4

Ecology and evolution: Populations, communities,

and biodiversity

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

adaptationadaptive traitage distributionage structureage structure diagramsallopatric speciationanthropogenicartificial selectionbiodiversitybiological diversitybiospherebiotic potentialcarnivorescarrying capacityclimax communityclumped distributioncommunitycompetitiondecomposers

density dependentdetritivoresectoparasitesEmigrationendemicendoparasitesenvironmental resistanceevolutionexponential growthextinctionfood chainfood webfossilfossil recordgrowth ratehabitat selectionhabitatsherbivoresheritable

hostimmigrationinterspecific competitionintraspecific competitioninvasive specieskeystone speciesK-strategistlimiting factorslogistic growthmass extinctionmutationsmutualismnatural selectionnicheomnivoresparasiteparasitism

Key Words

phylogenetic treespioneer speciespollinationpopulation densitypopulation dispersionpopulation distributionpopulation growth curvespopulation sizepredationpredatorpreyprimary consumersprimary successionrandom distributionresource partitioningr-strategistssecondary consumerssecondary succession

sex ratiospeciationspeciessuccessionsymbiosestertiary consumerstrophic 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 reproduceand/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

Unknown finch ancestor

Insect and nectar eatersFruit and seed eaters

Greater Koa-finch

Kona Grosbeak

Akiapolaau

Maui Parrotbill

Kuai Akialoa

Amakihi

Crested Honeycreeper

Apapane

Natural selection

Evidence of natural selection is all around us:

… and in our domesticated organisms.

Figure 4.23b

Dog breeds, types of cattle, improved crop plants—all result from artificial selection (natural selection conducted by human breeders).

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.

Formationof the

earth’searly

crust andatmosphere

Small organic

moleculesform in

the seas

Large organic

molecules(biopolymers)

form inthe seas

First protocells

form inthe seas

Single-cellprokaryotes

form inthe seas

Single-celleukaryotes

form inthe seas

Variety ofmulticellularorganismsform, first

in the seas and lateron land

Chemical Evolution(1 billion years)

Biological Evolution(3.7 billion years)

Click to view animation.

Stanley Miller's experiment animation.

Stabilizing Selection

Click to view animation.

Disruptive Selection

Click to view animation.

Niches and Natural Selection

Region of niche overlap

Generalist specieswith a broad nicheSpecialist species

with a narrow nicheNiche

breadth

Nicheseparation

Num

ber

of in

divi

dual

s

Resource use

Various Niches and Their Adaptations

Black skimmerseizes small fishat water surface

Flamingofeeds on minuteorganismsin mud

Scaup and otherdiving ducks feed onmollusks, crustaceans,and aquatic vegetation

Brown pelican dives for fish,which it locates from the air

Avocet sweeps bill throughmud and surface water in search of small crustaceans,insects, and seeds

Louisiana heron wades intowater to seize small fish

Oystercatcher feeds onclams, mussels, and other shellfish into which it pries its narrow beak

Dowitcher probes deeplyinto mud in search ofsnails, marine worms,and small crustaceans

Knot (a sandpiper) picks upworms and small crustaceansleft by receding tide

Herring gull is atireless scavenger

Ruddy turnstone searchesunder shells and pebbles for small invertebrates

Piping plover feedson insects and tinycrustaceans on sandy beaches

Geographic Separation

Early foxpopulation

Spreadsnorthwardandsouthwardandseparates

Adapted to heatthrough lightweightfur and long ears, legs, and nose, whichgive off more heat.

Adapted to cold through heavier fur, short ears,short legs, short nose. White fur matches snow for camouflage.

Gray Fox

Arctic Fox

Different environmentalconditions lead to differentselective pressures and evolutioninto two different species.

Northernpopulation

Southernpopulation

Mimicry

Span worm Bombardier beetle

Viceroy butterfly mimicsmonarch butterfly

Foul-tasting monarch butterfly

Poison dart frog When touched, the snake caterpillar changes shape to look like the head of a snake

Wandering leaf insect

Hind wings of io mothresemble eyes of a much larger animal

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

Figure 5.8

Random

Clumped

Uniform

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

Figure 5.9

Pyramid weighted toward young: population growing

Pyramid weighted toward old: population declining

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.

Biotic PotentialPOPULATION SIZE

Growth factors(biotic potential)

Favorable lightFavorable temperatureFavorable chemical environment (optimal level of critical nutrients)

Abiotic

BioticHigh reproductive rate

Generalized niche

Adequate food supply

Suitable habitat

Ability to compete for resources

Ability to hide from or defend against predatorsAbility to resist diseases and parasitesAbility to migrate and live in other habitatsAbility to adapt to environmental change

Decrease factors(environmental resistance)

Too much or too little lightTemperature too high or too lowUnfavorable chemical environment (too much or too little of critical nutrients)

Abiotic

BioticLow reproductive rate

Specialized niche

Inadequate food supply

Unsuitable or destroyed habitat

Too many competitorsInsufficient ability to hide from or defend against predatorsInability to resist diseases and parasitesInability to migrate and live in other habitatsInability to adapt to environmental change

SurvivorshipP

erce

nta

ge

surv

ivin

g (

log

sca

le)

100

10

1

0

Age

Early loss

Constant loss

Late loss

K-strategists

Terms come from:

K = symbol for carrying capacity. (Populations tend to stabilize near K.)

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 tocarrying capacity (K)

Specialist niche

High ability to compete

Late successional species

ElephantSaguaro

K-Selected Species

r-Selected

r = intrinsic rate of population increase. (Populations can potentially grow fast, have high r.)

r-Selected Species

Cockroach

Dandelion

Many small offspring

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.

Figure 5.15a

A “keystone” holds an arch together.

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 Clown fish and sea anemone

Mycorrhizae fungi on juniper seedlings in normal soil

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).

1. Open pond

2. Plants begin to cover surface; sediment deposited

3. Pond filled by sediment; vegetation grows over site

Figure 5.24

Primary aquatic succession

Secondary terrestrial succession

Figure 5.23

Succession

Click to view animation.

Table 8-1Page 158

Ecosystem Characteristics at Immature and Mature Stages of Ecological Succession

Characteristic

Ecosystem Structure

Plant size

Species diversity

Trophic structure

Ecological niches

Community organization(number of interconnecting links)

Ecosystem Function

Biomass

Net primary productivity

Food chains and webs

Efficiency of nutrient recycling

Efficiency of energy use

Immature Ecosystem(Early Successional Stage)

Small

Low

Mostly producers, few decomposers

Few, mostly generalized

Low

Low

High

Simple, mostly plant herbivorewith few decomposers

Low

Low

Immature Ecosystem(Late Successional Stage)

Large

High

Mixture of producers, consumers, and decomposers

Many, mostly specialized

High

High

Low

Complex, dominated by decomposers

High

High

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.

From The Science behind the Stories

Number of dry days rose Stream flow fell

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

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.”

“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.”

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 DataHow 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 DataThis 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.