CHAPTER 35 POPULATIONS AND COMMUNITIES. WHAT IS ECOLOGY? Ecology is the study of how organisms interact with each other and with their environment. Ecology

CHAPTER 35

POPULATIONS AND COMMUNITIES

WHAT IS ECOLOGY?

• Ecology is the study of how organisms interact with each other and with their environment.

• Ecology also includes the study of the distribution and abundance of organisms; ecology can be studied at progressively more encompassing levels of organization.

WHAT IS ECOLOGY?

• Levels of ecological organization:• 1. Populations - individuals

of the same organism that live together are members of a population.

• 2. Species - consists of all the populations of a particular organism.

• 3. Communities - populations of different species that live together in the same place constitute a community.

WHAT IS ECOLOGY?

• 4. Ecosystems - a community and the nonliving factors with which it interacts is called an ecosystem.

• 5. Biomes - major terrestrial assemblages of plants, animals, and microorganisms that occur over wide geographic areas and have distinctive physical characteristics are called biomes.

• 6. Biosphere- all the world’s biomes, along with its marine and freshwater assemblages, together constitute an interactive system called the biosphere.

WHAT IS ECOLOGY?

• The nature of the physical environment determines to a large extent which organisms live in a certain climate or region.• Key elements of the environment include:

• Temperature• Water• Sunlight• Soil

WHAT IS ECOLOGY?

• Many organisms are able to adapt to environmental changes by making morphological, physiological, or behavioral adaptations.• For example:

• The gray wolf grows a thicker coat of fur in the winter.

• The green iguana lizard escapes to the shade in the heat of the day.

POPULATION RANGE

• Organisms live as members of populations, groups of individuals that occur together at one place or time.

• Five aspects of populations are particularly important:• Population range• Population distribution• Population size• Population density• Population growth

• Most species have relatively limited geographical ranges.• Organisms must be adapted for the

environment in which they occur.

Species that occur in only one place

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Devil's holepupfish

IiwiHawaiian bird

Socorroisopod

Northern white rhinoceros

New Guineatree kangaroo

Iriomote cat

Catalina Islandmahogany tree

POPULATION RANGE

• Population ranges are not static; rather, they change through time.• These changes occur for two reasons:

• In some cases, the environment changes• For example, the range for trees that survive

better in colder temperatures shifts farther up a mountain when temperature increases in an area.

Present

Grassland,chaparral, and

desert scrub

Alpine tundra

Spruce-fir forests

Mixed conifer forest

Woodlands

Ele

vati

on

(km

)

3 km

2 km

1 km

0 kmGrassland, chaparral,

and desert scrub

Woodlands

Mixed conifer forest

Spruce-fir forests

Alpine tundra

15,000 years ago

RANGE EXPANSION OF THE CATTLE EGRET

• In addition, populations can expand their ranges when they are able to move from inhospitable habitats to suitable, previously unoccupied areas

• For example, cattle egret expansion

1966

1964

1965

1970

1960

1961

Equator1956

1970

1937

19431951

Immigrationfrom Africa

1958

POPULATION DISTRIBUTION

• A key characteristic affecting a species’ range is the way in which individuals of its populations are distributed; they may be:• Randomly spaced

• Individuals do not interact strongly with one another.

• Uniformly spaced• Often results from competition for resources.

• Clumped• Clumped spacing results from uneven distribution of

resources in the individuals’ immediate environment.

POPULATION GROWTH

• A population is a group of individuals of a species that live together and influence each other’s survival.

POPULATION GROWTH

• Populations have several properties:• population size is the number of individuals in

the population.• population density is the population size that

occurs in a given area.

POPULATION GROWTH

• Another characteristic about any population is its capacity to grow.• Population growth can be modeled in

different ways that identify what factors in nature limit growth.

POPULATION GROWTH

• Biotic potential, symbolized by r, is the rate at which a population of a given species will increase when no limits are placed on its rate of growth.

• The simplest model of population growth assumes a population growing without limits at its maximal rate.

POPULATION GROWTH

• The exponential growth model is defined by the following formula:

growth rate = G = riN

N is the population sizeG is the change in its numbers over timeri is the intrinsic rate of natural increase for that population

POPULATION GROWTH

• The actual rate of population increase, r, is defined as:

r = (b – d) + (i – e)

• b is the birthrate, d is the death rate.• e is the amount of emigration out of the

area and i is the amount of immigration into the area.

POPULATION GROWTH

• The innate capacity for growth of any population is exponential.• even when the rate of increase remains

constant, the actual increase in the number of individuals accelerates rapidly as the population grows.

• in practice, such patterns prevail for only short periods, usually when an organism reaches a new habitat with abundant resources.

POPULATION GROWTH

• No matter how rapidly populations grow, they eventually reach a limit imposed by shortages of important environmental factors.

• A population ultimately stabilizes at a certain size, called the carrying capacity.• the carrying capacity is symbolized by K and is

defined as the maximum number of individuals that an area can support.

POPULATION GROWTH

• The growth curve of a population that is approaching its carrying capacity can be approximated by the logistic growth equation:

G = rN [(K – N)/K]

• As N approaches K, the rate of population growth (G) begins to slow, until it reaches zero at N = K.

TWO MODELS OF POPULATION GROWTH

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Exponentialgrowth model

Logisticgrowth model

00 5

Po

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)Carryingcapacity

Number of generations (t)

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POPULATION GROWTH

• The sigmoid growth curve is characteristic of most biological populations.

• The processes of competition and emigration tend to increase as a population approaches its carrying capacity. Most natural populations exhibit

logistic growth

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2

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Time (years)

1945193519251915

10

Bre

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ma

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ur

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s)

THE INFLUENCE OF POPULATION DENSITY

• Many factors act to regulate the growth of populations in nature:• density-independent effects

• these effects regulate population growth regardless of population size.

• for example, weather effects or geological events (i.e., volcanoes).

• density-dependent effects• the effect that these factors have on population

growth depends on population size.• these effects grow stronger as the population

size increases.

DENSITY-DEPENDENT EFFECTS

0

Number of breeding females

8070605040302010

Nu

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rviv

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yo

un

g p

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emal

e 5.0

4.0

3.0

2.0

1.0

LIFE HISTORY ADAPTATIONS

• Life history describes the complete life cycle of an organism.• r-selected adaptations

• Favor rapid growth in a habitat with unlimited resources or in unpredictable environments - take advantage of resources when they are available.

• K-selected adaptations• Favor reproduction near the carrying capacity of

the environment.• help survival in an environment in which

individuals are competing for limited resources.

POPULATION DEMOGRAPHY

• Demography is the statistical study of populations.• measures characteristics of populations and

helps predict how population sizes will change in the future.• populations grow if births outnumber deaths

and shrink if deaths outnumber births.• birth and death rates are dependent on age

and sex.


• A cohort is a group of individuals of the same age.• Within a population, every cohort has the

following characteristics:• fecundity, or birthrate, which is defined as

the number of offspring produced in a standard time.

• mortality, or deathrate, which is the number of individuals that die in that period.

• The relative number of individuals in each cohort defines a population’s age structure.


• Sex ratio is the proportion of males and females in a population.• the number of births is usually directly related

to the number of females.

• Age distribution is the proportion of individuals in different age categories.• when a population lives in a constant

environment for a few generations, its age distribution tends to stabilize.


• A survivorship curve is one way to express the age distribution characteristics of a population.• Survivorship is defined as the percentage of an

original population that survives to a given age.• There are three types of survivorship curves:

• type I has the highest mortality for the oldest individuals

• type II has relatively the same mortality risk for all ages

• type III has the highest mortality for the youngest individuals

SURVIVORSHIP CURVES

0

Su

rviv

al p

er t

ho

usa

nd

1

Percent of maximum life span

1,000

100

10

100755025

Human(type I)Hydra

(type II)

Oyster(type III)

COMMUNITIES

• Community refers to the species that occur at any given locality.• Interactions among community members

govern many ecological and evolutionary processes.• for example, predation, competition, and mutualism

affect the population biology of a particular species, as well as the way in which energy and nutrients cycle through the ecosystem.

THE NICHE AND COMPETITION

• The niche an organism occupies is the sum total of all the ways it utilizes the resources of its environment.• Sometimes species are not able to occupy their

entire niche because of the presence or absence of other species.

• Competition describes the interaction when two organisms attempt to use the same resource when there is not enough of the resource to satisfy both.

• interspecific competition occurs between individuals of different species.

• intraspecific competition occurs between individuals of the same species.


• Fundamental niche is the entire niche that an organism may theoretically occupy.

• Realized niche is the actual niche that the organism is able to occupy because of competition.

Competition among two species of barnacles limits niche use

Chthamalus

Semibalanus

Realizedniches

Fundamentalniches


• G. F. Gause demonstrated the principle of competitive exclusion.• If two species are competing for a resource, the

species that uses the resource more efficiently will eventually eliminate the other locally.

• In other words, no two species with the same niche can coexist.

COMPETITIVE EXCLUSION AMONG THREE SPECIES OF PARAMECIUM

0

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Po

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(mea

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40 8 40 8 40 8

0 0

Days Days Days

P.bursaria

P.caudatumP.aurelia

12 16 20 24

100

50

200

12 16 20 24 24201612

200200

150

100

5050

(a)

00 0

04 8 84

Po

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eas

ure

d b

y v

olu

me)

50

75

25

P.caudatumP.aurelia

P.caudatumP.bursaria

Days

(c)(b)

200

50

100

50

Days16 20 2412 12 16 20


• Species in communities act to avoid competition whenever possible.• When niches overlap, two outcomes are

possible:• Competitive exclusion (i.e., winner takes all).• Resource partitioning, which divides up

resources to create two niches.• Thus, persistent competition between two

species is rare in natural communities • Either one species drives the other to

extinction or natural selection reduces the competition between the them.

RESOURCE PARTITIONING AMONG LIZARD SPECIES


• Resource partitioning can often be seen in similar species that occupy the same geographical area.• Such species are sympatric.• When a pair of species occupy the same habitat

(i.e., when they are sympatric), they tend to exhibit greater differences in morphology and behavior than the same two species do when living in different habitats (i.e., when they are allopatric).• The evident differences are called character

displacement and are favored by natural selection to facilitate habitat partitioning and reduce competition.

COEVOLUTION AND SYMBIOSIS

• Coevolution is the adaptation of two or more species to each other.• Examples of coevolution

include:• plants and animal

pollinators• predator-prey

interactions• symbiotic relationships


• In a symbiotic relationship, two or more kinds of organisms live together in often elaborate and more or less permanent relationships.• There are three major kinds of symbiotic

relationships:

• mutualism • parasitism • commensalism


• Mutualism is a symbiotic relationship in which both species benefit.• For example, ants tend to aphids, feeding on

the honeydew that aphids excrete continuously, moving the aphids around, and protecting them from potential predators.


• Parasitism is a symbiotic relationship in which one species benefits while the other is harmed.• This interaction is really a form of predator-prey

relationship but a parasite usually does not kill its host.

• The parasite is much smaller than the host and remains closely associated with it.


• There are many forms of parasitism in nature:• External parasites - also known as ectoparasites,

these parasites feed on the exterior surface of a host.• parasitoids are insects that lay eggs on living hosts.

• Internal parasites - also known as endoparasites, these parasites feed internally on their hosts.

• Brood parasitism is a form of parasitism in which the parasite does not consume the body of its host.• brood parasites are birds, such as cowbirds and cuckoos,

that lay their eggs in the nest of other species for the host to raise.


• Commensalism is a symbiotic relationship that benefits one species but neither hurts nor helps the other.

• Note: There is no clear-cut boundary between commensalism and mutualism

PREDATOR-PREY INTERACTIONS

• Predation is the consuming of one organism by another.• Under laboratory conditions, predators may

exhaust their prey species and then starve.• In nature, predators often have large effects on

prey populations.

0

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th

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(a)

Year

1935192519151905189518851875186518551845

120

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80

40

Snowshoe hareLynx


• Population cycles may be, in some situations, stimulated by predators.• A classic example is

the “10-year cycle” of the snowshoe hare, Lepus americanus, that appears to be under the influence of food plants and predators.


• Predator-prey interactions are an essential factor in the maintenance of communities that are rich and diverse in species.• Predators prevent or greatly reduce competitive

exclusion by reducing the number of individuals of competing species.

• Examples of key predators include sea stars, wolves, and mountain lions.

MIMICRY

• Batesian mimicry is when a palatable species resembles a poisonous one.

• Müllerian mimicry is when several unrelated, but protected, species come to resemble one another.• For example, the colors

black, yellow, and red tend to be common color patterns that warn predators relying on vision.

ECOLOGICAL SUCCESSION

• Succession is the orderly replacement of one community with another.• Primary succession occurs on bare, lifeless

substrates, such as those left behind when a glacier retreats or when a volcanic island emerges.• Pioneering community is the first to

become established.• Secondary succession occurs after an

already established community has been disturbed.

ECOLOGICAL SUCCESSION

• Succession happens because species alter the habitat and the resources available in it, often in ways that favor other species.

• Three dynamic concepts are of critical importance:• Tolerance - early successional stages are characterized

by weedy r-selected species that tolerate harsh conditions but do not compete well.

• Facilitation - the weedy species introduce local changes in the habitat that favor nonweedy species.

• Inhibition - sometimes the changes in habitat caused by one species may inhibit the growth of the species that caused them.

PLANT SUCCESSION PRODUCES PROGRESSIVE CHANGES IN THE SOIL

Year 100

Nit

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(g/m

2 o

f su

rfac

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a

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300

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Year 1

Pioneer mosses

Year 200

Invadingalders

Alderthickets

Spruceforest

Nitrogenin mineral soil

Nitrogenin forest floor

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CHAPTER 35 POPULATIONS AND COMMUNITIES. WHAT IS ECOLOGY? Ecology is the study of how organisms interact with each other and with their environment. Ecology