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Chapter Three: Populations,
Communities, & Species
Interactions
Principles of Environmental
Science - Inquiry and Applications,
2nd Edition
by William and Mary Ann Cunningham
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Chapter Three Readings
Required Reading
Cunningham & Cunningham, Chapter Three:
Populations, Communities
and Species Interactions
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Chapter Three Objectives
Objectives
After studying this chapter, you should be able to
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describe how environmental factors determine which species live in a given ecosystem
and where or how they live;
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understand how random genetic variation and natural selection lead to evolution,
adaptation, niche specialization, and partitioning of resources in biological
communities;
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compare and contrast interspecific predation, competition, symbiosis, commensalism,
mutualism, and coevolution;
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explain population growth rates, carrying capacity, and factors that limit population
growth;
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discuss productivity, diversity, complexity, and structure of biological communities and
how these characteristics might be connected to resilience and stability;
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explain how ecological succession results in ecosystem development and allows one
species to replace another; and,
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list some examples of exotic species introduced into biological communities, and
describe the effects such introductions can have on indigenous species.
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Chapter 3. Key Terms McGraw-Hill Course Glossary
 adaptation
Batesian mimicry
 biotic potential
 carrying capacity
 coevolution
 commensalism
 complexity
 convergent evolution
 divergent evolution
 diversity
 ecological development
 ecological niche
 ecotones
 edge effects
 overshoots
environmental resistance
 pioneer species
 evolution
predator
 exponential growth
 primary productivity
 habitat
 primary succession
 J curve
 r-adapted species
K-adapted species
 resource partitioning
 keystone species
 S-curve
 logistic growth
 secondary succession
 Mullerian mimicry
 selective pressure
 mutualism
 symbiosis
 natural selection
 tolerance limits
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Chapter 3 - Topics
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Who Lives Where, and Why?
Species Interactions
Population Dynamics
Community Properties
Communities in Transition
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Part 1: Who Lives Where,
and Why?
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Tolerance Limits
Each environmental factor (temperature, nutrient supply,
etc.) has both minimum and maximum levels beyond which
a species cannot survive or is unable to reproduce.
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Abundance and Distribution of Species
• Liebig - proposed that the
single environmental factor
in shortest supply relative to
demand is the critical
determinant in species
distribution
• Shelford - added to Liebig's
work by proposing that the
single environmental factor
closest to tolerance limits
determines where a
particular organism can live
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• Today we know that for
many species the
interaction of several
factors, rather than a single
limiting factor, determines
biogeographical distribution.
• Sometimes, the
requirements and
tolerances of species are
useful indicators of specific
environmental
characteristics.
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Adaptation and Natural Selection
Two types of adaptation:
• Acclimation - changes in an individual organism due to
non-permanent physiological modifications
• Evolution - gradual changes in a species due to
changes in genetic material and competition
Theory of evolution - developed by Charles Darwin and
Alfred Wallace.
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Natural selection - genetic combinations best adapted
for present environmental conditions tend to become
abundant
• Spontaneous, random mutations
• Selective pressure - physiological stress, predation,
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competition, luck
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Speciation
These 13 species of finches are descendents of a single
seed-eating species.
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The Taxonomic Naming System
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The
Ecological Niche
Habitat - the place or
set of environmental
conditions in
which a particular
organism lives
Ecological niche - the
role played by a
species in a biological
community
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Resource
Partitioning
Over time, niches can
evolve as species
develop new strategies
to exploit resources.
Law of Competitive
Exclusion:
No two species will
occupy the same niche
and compete for the
same resources in the
same habitat for very
long.
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Part 2: Species Interactions
Most obvious are Predation and Competition
- antagonistic relationships
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Three Types of Symbiosis:
• Commensalism - one member benefits, while the
other is neither benefited nor harmed
• Mutualism - both members of the partnership benefit;
the lichens (algae and fungi) above show mutualism
• Parasitism - a form of predation where one species
benefits and the other is harmed
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Defensive Mechanisms
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Batesian Mimicry
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Keystone species species that play
essential community
roles (examples:
mycorrhizae, giant
kelp)
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Part 3: Population Dynamics
Exponential growth - the unrestricted increase in
a population (also called the biotic potential of a
population)
Carrying capacity - the maximum number of
individuals of any species that can be supported by
a particular ecosystem on a sustainable basis
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Overshoots and Diebacks
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Population Oscillations
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Growth to a Stable Population
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Environmental Resistance
Environmental resistance - factors that tend to reduce
population growth rates:
• Density-dependent
• - linked to population size
• - disease, lack of food
• Intrinsic
• - attributes of a species
• - slow reproduction
• Density-independent
• - often environmental
• - droughts, floods, habitat
• destruction
• Extrinsic
• - external to a species
• - predators, competitors,
• environmental risks
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Part 4: Community Properties
• Primary productivity - a community's rate of
biomass production, or the conversion of solar
energy into chemical energy stored in living (or
once-living organisms)
• Net primary productivity - primary productivity
minus the energy lost in respiration
• Productivity depends on light levels,
temperature, moisture, and nutrient availability.
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Relative biomass accumulation
of major world ecosystems.
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Abundance and Diversity
Abundance - the number of individuals of a species
in an area
Diversity - the number of different species in an area
• A useful measure of the variety of ecological niches
or genetic variation in a community
• Decreases as we go from the equator towards the
poles
Abundance and diversity depend on total resource
availability in an ecosystem.
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Antarctic
Marine Food
Web
Complexity - the
number of species
at each trophic
level and the
number of trophic
levels in a
community
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Stability and Resilience
Stability - a dynamic equilibrium among the physical
and biological factors in an ecosystem or a
community
Resiliency - the ability to recover from disturbance
Three kinds of stability or resiliency in ecosystems:
• Constancy - lack of fluctuations in composition or
functions
• Inertia - resistance to perturbations
• Renewal - ability to repair damage after disturbance
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Community structure
Distribution of members of a
population in a given space can be:
• Random - individuals live wherever
resources are available
• Ordered - often the result of
biological competition
• Clustered - individuals of a species
cluster together for protection,
mutual assistance, reproduction, or
to gain access to a particular
environmental resource
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Edges and Boundaries
• Ecotones- the
boundaries between
adjacent habitats
• Often rich in species
diversity
• Example: the
boundary between a
forest and a meadow
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Edge vs. Core
Edge effects - the environmental and biotic conditions
at the edge of a habitat
• Temperature, moisture levels, predator species, etc.
• Edge effects associated with habitat fragmentation
are generally detrimental to species diversity.
Core habitat - the interior area of a habitat
• Habitat not impacted by edge effects
• Some species avoid edges and ecotones and prefer
interior environments.
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Part 5: Communities in
Transition
Ecological succession - the process by which
organisms occupy a site and gradually change
environmental conditions by creating soil, shelter,
shade, or increasing humidity
• Primary succession - occurs when a
community begins to develop on a site
previously unoccupied by living organisms
• Secondary succession - occurs when an
existing community is disrupted and a new one
subsequently develops at the site
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Primary
Succession
on Land
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Exotic Species
• Sometimes communities can be completely altered
by the introduction of exotic species.
• Exotic species are often introduced by humans.
• Successful exotics tend to be prolific, opportunistic
species, such as goats, cats, and pigs.
• Many ecologists consider exotic species
invasions the most pressing hazard for biological
communities in the coming century.
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Introduced Species and Community
Change
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Summary
• Communities consist of an assemblage of populations (a
population being a group of individuals of the same
species that interbreed).
• Individuals with adaptations that improve their
reproductive fitness are selected for.
• Natural selection works through inter- and intra-specific
interactions.
• Populations oscillate before reaching equilibrium at the
carrying capacity of the system.
• Communities often have repeatable structures and
properties that exhibit parallel chronosequences following
disturbances (primary and secondary succession).
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