Ecology and evolution:
Populations, communities,
and biodiversity
4
This lecture will help you understand:
• How evolution generates
biodiversity
• Speciation, extinction, and
the “biodiversity crisis”
• Population ecology
• Community ecology
• Species interactions
• Conservation challenges
• Evolution by natural
selection
Key Words
adaptation
adaptive trait
age distribution
age structure
age structure diagrams
allopatric speciation
anthropogenic
artificial selection
biodiversity
biological diversity
biosphere
biotic potential
carnivores
carrying capacity
climax community
clumped distribution
community
competition
decomposers
density dependent
detritivores
ectoparasites
Emigration
endemic
endoparasites
environmental resistance
evolution
exponential growth
extinction
food chain
food web
fossil
fossil record
growth rate
habitat selection
habitats
herbivores
heritable
host
immigration
interspecific competition
intraspecific competition
invasive species
keystone species
K-strategist
limiting factors
logistic growth
mass extinction
mutations
mutualism
natural selection
niche
omnivores
parasite
parasitism
Key Words
phylogenetic trees
pioneer species
pollination
population density
population dispersion
population distribution
population growth curves
population size
predation
predator
prey
primary consumers
primary succession
random distribution
resource partitioning
r-strategists
secondary consumers
secondary succession
sex ratio
speciation
species
succession
symbioses
tertiary consumers
trophic levels
uniform distribution
Central Case: Striking Gold in a Costa Rican
Cloud Forest
• The golden toad of Monteverde, discovered in 1964, had
disappeared 25 years later.
• Researchers determined that warming and drying of the forest was
most likely responsible for its extinction.
• As the global climate changes, more such events can be expected.
Biodiversity
Biodiversity, or biological diversity, is the sum of an
area’s organisms, considering the diversity of species, their
genes, their populations, and their communities.
A species is a particular type of organism; a population or
group of populations whose members share certain
characteristics and can freely breed with one another and
produce fertile offspring.
Biodiversity
Costa Rica’s Monteverde cloud forest is home to many
species and possesses great biodiversity.
Figure 5.1
Natural selection
Natural selection rests on three indisputable facts:
• Organisms produce more offspring than can survive.
• Individuals vary in their characteristics.
• Many characteristics are inherited by offspring from
parents.
Natural selection
THEREFORE, logically…
• Some individuals will be better suited to their
environment; they will survive and reproduce
more successfully.
• These individuals will transmit more genes to
future generations.
• Future generations will thus contain more genes
from better-suited individuals.
• Thus, characteristics will evolve over time to
resemble those of the better-suited ancestors.
Natural selection
Fitness = the likelihood that an individual will reproduce
and/or
the number of offspring an individual
produces over its lifetime
Adaptive trait,
or adaptation = a trait that increases an individual’s
fitness
Natural selection
Evidence of natural
selection is all around us:
In nature …
Diverse bills have
evolved among species of
Hawaiian honeycreepers.
Figure 4.23a
Beak Types Resulting From Natural Selection
Fruit and seed eaters
Insect and nectar eaters
Greater Koa-finch
Kuai Akialoa
Amakihi
Kona Grosbeak
Crested Honeycreeper
Akiapolaau
Maui Parrotbill
Apapane
Unknown finch ancestor
Natural selection
Evidence of
natural
selection is all
around us:
… and in our
domesticated
organisms.
Dog breeds, types of cattle, improved
crop plants—all result from artificial
selection (natural selection conducted
by human breeders).
Figure 4.23b
Speciation
The process by which new species come into being
Speciation is an evolutionary process that has given Earth
its current species richness—more than 1.5 million
described species and likely many million more not yet
described by science.
Allopatric speciation is considered the dominant mode of
speciation, and sympatric speciation also occurs.
Allopatric speciation
1. Single interbreeding
population
2. Population divided by a
barrier; subpopulations
isolated
Figure 5.2
Allopatric speciation
3. The two populations
evolve independently,
diverge in their traits.
4. Populations reunited
when barrier removed,
but are now different
enough that they don’t
interbreed.
Figure 5.2
Allopatric speciation
Many geological and climatic events can serve as barriers
separating populations and causing speciation.
on.
Chemical Evolution
(1 billion years)
Formation
of the
earth’s
early
crust and
atmosphere
Small
organic
molecules
form in
the seas
Large
organic
molecules
(biopolymers)
form in
the seas
Biological Evolution
(3.7 billion years)
Single-cell
prokaryotes
form in
the seas
Single-cell
eukaryotes
form in
the seas
Variety of
multicellular
organisms
form, first
in the seas
and later
on land
First
protocells
form in
the seas
Stanley Miller's experiment animation.
Click to view
animation.
Stabilizing Selection
Click to view
animation.
Disruptive Selection
Click to view
animation.
Number of individuals
Niches and Natural Selection
Niche
separation
Specialist species
with a narrow niche
Niche
breadth
Region of
niche overlap
Resource use
Generalist species
with a broad niche
Various Niches and Their Adaptations
Herring gull is a
tireless scavenger
Black skimmer
seizes small fish
at water surface
Flamingo
feeds on
minute
organisms
in mud
Scaup and other
diving ducks feed on
mollusks, crustaceans,
and aquatic vegetation
Brown pelican dives for fish,
which it locates from the air
Avocet sweeps bill through
mud and surface water in
search of small crustaceans,
insects, and seeds
Dowitcher probes deeply
into mud in search of
snails, marine worms,
and small crustaceans
Oystercatcher feeds on
clams, mussels, and
Louisiana heron wades into other shellfish into which
water to seize small fish
it pries its narrow beak
Ruddy turnstone
searches
under shells and
pebbles for small
invertebrates
Knot (a sandpiper) picks up
worms and small crustaceans
left by receding tide
Piping plover feeds
on insects and tiny
crustaceans on
sandy beaches
Geographic Separation
Arctic Fox
Northern
population
Early fox
population
Spreads
northward
and
southward
and
separates
Different environmental
conditions lead to different
selective pressures and evolution
into two different species.
Adapted to cold through
heavier fur, short ears,
short legs, short nose.
White fur matches
snow for camouflage.
Southern
population
Gray Fox
Adapted to heat
through lightweight
fur and long ears,
legs, and nose, which
give off more heat.
Mimicry
Span worm
Wandering leaf insect
Poison dart frog
Viceroy butterfly mimics
monarch butterfly
Bombardier beetle
Hind wings of io moth
resemble eyes of a
much larger animal
Foul-tasting monarch
butterfly
When touched, the
snake caterpillar
changes shape to look
like the head of a snake
Phylogenetic trees
Life’s diversification results from countless speciation
events over vast spans of time.
Evolutionary history of divergence is shown with diagrams
called phylogenetic trees.
Similar to family genealogies, these show relationships
among organisms.
Phylogenetic trees
These trees are constructed by analyzing patterns of
similarity among present-day organisms.
This tree shows all of life’s major groups.
Figure 5.4
Phylogenetic trees
Within the group Animals in the previous slide, one can
infer a tree of the major animal groups.
Figure 5.4
Phylogenetic trees
And within the group Vertebrates in the previous slide, one
can infer relationships of the major vertebrate groups, and
so on…
Figure 5.4
Extinction
Extinction is the disappearance of an entire species from
the face of the Earth.
Average time for a species on Earth is ~1–10 million years.
Species currently on Earth = the number formed by
speciation minus the number removed by extinction.
Extinction
Some species are more vulnerable to extinction than others:
• Species in small populations
• Species adapted to a narrowly specialized resource
or way of life
Monteverde’s golden toad was apparently such a specialist,
and lived in small numbers in a small area.
Extinction
Some species are more vulnerable to extinction than others:
• Species in small populations
• Species adapted to a narrowly specialized resource
or way of life
Monteverde’s golden toad was apparently such a specialist,
and lived in small numbers in a small area.
Life’s hierarchy of levels
Life occurs in levels:
from the atom up to
the molecule to
the cell to
the tissues to
the organs to
the organism…
Figure 5.7
Life’s hierarchy of levels
… and from the organism
to the population to
the community to
the ecosystem to
the biosphere.
Ecology deals with these
levels, from the organism
up to the biosphere.
Figure 5.7
Ecology
The study of:
the distribution and abundance of organisms,
the interactions among them,
and the interactions between organisms and
their abiotic environments
Ecology is NOT environmental advocacy!
(= a common MISUSE of the term)
Habitat and niche
Habitat = the specific environment where an organism lives
(including living and nonliving elements: rocks, soil, plants,
etc.)
Habitat selection = the process by which organisms choose
habitats among the options encountered
Niche = an organism’s functional role in a community
(feeding, flow of energy and matter, interactions with other
organisms, etc.)
Population ecology
Population = a group of individuals of a species that live in
a particular area
Several attributes help predict population dynamics
(changes in population):
• Population size
• Population density
• Population distribution
• Age structure
• Sex ratio
Population size
Number of individuals present at a given time
Population size for the golden toad was 1,500+ in 1987,
and zero a few years later.
Population density
Number of individuals per unit area or,
Number of individuals per unit volume
Population density for the harlequin frog increased locally
as streams dried and frogs clustered in splash zones.
Population distribution
Spatial arrangement of individuals
Clumped
Random
Uniform
Figure 5.8
Age structure
Or age distribution =
relative numbers of
individuals of each age or
age class in a population
Age structure diagrams,
or age pyramids, show
this information.
Figure 5.9
Age structure
Pyramid weighted
toward young:
population growing
Pyramid weighted
toward old: population
declining
Figure 5.9
Sex ratio
Ratio of males to females in
a population
Even ratios (near 50/50) are
most common.
Fewer females causes slower
population growth.
Note human sex ratio
biased toward females at
oldest ages.
Population growth
Populations grow, shrink, or remain stable,
depending on rates of birth, death, immigration,
and emigration.
(birth rate + immigration rate) –
(death rate + emigration rate)
= population growth rate
Exponential growth
Unregulated populations increase by exponential growth:
Growth by a fixed
percentage, rather
than a fixed amount.
Similar to growth
of money in a
savings account
Exponential growth in a growth curve
Population
growth curves
show change in
population size
over time.
Scots pine shows
exponential
growth
Figure 5.10
Limits on growth
Limiting factors restrain exponential population growth,
slowing the growth rate down.
Population growth levels off at a carrying capacity—the
maximum population size of a given species an
environment can sustain.
Initial exponential growth, slowing, and stabilizing at
carrying capacity is shown by a logistic growth curve.
Logistic growth curve
Figure 5.11
Population growth: Logistic growth
Logistic growth (shown here in yeast from the lab) is only
one type of growth curve, however.
Figure 5.12a
Population growth: Oscillations
Some populations fluctuate continually above and below
carrying capacity, as with this mite.
Figure 5.12b
Population growth: Dampening oscillations
In some populations, oscillations dampen, as population
size settles toward carrying capacity, as with this beetle.
Figure 5.12c
Population growth: Crashes
Some populations that rise too fast and deplete resources
may then crash, as with reindeer on St. Paul Island.
Figure 5.12d
Density dependence
Often, survival or reproduction lessens as populations
become more dense.
Density-dependent factors (disease, predation, etc.)
account for the logistic growth curve.
Biotic potential and reproductive strategies
Species differ in strategies for producing young.
Species producing lots of young (insects, fish, frogs, plants)
have high biotic potential.
Others, such as mammals and birds, produce few young.
However, those with few young give them more care,
resulting in better survival.
POPULATION SIZE
Biotic Potential
Growth factors
(biotic potential)
Abiotic
Favorable light
Favorable temperature
Favorable chemical environment
(optimal level of critical nutrients)
Biotic
High reproductive rate
Generalized niche
Adequate food supply
Suitable habitat
Ability to compete for resources
Ability to hide from or defend
against predators
Ability to resist diseases and parasites
Ability to migrate and live in other
habitats
Ability to adapt to environmental
change
Decrease factors
(environmental resistance)
Abiotic
Too much or too little light
Temperature too high or too low
Unfavorable chemical environment
(too much or too little of critical
nutrients)
Biotic
Low reproductive rate
Specialized niche
Inadequate food supply
Unsuitable or destroyed habitat
Too many competitors
Insufficient ability to hide from or defend
against predators
Inability to resist diseases and parasites
Inability to migrate and live in other
habitats
Inability to adapt to environmental
change
Survivorship
Late loss
Percentage surviving (log scale)
100
10
1
0
Early loss
K-strategists
Terms come from:
K = symbol for carrying
capacity. (Populations
tend to stabilize near K.)
K-Selected Species
Saguaro
Elephant
Fewer, larger offspring
High parental care and protection of offspring
Later reproductive age
Most offspring survive to reproductive age
Larger adults
Adapted to stable climate and environmental conditions
Lower population growth rate (r)
Population size fairly stable and usually close to
carrying capacity (K)
Specialist niche
High ability to compete
Late successional species
r-Selected
r-Selected Species
Dandelion
Cockroach
Many small offspring
r = intrinsic rate of
population increase.
(Populations can
potentially grow fast,
have high r.)
Little or no parental care and protection of offspring
Early reproductive age
Most offspring die before reaching reproductive age
Small adults
Adapted to unstable climate and environmental
conditions
High population growth rate (r)
Population size fluctuates wildly above and below
carrying capacity (K)
Generalist niche
Low ability to compete
Early successional species
Community ecology
Ecologists interested in how populations or species interact
with one another study community ecology.
Community = a group of populations of different species
that live in the same place at the same time
e.g., Monteverde cloud forest community–golden
toads, quetzals, trees, ferns, soil microbes, etc.
Roles in communities: Producers
By eating different foods, organisms are at different trophic
levels, and play different roles, in the community
Plants and other photosynthetic organisms are producers.
Figure 5.14b
Primary consumers
Animals that eat plants are primary consumers, or
herbivores, and are at the second trophic level.
Figure 5.14b
Secondary consumers
Animals that eat herbivores are secondary consumers, at
the third trophic level.
Figure 5.14b
Detritivores and decomposers
Detritivores and
decomposers eat
nonliving organic
matter; they recycle
nutrients.
Figure 5.14b
Trophic levels
Together these comprise trophic levels.
Figure 5.14b
Food chains and webs
We can represent feeding interactions (and thus energy
transfer) in a community:
Food chain: Simplified linear diagram of who eats
whom
Food web: Complex network of who eats whom
Food web for an eastern deciduous forest
Figure 5.14a
Keystone species
Species that have especially great impacts on other
community members and on the community’s identity
If keystone species are removed, communities
change greatly.
A “keystone” holds an arch together.
Figure 5.15a
Keystone species
When the keystone sea otter is removed, sea urchins
overgraze kelp and destroy the kelp forest community.
Figure 5.15b
Balance of Life
(a) Southern sea otter
(b) Sea Urchin
(c) Kelp bed
Predation
One species, the predator, hunts, kills, and consumes the
other, its prey.
Figure 5.16
Predation drives adaptations in prey
Cryptic coloration:
Camouflage to hide
from predators
Warning coloration:
Bright colors warn
that prey is toxic
Mimicry:
Fool predators
(here,
caterpillar
mimics snake)
Figure 5.18
Competition
When multiple species seek the same limited resource
Interspecific competition is between two or
more species.
Intraspecific competition is within a species.
Usually does not involve active fighting, but subtle contests
to procure resources.
Interspecific competition
Different outcomes:
Competitive exclusion = one species excludes the other
from a resource.
Species coexistence = both species coexist at a ratio of
population sizes, or stable equilibrium.
Competitive Exclusion Principle
Click to view
animation.
Interspecific competition
Adjusting resource use, habitat use, or way of life over
evolutionary time leads to:
Resource partitioning = species specialize in different
ways of exploiting a resource.
Character displacement = physical characters evolve
to become different to better differentiate resource use.
Resource partitioning
Tree-climbing bird
species exploit
insect resources in
different ways.
Figure 5.20
Parasitism
One species, the parasite,
exploits the other species,
the host, gaining benefits
and doing harm.
Figure 5.21
Mutualism
Both species benefit one another.
Hummingbird pollinates flower while gaining nectar for itself.
Figure 5.22
Mutualism
Oxpeckers and black rhinoceros
Mycorrhizae fungi on juniper
seedlings in normal soil
Clown fish and sea anemone
Lack of mycorrhizae fungi on
juniper seedlings in sterilized soil
Succession
A series of regular, predictable, quantifiable changes
through which communities go
• Primary succession: Pioneer species colonize a
newly exposed area (lava flows, glacial retreat,
dried lake bed).
• Secondary succession: The community changes
following a disturbance (fire, hurricane, logging).
Primary aquatic succession
1. Open pond
2. Plants begin to cover
surface; sediment
deposited
3. Pond filled by sediment;
vegetation grows over
site
Figure 5.24
Secondary terrestrial succession
Figure 5.23
Succession
Click to view
animation.
Ecosystem Characteristics at Immature and Mature Stages of Ecological Succession
Characteristic
Immature Ecosystem
(Early Successional Stage)
Immature Ecosystem
(Late Successional Stage)
Small
Large
Low
High
Mostly producers, few decomposers
Mixture of producers, consumers,
and decomposers
Ecosystem Structure
Plant size
Species diversity
Trophic structure
Ecological niches
Community organization
(number of interconnecting
links)
Few, mostly generalized
Many, mostly specialized
Low
High
Ecosystem Function
Biomass
Net primary productivity
Food chains and webs
Low
High
High
Low
Simple, mostly plant
with few decomposers
herbivore
Complex, dominated by
decomposers
Efficiency of nutrient recycling Low
High
Efficiency of energy use
High
Low
Table
Invasive species
A species that spreads widely and rapidly becomes
dominant in a community, changing the community’s
normal functioning
Many invasive species
are non-native, introduced
from other areas.
Purple loosestrife invades
a wetland.
Figure 5.25
Climate change and Monteverde
Monteverde’s cloud forest become drier in the 1970s–
1990s.
Number of dry days rose
Stream flow fell
From The Science behind the Stories
Climate change and Monteverde
Cool ocean; low
clouds; mountains
receive moisture
Warm ocean; high
clouds; mountains
get less moisture
From The Science behind the Stories
Viewpoints: Conservation of Monteverde?
Robert
Lawton
“A few committed people
can have an impact.
Conservation efforts must
take into account local social
aspirations. Conservation
can lead to economic
success. But local
conservation is not enough.”
Nathaniel
Wheelwright
“Whatever negative local impact
the steady onslaught of ecotourists
may have on resplendent quetzals
and howler monkeys, it is more
than compensated for by inspiring
people to appreciate tropical forests
and their own natural heritage.”
From Viewpoints
Conclusions: Challenges
Earth’s biodiversity faces a mass extinction event caused by
human actions.
Climate change may alter communities and cause species
extinctions.
Invasive species pose a new threat to community stability.
Conservation efforts need to consider local economies and
social conditions in order to succeed.
Evolution and natural selection provide a strong explanation
for how Earth’s life diversified.
Conclusions: Solutions
There is still time to avoid most species extinctions
threatened by human actions.
Studies like those at Monteverde are clarifying the effects of
climate change.
Ecological restoration efforts can remove invasive species
and restore original communities.
Many conservation efforts today are locally run or promote
local economies.
QUESTION: Review
Allopatric speciation requires…?
a. Natural selection
b. More than two populations
c. Some kind of barrier separating populations
d. Sex ratio bias in one population
QUESTION: Review
Which is a K-strategist?
a. A dragonfly that lays 300 eggs and flies away
b. An oak tree that drops its acorns each year
c. A bamboo plant that flowers only once every 20
years
d. A human who raises three children
e. A fish on the second trophic level
QUESTION: Review
Which of the following lists of trophic levels is in the
correct order?
a. Producer, secondary consumer, herbivore
b. Producer, herbivore, secondary consumer
c. Secondary consumer, producer, detritivore
d. Herbivore, carnivore, producer
QUESTION: Review
Primary succession would take place on all of the following
EXCEPT…?
a. The slopes of a Hawaiian volcano’s new lava
flow
b. A South Carolina coastal forest after a hurricane
c. Alaskan land just uncovered as a glacier melts
d. A new island formed by falling levels of a
reservoir in Ohio
QUESTION: Weighing the Issues
Can we continue raising the Earth’s carrying capacity for
humans by developing technology and using resources more
efficiently?
a. Yes, our growth can continue indefinitely.
b. Our growth can continue some more, but will
eventually be halted by limiting factors.
c. No, we cannot raise Earth’s carrying capacity
for ourselves any longer.
QUESTION: Weighing the Issues
Are national parks and preserves the best way to conserve
biodiversity?
a. Yes, because species depend on their habitats
and intact communities being protected.
b. No, because climate change can ruin
conservation efforts if it changes conditions
inside preserves.
c. Ecotourism and encouraging local interest in
conservation is more important than
establishing parks.
QUESTION: Interpreting Graphs and Data
You would expect this population to be…?
a. Growing rapidly
b. Shrinking rapidly
c. Stable in size
d. Oscillating in size
Figure 5.9
QUESTION: Interpreting Graphs and Data
How can you tell that this population growth curve shows exponential
growth?
a. Population is increasing.
b. Data points match curve
closely.
c. Population is rising by the
same number during each
interval.
d. Population is rising by the
same percentage during each
interval.
Figure 5.10
QUESTION: Interpreting Graphs and Data
This shows
growth ending at a(n)
.
a. exponential… carrying
capacity
b. intrinsic… equilibrium
c. logistic… carrying
capacity
d. runaway…
equilibrium
e. logistic… extinction
Figure 5.12a
QUESTION: Viewpoints
What is the most important lesson we can learn from the
Monteverde preserve?
a. Preserves do little good if species can become
extinct inside them.
b. Climate change means that we will need more
than preserves to save all species.
c. Ecotourism and local participation can make for
successful conservation.