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Biodiversity, Species Interactions,
and Population Control
Chapter 5
Core Case Study: Southern Sea Otters: Are
They Back from the Brink of Extinction?
 Habitat
 Hunted: early 1900s
 Partial recovery
 Why care about sea otters?
• Ethics
• Keystone species
• Tourism dollars
5-1 How Do Species Interact?
 Concept 5-1 Five types of species
interactions—competition, predation, parasitism,
mutualism, and commensalism—affect the
resource use and population sizes of the
species in an ecosystem.
Species Interact in Five Major Ways
 Interspecific Competition
 Predation
 Parasitism
 Mutualism
 Commensalism
Most Species Compete with One Another
for Certain Resources
 Competition
 Competitive exclusion principle
4. Competition & Predation
 Interspecific competion results because of
niche overlap = overlap in requirements for
limited resources.
 Types of Competition:
 interference competition: one species limits
another species' access to a resource; e.g.,
hummingbirds defending feeding territories.
 exploitation competition: competing species
both have access to a limited resource, but one
exploits the resource more quickly or
efficiently.
Principle of Competitive
Exclusion
G.P. Gausse, in a classical
experiment (1934),
showed that two species
with identical niches can
not coexist indefinitely.
This is called the principle
of competitive
exclusion. Note that
when grown together,
Paramecium aurelia
outcompetes
Paramecium caudatum.
Most Consumer Species Feed on Live
Organisms of Other Species (1)
 Predators may capture prey by
• Walking
• Swimming
• Flying
• Pursuit and ambush
• Camouflage
• Chemical warfare
Predators & Prey
What features characterize them?
Predators tend to evolve characteristics for
efficient capture of prey (keen eyesight,
speed, etc.).
Prey tend to evolve chacteristics to avoid being
eaten (camoflauge, chemical defenses,
behaviors that startle predators, etc.).
Most Consumer Species Feed on Live
Organisms of Other Species (2)
 Prey may avoid capture by
• Camouflage
• Chemical warfare
• Warning coloration
• Mimicry
• Deceptive looks
• Deceptive behavior
Science Focus: Why Should We Care
about Kelp Forests?
 Kelp forests: biologically diverse marine habitat
 Major threats to kelp forests
• Sea urchins
• Pollution from water run-off
• Global warming
Predator and Prey Species Can Drive
Each Other’s Evolution
 Intense natural selection pressures between
predator and prey populations
 Coevolution
Types of Species Interactions
 major types of biotic interactions (continued):
 symbiosis: a long–lasting relationship in which
species live together in intimate association:
 parasitism: one organism (parasite) lives on part
of another organism (host), e.g., flea living on a
dog
 mutualism: two species interacting in a way that
benefits both, e.g., lichens consist of algae &
fungi that benefit each other (in this example
can't live apart);
 commensalism: one organism benefits from
another, but neither helps nor harm that other
organism, e.g., epiphyte growing on a tree
(epiphyte benefits & tree not effected, unless
there are many epiphytes).
Some Species Feed off Other Species by
Living on or in Them
 Parasitism
 Parasite-host interaction may lead to coevolution
 Parasitism can be viewed as a special type of
predation wherein the parasite:
 1) is usually smaller than the prey,
 2) remains closely associated with the prey over
time, &
 3) rarely kills its host.
5. Symbiotic Species
Interactions
 Endoparasites
 live inside their host, e.g., tapeworm living in the gut of
a mammal; plasmodium living inside a vertebrate &
causing malaria.
 Ectoparasites
 live outside their host, e.g., mosquito feeding on the
blood of mammal; lamprey attaching to outside of a
host fish (see Fig. 9–13).
In Some Interactions, Both Species
Benefit
 Mutualism
 Nutrition and protection relationship
 Gut inhabitant mutualism
Mutualism
Mutualism involves a relationship in which two
interacting species benefit.
 obligatory mutualism results when two organisms
can not live without each other;
 example: in lichens an algae provides photosynthesis & a fungi
provides a home for the algae;
 example: Rhizobium bacteria, in legume plant root nodules, fix
nitrogen & legume provides carbohydrates & home;
 example: termites have gut organism that can digest cellulose.
 In other mutualisms the organisms can live apart, but
there is strong mutual benefit in the relationship;
 example: flowering plants & their pollinators, plant gets
pollinated, pollinator gets nectar or pollen to eat;
Mutualism
There are many more classic examples of mutualism.
example: oxpeckers, a type of bird, feeds on the parasitic tics of
various large mammals in Africa, such as the black rhinoceros
(see Fig. 9–14);
example: mycorrhizal fungi live in the roots of various plants; the
fungus gets carbohydrates & the plant gets better absorption of
nutrients by the fungal mat that extends beyond the roots (see
Fig. 9–15);
example: the clownfish in the coral reefs of Australia lives among
the tentacles of sea anemones; the clownfish gains protection
from the stinging tentacles & food scraps when the anemone
feeds; the anemone gains protection from various fish that feed
on sea anemones (see Fig. 9–16);
example: certain species of stinging ants live in acacias; the ants
get a home and food in the form of nectar; the acacias get
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protection from various herbivores.
In Some Interactions, One Species
Benefits and the Other Is Not Harmed
 Commensalism
 Epiphytes
 Birds nesting in trees
Commensalism
Commensalism involves a symbiotic relationship in
which one species beneifits while another is neither
helped not harmed to a significant degree.
example: redwood sorrel, a small herbaceous plant,
benefits from growing in the shade of tall redwoods, but the
redwoods are not affected;
example: epiphytes (such as orchids & bromeliads) that
grow on trunk & branches of trees in the tropical rain forest
gain a favorable place to live; whereas, at least when
epiphytes are not overly abundant, the tree is not affected
(see Fig. 9–17).
Note that if epiphytes become sufficiently abundant to block light, the
tree can be negatively affected, and this becomes an example of
competition.
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5-2 How Can Natural Selection Reduce
Competition between Species?
 Concept 5-2 Some species develop
adaptations that allow them to reduce or avoid
competition with other species for resources.
Some Species Evolve Ways to Share
Resources
 Resource partitioning
 Reduce niche overlap
 Use shared resources at different
• Times
• Places
• Ways
Resource Partitioning
Species with similar
resource requirements
can coexist because
they use limited
resources at different
times, in different ways,
or in different places.
For example, specialized
feeding niches of various
birds of coastal wetland
enable coexistence of
many species.
Resource Partitioning
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Resource Partitioning
Five species of insect–eating warblers are able to
coexist in spruce forest of Maine. Each species
minimizes competition with others for food by spending
at least half its feeding time in a distinct portion of
spruce trees (shaded areas); each also consumes
somewhat different insect species.
Fig. 9–5
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Resource Partitioning
Fig. 9–5 (continued)
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Character Displacement
Over many years
coexisting species with
similar niches tend to
evolve physical &
behavioral adaptations
to minimize
competition.
For example on islands
where they co–ocurr,
species of Darwin's
finch have evolved
different bill sizes &
eat different size prey.
5-3 What Limits the Growth of
Populations?
 Concept 5-3 No population can continue to
grow indefinitely because of limitations on
resources and because of competition among
species for those resources.
1. Characteristics of
Populations
 population
dynamics
 population size
 population
density
 Dispersion
 age structure
 . is the number of individuals in a
population at a given time;
 is the number of individuals per unit
area in terrestrial ecosystems or
per unit volume in aquatic
ecosystems;
 is the proportion of individuals in
each age group (e.g.,
prereproductive, reproductive, &
postreproductive) of a population.
 is the spatial patterning individuals;
 Changes in population size,
density, dispersion, & age
distribution are known as
Populations Have Certain
Characteristics (1)
 Populations differ in
• Distribution
• Numbers
• Age structure
 Population dynamics
Populations Have Certain
Characteristics (2)
 Changes in population characteristics due to:
• Temperature
• Presence of disease organisms or harmful
chemicals
• Resource availability
• Arrival or disappearance of competing species
Most Populations Live Together in
Clumps or Patches (1)
 Population distribution
• Clumping
• Uniform dispersion
• Random dispersion
Most Populations Live Together in
Clumps or Patches (2)
 Why clumping?
• Species tend to cluster where resources are
available
• Groups have a better chance of finding clumped
resources
• Protects some animals from predators
• Packs allow some to get prey
• Temporary groups for mating and caring for
young
Characteristics of Populations
What is the difference between clumped, uniform &
random dispersion?
Fig. 10–2
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Populations Can Grow, Shrink, or
Remain Stable (1)
 Population size governed by
•
•
•
•
Births
Deaths
Immigration
Emigration
 Population change =
(births + immigration) – (deaths + emigration)
2. Population Dynamics & Carrying
Capacity
 Population size is governed by births, deaths,
immigration, and emigration:
 [Population Change] =
 [Births + Immigration] – [Deaths + Emigration]
 If the number of individuals added by births &
immigration are balanced by those lost by deaths &
emigration then there is zero population growth;
 populations vary in their capacity for growth, also
known as biotic potential;
 the intrinsic rate of growth (r) is the rate at which a
population will grow if it had unlimited resources.
Population Dynamics
What are
the most
important
factors that
tend to
increase or
decrease
population
size?
Fig. 10–3
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Carrying Capacity
 There are always limits to population growth
in nature.
 carrying capacity (K) is the number of
individuals that can be sustained in a given
space;
 the concept of carrying capacity is of central
importance in environmental science;
 if the carrying capacity for an organism is
exceeded, resources are depleted,
environmental degradation results, & the
population declines.
Populations Can Grow, Shrink, or
Remain Stable (2)
 Age structure
• Pre-reproductive age
• Reproductive age
• Post-reproductive age
No Population Can Grow Indefinitely:
J-Curves and S-Curves (1)
 Biotic potential
• Low
• High
 Intrinsic rate of increase (r)
 Individuals in populations with high r
•
•
•
•
Reproduce early in life
Have short generation times
Can reproduce many times
Have many offspring each time they reproduce
No Population Can Grow Indefinitely:
J-Curves and S-Curves (2)
 Size of populations limited by
•
•
•
•
•
Light
Water
Space
Nutrients
Exposure to too many competitors, predators or
infectious diseases
No Population Can Grow Indefinitely:
J-Curves and S-Curves (3)
 Environmental resistance
 Carrying capacity (K)
 Exponential growth
 Logistic growth
Exponential vs. Logistic
Growth
 What’s the difference
between Exponential &
Logistic Growth?
 Exponential growth
occurs when resources
are not limiting.
 Logistic growth occurs
when resources become
more and more limiting
as population size
increases.
Exponential Population Growth
 Exponential growth
occurs when resources
are not limiting.
 during exponential growth
population size increases
faster & faster with time;
 currently the human
population is undergoing
exponential growth;
 exponential growth can
not occur forever because
eventually some factor
limits population growth.
Logistic Population Growth
 Logistic population growth
occurs when the
population growth rate
decreases as the
population size increases.
 note that when the
population is small the
logistic population growth
curve looks like
exponential growth;
 over time, the population
size approaches a carrying
capacity (K).
Exceeding the Carrying
Capacity
During the mid–1800s sheep populations exceeded the carrying
capacity of the island of Tasmania. This "overshoot" was
followed by a "population crash". Numbers then stabilized, with
oscillation about the carrying capacity.
Exceeding the Carrying
Capacity
Reindeer introduced to a small island off of Alaska in
the early 1900s exceeded the carrying capacity, with
an "overshoot" followed by a "population crash" in
which the population was totally decimated by the
mid–1900s.
Population Curves in Nature
Natural populations display a broad diversity of population curves.
Stable populations are relatively constant over time.
Cyclic curves are often associated with seasons or fluctuating
resource availability.
Irruptive curves are characteristic of species that only have high
numbers for only brief periods of times (e.g., seven–year
cicada).
Population Curves in Nature
Population cycles for the snowshoe hare &
Canadian lynx are believed to result because the
hares periodically deplete their food, leading to
first a crash of the hare population & then a crash
of the lynx population.
Science Focus: Why Are Protected Sea
Otters Making a Slow Comeback?
 Low biotic potential
 Prey for orcas
 Cat parasites
 Thorny-headed worms
 Toxic algae blooms
 PCBs and other toxins
 Oil spills
When a Population Exceeds Its Habitat’s
Carrying Capacity, Its Population Can Crash
 Carrying capacity: not fixed
 Reproductive time lag may lead to overshoot
• Dieback (crash)
 Damage may reduce area’s carrying capacity
Species Have Different Reproductive
Patterns
 r-Selected species, opportunists
 K-selected species, competitors
3. Reproductive Strategies &
Survival
 Organisms can be divided into two categories
of "strategies" for reproduction & survival:
 r–strategist species,
 tend to live in recently disturbed (early
successional) environments where resources
are not limiting; such species tend to have
high intrinsic rates of growth (high r);
 K–strategist species
 tend to live in environments where resources
are limiting (later succession) & tend to have
lower intrinsic rates of growth and
characteristics that enable them to live near
their carry capacity (population size near K).
r–Strategist Species
Characteristics of r–
strategists, including
production of many
small & unprotected
young, enable these
species to live in
places where
resources are
temporarily abundant.
These species are
typically "weedy" or
opportunistic.
K–Strategist Species
Characteristics of
K–strategists,
including
production of
few large & well
cared for young,
enable these
species to live in
places where
resources are
limited.
These species are
typically good
competitors.
Survivorship Curves
 Three kinds of
curves:
 late loss (usually
K–strategists), in
which high
mortality is late in
life;
 constant loss
(such as
songbirds), in
which mortality is
about the same
for any age;
 early loss
(usually r–
strategists), in
which high
mortality is early
in life.
Genetic Diversity Can Affect the Size
of Small Populations
 Founder effect
 Demographic bottleneck
 Genetic drift
 Inbreeding
 Minimum viable population size
Under Some Circumstances Population
Density Affects Population Size
 Density-dependent population controls
•
•
•
•
Predation
Parasitism
Infectious disease
Competition for resources
Several Different Types of Population
Change Occur in Nature
 Stable
 Irruptive
 Cyclic fluctuations, boom-and-bust cycles
• Top-down population regulation
• Bottom-up population regulation
 Irregular
Humans Are Not Exempt from Nature’s
Population Controls
 Ireland
• Potato crop in 1845
 Bubonic plague
• Fourteenth century
 AIDS
• Global epidemic
Case Study: Exploding White-Tailed Deer
Population in the U.S.
 1900: deer habitat destruction and uncontrolled
hunting
 1920s–1930s: laws to protect the deer
 Current population explosion for deer
• Lyme disease
• Deer-vehicle accidents
• Eating garden plants and shrubs
 Ways to control the deer population
5-4 How Do Communities and Ecosystems
Respond to Changing Environmental
Conditions?
 Concept 5-4 The structure and species
composition of communities and ecosystems
change in response to changing environmental
conditions through a process called ecological
succession.
Communities and Ecosystems Change
over Time: Ecological Succession
 Natural ecological restoration
• Primary succession
• Secondary succession
Some Ecosystems Start from Scratch:
Primary Succession
 No soil in a terrestrial system
 No bottom sediment in an aquatic system
 Early successional plant species, pioneer
 Midsuccessional plant species
 Late successional plant species
Primary Succession
Primary succession occurs with time in lifeless areas.
examples include succession newly formed islands &
succession after the retreat of a glacier;
• typically lichens & mosses first colonize bare rock;
• later small herbs & shrubs colonize;
• finally tree species colonize;
• the first species to colonize are termed pioneer
species;
• the progression of species that colonize with time are
commonly termed early, mid, & late successional
species.
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Primary Succession
Generalized
physical
appearance
showing the
types, relative
sizes, and
stratification of
plant species in
various
terrestrial
communities or
ecosystems.
Fig. 9–18
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Primary Succession
Primary
succession
over several
hundred years
on bare rock
exposed by a
retreating
glacier on Isle
Royal in
northern Lake
Superior.
Fig. 9–19
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Primary Succession
Greatly
simplified view
of primary
succession in
a newly
created pond
in a temperate
area. Nutrient
rich bottom
sediment is
shown in dark
brown.
Fig. 9–20a
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Some Ecosystems Do Not Have to Start
from Scratch: Secondary Succession (1)
 Some soil remains in a terrestrial system
 Some bottom sediment remains in an aquatic
system
 Ecosystem has been
• Disturbed
• Removed
• Destroyed
Secondary Succession
Secondary succession occurs where the natural
community of organisms has been disturbed, removed,
or destroyed.
example: "old field succession" in eastern North
America, where agricultural fields go through
succession from herbaceous plants, to shrubs & early
successional trees, to mid–successional forest, to oak–
hickory forest;
•according to the classic view, succession proceeds
until an area is occupied by a climax community,
however recent views recognize that succession is
influenced by variability & chaotic events such that a
single climax is not predictable.
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Secondary Succession
Secondary
succession
over 150–200
years in an
abandoned
farm field in
North
Carolina.
Fig. 9–21
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Secondary Succession
Successional changes in the animal community
accompany successional changes in the plant
community.
Fig. 9–22
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Some Ecosystems Do Not Have to Start
from Scratch: Secondary Succession (2)
 Primary and secondary succession
• Tend to increase biodiversity
• Increase species richness and interactions
among species
 Primary and secondary succession can be
interrupted by
•
•
•
•
•
Fires
Hurricanes
Clear-cutting of forests
Plowing of grasslands
Invasion by nonnative species
Disturbance
What is the role of disturbance in succession?
disturbance: a discrete event that disrupts an
ecosystem or community;
examples of natural disturbance: fires, hurricanes,
tornadoes, droughts, & floods;
examples of human–caused disturbance: deforestation,
overgrazing, plowing;
disturbance initiates secondary succession by eliminating
part or all of the existing community, & by changing
conditions & releasing resources.
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Science Focus: How Do Species Replace
One Another in Ecological Succession?
 Facilitation
 Inhibition
 Tolerance
Mechanisms of Succession
Both primary & secondary succession are driven by
three mechanisms:
facilitiation: a process by which an earlier successional
species makes the environment suitable for latter
successional species; e.g., legumes fixing nitrogen can
enable later successional species;
inhibition: a process whereby one species hinders the
establishment & growth of other species; e.g., shade of
late successional trees inhibits the growth of early
successional trees;
tolerance: a process whereby later successional
species are unaffected by earlier successional species.
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Changes During Succession
During succession species diversity & stratification
tend to increase, while growth rates & primary
productivity tend to decrease.
Fig. 9–23
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Ecosystem Changes During Succession
Characteristic
Plant size
Species diversity
Trophic structure
Early Succession
small
low
mostly producers
Ecological niches
few, more
generalized
low
Late Succession
large
high
mixture of producers,
consumers, &
decomposers
many, more
specialized
high
low
high
high
low
simple
low
complex
high
low
high
Community
organization (# links)
Biomass
Net Primary
Productivity
Food web
Efficiency for nutrient
cycling
Efficiency of energy
use
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Secondary Succession
Successional changes in the animal community
accompany successional changes in the plant
community.
Fig. 9–22
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Succession Doesn’t Follow a
Predictable Path
 Traditional view
• Balance of nature and a climax community
 Current view
• Ever-changing mosaic of patches of vegetation
• Mature late-successional ecosystems
• State of continual disturbance and change
Living Systems Are Sustained through
Constant Change
 Inertia, persistence
• Ability of a living system to survive moderate
disturbances
 Resilience
• Ability of a living system to be restored through
secondary succession after a moderate
disturbance
 Tipping point
8. Stability & Sustainability
Stability has three aspects:
inertia (or persistence): the ability of a system to resist being
disturbed or altered;
constancy: the abilty of a living system to maintain a certain
size or state;
resilience: the ability of a living system to recover after a
disturbance;
Signs of poor health or stressed ecosystems:
decrease in primary productivity;
increased nutrient losses;
decline or extinction of indicator species;
increased populations of pests or disease organisms;
decline in species diversity;
•presence of contaminants.
Through an understanding of ecology we can grapple with what
it means to have sustainable ecosystems.
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