Linzey - Vertebrate Biology - Chapter 10 docx

Every habitat has a theoretical maximum number of individuals of a given species that it can support for an extended period of time.. Such irruptions, which cause the population to excee

C H A P T E R 1 0

Population Dynamics

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 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

Environmental resistance

Biotic (reproductive) potential Carrying capacity

Irruptive

Cyclic Stable

(c)

Year

400

300

200

100

0

(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 poppop-ulation 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

10

8

6

4

2

0

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

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

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 conparaly-sistent 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.

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 texvirgini-anus) herd (Fig 10.5) Fawn survival was 30 percent

Year

Inside

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.

TABLE 10.1

Number of Predators Removed from Kaibab Deer Habitat

for fur, sport and lion bounty

1964–1976 Private hunters and trappers

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 reproduccondi-tion 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 imNor-plants 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

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

Fawning Shrubland

Chaparral

100

90

80

70

60

50

40

30

20

10

0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

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

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 numbers will 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

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 km2 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

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 fishfish-ers 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)

40

30

20

10

0

6

5

4

3

Clutch

Field vole abundance

(b)

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