Chapter 7: Community Ecology
Community Structure
- described by overall physical appearance
o relative sizes, stratification, and distribution of populations and
species in each community
Species Diversity
- species diversity = species richness (# of different species it contains) +
species evenness (the abundance of individuals within each of those species)
- niche structure: how many potential ecological niches occur, how they
resemble or differ from one another, and how the species occupying different
niches interact.
- Geographical location: species diversity is highest in tropics and declines as
we move from the equator toward the polls.
Types of Species
- native species: species that normally live and thrive in a particular
community.
- nonnative species: species that migrate into or are deliberately or
accidently introduced into a community, also called invasive or alien
species.
o Some can be beneficial to help keep away pests or other annoyances.
o Some can be harmful and wipe out native species
- Indicator species: species that serve as early warnings of damage to a
community or an ecosystem.
o Birds and butterflies are excellent indicators because they are found
almost everywhere and are affected quickly by environmental change.
- Keystone species: species that play roles affecting many other organisms in
an ecosystem.
o If this species disappears, the whole ecosystem will most likely fall
apart.
- Foundation species: species that play a major role in shaping communities
by creating and enhancing their habitats in ways that benefit other species.
Species Diversity on Islands
- Factors that affect the theory of island biogeography:
o Size
 Smaller island = fewer different species and lower immigration
rate because it is a smaller target
 Larger island = more diversity within species and a higher
immigration rate
o Distance from the mainland
 Islands closer to the mainland have a higher immigration rate
because less distance must be travelled to get to it. Because of
this it will have a more species diversity
Potential Reasons for Species Decline
- Scientists are noticing a recent decline in amphibians and are researching
what factors are making the species disappear.
o Habitat loss and fragmentation
Prolonged drought
Pollution
Increases in UV radiation
Parasites
Viral and fungal diseases
Climate change
Overhunting
Natural immigration or deliberate introduction of nonnative
predators and competitors
Why are Sharks Important?
It is not just sharks that are important. Other predators like sharks are important as
well. All predators help with natural selection and regulating the food chain in their
ecosystems and communities.
Species Interactions
- Interspecific competition: competition for shared or limited resources.
Instead of fighting for these, most competition involves ability of one species
to become more efficient in acquiring food or other resources.
o When competition arises, one of the competing species must migrate
to another area, shift its feeding habits or behavior through natural
selection, suffer a sharp population decline, or become extinct in the
area.
o Resource partitioning: occurs when species competing for similar
scarce resources evolve more specialized traits that allow them to use
shared resources at different times, in different ways, or in different
places.
- Predation: members of one species (the predator) feed directly on all or part
of a living organism of another species (the prey).
o The two kinds of organisms from a predator-prey relationship.
o In order to find food/mates/prey animals use:
 Electromagnetic radiation- visible light, but also some detect
UV light and many have acute night vision
 Sound- enhanced by ear shapes and ability of organisms to use
objects to bounce the sound off of
 Smelling- some can smell chemicals their prey gives off to
locate them
 Prey defend themselves by using: protective shells,
camouflage, chemical warfare, bad tastes, or bright colors and
shapes
- Parasitism: when one species (the parasite) feeds on part of another
organism, usually by living on or in the host.
o The parasite benefits and the host is harmed.
o The parasite is usually smaller than the host and rarely kills the host.
 Worms, ticks, fleas, mosquitoes, mistletoe plants, and sea
lampreys, among others.
- Mutualism: two species of a network of species interact in a way that
benefits both.
o
o
o
o
o
o
o
o
o Combines nutrition and protection
- Commensalism: interaction that benefits one species, but has little effect on
the other species.
Succession
Ecological succession: gradual change in species composition of a given area.
- Primary succession: involves the gradual establishment of various biotic
communities in lifeless areas where there is no soil in a terrestrial
community or no bottom sediment in an aquatic community.
o bare rock exposed by a retreating glacier or severe soil erosion, newly
cooled lava, abandoned highway or a newly created shallow pond or
reservoir.
o Takes a long time, and begins with a pioneer species.
o After hundreds of years, there might be enough soil for grow of herbs,
grasses, and low shrubs.
o As these grow, trees will start to grow.
- Secondary succession: Begins in an area where the natural community of
organisms has been disturbed, removed, or destroyed, but some soil or
bottom sediment remains.
o Abandoned farmland, burned or cut forests, heavily polluted streams,
and land that has been flooded.
o New vegetation can germinate within weeks.
o Secondary succession locations often have high resilience: the ability
to recover and be restored after being disturbed.
Ecological Stability
- Ecologists disagree on whether or not a community needs both high inertia
and high resilience to be considered stable.
- Evidence suggests most have one but no the other
o Tropical rain forests have high species diversity and high inertia and
thus are resistant to significant alteration/destruction
 When a part is degraded, the resilience becomes so low it
cannot be restored
o Grasslands are less diverse than most forests and have low inertia
because they burn easily.
 Most of their plant matter is underground in the roots so they
have a high resilience and recover quickly.
- Populations, communities, and ecosystems are rarely, if ever, in equilibrium.
They are in a continuing state of disturbance, fluctuation, and change.
Why Should we Bother to Protect Natural Systems?
- Human activities and disturbances are disrupting, destroying, degrading and
simplifying many of the world’s ecosystems.
o We must use great caution in making potentially harmful changes to
communities and ecosystems.
Practice Questions:
1. What is the most influential factor on terrestrial species diversity?
a. longitude
b. latitude
c. depth
d. elevation
2. Which is NOT a characteristic that an ecologist uses to describe a biological
community?
a. niche structure
b. physical appearance
c. species diversity
d. elevation
3. A species that plays a role in affecting many other organisms in an ecosystem is a
(an):
a. native species
b. indicator species
c. keystone species
d. foundation species
4. Parasitism occurs when:
a. one species feeds on part of another organism
b. the parasite benefits and the host is harmed
c. all of the above
d. none of the above
5. Which of the following is a pioneer species?
a. lichen
b. holly shrub
c. redwood tree
d. oak tree
6. As succession proceeds, ________ increases and _______ decreases.
a. plant growth rate, net primary productivity
b. stratification, species diversity
c. net primary productivity, stratification
d. species diversity, net primary productivity
7. The ability of a living system to bounce back after a disturbance that is not too
drastic is called
a. constancy
b. resilience
c. inertia
d. primary succession
8. True or false: Scientists now believe that succession continues along a predictable
path until a stable climax community is reached.
a. true
b. false
9. True or false: Extinction is an example of an aspect of stability in living systems.
a. true
b. false
10. True or false: a grassland has low inertia.
a. true
b. false
11. According to the theory of island biography, a _______ will have the greatest
number of species. (2 things)
12. An amphibian is an important _________ species.
13. What are 4 factors that can affect a species and cause a decline in a population?
14. Explain the concept of resource partitioning.
15. What is the difference between a foundation species and a native species?
16. What are the 5 basic types of interactions between species?
17. What is the difference between primary and secondary succession?
18. Explain the concept of mutualism and explain how this occurs between a
clownfish and a sea anemone.
19. What are some effects of nonnative species?
20. What is a niche structure?
Answer Key
1. b. Latitude; this is because latitude determines the biome which in return
determines the habitats which species live in and determine which types of species
can survive.
2. d. elevation; elevation is not important when describing a biological community.
The main characteristics that determine this are niche structure, physical
appearance, and species diversity.
3. c. keystone species; a keystone species is like the “key stone” in the arch of rocks.
If this stone falls out, the other stones fall. This is the same with an ecosystem. The
keystone species regulates the ecosystem.
4. c. all of the above; parasitism is characterized by one species feeding on a part of
another organism and that the host is harmed while the parasite benefits.
5. a. lichen; a pioneer species is a species that arrives and attaches themselves to
inhospitable patches of bare rock. They are the catalyst for primary succession. A
holly shrub, redwood tree, and oak tree are all developed species, while lichen is a
microorganism.
6. d. species diversity, net primary productivity; Succession is the gradual change in
species composition of a given area. When succession changes there are more
species which causes an increase in species diversity. At the same time, the net
primary productivity decreases because there are more species consuming the
plants.
7. b. resilience; When a species can bounce back from a disturbance, it means they
have a high resilience factor, the ability to recover.
8. b. false; Scientists believe that succession is always occurring and is never stable.
It is constantly changing.
9. b. false; Extinction is not an example of stability in living systems because when a
species becomes extinct the ecosystem will fall apart.
10. a. true; a grassland has a low inertia.
11. large island, close to the mainland; this is because when it has more space, more
species can live on the island. Also, if it is close to a mainland, the migration from
the mainland to the island is greater than an island further away from a mainland.
12. indicator species. Scientists have recently taken interest in amphibians as
indicator species because they live part of their life on water, and part of their life on
land. Because of the great decline in the amphibian populations scientists have
began to look at factors in their life cycles. Some of these are habitat loss and
fragmentation, prolonged drought, pollution, increases in ultraviolet radiation,
parasites, diseases, climate change, overhunting, natural immigration or deliberate
introduction of nonnative predators and competitors.
13. 4 factors that can affect a species and cause its decline are:
1. habitat loss and fragmentation, especially from draining and filing of inland
wetlands, deforestation, and development.
2. Pollution, especially exposure to pesticides, which can make animals more
vulnerable to bacterial, viral, and fungal disease and can cause sexual abnormalities.
3. Climate change, global warming evaporates water and increases cloud cover in
tropical forests. This lowers daytime temperatures and makes nights warmer and
creates favorable conditions for the spread of fungi.
4. Viral and fungal diseases that affect the health of species.
14. Resource partitioning is when species competing for similar scarce resources
evolve more specialized traits that allow them to used shared resources at different
times, in different ways, or in different places. This occurs because of natural
selection of two competing species that become more specialized.
15. A foundation species is a species that plays a major role in communities by
shaping them by creating and enhancing their habitats in a way that benefits others.
A native species is a species that normally lives or thrives in a particular community.
Foundation species create a foundation for other species or provide new resources
that the other species may not have had access to, while a native species is the
original species that was in the area.
16. The 5 basic types of interactions between species are:
1. interspecific competition is when species compete for shared or limited resources
such as space and food. This is the most common interaction between species.
Instead of fighting for resources, most competition involves the ability of one
species to become more efficient in acquiring food or other resources.
2. Predation: members of one species (the predator) feed directly on all or part of a
living organism or another species (the prey). Together, the two kinds of organisms
form a predator-prey relationship. At the individual level, members of the prey
species are clearly harmed. At the population level, predation plays a role in
evolution by natural selection.
3. Parasitism: occurs when one species (the parasite) feeds on part of another
organism (the host), usually by living on or in the host. The parasite benefits and
the host is harmed.
4. Mutualism: when two species or a network or species interact in a way that
benefits both.
5. Commensalism: An interaction that benefits one species but has little, if any effect
on the other species.
17. Primary succession involves the gradual establishment of various biotic
communities in lifeless areas where there is no soil in a terrestrial community or no
bottom sediment in an aquatic community. Secondary succession is when a series
of communities with different species develop in places containing soil or bottom
sediment.
18. Mutualism is when two species or a network or species interact in a way that
benefits both. An example is the clownfish species which live within sea anemones,
whose tentacles sing and paralyze most fish that touch them. The clownfish that are
not harmed by the tentacles, gain protection from predators and feed on the detritus
left from the meals of the anemones.
19. Nonnative species are other species that migrate into or are deliberately or
accidently introduced into a community. A lot of people see them as villains but
most are introduced and domesticated species of crops and animals are beneficial.
A nonnative species can reduce some or most of a community’s native species.
20. A niche structure is how many potential ecological niches occur, how they
resemble or differ from one another, and how the species occupying different niches
interact.