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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
Video: Kelp forest (Channel Islands)
Purple Sea Urchin
Video: Otter feeding
Video: Coral spawning
Science Focus: Why Should We Care
about Kelp Forests?
 Kelp forests: biologically diverse marine habitat
 Major threats to kelp forests
• Sea urchins (male sea otters can eat ~50 sea
urchins/ day – equivalent of a 150lb person
eating 160 burgers/day)
• Pollution from water run-off – pesticides
herbicides
• Global warming
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 – members of 2 or more
species interact for limited resources
 Predation – member of 1 species feeds directly on another
living species
 Parasitism – 1 organism feeds on another; causing harm
to the host
 Mutualism – interaction between 2 species in which both
benefit
 Commensalism – interaction between 2 species that
benefits 1 & causes no harm to the other
Most Species Compete with One Another
for Certain Resources
 Competition – ability of 1 species to be more
efficient at finding food or other resources than
another species
 Competitive exclusion principle – no 2
species can occupy same niche for very long
• Both can suffer OR
• 1 must either: migrate, change its habits, suffer a
population decline, or become extinct
Name the species that has out-competed most
other species on Earth.
Most Consumer Species Feed on Live
Organisms of Other Species (1)
 Predators may capture prey by
• Walking
• Swimming
• Flying
Crazy animal
• Pursuit and ambush
behaviour Seahorse
• Camouflage
attacked by
• Chemical warfare - venom Flounder YouTube
Owl Attacks Camera in Slow Motion Made For Spirtaway1
and Mpenziwe And Haifahaifa12 - YouTube
Most Consumer Species Feed on Live
Organisms of Other Species (2)

Prey may avoid capture by
• Camouflage
• Chemical warfare - skunks
• Warning coloration
Cephalopod, master of
• Mimicry
camouflage - YouTube
• Deceptive looks
• Deceptive behavior
Biologist E.O. Wilson’s 2 rules for coloration:
1. small & strikingly beautiful – probably poisonous
2. strikingly beautiful & easy to catch – probably
deadly
(a) Span worm
(c) Bombardier beetle
(e) Poison dart frog
(b) Wandering leaf insect
(d) Foul-tasting monarch butterfly
(f) Viceroy butterfly mimics
monarch butterfly
Spicebush caterpillar defense – YouTube
Snake Caterpillar - YouTube
(g) Hind wings of Io moth
resemble eyes of a much
larger animal.
(h) When touched,
snake caterpillar changes
shape to look like head of snake.
Stepped Art
Fig. 5-2, p. 103
Predator and Prey Species Can Drive
Each Other’s Evolution
 Intense natural selection pressures between
predator and prey populations
 Predation helps increase biodiversity by promoting
natural selection leading to species evolving ability
to share limited resources by reducing niche overlap
 Coevolution – pred-prey populations interact long
enough (100’s – 1000’s of years) that changes in
gene pool of 1 leads to changes in the other
• Bats and moths
An evolutionary arms race (coevolution) YouTube
Coevolution: A Langohrfledermaus
Bat Hunting a Moth
Some Species Feed off Other Species by
Living on or in Them
 Parasitism
Parasite usually much smaller than host and doesn’t kill
them
 Parasite-host interaction may lead to coevolution
• Malaria parasite evolved methods to stick to
blood vessels and counteract host’s immune
system
Parasitism: Tree with Parasitic Mistletoe,
Trout with Blood-Sucking Sea Lampreys
Healthy tree on left
Tree infested w/ parasitic mistletoe
on right
Sea Lamprey attached to lake
trout from Great Lakes
In Some Interactions, Both Species
Benefit
 Mutualism
 Nutrition and protection relationship
 Gut inhabitant mutualism
 Each species benefits by unintentionally
exploiting the other as a result of traits obtained
through natural selection
Mutualism: Oxpeckers Clean Rhinoceros;
Anemones Protect and Feed Clownfish
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. (Oxpeckers and black rhinoceros: Joe McDonald/Tom Stack &
Associates; clownfish and sea anemone: Fred Bavendam/Peter Arnold, Inc.)
In Some Interactions, One Species
Benefits and the Other Is Not Harmed
 Commensalism
 Epiphytes
 Birds nesting in trees
Commensalism: Bromiliad Roots on Tree
Trunk Without Harming Tree
This bromeliad (epiphyte) roots
on the trunk of a tree, rather
than in soil, without harming or
penetrating the tree. It gains
access to water, sunlight, &
nutrient debris without any
harm or benefit to the tree.
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
Competing Species Can Evolve to
Reduce Niche Overlap
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.
Blackburnian
Warbler
Black-throated
Green Warbler
Cape May
Warbler
Bay-breasted
Warbler
Yellow-rumped
Warbler
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.)
Stepped Art
Fig. 5-8, p. 107
Specialist Species of Honeycreepers
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
 Population dynamics – study of how these
characteristics of populations change in
response to changes in environmental
conditions
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 – most populations live in clumps or
patches
• Ex. Desert veg around a spring; wolf packs
• 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
Battle at Kruger (shorter version) YouTube
Populations Can Grow, Shrink, or
Remain Stable (1)
 Population size governed by
•
•
•
•
Births
Deaths
Immigration
Emigration
 Population change =
(births + immigration) – (deaths + emigration)
Populations Can Grow, Shrink, or
Remain Stable (2)
 Age structure – can have strong effect on how
rapidly a population changes
• Pre-reproductive age
• Reproductive age
• Post-reproductive age
Populations w/ even distributions among 3 groups
tend to be stable
No Population Can Grow Indefinitely:
J-Curves and S-Curves (1)
 Biotic potential – capacity for pop growth under ideal
conditions
• Low – large individuals (elephants, whales)
• High – small individuals (bacteria, insects)
 Intrinsic rate of increase (r) – rate at which pop
will grow if 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 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 - combination of all factors
that act to limit growth of population
 Carrying capacity (K) – max pop of given species that
particular habitat can sustain indefinitely w/o being degraded
• Determined by biotic potential & env resistance
 Exponential growth – a pop w/ few limitations on
resource supplies grows exponentially “J” growth curve
 Logistic growth – rapid exponential pop growth followed
by steady decrease in growth until pop size levels off
• Encounters env resistance
• Pop typically fluctuates above & below K
• S shaped curve
No Population Can Continue to Increase
in Size Indefinitely
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. See an
animation based on this figure at
CengageNOW. Question: What is
an example of environmental
resistance that humans have not
been able to overcome?
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
To keep from drifting apart, sea otters may sleep
holding hands - Author: joemess from austin
Population Size of Southern Sea Otters
Off the Coast of So. California (U.S.)
Logistic Growth of a Sheep Population
on the island of Tasmania, 1800–1925
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.
Active Figure: Exponential growth
Animation: Logistic growth
When a Population Exceeds Its Habitat’s
Carrying Capacity, Its Population Can Crash
 Carrying capacity: not fixed
• Can increase or decrease seasonally
• Weather conditions (drought)
• Abundance of predators or competitors
 Reproductive time lag (period needed for birth
rate to fall & death rate to rise) may lead to
overshoot K
• Dieback (crash)
 Damage may reduce area’s carrying capacity
• Overgrazing cattle – reduced grass cover – increased
sagebrush & replace grass – decrease K for cattle
Exponential Growth, Overshoot, and
Population Crash of a Reindeer
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
 r-Selected species, opportunists – high rate of
reproduction
• Insects, bacteria, annuals, rodents, frogs
• Easily gain foothold in disturbed areas
• Often experience boom & bust cycles as competitors
move in
 K-selected species, competitors
• Reproduce later in life w/ fewer offspring
• Strong offspring can compete for resources
• Do well in competitive conditions when pop near carrying
capacity (K)
• Humans, whales, sharks, long lived plants, birds of prey
Positions of r- and K-Selected Species on
the S-Shaped Population Growth Curve
Positions of rselected and Kselected species on
the sigmoid (Sshaped) population
growth curve.
Genetic Diversity Can Affect the Size
of Small Populations
 Founder effect – a few indiv. Colonize new habitat that’s geogr.
Isolated from other members
• Limited gen diversity may threaten colony’s survival
 Demographic bottleneck – only a few indiv survive catastrophe
• Lack of gen diversity may limit ability to rebuild population
 Genetic drift – random genetic changes lead to unequal
reproductive success
• Some indiv breed more & their genes dominate
 Inbreeding – can occur from bottleneck; freq of defective genes
increase limiting survival
 Minimum viable population size – minimum # of indiv
certain populations (rare/ endangered) need for long-term
survival
Under Some Circumstances Population
Density Affects Population Size
 Population Density - # of indiv in a population
found in a particular area or volume
 Density-dependent population controls
• Predation
• Parasitism
• Infectious disease
• Competition for resources
These factors tend to regulate a pop at a constant
size often near carrying capacity
Density Independent controls- severe freeze, floods,
hurricanes, fire, pollution, habitat destruction
Several Different Types of Population
Change Occur in Nature
 Stable – pop size flux slightly above & below K
• Found in undisturbed tropical rain forests
 Irruptive – pop explodes then crashes
• Insects explode in summer & crash in winter
 Cyclic fluctuations, boom-and-bust cycles – pop
cycles every few years
• Top-down population regulation – through predation
• Bottom-up population regulation – pred/prey pops
controlled by scarcity of resources
 Irregular - no pattern to pop size
• May be due to flux in response to periodic
catastrophic pop crashes
Population Cycles for the Snowshoe
Hare and Canada Lynx
Canadian Lynx V's A Hare - YouTube
Humans Are Not Exempt from Nature’s
Population Controls
 Ireland
• Potato crop in 1845 destroyed by fungus
• ~1 million died from hunger or disease related to
malnutrition
• About 3 million migrated
 Bubonic plague
• Fourteenth century – killed ~25 million
• Spread quickly in densely populated cities w/ poor
sanitation & plenty of rats
 AIDS
• Global epidemic – killed 25 million btwn 1981 & 2007
• ~2.1 million die each year
• We have extended Earth’s carrying capacity for
humans
•
technology
•Cultural and social changes
•Increased food production
•Inhabited otherwise uninhabitable lands
Can we continue to expand???
Case Study: Exploding White-Tailed Deer
Population in the U.S.
 1900: deer habitat destruction and uncontrolled hunting; less
than 500,000 animals in USA
 1920s–1930s: laws to protect the deer
• Natural predators (wolves, mtn lions) decimated
 Current population explosion for deer (25-30 million)
• We’ve made perfect edge habitat for them
•
•
•
•
Lyme disease
Deer-vehicle accidents (~14,000 people killed annually)
Eating garden plants and shrubs
Destroy native groundcover and allow weeds to move in
 Ways to control the deer population
 Example : Monmouth County Parks System
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 – gradual establishment of
biotic communities in lifeless areas where there’s no
soil
• Secondary succession – more common; series of
communities or ecosys develop in places w/ soil or
bottom sediment
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
lichens & moss – start soil formation
 Midsuccessional plant species
Herbs, grasses, low shrubs, few trees
 Late successional plant species
mostly trees
Primary Ecological Succession
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)
 Some soil remains in a terrestrial system
 Some bottom sediment remains in an aquatic
system
 Ecosystem has been
• Disturbed
• Removed
• Destroyed
Burned forest; Jim Peaco; 1988
 Abandoned farms, burned or cut forests, flooded
areas
Natural Ecological Restoration of
Disturbed Land
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? See an
animation based on this
figure at CengageNOW.
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
Life After People - Chernobyl - YouTube
Science Focus: How Do Species Replace
One Another in Ecological Succession?
3 factors determine how & rate of succession
 Facilitation – one set of species makes area suitable for
different species & then less suitable for itself
• Lichens & mosses
 Inhibition – some early species hinder establishment &
growth of others
• Plants use chemical warfare (pine – acidic soil)
• Succession only occurs with disturbance
 Tolerance late successional plants unaffected by plants of
earlier stages b/c not in direct competition w/ them
• Shade tolerant trees
Succession Doesn’t Follow a
Predictable Path
 Traditional view
• Balance of nature and a climax community – stable
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
Ecosystems tend to have one or the other
tropical rain forest is persistent but not resilient
 Tipping point – change is abrupt & irreversible
• If certain # trees eliminated from rain forest; could
crash & become grassland