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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 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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Exponentialgrowth model
Logisticgrowth model
00 5
Po
pu
lati
on
siz
e (N
)Carryingcapacity
Number of generations (t)
1510
250
500
750
1,000
1,250
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
8
6
4
2
0
Time (years)
1945193519251915
10
Bre
ed
ing
ma
le f
ur
se
als
(th
ou
sa
nd
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
mb
er o
f su
rviv
ing
yo
un
g p
er f
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.
POPULATION DEMOGRAPHY
• 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.
POPULATION DEMOGRAPHY
• 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.
POPULATION DEMOGRAPHY
• 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.
THE NICHE AND COMPETITION
• 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
THE NICHE AND COMPETITION
• 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
50
100
150
Po
pu
lati
on
den
sity
(mea
sure
d b
y vo
lum
e)
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
pu
lati
on
den
sit
y(m
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
THE NICHE AND COMPETITION
• 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
THE NICHE AND COMPETITION
• 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
COEVOLUTION AND SYMBIOSIS
• 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
COEVOLUTION AND SYMBIOSIS
• 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.
COEVOLUTION AND SYMBIOSIS
• 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.
COEVOLUTION AND SYMBIOSIS
• 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.
COEVOLUTION AND SYMBIOSIS
• 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
Nu
mb
er o
f p
elts
(in
th
ou
san
ds)
(a)
Year
1935192519151905189518851875186518551845
120
160
80
40
Snowshoe hareLynx
PREDATOR-PREY INTERACTIONS
• 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
• 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
rog
en c
on
cen
trat
ion
(g/m
2 o
f su
rfac
e)
a
b
c
d
300
250
200
150
100
50
Year 1
Pioneer mosses
Year 200
Invadingalders
Alderthickets
Spruceforest
Nitrogenin mineral soil
Nitrogenin forest floor