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AP Biology
Ecology Unit
Chapters 52-54
Chapter 52
• Ecology is the scientific study of the
interactions between organisms and the
environment
• These interactions determine distribution of
organisms and their abundance
• Ecology reveals the richness of the biosphere
The Scope of Ecological Research
Ecologists work at levels ranging from individual
organisms to the planet
1-Organismal ecology studies how an organism’s structure, physiology, and (for
animals) behavior meet environmental challenges
2-Population ecology focuses on factors affecting how many individuals of a species
live in an area
•A population is a group of individuals of the same species living in an area
3-Community ecology deals with the whole array of interacting species in a community
•A community is a group of populations of different species in an area
4-Ecosystem ecology emphasizes energy flow and chemical cycling among the
various biotic and abiotic components
•An ecosystem is the community of organisms in an area and the physical factors with
which they interact
5-Landscape ecology deals with arrays of ecosystems and how they
are arranged in a geographic region
•A landscape is a mosaic of connected ecosystems
6-The biosphere is the global ecosystem, the sum of all the planet’s
ecosystems
•Global ecology examines the influence of energy and materials on
organisms across the biosphere
Concept 52.2: Interactions between organisms and
the environment limit the distribution of species
• Ecologists have long recognized global and regional
patterns of distribution of organisms within the biosphere
• Biogeography is a good starting point for understanding
what limits geographic distribution of species
• Ecologists recognize two kinds of factors that determine
distribution: biotic, or living factors, and abiotic, or
nonliving factors
Fig. 52-6
Ecologists consider multiple factors when attempting to explain the
distribution of species
Why is species X absent
from an area?
Yes
Does dispersal
limit its
distribution?
No
Area inaccessible
or insufficient time
Does behavior
limit its
distribution?
Yes
Habitat selection
Yes
No
Do biotic factors
(other species)
limit its
distribution?
No
Predation, parasitism, Chemical
competition, disease factors
Do abiotic factors
limit its
distribution?
Water
Oxygen
Salinity
pH
Soil nutrients, etc.
Temperature
Physical Light
factors Soil structure
Fire
Moisture, etc.
Flowchart of factors limiting geographic distribution
Dispersal and Distribution
• Dispersal is movement of individuals
away from centers of high population
density or from their area of origin
• Dispersal contributes to global distribution
of organisms
Species Transplants
• Species transplants include organisms
that are intentionally or accidentally
relocated from their original distribution
• Species transplants can disrupt the
communities or ecosystems to which they
have been introduced
Fig. 52-8
RESULTS
100
Seaweed cover (%)
80
Both limpets and urchins
removed
Sea urchin
Only urchins
removed
60
Limpet
40
Only limpets removed
Control (both urchins
and limpets present)
20
0
August
1982
February
1983
August
1983
February
1984
Climate
• Four major abiotic components of climate are
temperature, water, sunlight, and wind
• The long-term prevailing weather conditions in
an area constitute its climate
• Macroclimate consists of patterns on the
global, regional, and local level
• Microclimate consists of very fine patterns,
such as those encountered by the community
of organisms underneath a fallen log
Fig. 52-10c
60ºN
30ºN
March equinox
0º (equator)
June solstice
30ºS
Constant tilt
of 23.5º
December solstice
September equinox
Seasonal variations of light and temperature increase steadily toward the
poles
Seasonality
• The angle of the sun leads to many seasonal
changes in local environments
• Lakes are sensitive to seasonal temperature
change and experience seasonal turnover
Concept 52.3: Aquatic biomes are diverse and
dynamic systems that cover most of Earth
• Biomes are the major ecological
associations that occupy broad
geographic regions of land or water
• Varying combinations of biotic and abiotic
factors determine the nature of biomes
• Aquatic biomes account for the largest
part of the biosphere in terms of area
• They can contain fresh water or salt
water (marine)
• Oceans cover about 75% of Earth’s
surface and have an enormous impact on
the biosphere
Fig. 52-16
Stratification of Aquatic Biomes
Intertidal zone
Oceanic zone
Neritic zone
Littoral
zone
Limnetic
zone
0
Photic zone
200 m
Continental
shelf
Benthic
zone
Photic
zone
Benthic
zone
Pelagic
zone
Aphotic
zone
Pelagic
zone
Aphotic
zone
2,000–6,000 m
Abyssal zone
(a) Zonation in a lake
(b) Marine zonation
Many aquatic biomes are stratified into zones or layers defined by light
penetration, temperature, and depth
• In oceans and most lakes, a temperature
boundary called the thermocline separates the
warm upper layer from the cold deeper water
• Many lakes undergo a semiannual mixing of
their waters called turnover
• Turnover mixes oxygenated water from the
surface with nutrient-rich water from the bottom
Fig. 52-17-5
Winter
Summer
Spring
2º
4º
4º
4º
4ºC
0º
4º
4º
Autumn
20º
18º
8º
6º
5º
4ºC
4º
4º
4º
4ºC
Thermocline
22º
4º
4º
4º
4º
4ºC
4º
Lakes
• Oligotrophic lakes are nutrient-poor and generally
oxygen-rich
• Eutrophic lakes are nutrient-rich and often depleted of
oxygen if ice covered in winter
• Rooted and floating aquatic plants live in the shallow and
well-lighted littoral zone
• Water is too deep in the limnetic zone to support rooted
aquatic plants; small drifting animals called zooplankton
graze on the phytoplankton
Wetlands
• A wetland is a habitat that is inundated by water at least
some of the time and that supports plants adapted to
water-saturated soil
• Wetlands can develop in shallow basins, along flooded
river banks, or on the coasts of large lakes and seas
• Wetlands are among the most productive biomes on earth
and are home to diverse invertebrates and birds
Streams and Rivers
• The most prominent physical characteristic of
streams and rivers is current
• A diversity of fishes and invertebrates inhabit
unpolluted rivers and streams
• Damming and flood control impair natural
functioning of stream and river ecosystems
Estuaries
• An estuary is a transition area between river
and sea
• Salinity varies with the rise and fall of the tides
• Estuaries are nutrient rich and highly
productive
• An abundant supply of food attracts marine
invertebrates and fish
Video: Flapping Geese
Oceanic Pelagic Zone
• The oceanic pelagic biome is a vast
realm of open blue water, constantly
mixed by wind-driven oceanic currents
• This biome covers approximately 70% of
Earth’s surface
• Phytoplankton and zooplankton are the
dominant organisms in this biome; also
found are free-swimming animals
Video: Shark Eating a Seal
Coral Reefs
• Coral reefs are formed from the calcium
carbonate skeletons of corals (phylum
Cnidaria)
• Corals require a solid substrate for attachment
• Unicellular algae live within the tissues of the
corals and form a mutualistic relationship that
provides the corals with organic molecules
Video: Coral Reef
Video: Clownfish and Anemone
Concept 52.4: The structure and distribution of terrestrial
biomes are controlled by climate and disturbance
• Climate is very important in determining why
terrestrial biomes are found in certain areas
• Biome patterns can be modified by
disturbance such as a storm, fire, or human
activity
Fig. 52-19
Tropical forest
Savanna
Desert
30ºN
Tropic of
Cancer
Equator
Tropic of
Capricorn
30ºS
Chaparral
Temperate
grassland
Temperate
broadleaf forest
Northern
coniferous forest
Tundra
High mountains
Polar ice
General Features of Terrestrial Biomes
and the Role of Disturbance
• Terrestrial biomes are often named for major
physical or climatic factors and for vegetation
• Terrestrial biomes usually grade into each
other, without sharp boundaries
• The area of intergradation, called an ecotone,
may be wide or narrow
• Vertical layering is an important feature of
terrestrial biomes, and in a forest it might
consist of an upper canopy, low-tree
layer, shrub understory, ground layer of
herbaceous plants, forest floor, and root
layer
• Layering of vegetation in all biomes
provides diverse habitats for animals
• Biomes are dynamic and usually exhibit
extensive patchiness
Terrestrial Biomes
• Terrestrial biomes can be characterized
by distribution, precipitation, temperature,
plants, and animals
Tropical Forest
• In tropical rain forests, rainfall is
relatively constant, while in tropical dry
forests precipitation is highly seasonal
• Tropical forests are vertically layered and
competition for light is intense
• Tropical forests are home to millions of
animal species, including an estimated 5–
30 million still undescribed species of
insects, spiders, and other arthropods
Desert
• Precipitation is low and highly variable,
generally less than 30 cm per year; deserts
may be hot or cold
• Desert plants are adapted for heat and
desiccation tolerance, water storage, and
reduced leaf surface area
• Common desert animals include many kinds of
snakes and lizards, scorpions, ants, beetles,
migratory and resident birds, and seed-eating
rodents; many are nocturnal
Savanna
• Savanna precipitation and temperature
are seasonal
• Grasses and forbs make up most of the
ground cover
• Common inhabitants include insects and
mammals such as wildebeests, zebras,
lions, and hyenas
Chaparral
• Chaparral climate is highly seasonal,
with cool and rainy winters and hot dry
summers
• The chaparral is dominated by shrubs,
small trees, grasses, and herbs; many
plants are adapted to fire and drought
• Animals include amphibians, birds and
other reptiles, insects, small mammals
and browsing mammals
Fig. 52-21d
An area of chaparral
in California
Northern Coniferous Forest
• The northern coniferous forest, or taiga,
extends across northern North America and
Eurasia and is the largest terrestrial biome on
Earth
• Winters are cold and long while summers may be
hot
• The conical shape of conifers prevents too much
snow from accumulating and breaking their
branches
• Animals include migratory and resident birds, and
large mammals
Temperate Broadleaf Forest
• Winters are cool, while summers are hot and
humid; significant precipitation falls year round as
rain and snow
• A mature temperate broadleaf forest has vertical
layers dominated by deciduous trees in the
Northern Hemisphere and evergreen eucalyptus in
Australia
• Mammals, birds, and insects make use of all
vertical layers in the forest
• In the Northern Hemisphere, many mammals
hibernate in the winter
Tundra
• Tundra covers expansive areas of the Arctic;
alpine tundra exists on high mountaintops at all
latitudes
• Winters are long and cold while summers are
relatively cool; precipitation varies
• Permafrost, a permanently frozen layer of soil,
prevents water infiltration
• Vegetation is herbaceous (mosses, grasses,
forbs, dwarf shrubs and trees, and lichen) and
supports birds, grazers, and their predators
Fig. 52-21h
Denali National Park, Alaska,
in autumn
Fig. 52-UN1
Why is species X absent from an area?
Does dispersal limit its distribution?
Yes
Area inaccessible or
insufficient time
No
Does behavior limit its distribution?
Yes
Habitat selection
No
Do biotic factors (other species)
limit its distribution?
No
Yes
Predation, parasitism,
competition, disease
Chemical
factors
Water, oxygen, salinity, pH,
soil nutrients, etc.
Do abiotic factors limit its distribution?
Physical
factors
Temperature, light, soil
structure, fire, moisture, etc.
Fig. 52-T1
Fig. 52-UN3
Mean height (cm)
Fig. 52-UN2
100
50
Altitude (m)
0
3,000
2,000
Sierra Nevada
1,000
0
Seed collection sites
Great Basin
Plateau
Chapter 53
Population ecology is the study of populations
in relation to environment, including
environmental influences on density and
distribution, age structure, and population size
Concept 53.1: Dynamic biological processes
influence population density, dispersion, and
demographics
• A population is a group of individuals of a single
species living in the same general area
• Density is the number of individuals per unit area
or volume
• Dispersion is the pattern of spacing among
individuals within the boundaries of the population
• Density is the result of an interplay between processes that
add individuals to a population and those that remove
individuals
• Immigration is the influx of new individuals from other
areas
• Emigration is the movement of individuals out of a
population
Fig. 53-3
Births
Births and immigration
add individuals to
a population.
Immigration
Deaths
Deaths and emigration
remove individuals
from a population.
Emigration
Patterns of Dispersion
• Environmental and social factors influence
spacing of individuals in a population
• Three types of Dispersion:
• Clumping is most common, individuals aggregate in
patches, may be influenced by resource availability and
behavior
• Uniform is one in which individuals are evenly
distributed, may be influenced by social interactions such
as territoriality
• Random is the position of each individual is
independent of other individuals, it occurs in the absence
of strong attractions or repulsions
Fig. 53-4
(a) Clumped
(b) Uniform
(c) Random
Demographics
• Demography is the study of the vital statistics of
a population and how they change over time
• Death rates and birth rates are of particular
interest to demographers
• A life table is an age-specific summary of the
survival pattern of a population
• It is best made by following the fate of a cohort, a
group of individuals of the same age
• The life table of Belding’s ground squirrels reveals
many things about this population
Table 53-1
Survivorship Curves
• A survivorship curve is a graphic way of
representing the data in a life table
• The survivorship curve for Belding’s ground
squirrels shows a relatively constant death rate
Fig. 53-5
Number of survivors (log scale)
1,000
100
Females
10
Males
1
0
2
4
6
Age (years)
8
10
• Survivorship curves can be classified into
three general types:
– Type I: low death rates during early and middle
life, then an increase among older age groups
– Type II: the death rate is constant over the
organism’s life span
– Type III: high death rates for the young, then a
slower death rate for survivors
Number of survivors (log scale)
Fig. 53-6
1,000
I
100
II
10
III
1
0
50
Percentage of maximum life span
100
Reproductive Rates
• For species with sexual reproduction,
demographers often concentrate on
females in a population
• A reproductive table, or fertility
schedule, is an age-specific summary of
the reproductive rates in a population
• It describes reproductive patterns of a
population
Table 53-2
Concept 53.2: Life history traits are
products of natural selection
• An organism’s life history comprises the traits
that affect its schedule of reproduction and
survival:
– The age at which reproduction begins
– How often the organism reproduces
– How many offspring are produced during each
reproductive cycle
• Life history traits are evolutionary outcomes
reflected in the development, physiology, and
behavior of an organism
Evolution and Life History Diversity
• Life histories are very diverse
• Species that exhibit semelparity, or big-bang
reproduction, reproduce once and die
• Species that exhibit iteroparity, or repeated
reproduction, produce offspring repeatedly
• Highly variable or unpredictable environments
likely favor big-bang reproduction, while
dependable environments may favor repeated
reproduction
Fig. 53-7
“Trade-offs” and Life Histories
• Organisms have finite resources, which
may lead to trade-offs between survival
and reproduction
• In animals, parental care of smaller
broods may facilitate survival of offspring
Fig. 53-8
Parents surviving the following winter (%)
RESULTS
100
Male
Female
80
60
40
20
0
Reduced
brood size
Normal
brood size
Enlarged
brood size
• Some plants produce a large number of small
seeds, ensuring that at least some of them will
grow and eventually reproduce
• Other types of plants produce a moderate
number of large seeds that provide a large
store of energy that will help seedlings become
established
Fig. 53-9
(a) Dandelion
(b) Coconut palm
Concept 53.3: The exponential model
describes population growth in an idealized,
unlimited environment
• It is useful to study population growth in an
idealized situation
• Idealized situations help us understand the
capacity of species to increase and the
conditions that may facilitate this growth
Per Capita Rate of Increase
• If immigration and emigration are ignored, a
population’s growth rate (per capita increase)
equals birth rate minus death rate
• Zero population growth occurs when
the birth rate equals the death rate
• Most ecologists use differential calculus
to express population growth as growth
rate at a particular instant in time:
N 
t rN
where N = population size, t = time, and r = per
capita rate of increase = birth – death
Exponential Growth
• Exponential population growth is population
increase under idealized conditions
• Under these conditions, the rate of
reproduction is at its maximum, called the
intrinsic rate of increase
• Equation of exponential population growth:
dN 
rmaxN
dt
Exponential population growth results in a
J-shaped curve
• The J-shaped curve of exponential
growth characterizes some
rebounding populations
Fig. 53-10
2,000
Population size (N)
dN
= 1.0N
dt
1,500
dN
= 0.5N
dt
1,000
500
0
0
5
10
Number of generations
15
Concept 53.4: The logistic model describes
how a population grows more slowly as it
nears its carrying capacity
• Exponential growth cannot be sustained for long in any
population
• A more realistic population model limits growth by
incorporating carrying capacity
• Carrying capacity (K) is the maximum population size the
environment can support
• In the logistic population growth model, the per capita
rate of increase declines as carrying capacity is reached
• The logistic model of population growth produces
a sigmoid (S-shaped) curve
Fig. 53-12
Exponential
growth
Population size (N)
2,000
dN
= 1.0N
dt
1,500
K = 1,500
Logistic growth
1,000
dN
= 1.0N
dt
1,500 – N
1,500
500
0
0
5
10
Number of generations
15
The Logistic Model and Real Populations
• The growth of laboratory populations of
paramecia fits an S-shaped curve
• These organisms are grown in a constant
environment lacking predators and competitors
Number of Paramecium/mL
Fig. 53-13a
1,000
800
600
400
200
0
0
5
10
Time (days)
15
(a) A Paramecium population in the lab
The Logistic Model and Life Histories
• Life history traits favored by natural selection may
vary with population density and environmental
conditions
• K-selection, or density-dependent selection,
selects for life history traits that are sensitive to
population density
• r-selection, or density-independent selection,
selects for life history traits that maximize
reproduction
Population Change and Population
Density
• In density-independent populations, birth rate
and death rate do not change with population
density
• In density-dependent populations, birth rates
fall and death rates rise with population density
Density-Dependent Population
Regulation
• Density-dependent birth and death rates are an
example of negative feedback that regulates
population growth
• They are affected by many factors, such as
competition for resources, territoriality, disease,
predation, toxic wastes, and intrinsic factors
Competition for Resources
• In crowded populations, increasing population
density intensifies competition for resources
and results in a lower birth rate
Territoriality
• In many vertebrates and some invertebrates, competition
for territory may limit density
• Cheetahs are highly territorial, using chemical
communication to warn other cheetahs of their boundaries
• Oceanic birds exhibit territoriality in nesting behavior
Fig. 53-17
(a) Cheetah marking its territory
(b) Gannets
Disease
• Population density can influence the
health and survival of organisms
• In dense populations, pathogens can
spread more rapidly
Predation
• As a prey population builds up, predators
may feed preferentially on that species
Toxic Wastes
• Accumulation of toxic wastes can contribute to
density-dependent regulation of population size
Intrinsic Factors
• For some populations, intrinsic (physiological)
factors appear to regulate population size
Population Dynamics
• The study of population dynamics focuses on the
complex interactions between biotic and abiotic factors that
cause variation in population size
• Stability and Fluctuation
• Long-term population studies have challenged
the hypothesis that populations of large
mammals are relatively stable over time
• Weather can affect population size over time
• Changes in predation pressure can drive
population fluctuations-Next slide
Fig. 53-19
2,500
50
Moose
40
2,000
30
1,500
20
1,000
10
500
0
1955
1965
1975
1985
Year
1995
0
2005
Number of moose
Number of wolves
Wolves
Concept 53.6: The human population is no
longer growing exponentially but is still
increasing rapidly
• No population can grow indefinitely, and
humans are no exception
• Though the global population is still growing,
the rate of growth began to slow during the
1960s
Regional Patterns of Population Change
• To maintain population stability, a regional
human population can exist in one of two
configurations:
– Zero population growth =
High birth rate – High death rate
– Zero population growth =
Low birth rate – Low death rate
• The demographic transition is the move
from the first state toward the second
state
Birth or death rate per 1,000 people
Fig. 53-24
50
40
30
20
10
Sweden
Birth rate
Death rate
0
1750
1800
Mexico
Birth rate
Death rate
1850
1900
Year
1950
2000 2050
• The demographic transition is associated with
an increase in the quality of health care and
improved access to education, especially for
women
• Most of the current global population growth is
concentrated in developing countries
Age Structure
• One important demographic factor in present and
future growth trends is a country’s age structure
• Age structure is the relative number of individuals
at each age
• Age structure diagrams can predict a population’s
growth trends
• They can illuminate social conditions and help us
plan for the future
Fig. 53-25
Rapid growth
Afghanistan
Male
Female
10 8
6 4 2 0 2 4 6
Percent of population
Age
85+
80–84
75–79
70–74
65–69
60–64
55–59
50–54
45–49
40–44
35–39
30–34
25–29
20–24
15–19
10–14
5–9
0–4
8 10
8
Slow growth
United States
Male
Female
6 4 2 0 2 4 6
Percent of population
Age
85+
80–84
75–79
70–74
65–69
60–64
55–59
50–54
45–49
40–44
35–39
30–34
25–29
20–24
15–19
10–14
5–9
0–4
8
8
No growth
Italy
Male
Female
6 4 2 0 2 4 6 8
Percent of population
Limits on Human Population Size
• The ecological footprint concept
summarizes the aggregate land and
water area needed to sustain the people
of a nation
• It is one measure of how close we are to
the carrying capacity of Earth
• Countries vary greatly in footprint size
and available ecological capacity
• The carrying capacity of Earth for humans is
uncertain
• The average estimate is 10–15 billion
• Our carrying capacity could potentially be
limited by food, space, nonrenewable
resources, or buildup of wastes
Chapter 54
• A biological community is an
assemblage of populations of various
species living close enough for potential
interaction
Concept 54.1: Community interactions are
classified by whether they help, harm, or have no
effect on the species involved
• Ecologists call relationships between species in a
community interspecific interactions
• Examples are competition, predation, herbivory,
and symbiosis (parasitism, mutualism, and
commensalism)
• Interspecific interactions can affect the survival
and reproduction of each species, and the effects
can be summarized as positive (+), negative (–),
or no effect (0)
Competition
• Interspecific competition (–/– interaction) occurs when
species compete for a resource in short supply
• Strong competition can lead to competitive
exclusion, local elimination of a competing
species
• The competitive exclusion principle states that
two species competing for the same limiting
resources cannot coexist in the same place
Ecological Niches
• The total of a species’ use of biotic and
abiotic resources is called the species’
ecological niche
• An ecological niche can also be thought of
as an organism’s ecological role
• Ecologically similar species can coexist in a
community if there are one or more
significant differences in their niches
• Resource partitioning is differentiation of
ecological niches, enabling similar species
to coexist in a community
Fig. 54-2
A. distichus perches on fence
posts and other sunny surfaces.
A. insolitus usually perches
on shady branches.
A. ricordii
A. insolitus
A. aliniger
A. distichus
A. christophei
A. cybotes
A. etheridgei
• As a result of competition, a species’
fundamental niche may differ from its
realized niche
Fig. 54-3
EXPERIMENT
Chthamalus
Balanus
High tide
Chthamalus
realized niche
Balanus
realized niche
Ocean
Low tide
RESULTS
High tide
Chthamalus
fundamental niche
Ocean
Low tide
Character Displacement
• Character displacement is a tendency for
characteristics to be more divergent in sympatric
populations of two species than in allopatric
populations of the same two species
• An example is variation in beak size between
populations of two species of Galápagos finches
Fig. 54-4
G. fuliginosa G. fortis
Percentages of individuals in each size class
Beak
depth
60
Los Hermanos
40
G. fuliginosa,
allopatric
20
0
60
Daphne
40
G. fortis,
allopatric
20
0
60
Sympatric
populations
Santa María, San Cristóbal
40
20
0
8
10
12
Beak depth (mm)
14
16
Predation
• Predation (+/– interaction) refers to
interaction where one species, the predator,
kills and eats the other, the prey
• Some feeding adaptations of predators are
claws, teeth, fangs, stingers, and poison
• Prey display various defensive adaptations
• Behavioral defenses include hiding, fleeing,
forming herds or schools, self-defense, and
alarm calls
• Animals also have morphological and
physiological defense adaptations
• Cryptic coloration, or camouflage, makes
prey difficult to spot
Video: Seahorse Camouflage
Fig. 54-5
(a) Cryptic
coloration
Canyon tree frog
(b) Aposematic
coloration
Poison dart frog
(c) Batesian mimicry: A harmless species mimics a harmful one.
Hawkmoth
larva
Green parrot snake
(d) Müllerian mimicry: Two unpalatable species
mimic each other.
Cuckoo bee
Yellow jacket
• Animals with effective chemical defense often
exhibit bright warning coloration, called
aposematic coloration
• Predators are particularly cautious in dealing with
prey that display such coloration
• In some cases, a prey species may gain
significant protection by mimicking the
appearance of another species
• In Batesian mimicry, a palatable or
harmless species mimics an unpalatable or
harmful model
• In Müllerian mimicry, two or more
unpalatable species resemble each other
Herbivory
• Herbivory (+/– interaction) refers to an
interaction in which an herbivore eats parts
of a plant or alga
• It has led to evolution of plant mechanical
and chemical defenses and adaptations by
herbivores
Symbiosis
• Symbiosis is a relationship where two or
more species live in direct and intimate
contact with one another
Parasitism
• In parasitism (+/– interaction), one
organism, the parasite, derives nourishment
from another organism, its host, which is
harmed in the process
• Parasites that live within the body of their
host are called endoparasites; parasites
that live on the external surface of a host are
ectoparasites
Mutualism
• Mutualistic symbiosis, or mutualism (+/+
interaction), is an interspecific interaction
that benefits both species
• A mutualism can be
– Obligate, where one species cannot survive
without the other
– Facultative, where both species can survive
alone
Video: Clownfish and Anemone
Commensalism
• In commensalism (+/0 interaction), one
species benefits and the other is apparently
unaffected
• Commensal interactions are hard to
document in nature because any close
association likely affects both species
Concept 54.2: Dominant and keystone
species exert strong controls on community
structure
• In general, a few species in a community exert
strong control on that community’s structure
• Two fundamental features of community structure
are species diversity and feeding relationships
Trophic Structure
• Trophic structure is the feeding
relationships between organisms in a
community
• It is a key factor in community dynamics
• Food chains link trophic levels from
producers to top carnivores
Video: Shark Eating a Seal
Fig. 54-11
Quaternary
consumers
Carnivore
Carnivore
Tertiary
consumers
Carnivore
Carnivore
Secondary
consumers
Carnivore
Carnivore
Primary
consumers
Herbivore
Zooplankton
Primary
producers
Plant
Phytoplankton
A terrestrial food chain
A marine food chain
Food Webs
• A food web is a branching food chain with
complex trophic interactions
• Species may play a role at more than one
trophic level
• Food webs can be simplified by isolating a
portion of a community that interacts very
little with the rest of the community
Fig. 54-12
Humans
Smaller
toothed
whales
Baleen
whales
Crab-eater
seals
Birds
Leopard
seals
Fishes
Sperm
whales
Elephant
seals
Squids
Carnivorous
plankton
Euphausids
(krill)
Copepods
Phytoplankton
Limits on Food Chain Length
• Each food chain in a food web is usually
only a few links long
• Two hypotheses attempt to explain food
chain length: the energetic hypothesis and
the dynamic stability hypothesis
• The energetic hypothesis suggests that
length is limited by inefficient energy transfer
• The dynamic stability hypothesis
proposes that long food chains are less
stable than short ones
• Most data support the energetic hypothesis
Species with a Large Impact
• Certain species have a very large impact on
community structure
• Such species are highly abundant or play a
pivotal role in community dynamics
Dominant Species
• Dominant species are those that are most
abundant or have the highest biomass
• Biomass is the total mass of all individuals
in a population
• Dominant species exert powerful control
over the occurrence and distribution of other
species
• One hypothesis suggests that dominant
species are most competitive in exploiting
resources
• Another hypothesis is that they are most
successful at avoiding predators
• Invasive species, typically introduced to a
new environment by humans, often lack
predators or disease
Keystone Species
• Keystone species exert strong control on a
community by their ecological roles, or niches
• In contrast to dominant species, they are not
necessarily abundant in a community
Foundation Species (Ecosystem
“Engineers”)
• Foundation species (ecosystem “engineers”)
cause physical changes in the environment that
affect community structure
• For example, beaver dams can transform
landscapes on a very large scale
• Some foundation species act as facilitators that
have positive effects on survival and reproduction
of some other species in the community
Concept 54.3: Disturbance influences
species diversity and composition
• Decades ago, most ecologists favored the
view that communities are in a state of
equilibrium
• This view was supported by F. E. Clements
who suggested that species in a climax
community function as a superorganism
• Other ecologists, including A. G. Tansley
and H. A. Gleason, challenged whether
communities were at equilibrium
• Recent evidence of change has led to a
nonequilibrium model, which describes
communities as constantly changing after
being buffeted by disturbances
Characterizing Disturbance
• A disturbance is an event that changes a
community, removes organisms from it, and
alters resource availability
• Fire is a significant disturbance in most
terrestrial ecosystems
• It is often a necessity in some communities
• The large-scale fire in Yellowstone National
Park in 1988 demonstrated that
communities can often respond very rapidly
to a massive disturbance
Fig. 54-21
(a) Soon after fire
(b) One year after fire
Ecological Succession
• Ecological succession is the sequence of
community and ecosystem changes after a
disturbance
• Primary succession occurs where no soil
exists when succession begins
• Secondary succession begins in an area
where soil remains after a disturbance
• Early-arriving species and later-arriving
species may be linked in one of three
processes:
– Early arrivals may facilitate appearance of later
species by making the environment favorable
– They may inhibit establishment of later species
– They may tolerate later species but have no
impact on their establishment
• Retreating glaciers provide a valuable fieldresearch opportunity for observing succession
• Succession on the moraines in Glacier Bay,
Alaska, follows a predictable pattern of change in
vegetation and soil characteristics
Fig. 54-22-1
1941
1907
1
Pioneer stage, with
fireweed dominant
0
1860
Glacier
Bay
Alaska
1760
5 10 15
Kilometers
Fig. 54-22-2
1941
1907
2
1
Pioneer stage, with
fireweed dominant
0
1860
Glacier
Bay
Alaska
1760
5 10 15
Kilometers
Dryas stage
Fig. 54-22-3
1941
1907
2
1
Pioneer stage, with
fireweed dominant
0
1860
Dryas stage
5 10 15
Kilometers
Glacier
Bay
Alaska
1760
3
Alder stage
Fig. 54-22-4
1941
1907
2
1
Pioneer stage, with
fireweed dominant
0
1860
Dryas stage
5 10 15
Kilometers
Glacier
Bay
Alaska
1760
4
Spruce stage
3
Alder stage
Human Disturbance
• Humans have the greatest impact on
biological communities worldwide
• Human disturbance to communities usually
reduces species diversity
• Humans also prevent some naturally
occurring disturbances, which can be
important to community structure
Concept 54.4: Biogeographic factors
affect community biodiversity
• Latitude and area are two key factors that
affect a community’s species diversity
Latitudinal Gradients
• Species richness generally declines along
an equatorial-polar gradient and is
especially great in the tropics
• Two key factors in equatorial-polar gradients
of species richness are probably
evolutionary history and climate
• The greater age of tropical environments
may account for the greater species
richness
• Climate is likely the primary cause of the latitudinal
gradient in biodiversity
• Two main climatic factors correlated with
biodiversity are solar energy and water availability
• They can be considered together by measuring a
community’s rate of evapotranspiration
• Evapotranspiration is evaporation of water from
soil plus transpiration of water from plants
Area Effects
• The species-area curve quantifies the idea
that, all other factors being equal, a larger
geographic area has more species
• A species-area curve of North American
breeding birds supports this idea
Island Equilibrium Model
• Species richness on islands depends on
island size, distance from the mainland,
immigration, and extinction
• The equilibrium model of island
biogeography maintains that species
richness on an ecological island levels off at
a dynamic equilibrium point
• Studies of species richness on the Galápagos
Islands support the prediction that species
richness increases with island size
Concept 54.5: Community ecology is useful
for understanding pathogen life cycles and
controlling human disease
• Ecological communities are universally
affected by pathogens, which include
disease-causing microorganisms, viruses,
viroids, and prions
• Pathogens can alter community structure
quickly and extensively
Pathogens and Community Structure
• Pathogens can have dramatic effects on
communities
• Human activities are transporting pathogens
around the world at unprecedented rates
• Community ecology is needed to help study and
combat them
Community Ecology and Zoonotic
Diseases
• Zoonotic pathogens have been transferred
from other animals to humans
• The transfer of pathogens can be direct or
through an intermediate species called a
vector
• Many of today’s emerging human diseases
are zoonotic