of native Aus-
tralian marsupial species have been endangered or totally
eliminated through competition with, or by having their
habitats destroyed by, the Old World rabbit. Competition
with rabbits for burrows has caused the extinction of one
species of bilbie, or rabbit-eared bandicoot (Macrotis leucura),
and has caused a second species (Macrotis lagotis) to retreat
to northern Australia, where it is listed as endangered. Other
marsupials adversely affected by rabbits include mulgaras
(Dasycercus cristicauda), hairy-nosed wombats (Lasiorhinus

latifrons), long-nosed potaroos (Potorous tridactylus), and
banded hare wallabys (Lagostrophus fasciatus). Livestock,
including introduced sheep and cattle, struggle to compete
with the rabbits for pasture.
In the early 1950s, Australian government scientists
released myxomatosis, a rabbit-killing virus (Kaiser, 1995;
Adler, 1996a; Drollette, 1996; Seife, 1996). Although quite
successful at first, myxomatosis gradually became less effec-
tive, particularly in Australia’s dry rangelands. In 1991,
researchers began testing a calicivirus known as rabbit hem-
orrhagic disease (RHD) virus. It kills quickly and fairly pain-
lessly by causing blood clots in the lungs, heart, and kidneys.
In March 1995, following laboratory testing, it was injected
into rabbits quarantined on Wardang Island in Spencer Gulf,
South Australia. By late September, however, the virus had
evaded containment (possibly by flying insects) and spread
to the mainland, killing rabbits hundreds of kilometers
inland. It appears to kill 80 to 95 percent of the adult rab-
bits it encounters. In September 1996, the Australian gov-
ernment announced a nationwide campaign to reduce the
annual $472-million damage that rabbits cause to agriculture.
The lethal rabbit virus was to be released at 280 sites. The
expectation is that, after the calicivirus kills most of the rab-
bits, it will remain in the reduced population and act as a
long-term regulator of the rabbit population.
The virus appears to be working exactly as animal con-
trol and health officials had hoped (Drollette, 1997). The
wild rabbit population has dropped by 95 percent in some
regions, and native fauna and flora are already staging a
comeback.

Opponents fear that the virus could jump the species
barrier (Anonymous, 1996). For this reason, the New
Zealand Department of Agriculture decided not to intro-
duce the virus pending further study (Duston, 1997). How-
ever, in August 1997, officials confirmed that several dead
rabbits near Cromwell in New Zealand tested positive for the
rabbit calicivirus (Pennisi, 1997d). It is suspected that the
virus may have been released intentionally. The virus quickly
spread across hundreds of miles, making containment and
eradication impossible.
Density-Independent Factors
Climatic factors such as rainfall, flooding, drought, and tem-
perature often play a major role in limiting population
growth. Fires and volcanic eruptions also affect populations
without regard to their density.
Most species in temperate areas are seasonal breeders,
with temperature being a major factor affecting reproduction.
They produce their young during the time of year that is
most favorable for their survival. Most fishes, amphibians,
and reptiles breed in late winter or spring. Birds breed and
raise their young during the warmer months of the year. Most
mammals produce their young during the same optimum
period. Most bats breed in the fall, but because of delayed
fertilization (see Chapter 9), the ova are not fertilized until late
winter or early spring, and young are born shortly thereafter.
Linzey: Vertebrate Biology 10. Population Dynamics Text © The McGraw−Hill
Companies, 2003
Population Dynamics 331
Months
Fawning

Shrubland
Chaparral
Deer per square mile
100
90
80
70
60
50
40
30
20
10
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov De
c
FIGURE 10.6
Comparison of mule deer (Odocoileus hemionus) population density
through the year on poor range (chaparral) and good range (shrubland)
in California. Females living in shrubland produced an average of 1.65
fawns annually, whereas does in chaparral habitat averaged 0.77
fawns annually. Over a 4-year period, shrubland does will produce an
average of 6.48 fawns, whereas does in chaparral habitat will produce
an average of only 3.08 fawns.
Source: Data from R. D. Taber, Transactions of the 21st North American
Wildlife Conference, 1956.
Temperature controls the food supply for many species.
A late spring freeze that kills flying insects or forces them to
become dormant can have disastrous effects on insectivorous
birds such as swallows and purple martins (Progne subis), as

well as on bats. A freeze that kills the buds of oak, hickory,
and other mast-bearing trees can create hardship for many
animals in late summer, fall, and winter. For example, turkeys,
squirrels, deer, bears, and others depend on acorns, hickory
nuts, and other mast for their late summer food supply. Mass
emigrations of some forms such as gray squirrels (Sciurus car-
olinensis) have been reported during years of poor food sup-
ply (Seton, 1920; Flyger, 1969; Gurnell,1987). During such
mass movements, more individuals are susceptible to preda-
tion, and many more than normal are struck and killed by
vehicles; natural mortality probably also increases. Some
species, such as black bears, often leave the protective con-
fines of parks and refuges in search of food. Many are shot
as nuisance bears when they wander into civilization; others
become victims of hunters or motor vehicles.
Members of a species living in an optimal habitat gener-
ally produce more young than members of the same species liv-
ing in a poor habitat. A study of mule deer (Odocoileus
hemionus) in California revealed that does in good shrubland
habitat produced an average of 1.65 fawns annually, whereas
does in poor chaparral habitat averaged 0.77 fawns each (Taber,
1956) (Fig. 10.6). At this rate over a 4-year period, shrubland
does will produce an average of 6.48 fawns, whereas does in
chaparral habitat will produce only an average of 3.08 fawns.
The breeding season following a poor food year also
usually results in fewer young being born. Litter and clutch
sizes will be smaller in many species. Depending on the
severity of the food shortage, female white-tailed deer
(Odocoileus virginianus), for example, may resorb a develop-
ing fetus or give birth to no more than one young. Herd

sizes obviously will decrease as the average production per
female decreases.
Rainfall, or the lack thereof, can drastically affect the
breeding of certain groups, especially amphibians and water-
fowl. If breeding ponds and pools dry up before the larvae
and tadpoles can successfully metamorphose, annual recruit-
ment may approach zero. Many nesting waterfowl are much
more susceptible to predators during periods of drought.
Extensive periods of rainfall and flooding also can be disas-
trous for many species.
The deaths of 158 manatees along Florida’s Gulf Coast
between Naples and Fort Myers during a 3-month period in
the spring of 1996 was caused by red tide algae. A red tide
is a natural algal (Gymnodinium breve) bloom that sporadi-
cally occurs along the coast and produces brevitoxin, a pow-
erful neurotoxin. Unseasonably cold weather farther north
brought a large concentration of manatees to Florida’s Gulf
Coast, and a strong northwest wind blew a potent strain of
the red tide algae deep into manatee feeding areas (Fig. 10.7).
Manatees swam in contaminated water, drank it, and ate sea
grass infected with it. When the toxin level got high enough,
it attacked the manatees’ nervous system. One of the first
nerve centers to be incapacitated was the one that regulates
FIGURE 10.7
During the winter months, manatees (Trichechus manatus) congregate in
the Crystal River in Florida, a sanctuary of warm water with an abun-
dance of water hyacinth.
Linzey: Vertebrate Biology 10. Population Dynamics Text © The McGraw−Hill
Companies, 2003
332 Chapter Ten

the diaphragm—the major muscle used by mammals for
breathing. Many manatees suffocated. Levels of brevitoxin 50
to 100 times normal were found in tissues from the lungs,
stomachs, kidneys, and livers (Holden, 1996b). The result
was the greatest number of manatee deaths from a single
event since record keeping began in 1974. This deadly red
tide, along with deaths from other natural causes, cold
weather stress, boats on Florida’s waterways, and other unde-
termined factors caused 415 manatee deaths in 1996, more
than twice as many as the previous record of 206 deaths in
1990 (Anonymous, 1997d). The total Florida manatee pop-
ulation in 1996 was 2,639.
Cycles
Populations of some species such as lemmings and voles show
rhythmic fluctuations (Fig. 10.8). Their populations increase for
several years and then fall dramatically. This cycle is repeated
with some regularity. Three- or 4-year cycles are characteristic
of certain species inhabiting tundra and northern boreal forests,
such as lemmings (Lemmus and Dicrostonyx), voles (Microtus),
ptarmigan (Lagopus), and spruce grouse (Dendragopus), as well
as some of the birds and mammals that prey on these species.
Some species inhabiting the northern coniferous forests, such
as lynx (Lynx canadensis), hares (Lepus americanus), and ruffed
grouse (Bonasa umbellus), have a longer 10-year cycle.
Due to the intricacies of most food webs, anything affect-
ing one species also will affect one or more additional species.
When a prey species is abundant, its numberswill be reflected
in increasing numbers of the predatory species (Fig. 10.9).
Better-nourished females will be able to produce and suc-
cessfully care for a larger number of offspring than if they

Year
1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971
New York
Percent of stems cut
100
80
60
40
20
Wisconsin
Ohio
FIGURE 10.8
Comparison of cyclic population fluctuations of the meadow vole (Microtus penn-
sylvanicus) in Wisconsin, Ohio, and New York.
Source: Data from U.S. Fish and Wildlife Service, 1971.
BIO-NOTE 10.2
Invasion of the Brown Tree Snakes
The U.S. territory of Guam is being overrun by brown
tree snakes (Boiga irregularis), a nocturnal, tree-climbing,
bird-eating, egg-gobbling, mildly poisonous reptile that
can reach 3 m in length. Brown tree snakes, which origi-
nally found their way to Guam some 50 years ago,
encountered no natural predators and an abundant food
supply. The population of these snakes has soared to an
estimated 2,000,000 or more—about 10,000 per 1.6 km
2
.
The snakes hang like vines from trees, fences, and power
poles. Power outages caused by electricity arcing across
snakes spanning power lines have become a frequent prob-

lem. These snakes have eliminated Guam’s native lizards
and 9 of 18 species of Guam’s native forest birds; 6 of the
remaining species are endangered, and the other 3 are
rare. Research is under way to control the snake popula-
tion by using a strain of virus that will kill the snakes
without affecting other animal life. Extensive efforts are
being taken to prevent this snake from invading Hawaii,
which is home to 40 percent of the nation’s endangered
birds (many of which are already threatened by introduced
wildlife). Snake-sniffing beagles and their handlers closely
inspect every commercial and military flight from Guam.
Douglas, 1997
Allen, 1998
Fritts and Rodda, 1999
were malnourished and/or emaciated (Madsen and Shine,
1992). In addition, many predators will turn their efforts to
a secondary prey if their primary prey becomes scarce. Erlinge
Linzey: Vertebrate Biology 10. Population Dynamics Text © The McGraw−Hill
Companies, 2003
Population Dynamics 333
et al. (1991) suggested that predation has a significant influ-
ence on the pattern of change in a population. In ecosystems
dominated by predators specializing on a single species, a
cyclic pattern is promoted, whereas in ecosystems dominated
by switching “generalist” predators, cyclicity is limited.
Numerous studies of snowshoe (varying) hares (Lepus
americanus) and a variety of predators have shown significant
predator responses to hare cycles (Brand et al., 1976; Brand
and Keith, 1979; Powell, 1980; Todd et al., 1981; Thompson
and Colgan, 1987) (see Chapter 13). For example, snowshoe

hares are the primary prey of many fisher (Martes americana)
populations. Bulmer (1974, 1975) examined fur sale records
in Canada and concluded that population fluctuations of fish-
ers were linked to hare cycles. However, a study of fishers in
Minnesota during eight winters when the snowshoe hare pop-
ulation declined revealed that fishers consumed less hare as
the hare population declined (33% of the diet during 1977–79,
but only 3% in 1984). Consumption of small mammals (deer
mice, Peromyscus; voles, Microtus, Clethrionomys; lemmings,
Synaptomys; shrews, Blarina, Sorex; and moles, Condylura),
however, increased from 4 to 5 percent during 1977–79, to 19
percent of the weight of the stomach contents in 1984. Fat
deposits and reproduction (proportion of pregnant females,
mean number of corpora lutea, and proportion of juveniles in
the fisher harvest) by the fishers did not decrease during the
period of the study (Kuehn, 1989).
MacLulich (1937) presented the original data on cyclic
fluctuations of snowshoe hare and Canadian lynx (Lynx
canadensis) populations obtained from records of pelts
received by the Hudson Bay Company and covering the
period from 1845 to 1935 (Fig. 10.10). These data show that
these cycles have been going on for as long as records have
been kept in North America. It now serves as a classic study
of how the cyclic fluctuations of one species (prey) appar-
ently affect another species (predator). More recent studies
have shown, however, that lynx are not the primary cause of
periodic drops in hare populations, although they may be a
contributing factor in the decline. Furthermore, Stenseth et
al. (1999) found that the dynamics of lynx populations could
be grouped according to three geographical regions of

Canada that differed in climate and proposed that external
factors such as weather influence lynx population density.
In reference to snowshoe hares, Lack (1954) stated: “It
is suggested that the basic cause of the cycles is the domi-
nant rodent [snowshoe hare] interacting with its vegetable
food to produce a predator-prey oscillation. When the pri-
mary consumers decline in numbers, their bird and mammal
predators become short of food, prey upon and cause the
decrease of the gallinaceous birds of the same region, and
themselves die of starvation and/or emigrate.” Keith (1974)
and Keith and Windberg (1978) proposed an essentially
identical theory to explain the 10-year snowshoe hare and
grouse cycles. A similar theory was also proposed to explain
the 3- to 4-year vole–predator–small game cycle in Sweden
(Hornfeldt, 1978).
Hares normally feed on the bark and twigs of birch,
poplar, alder, and black spruce (Fig. 10.11). As hare popula-
tions increase, food becomes scarcer, and the hares are forced
to feed on the young shoots of these plants, which contain
large amounts of toxins (see Chapter 13). The plant toxins
act as antifeedants, resulting in a loss of weight and a decline
in health in the hares, which causes them to be more sus-
ceptible to predation (Joggia et al., 1989; Reichardt et al.,
1990a, b). Thus, it appears that the chemical defenses of cer-
tain plants serve as a density-dependent means of regulating
hare populations, at least indirectly. While hare populations
are low, the vegetation recovers, stimulating a resurgence of
hare populations and initiating another cycle. It may well be
a combination of limited food resources, climatic conditions,
and predation—rather than any single phenomenon alone—

that explains cycles in hare populations.
Some researchers feel that some cycles can be explained
by another type of nutrient recovery, namely, seed produc-
tion (Pitelka, 1964). Many northern plants have seed cycles
of approximately 3 1/2 years. These plants require this time
to build up sufficient nutrient material to produce seeds.
Year
1978
(a)
1980 1982 1984 1986 1988 1990 1992
Mean clutch size
Field vole abundance
7
50
40
30
20
10
0
6
5
4
3
Clutch
Field vole abundance
01020304050
(b)
Mean clutch size
7
6

5
4
3
r
= 0.87,
p
< 0.001
Voles
(a) Average annual clutch sizes of barn owls (Tyto alba) show a cyclic
pattern clearly in synchrony with the field vole (Microtus agrestis) cycle
near Esk, Scotland. (b) Average barn owl clutch sizes in the Esk study
area were closely correlated with spring field vole abundance.
Data from Taylor, Barn Owls, 1994, Cambridge University Press.
FIGURE 10.9
Linzey: Vertebrate Biology 10. Population Dynamics Text © The McGraw−Hill
Companies, 2003
334 Chapter Ten
Such a cycle corresponds closely to the population cycles of
many small mammals.
In northern Scandinavia, microtine rodent populations
are cyclic. Interestingly, the production of willow ptarmigan
(Lagopus lagopus) chicks is usually low in microtine rodent
crash years. Although some plants in this region produce
compounds (estrogens) that have negative effects on bird and
mammal reproduction, it could not be found that intense
herbivory from the microtine rodents induced production of
plant estrogens in the spring food plants of the ptarmigan
(Hanssen et al., 1991).
Gliwicz (1990) proposed that an important intrinsic fac-
tor regulating population diversity is a regular dispersal of first-

born young of the year from their natal habitats. Population
cycles in microtine rodents are an ecological consequence of
this dispersal behavior, which normally results in annual cycles.
Multiannual cycles occur only under certain sets of extrinsic
factors. Low abundance of predators or sufficient snow cover
to reduce predation is required for cycles to occur.
Cyclicity only occurs in communities with few predators
(Hansson and Henttonen, 1988). High predation pressure
normally prevents small rodents from population cycling by
Years
Hare
Lynx
Number (thousands)
140
120
100
80
60
40
20
1845 1855 1865 1875 1885 1895 1905 1915 1925 1935
(b)
(a)
Population cycles for the snowshoe hare (Lepus americanus) and its major predator, the Canadian lynx (Lynx
canadensis). The 9- to 10-year cycles are based on records of the Hudson Bay Company. Increases and
decreases in the hare population are followed by corresponding increases and decreases in the lynx population.
Source: Data based on the number of lynx and snowshoe hare pelts received by the Hudson Bay Company in the
years indicated.
FIGURE 10.10
Linzey: Vertebrate Biology 10. Population Dynamics Text © The McGraw−Hill

Companies, 2003
Population Dynamics 335
FIGURE 10.11
Snowshoe hare (Lepus americanus) browsing intensively on an early
successional shrub. As the density of hare populations increases, trees
such as alder, poplar, black spruce, and birch become overbrowsed.
The new shoots that emerge contain high concentrations of toxins,
which result in a loss of weight and decline in health of the hares, caus-
ing them to be more susceptible to predation. As the hare populations
decline, the vegetation recovers.
keeping their densities low, especially during winter and early
spring (Hansson, 1979; Erlinge et al., 1983; Erlinge, 1987).
Although many possible explanations, including
extraterrestrial events such as sunspot cycles, have been pro-
posed to explain cycles (reviewed by Keith, 1963, and Rose-
berry and Klimstra, 1984), their most immediate and
probable cause is the interaction of the population and its
environment. In addition, some species may possess an inher-
ent self-regulation (biological clock) that triggers the cyclic
events. Cyclic trends in local populations are poorly docu-
mented primarily because intensive, long-term data for such
populations are generally lacking.
Irruptions
Enormous numbers of animals sometimes occur in a given
area for brief periods of time because of certain environmental
and climatic conditions. Abnormally high numbers of a given
species are known as irruptions and are of limited duration
(Figs. 10.1b and 10.12). Conditions leading to irruptions
may involve a combination of mild winters, an abundant food
supply, and the removal of predators (as with Kaibab deer).

The greatest irruption ever recorded in North America
involved the montane vole (Microtus montanus). This spec-
tacular irruption occurred in 1906–1908 in Nevada and Cal-
ifornia (Piper, 1909). In some areas, estimated population
density exceeded 25,000 voles/ha. Approximately 10,000 ha
of alfalfa—stems, leaves, and roots—were destroyed in
Humboldt County, Nevada. In 1957–58, a smaller, but more
extensive, outbreak of montane voles occurred in California,
Oregon, Washington, Idaho, Nevada, Utah, southwestern
Montana, and western Wyoming (Spencer, 1959; White,
1965). Although most densities per hectare were in the hun-
dreds, maximum population density in some areas was esti-
mated to be between 5,000 and 7,500 individuals/ha.
Predator populations (owls, hawks, gulls) increased (by
immigration) and helped to lower vole densities. A high
incidence of tularemia, caused by the bacterium Pasturella
tularensis, was found among these animals during the sub-
sequent decline and may have been a (density-dependent)
factor in the population decline. Another factor in the
decline of high populations may be the exhaustion of the
adrenal gland (see the Endocrine System section in Chap-
ter 9) brought on by stress caused by the high level of
intraspecific interaction (Christian, 1950, 1959, 1963;
Autrum and Holst, 1968; Holst, 1969). Stress stimulates
the adrenal medulla to secrete epinephrine (adrenalin) and
norepinephrine (noradrenalin). These adrenal secretions
assist in preparing an animal’s body for stressful situations
by altering blood flow, adjusting heart and breathing rates,
decreasing action of the digestive system, and so forth.
Under conditions of continuous stress, this chemical control

system may cease functioning.
Year
Number of raccoons taken
24,000
20,000
16,000
12,000
8,000
4,000
0
1850 1860 1870 1880 1890 1900 1910
FIGURE 10.12
A 60-year record of raccoon pelts taken in the same general area
and purchased by the Hudson Bay Company illustrates the irruptive
changes in the population. These irruptive changes were caused by
occasional improvements in the weather followed by returns to more
normal conditions.
Source: Data from Miller, Resource Conservation and Management, 1990,
Wadsworth, Inc., Belmont, CA.
Linzey: Vertebrate Biology 10. Population Dynamics Text © The McGraw−Hill
Companies, 2003
336 Chapter Ten
Supplemental Reading
Chitty, D. 1996. Do Lemmings Commit Suicide? New York:
Oxford University Press.
Gunderson, H. L. 1978. A mid-continental irruption of Canada
lynx, 1962–1963. Prairie Naturalist 10:71–80.
Morris, D. W., Z. Abramsky, B. J. Fox, and M. R. Willig. 1989.
Patterns in the Structure of Mammalian Communities. Lubbock:
Texas Tech University Press.

Slobodkin, L. B. 1980. Growth and Regulation of Animal Populations.
New York: Dover Publications.
Thompson, H. V., and C. M. King (eds.). 1994. The European
Rabbit: History and Biology of a Successful Colonizer.New York:
Oxford University Press.
Wildlife Conservation. May–June 1996 issue. Entire issue devoted
to cats—bobcats, lynx, ocelots, jaguars, cougars, tigers, lions,
cheetahs, etc.
Vertebrate Internet Sites
Visit the zoology website at to find live
Internet links for each of the references listed below.
1. T
errestrial Mammals of the Arctic.
Information from a text on Arctic mammals; data on 48 species
from the book’s appendix.
2. T
he Lynx and the Hare.
An analysis of the interrelationship between lynx and hares in
the Northwest Territory of Canada.
3. P
opulation Ecology.
This site provides on-line data, information from lecture
courses, and the names of organizations, people, and journals
involved in population ecology.
4. P
redator–Prey Dynamics.
Links to information from a class in population dynamics.
5. Beklemishe
v.
Information on this famous Russian population biologist and

his theories.
Review Questions
1. What defines the carrying capacity for a particular environ-
ment? How are carrying capacities for game species regulated
by state wildlife agencies?
2. List several factors that affect carrying capacity.
3. How does carrying capacity relate to humans? List several ways
in which humans have increased the carrying capacities for
certain regions.
4. Differentiate between density-dependent and density-indepen-
dent factors. Give examples of limiting factors that come into
play when a population of mammals reaches very high density.
5. List some modern techniques that are being used to control
exploding vertebrate populations.
6. The release of the rabbit hemorrhagic disease virus in Aus-
tralia has been very controversial. If the virus should jump the
species barrier, what steps would you undertake to keep it
under control?
7. List several theories that have been proposed to explain cyclic
fluctuations in mammals.
8. How do irruptions differ from cyclic fluctuations?