Chapter 5 (Miller and Spoolman, 2010)
Figure 5.1
An endangered southern sea otter in Monterey Bay, California (USA), uses a stone to
crack the shell of a clam (insert). It lives in a giant kelp bed near San Clemente Island,
California (background). Scientific studies indicate that the otters act as a keystone
species in a kelp forest system by helping to control the populations of sea urchins and
other kelp-eating species.
Core Case Study: Southern Sea Otters: Are
They Back from the Brink of Extinction?
 Habitat: giant kelp forest of Pacific Coast of N.A.
 Fast agile swimmers that eat about ¼ their weight in
shellfish: clams, mussels, crabs, sea urchins, abalone and
40 other benthic animals.
 Hunted to near extinction by early 1900s
 Partial recovery from 1938 to 2007, pop. increased from 50
to about 3000.
 Helped in part by ESA listing in 1977
 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
 These interactions have significant effects on the resources
use and pop. sizes of species in an ecosystem
 Also influence the abilities of the interacting species to
survive, thus the interactions are agents of natural
selection.
Most Species Compete with One
Another for Certain Resources
 Competition is the most common interaction
 The greater the niche overlap, the greater the
competition.
 Competitive exclusion principle – no two species
can occupy the exact same niche.
 Competition would be too intense
 Humans are outcompeting other species for space food
and other resources as our ecological footprint
increases.
Most Consumer Species Feed on
Live Organisms of Other Species (1)
 Predation is when a member of species feeds directly on
all or part of a living organism of another plant or animal.
 Predator and prey form a predator-prey relationship.
 Herbivores, carnivores, and omnivores are predators.
 Methods of Prey Capture by Predators:
 Herbivores

walk, swim, or fly up to plants they feed on.
 Carnivores


Pursuit requires speed and agility on ground, in water, or in the air
Ambush predators use stealth and camouflage.
 Chemical warfare
 Venom
Most Consumer Species Feed on
Live Organisms of Other Species (2)
 Prey escape/avoidance methods:
 Highly developed senses, sight and smell (so do predators!)
 Flight response

Run, swim, and fly fast
 Protective armor
 Shells, bark, spines, thorns
 Camouflage to hide
 Chemical warfare
 Poisons (oleander plants, toads), irritants (poison ivy, bombardier
beetle), foul odor (skunk, stink bug), bad taste (monarch butterfly)
 Warning coloration
 Mimicry (viceroy butterfly, milk snake)
 Deceptive looks
 Deceptive behavior
 Schooling or herding behaviors
Most Consumer Species Feed on
Live Organisms of Other Species (3)
 At the individual level
 Predator benefits
 Prey species is harmed
 At the population level
 Predation plays a role in natural selection

Predators take the sick, weak, old, and less fit members of the
prey species.
 Some people view predators with contempt. If you
were an ambassador for nature, what would you tell
these people?
Figure 5.2
Some ways in which prey species avoid their predators: (a, b) camouflage, (c–e)
chemical warfare, (d, e) warning coloration, (f) mimicry, (g) deceptive looks, and (h)
deceptive behavior.
Science Focus: Why Should We
Care about Kelp Forests?
 Kelp forests
 Restricted to cold, nutrient-rich, and fairly shallow coastal
waters.
 One of most biologically diverse marine ecosystems
supporting large numbers of marine plants and animals.
 Help reduce shore erosion.
 Harvested as a renewable resource for algin found in blades.

Used in toothpaste, ice cream, and many other products.
 Major threats to kelp forests
 Sea urchins
 Pollution from water run-off
 Global warming
Figure 5-A Purple sea urchin in coastal waters of the U.S. state of CA.
Predator and Prey Species Can
Drive Each Other’s Evolution
 To survive, predators must eat and prey must avoid
being eaten
 Predators and prey populations exert intense natural
selection pressures on one another.
 Coevolution occurs when populations of two
different species interact over such a long period of
time, changes in the gene pool of one species can lead
to changes in the gene pool of the other species.
Figure 5.3
Coevolution. A Langohrfledermaus bat hunting a moth. Long-term interactions between
bats and their prey such as moths and butterflies can lead to coevolution, as the bats
evolve traits that increase their chances of getting a meal and the moths evolve traits
that help them avoid being eaten.
Some Species Feed off Other
Species by Living on or in Them (1)
 Parasitism occurs when one species (the parasite)
feeds on the body of, or the energy used by, another
organism, usually by living on or in the host.
 Parasites rarely kill host, but may gradually weaken
them.
 Endoparasites, some pathogenic

Tapeworms, liver fluke, Trypanosoma
 Ectoparasites
 Mosquitoes, fleas, ticks, mistletoe, and sea lamprey
 Other forms of parasitism: Brood parasitism and klepto-
parasitism
Some Species Feed off Other
Species by Living on or in Them (2)
 At the individual level
 For host, parasites are harmful.
 Parasites benefit.
 But at the population level
 Parasites can promote biodiversity by increasing species
richness.
 Help keep a host’s population size in check.
 Parasite-host relationship may lead to coevolution
Figure 5.4
Parasitism: (a) Healthy tree on the left and an unhealthy one on the right, which is
infested with parasitic mistletoe. (b) Blood-sucking parasitic sea lampreys attached to an
adult lake trout from the Great Lakes (USA).
In Some Interactions, Both Species
Benefit
 Mutualism occurs when two species behave in ways
that benefit both by providing each with food, shelter,
or some other resource.
 Flower s and their pollinators
 Nutrition and protection
 Gut inhabitant mutualism
Figure 5.5
Examples of mutualism. (a) Oxpeckers (or tickbirds) feed on parasitic ticks that infest
large, thick-skinned animals such as the endangered black rhinoceros. (b) A clownfish
gains protection and food by living among deadly stinging sea anemones and helps
protect the anemones from some of their predators.
In Some Interactions, One Species
Benefits and the Other Is Not Harmed
 Commensalism is an interaction that benefits one
species but has little, if any, effect on the other.
 Epiphytes
 Birds nesting in trees
Figure 5.6
In an example of commensalism, this bromeliad—an epiphyte, or air plant, in Brazil’s
Atlantic tropical rain forest—roots on the trunk of a tree, rather than in soil, without
penetrating or harming the tree. In this interaction, the epiphyte gains access to water,
other nutrient debris, and sunlight; the tree apparently remains unharmed.
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 occurs when species
competing for similar scarce resources evolve
specialized traits that allow them to use shared
resources
 at different times
 in different ways
 in different places
 Niche overlap can be reduced when natural selection
reduces broad and overlapping niches.
 Species become more specialized.
Figure 5.7
Competing species can evolve to reduce niche overlap. The top diagram shows the
overlapping niches of two competing species. The bottom diagram shows that through
natural selection, the niches of the two species become separated and more specialized
(narrower) as the species develop adaptations that allow them to avoid or reduce
competition for the same resources.
Figure 5.8
Sharing the wealth: resource partitioning of five species of insect-eating warblers in the
spruce forests of the U.S. state of Maine. Each species minimizes competition for food
with the others by spending at least half its feeding time in a distinct portion (shaded
areas) of the spruce trees, and by consuming different insect species. (After R. H.
MacArthur, “Population Ecology of Some Warblers in Northeastern Coniferous Forests,”
Ecology 36 (1958): 533–536.)
Figure 4.13
Specialized feeding niches of various bird species in a coastal wetland. This
specialization reduces competition and allows sharing of limited resources.
Figure 5.9
Specialist species of honeycreepers. Evolutionary divergence of honeycreepers into
species with specialized ecological niches has reduced competition between these
species. Each species has evolved a beak specialized to take advantage of certain types
of food resources.
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.
Populations Have Certain
Characteristics (1)
 Populations differ in
 Distribution
 Numbers
 Age structure
 Density
 Population dynamics is the study of how
characteristics of populations change in response to
changes in environmental conditions.
Populations Have Certain
Characteristics (2)
 Changes in population characteristics due, for
example, 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
 Three general patterns of population distribution or
dispersion in a habitat.
 Clumping, most common as resources are also clumped.
 Uniform dispersion, when resources is even and scarce.
 Random dispersion, not as common.
 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.
Figure 5.10 Generalized dispersion patterns. The most common pattern is clumps of
members of a population scattered throughout their habitat, mostly because resources
are usually found in patches. Questions: Why do you think the creosote bushes are
uniformly spaced while the dandelions are not?
Populations Can Grow, Shrink, or
Remain Stable (1)
 Population size governed by
 Births
 Deaths
 Immigration
 Emigration
 Population change (N) =
(births + immigration) – (deaths + emigration)
Populations Can Grow, Shrink, or
Remain Stable (2)
 How fast a population grows or declines depends on its
age structure—the proportions of individuals at
various ages.
 Prereproductive age: not mature enough to reproduce.
 Reproductive age: those capable of reproduction.
 Postreproductive age: those too old to reproduce.
No Population Can Grow Indefinitely:
J-Curves and S-Curves (1)
 Biotic potential is the capacity for population growth under
ideal conditions.
 Low, usu. in species with large individuals
 High, usu. in small species.
 Intrinsic rate of increase (r) is the rate at which the population
of a species grows if it had unlimited resources.
 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 is regulated by limiting factors.
 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 is the combination of all factors
that act to limit the growth of a population.
 Biotic potential and environmental resistance determine
carrying capacity (K)—the maximum population of a given
species that a particular habitat can sustain indefinitely
without being degraded.
 Exponential growth – growth at a fixed rate relative to the
population size, e.g. 2 % annually.
 Curve shape, J
 Logistic growth – rapid growth followed by a steady
decrease as a population encounters environmental
resistance.
 Curve shape, S (or sigmoid)
Figure 5.11
No population can continue to increase in size indefinitely. Exponential growth (left half of
the curve) occurs when resources are not limiting and a population can grow at its intrinsic
rate of increase (r) or biotic potential. Such exponential growth is converted to logistic
growth, in which the growth rate decreases as the population becomes larger and faces
environmental resistance. Over time, the population size stabilizes at or near the carrying
capacity (K) of its environment, which results in a sigmoid (S-shaped) population growth
curve. Depending on resource availability, the size of a population often fluctuates around
its carrying capacity, although a population may temporarily exceed its carrying capacity
and then suffer a sharp decline or crash in its numbers. Question: What is an example of
environmental resistance that humans have not been able to overcome?
Figure 5.12
Logistic growth of a sheep population on the island of Tasmania between 1800 and
1925. After sheep were introduced in 1800, their population grew exponentially, thanks
to an ample food supply. By 1855, they had overshot the land’s carrying capacity. Their
numbers then stabilized and fluctuated around a carrying capacity of about 1.6 million
sheep.
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
Figure 5.B
Population size of southern sea otters off the coast
of the U.S. state of California, 1983–2007.
According to the U.S. Fish and Wildlife Service,
the sea otter population would have to reach
about 8,400 animals before it can be removed
from the endangered species list. (Data from U.S.
Geological Survey)
When a Population Exceeds Its Habitat’s
Carrying Capacity, Its Population Can Crash
 Carrying capacity is not fixed.
 Reproductive time lag may lead to overshoot of K.
 The time lag is the period needed for the birth rate to
decrease and the death rate to increase in response to
resource overconsumption.
  Dieback, or crash
 Damage from overconsumption/use may reduce area’s
carrying capacity.
Figure 5.13
Exponential growth, overshoot, and population crash of reindeer introduced to the small
Bering Sea island of St. Paul. When 26 reindeer (24 of them female) were introduced in
1910, lichens, mosses, and other food sources were plentiful. By 1935, the herd size had
soared to 2,000, overshooting the island’s carrying capacity. This led to a population crash,
when the herd size plummeted to only 8 reindeer by 1950. Question: Why do you think
this population grew fast and crashed, unlike the sheep in Figure 5-12?
Species Have Different Reproductive
Patterns
Natural capital: generalized characteristics of r-selected (opportunist) species and K-selected
(competitor) species. Many species have characteristics between these two extremes.
Figure 5.14
Positions of r-selected and K-selected species on the sigmoid (S-shaped) population
growth curve.
When does death come? Survivorship curves for populations of different species, show
the percentages of the members of a population surviving at different ages. Most
members of a late loss population (such as elephants, rhinoceroses, and humans) live to
an old age. Members of a constant loss population (such as many songbirds) die at all
ages. In an early loss population (such as annual plants and many bony fish species),
most members die at a young age. These generalized survivorship curves only
approximate the realities of nature.
Genetic Diversity Can Affect the
Size of Small Populations
 When a population becomes so reduced, reduced genetic
diversity can affect the overall survival of the population.
 Genetic drift – random changes in gene (i.e., allele) frequencies
in a population that can lead to unequal reproductive success;
occurs more often in small populations.
 Founder effect – when a few members of a population colonize a
new area and become geographically isolated.
 Demographic bottleneck – occurs when only a few member of a
population survive a catastrophic die-off.

Inbreeding – occurs when individuals of small population mate with
each other  increase in the freq. of defective genes.
 Minimum viable population size – the number of individuals a
population needs for long-term survival.
Random Effects on Allele
Frequency in Small Populations
Genetic Bottleneck
http://www.newsc
ientist.com/article
/dn13490
Founder Effect
Under Some Circumstances Population
Density Affects Population Size
 Population density – the number of individuals in a
population found in a particular area or volume.
 Density-dependent population controls
 Predation
 Parasitism
 Infectious disease
 Competition for resources
 Density dependent factors tend to regulate a population at a
fairly constant size, often near carrying capacity of an area.
 Density independent factors are often abiotic
 Severe freeze, hurricanes, fires, pollution, habitat destruction,
wetland loss.
Several Different Types of
Population Change Occur in Nature
 Stable
 Hovers around K.
 Characteristic of species that live in undisturbed tropical rain forests.
 Irruptive
 Increase to a high peak and then crash
 Algae and insects display this type of population changes
 Linked to seasonal changes in weather and nutrient availability
 Cyclic fluctuations, boom-and-bust cycles
 Changes occur in regular cycles
 Examples:


Lemming populations rise and fall every 3-4 years
Lynx and snowshoe hare every 10 years
 Top-down population regulation
 Bottom-up population regulation
 Irregular
 Changes in pop. size with no recurring pattern.
Figure 5.15
Population cycles for the snowshoe hare and Canada lynx. At one time, scientists
believed these curves provided circumstantial evidence that these predator and prey
populations regulated one another. More recent research suggests that the periodic
swings in the hare population are caused by a combination of top-down population
control—through predation by lynx and other predators—and bottom-up population
control, in which changes in the availability of the food supply for hares help determine
hare population size, which in turn helps determine the lynx population size. (Data from
D. A. MacLulich)
Humans Are Not Exempt from
Nature’s Population Controls
 Ireland
 Potato crop in 1845
 Bubonic plague
 Fourteenth century
 AIDS
 Global epidemic
 So far, technological, social, and other cultural changes
have extended the earth’s carrying capacity for humans.
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
 Pop no 25-30 million
 Current population explosion for deer
 In some forests, they are consuming native ground
cover making way for non-native invaders.
 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
 Types and numbers of species change in a biological
community over time.
 Mature forests and other ecosystems do not spring up
from bare rock.
 Instead they go through changes in species composition
over long periods of time.
 Ecological succession
 Primary succession
 Secondary succession
Some Ecosystems Start from
Scratch: Primary Succession
 Primary succession begins with a lifeless area where
there is
 No soil in a terrestrial system
 No bottom sediment in an aquatic system
 Early successional plant species, called pioneer, or
colonizing species.
 Lichens and mosses
 Midsuccessional plant species
 Herbs, grasses and shrubs, and later trees
 Late successional plant species
 Other trees
Figure 5.16
Primary ecological succession. Over almost a thousand years, plant communities
developed, starting on bare rock exposed by a retreating glacier on Isle Royal, Michigan
(USA) in northern Lake Superior. The details of this process vary from one site to
another. Question: What are two ways in which lichens, mosses, and plants might get
started growing on bare rock?
Some Ecosystems Do Not Have to Start
from Scratch: Secondary Succession (1)
 Secondary succession begins in an area
 Disturbed
 Removed
 Destroyed
 Some soil remains in a terrestrial system
 Some bottom sediment remains in an aquatic system
Figure 5.17
Natural ecological restoration of disturbed land. Secondary ecological succession of plant communities
on an abandoned farm field in the U.S. state of North Carolina. It took 150–200 years after the farmland
was abandoned for the area to become covered with a mature oak and hickory forest. A new
disturbance, such as deforestation or fire, would create conditions favoring pioneer species such as
annual weeds. In the absence of new disturbances, secondary succession would recur over time, but not
necessarily in the same sequence shown here. Questions: Do you think the annual weeds (left) would
continue to thrive in the mature forest (right)? Why or why not?
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
 Are accompanied by succession of faunal species
 Primary and secondary succession can be interrupted by
 Fires
 Hurricanes
 Clear-cutting of forests
 Plowing of grasslands
 Invasion by nonnative species
Science Focus: How Do Species Replace
One Another in Ecological Succession?
 Facilitation is when one set of species makes an area
suitable for species with different niche requirements.
 Inhibition is when some early species hinder the
establishment and growth of other species.
 Some plants produce allopathic compounds.
 Tolerance may be observed when late successional plants
are largely unaffected by plants at earlier stages because
they are not in direct competition with them for key
resources.
 For example, shade tolerant trees.
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, or 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