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Living In Ecosystems
Chapter 30
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Outline
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Population Growth
Life History Adaptations
Population Demography
The Niche and Competition
 Resource Partitioning
Symbiotic Relationships
Plant and Animal Defenses
Predator-Prey Cycles
Ecological Succession
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Population Growth
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Populations are composed of groups of
individuals of the same species living
together.
 Critical Properties
- Population Size
- Population Density
- Population Dispersion
- Capacity for Growth
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Population Growth
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Exponential Growth Model
 Assumes population growing without limits
at its maximal rate. (r = biotic potential)
dN/dt=riN
- N = Number of individuals in population
- dN/dt = Rate of change in population
size over time
- ri = Intrinsic rate of increase
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Population Growth
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A population’s actual rate of increase is the
difference between birth rate and death rate
corrected for migration.
Innate capacity for growth of any population
is exponential.
 Even when rate of increase remains
constant, the actual increase in the number
of individuals accelerates rapidly as the
size of the population grows.
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Population Growth
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Carrying Capacity (K)
 Number of individuals an area can
indefinitely support.
Logistic Growth Model
 As population approaches its carrying
capacity, its growth rate slows as
resources become scarce.
dN/dt = rN (K-N/K)
- Sigmoid Growth Curve
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Two Models of Population Growth
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Life History Adaptations
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Exponential Model
 Describes species with r-selected
adaptations.
- Favor high rate of increase
Logistic Model
 Describes species with k-selected
adaptations.
- Favor reproduction near carrying
capacity.
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Influence of Population Density
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Density-Dependent Effects
 Effects are independent of population size
and act to regulate growth (weather).
Density-Dependent Effects
 Effects are dependent on size of
population and act to regulate growth
(resource competition).
- Have increasing effect as population
size increases.
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Population Demography
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Age Structure
 Cohort - Group of individuals of same age.
Each has a characteristic:
- Fecundity Rate - Number of offspring
produced during a standard time.
- Mortality Rate - Number of individuals
that die during a standard time.
 Relative number of individuals in each
cohort defines population’s age structure.
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Population Demography
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Sex Ratio
 Proportion of males and females in a
population.
- Usually directly related to number of
females in the population.
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Population Demography
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Survivorship Curves
 Graphically express age distribution
characteristics by plotting percentage of
original population that survives to a given
age.
- Type I - Mortality rises in postreproductive years.
- Type II - Mortality constant throughout life.
- Type III - Mortality low after
establishment.
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Survivorship Curves
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Population Demography
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Life Tables
 Indicate chance of survival at any age.
- Follow cohort from birth to death.
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The Niche and Competition
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Niche - Biological role in community.
 Fundamental - Theoretical role
 Realized - Actual role
Competition - Two or more organisms attempt
to use same resource.
 Interference - Fighting
 Exploitative - Consuming shared resources
 Interspecific - Different species
 Intraspecific - Same species
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Barnacle Competition
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Competitive Exclusion
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Gause - No two species can coexist in the
same niche indefinitely.
 When two species coexist on long-term
basis, their niches differ in one or more
features.
- Otherwise, one is eventually driven to
extinction.
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Resource Partitioning
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Sympatric Species - Occupy same
geographical area but avoid competition by
utilizing different portions of the habitat.
 Character Displacement - Differences arise
between species due to natural selection.
Allopatric Species - Do not occupy same
geographical area, thus are not usually in
competition.
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Symbiosis
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Symbiotic Relationship - Two or more
species of organisms live together, and at
least one gains benefit.
 Commensalism - One species benefits
while other neither benefits or is harmed.
 Mutualism - Both species benefit.
 Parasitism - One species benefits while the
other is harmed.
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Commensalism
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Symbiotic relationship that benefits one
species and neither hurts or helps the other.
 Oxpeckers and Rhinos
- No definite boundary between
commensalism and mutualism.
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Mutualism
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Symbiotic relationship among organisms in
which both species benefit.
 Ants and Aphids
 Ants an Acacias
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Parasitism
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Specialized form of symbiosis in which
predator is much smaller than prey.
Interaction is harmful to prey but beneficial to
predators.
 Ectoparasites - External parasites.
- Parasitoids - Lay eggs on living hosts.
 Endoparasites - Internal parasites.
 Brood Parasitism - Lay eggs in nests of
other species.
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Plant Defenses
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Predator-Prey Interactions
 Morphological Defenses
- Thorns, spines, plant hairs
 Chemical Defenses
- Secondary chemical compounds
Evolution of herbivores avoiding plant
defense allows access to a new resource
without competition from other herbivores.
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Animal Defenses
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Feeding on plants rich in secondary
compounds may have added benefit.
 Blue Jays and Monarch Butterflies
Defensive Coloration
 Aposomatic Coloration - Advertise
poisonous nature with bright coloration.
 Cryptic Coloration - Camouflage
Chemical Defenses
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Predator-Prey Cycles
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Predation is consumption of one organism by
another.
 Under simple laboratory conditions,
predators often exterminate their prey, and
then become extinct themselves when they
run out of food.
- If refuges are provided for the prey, a
few individuals usually exist, and then
repopulate after the predators die out.
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Predator-Prey Cycles
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Snowshoe Hares (Lepus americanus).
 Food - Willows
 Predators - Canada Lynx (Lynx canadensis)
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Mimicry
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Batesian Mimicry - Palatable individuals
mimic distasteful or toxic individuals.
 Mimics must be relatively rare.
Mullerian Mimicry - Unrelated but protected
(toxic) species come to resemble one
another.
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Ecological Succession
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Succession - Ecosystem change from simple
to more complex plant communities.
 Secondary Succession - Occurs in areas
where an existing community has been
disturbed.
 Primary Succession - Occurs on bare
rocks.
- Climax Community
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Ecological Succession
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Three Critical Concepts of Succession
 Tolerance
 Facilitation
 Inhibition
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Biodiversity
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Measure of number of different types of
species in an area.
 Crucial to ecosystem preservation.
Biodiversity Promotion
 Ecosystem Size
- Larger ecosystems, usually have higher
levels of biodiversity
 Latitude
- Length of growing season
- Climatic stability
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Latitudinal Cline in Species Richness
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Island Biogeography
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Equilibrium Model
 McArthur and Wilson proposed island
species richness is a dynamic equilibrium
between colonization and extinction.
- Island size and distance from mainland
play important roles.
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Equilibrium Model of Island Biogeography
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Review
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Population Growth
Life History Adaptations
Population Demography
The Niche and Competition
 Resource Partitioning
Symbiotic Relationships
Plant and Animal Defenses
Predator-Prey Cycles
Ecological Succession
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies