Chapter 35

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Transcript Chapter 35

POPULATIONS AND
COMMUNITIES
CHAPTER 35
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.
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Devil's hole
pupfish
Iiwi
Hawaiian bird
Socorro
isopod
Iriomote cat
New Guinea
tree kangaroo
Catalina Island
mahogany tree
Northern white rhinoceros
Species that occur in only one place
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
15,000 years ago
Alpine tundra
Elevation (km)
3 km
Spruce-fir forests
Spruce-fir forests
2 km
Mixed conifer forest
1 km
Alpine tundra
Woodlands
Grassland, chaparral,
0 km
and desert scrub
Mixed conifer forest
Woodlands
Grassland,
chaparral, and
desert scrub
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
1970
1964
1965
1960
Immigration
from Africa
1961
1958
Equator
1951 1943
1956
1970
1937
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 size
G is the change in its numbers over time
ri 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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Population size (N)
1,250
Carrying
capacity
Exponential
growth model
1,000
750
Logistic
growth model
500
250
0
0
5
10
Number of generations (t)
15
• 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.
Breeding male fur seals (thousands)
POPULATION GROWTH
10
8
6
4
2
0
1915
1925
1935
1945
Time (years)
Most natural populations exhibit
logistic growth
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.
Number of surviving young per female
DENSITY-DEPENDENT EFFECTS
5.0
4.0
3.0
2.0
1.0
0
10
20
30
40
50
60
Number of breeding females
70
80
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
Survival per thousand
1,000
Human
(type I)
Hydra
(type II)
100
Oyster
(type III)
10
1
0
25
50
75
100
Percent of maximum life span
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
FundamentalRealized
niches
niches
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.
Population density
(measured by volume)
COMPETITIVE EXCLUSION AMONG
THREE SPECIES OF PARAMECIUM
200
200
200
50
150
150
P.aurelia
P.caudatum
100
100
100
50
50
50
0
0
0
0
8
4
12
16
20
24
0
4
(a)
Population density
(measured by volume)
Days
8
12
16
20
24
0
4
12
8
Days
16
Days
P.caudatum
P.aurelia
200
P.bursaria
P.caudatum
P.bursaria
75
50
100
50
50
25
0
0
0
4
8
12 16
Days
20
24
0
4
8
12
Days
16
20
20
24
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.
PREDATOR-PREY INTERACTIONS
• Population cycles
may be, in some
situations, stimulated
by predators.
(a)
160
Number of pelts (in thousands)
• 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.
Snowshoe hare
Lynx
120
80
40
0
1845 1855 1865 1875 1885 1895 1905 1915 1925 1935
Year
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
c
Nitrogen concentration
(g/m2 of surface)
300
250
d
200
Nitrogen
in forest floor
150
100
b
50
a
Year 1
Pioneer mosses
Nitrogen
in mineral soil
Year 100
Invading Alder
alders
thickets
Year 200
Spruce
forest