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Community Ecology
Chapter 8
COMMUNITY STRUCTURE AND
SPECIES DIVERSITY
• Biological communities differ in their structure or
spatial distribution.
Figure 7-2
Community Structure has 4
characteristics:
• 1. Physical appearance: relative size,
stratification & distribution of its populations
and species
• 2. Species diversity or richness
• 3. Species abundance
• 4. Niche structure – number of ecological
niches, species interactions with similarities
and differences
Species Diversity and Niche
Structure: Different Species Playing
Different Roles
• Biological communities differ in the types and
numbers of species they contain and the
ecological roles those species play.
– Species diversity: the number of different species it
contains (species richness) combined with the
abundance of individuals within each of those species
(species evenness).
Species Diversity and Niche Structure
• Niche structure: how many potential
ecological niches occur, how they resemble or
differ, and how the species occupying different
niches interact.
• Geographic location: species diversity is
highest in the tropics and declines as we move
from the equator toward the poles.
What determines species diversity ?
• Edge Effect-the boundaries between two
ecosystems (ecotones) may have a
different combination of species than those
found in the two ecosystems.
• Forest-field ecotone and game species
• The most species rich areas:
– Coral reefs
– Large tropical lakes
Three main factors affect species
diversity:
• 1. Latitude – decreases as moves away from
equator
• 2. Depth in aquatic systems – increases from
surface to 2000 m, then decreases until the
sea bottom where diversity is usually high
• 3. Pollution in aquatic systems
Case Study:
Species Diversity on Islands
• MacArthur and Wilson proposed the species
equilibrium model or theory of island
biogeography in the 1960’s.
• Model projects that at some point the rates of
immigration and extinction should reach an
equilibrium based on:
– Island size
– Distance to nearest mainland
TYPES OF SPECIES
• Native, nonnative, indicator, keystone, and
foundation species play different ecological
roles in communities.
– Native: those that normally live and thrive in a
particular community.
– Nonnative species: those that migrate,
deliberately or accidentally introduced into a
community.
Indicator Species:
Biological Smoke Alarms
• Species that serve as early warnings of damage to
a community or an ecosystem.
– Presence or absence of trout species because they
are sensitive to temperature and oxygen levels.
– Canary in the Coal mine – they
sang untill increase in CO
– Salmon and Stoneflies
– Amphibians
Case Study:
Why are Amphibians Vanishing?
• Frogs serve as indicator species because different
parts of their life cycles can be easily disturbed.
Figure 7-3
Case Study:
Why are Amphibians Vanishing?
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Habitat loss and fragmentation.
Prolonged drought.
Pollution.
Increases in ultraviolet radiation.
Parasites.
Viral and Fungal diseases.
Overhunting.
Natural immigration or deliberate introduction of
nonnative predators and competitors.
Keystone Species• Strong interaction with other species affect
the health and survival of the
• other species. Significance is out of proportion
with their biomass…. In other words, a small
biomass of these organisms have a large effect
on the ecosystem.
• Pacific Northwest Millipede
• They process material
• out of proportion to
• their numbers or
• biomass
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Top PredatorsInteraction Between Species
Intraspecific CompetitionTerritorialityInterspecific CompetitionInterference CompetitionExploitation Competition
The Competitive Exclusion Principle
• Says that two species sharing the same Resource
cannot coexist indefinitely in an ecosystem that
does not have enough resources to meet the needs
of both species.
Strategies that Reduce Competition
• Resource Partitioning- share the wealth by
using a resource at
• Different times
• In different ways
• In different places
• Fundamental verses Realized
• Niche
– Hawks and Owls
– Lions and Leopards
– Chum and Coho Salmon
Predator-Prey Relationships
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On the population level predators are beneficial to prey
because the predator
Reduces the prey population giving remaining
prey greater access to the available food supply
Improve the genetic stock of the prey population
Examples of predator adaptations that maximize their
chances of getting a meal
Cheetahs-speed
Eagles
Wolves and Lions-teamwork
Snowy Owls
PREDATION
• Some prey escape their
predators or have outer
protection:
• Mimicry
• Deceptive Looks
• Deceptive Behavior
• Warning Coloration
• Camouflaged
• Chemicals warfare to
repel predators.
Figure 7-8
Symbiotic Relationships
• Parasitism- one benefits while the other is
harmed (host) Usually draws nutrients from
the host which weakens it, but seldom kills it.
• Some parasites live inside (tapeworm); others
live outside (fleas and mosquitoes)
Mutualism: Win-Win Relationship
• (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
& food by living among deadly
stinging sea anemones and helps
protect the anemones from some
of their predators.
• (c) Beneficial effects of mycorrhizal
fungi attached to roots of juniper
seedlings on plant growth
compared to
• (d) growth of such seedlings in
sterilized soil without mycorrhizal
fungi.
Figure 7-9
Mutualism-both benefit
• Lichens are a relationship between a
• fungus and an algae. The fungus supplies
_________ and receives___________ from
the algae. The algae supplies ____________
and receives ___________ from the fungus
– Rhizobium bacteria and the roots of legumes
– Zooanthellae and coral polyps
– Clownfish and sea anemones
– Mycorrhizae fungi and the roots
– of many trees
Commensalism: Using without Harming
• Some species
interact in a way
that helps one
species but has
little or no effect
on the other.
Figure 7-10
Ecological Succession –gradual change in
species composition in a specific area.
• 1. Primary succession – establishing life on lifeless
ground
– Begins where there is no soil in terrestrial ecosystems
– no bottom sediment in aquatic ecosystems
• Pioneer species start things off by getting a
foothold on bare surfaces like rocks. Lichens and
moss specialize at this.
• They secrete acids which begin to breakdown the
rock and they trap wind blown soil and bits of
organic matter.
Primary Succession:
Starting from Scratch
• Primary
succession begins
with an
essentially lifeless
are where there
is no soil in a
terrestrial
ecosystem
Figure 7-11
Early successional plant species follow. Typically
these:
 Grow close to the ground
 Grow quickly under harsh conditions
 Have short lives
Ex: include small perennial grasses, herbs, or ferns
Mid-successional species follow these which include
less hardy species of grasses, herbs, and low shrubs
Late successional species are typically those that are
adapted to the climate and soil type of the area and
are typically trees
Secondary succession-begins in
areas where an established natural
community has been disturbed
 Abandoned farmlands
 Burned or cut forests
 Heavily polluted streams
 Land that has been dammed or
flooded
Secondary Succession:
Starting Over with Some Help
• Secondary
succession
begins in an
area where the
natural
community has
been
disturbed.
Figure 7-12
Three species interactions involved in
succession
1. Facilitation- one species makes an area suitable
for another species with different niche
requirements. Example… lichens, legumes & N
2. Inhibition- early species hinder the
establishment and growth of other
species. Example… plants that release toxins
3. Tolerance- late successional stages are
unaffected by earlier successional
stages. Example…
Disturbances and species diversity
Intermediate disturbance hypothesis communities that experience fairly frequent,
moderate disturbances have the greatest species
diversity
Old View of Succession
Orderly progression of successional stages building
towards a stable community of a few long lived species
known as a ______________ community.
Instead, most disturbed communities result in everchanging mosaic of vegetation patches at different
successional stages.
ECOLOGICAL STABILITY AND
SUSTAINABILITY
• Living systems maintain some degree of stability
through constant change in response to
environmental conditions through:
– Inertia (persistence): the ability of a living system to
resist being disturbed or altered.
– Constancy: the ability of a living system to keep its
numbers within the limits imposed by available
resources.
– Resilience: the ability of a living system to bounce
back and repair damage after (a not too drastic)
disturbance.
ECOLOGICAL STABILITY AND
SUSTAINABILITY
• Having many different species appears to
increase the sustainability of many communities.
• Human activities are disrupting ecosystem
services that support and sustain all life and all
economies.
The Precautionary Principle
• When evidence indicates that our actions may
harm the environment, even though all the “cause
and effect” relationships have not been established
• between our actions and harm to the
environment, it is better to take precautionary
measures to prevent harm.
– It is easier to prevent pollution than it is to clean it up
once the harm of that pollution has been established.
– It is easier to protect ecosystems than it is to recreate
them once they have been destroyed.
• How much time and effort should we put into
preventing Global Warming
• when we are not even sure of the ultimate effects
of climate change ?