Download Notes Chapter 21 Community Ecology

21 Community Ecology

The five major types of close interactions, or symbioses, among species—predation,
parasitism, competition, mutualism, and commensalism.
 Interactions among various species comes from a long history of evolution in which the
species adjust to one another
 Coevolution: the back-and-fourth evolutionary adjustments between interacting members
of an ecosystem
1. Distinguish predation from parasitism.
Predation - the act of one animal feeding on another
1) Predation is a powerful force in a community.
a. In predation, one individual, the predator, captures, kills, and consumes another
individual, the prey – the thing that will be consumed
b. Predation influences where and how species live by determining the relationships
in the food web.
c. Predation is also an effective regulator of population size.
d. Predators, Prey, and Natural Selection - Natural selection, the major mechanism of
evolution, favors adaptations that improve the efficiency of predators at finding,
capturing, and consuming prey.
(1) A predator’s survival depends on its ability to capture food, but a prey’s
survival depends on its ability to avoid being captured
a. Predators often develop adaptations to catching their prey.
i. example: spider web, heat-sensitive pits of snakes, sharp teeth,
camouflage.
b. Prey often use mimicry.
i. Mimicry – a harmless species, looks like a poisonous one.
c. Prey species have evolved to escape, avoid or fight off predators
i. Plants use thorns, spines and prickles for protection
ii. Plants also use chemical compounds for protection which are
sometimes their primary defense against herbivores
(a) Secondary compounds: defensive chemical compound
produced by a plant
(b) Certain herbivores are able to feed on plants that are
protected by defensive chemicals
(c) For example, some herbivores have evolved to be able to
break down the defensive chemical
A. Parasitism
1) Parasitism is a species interaction that resembles predation in that one individual is
harmed while the other individual benefits - one organism feeds on and usually
lives on or in another
a. In parasitism, one individual, known as the parasite, feeds on another individual,
known as the host.
(1)
(2)
But while most forms of predation immediately remove an individual of the
prey species from the population, parasitism usually does not result in the
immediate death of the host.
Often, the parasite feeds on the host for a long time instead of killing it.
 Ectoparasites – external parasites. They live on their host but do not
enter.
i. ex: fleas, ticks, lice, leeches, etc.
 Endoparasites – internal parasites. They live inside their host’s body.
i. ex: protists such as malaria parasites and tapeworm.
2. Explain how competition can affect community structure.
A. Most competition does not involve fighting
1) Some competing species never encounter one another: they interact only by means of
their effect on the resource in question
B. Competition Research
1) Competition results from fundamental niche overlap—the use of the same limited
resource by two or more species.
a. Some species of plants release toxins into the soil that prevent individuals of other
species from growing nearby, restricting the living space of the other species.
b. More often, one organism will be able to use a resource more
efficiently, leaving less of the resource available to the other species.
(1) The principle of competitive exclusion to describe situations in which one
species is eliminated from a community because of competition for the same
limited resource.
(2) In competitive exclusion, one species uses the resource more efficiently and
has a reproductive advantage that eventually eliminates the other species.
1) Competition and Community Structure
a. Competition has the potential to be an important influence on the nature of a
community.
b. The composition of a community may change through competitive exclusion.
c. Competitors may also evolve niche differences or anatomical differences that
lessen the intensity of competition.
d. Natural selection favors differences between potential competitors.
e. These differences are often greatest where the ranges of potential competitors
overlap.
f. This phenomenon is called character displacement - the phenomenon that shows
that competitors may evolve niche differences or anatomical differences to lessen
the intensity of competition.
(d) If there is still too much competition, some species will use resource
partitioning, in which each species will only use a portion of their available
resources.
3. Contrast mutualism with commensalism, and give one example of each type of relationship.
A. Mutualism is a cooperative relationship in which both species derive some benefit.
1) Some mutualistic relationships are so close that neither species can survive without
the other.
2) Pollination is one of the most important mutualistic relationships on Earth.
a. Animals such as bees, butterflies, flies, beetles, bats, and birds pollinate many
flowering plants.
b. Animals that carry pollen are called pollinators.
c. A flower is a lure for pollinators, which are attracted by the flower’s color, pattern,
shape, or scent.
d. The plant usually provides food for its pollinators.
e. As the animal feeds in the flower, it picks up a load of pollen, which it will carry to
the next flower of the same species it visits.
B. Commensalism is an interaction in which one species benefits and the other is not
affected.
1) Some cases of commensalism may be mutualism in which the benefit to the second
organism hasn’t yet been identified.
2) An example of commensalism is the relationship between cattle egrets and Cape
buffalo in Tanzania.
a. The birds feed on small animals such as insects and lizards that are forced out of
their hiding places by the buffalo’s movement through the grass.
4. Distinguish between primary and secondary succession.
A. Successional Changes In Communities
1) Ecologists recognize two types of succession.
a. Primary succession is the development of a community in an area that has not
supported life previously, such as bare rock, a sand dune, or an island formed by a
volcanic eruption.
(1) Often proceeds very slowly because the minerals necessary for plant growth
are unavailable.
(2) The species that predominate early in succession—called the pioneer
species—tend to be small, fast-growing, and fast-reproducing.
(3) Pioneer species are well suited for invading and occupying a disturbed
habitat.
b. Secondary succession is the sequential replacement of species that follows
disruption of an existing community.
(1) Occurs where an existing community has been cleared by a disturbance,
such as agriculture, but the soil has been left intact.
(2) The disruption may stem from a natural disaster, such as a forest fire or a
strong storm, or from human activity, such as farming, logging, or mining.
(3) In secondary succession, it commonly takes about 100 years for the original
ecosystem to return through a series of well-defined stages.
2) The Complexity of Succession
a. The traditional description of succession is that the community proceeds through a
predictable series of stages until it reaches a stable end-point, called the climax
community.
(1) The organisms in each stage alter the physical environment in ways that
make it less favorable for their own survival but more favorable for the
organisms that eventually succeed them.
(2) Each stage paves the way for the next, leading ultimately to the climax
community, which remains constant for a long period of time.
(3) Climax communities are not stable and continue to change.
(4) Instead of proceeding inevitably toward the climax community, succession
may be regularly “reset” by disturbances.
Chapter 21 Community Ecology
 Ecologists recognize five major kinds of species interactions in communities: predation,
parasitism, competition, mutualism, and commensalism.
 The interaction in which one organism kills and eats another is predation. Predators have
evolved many ways to efficiently find and capture prey. Prey have evolved many ways to
defend themselves against predators.
 Mimicry is a defense mechanism in which a harmless species gains protection by its
resemblance to a poisonous or distasteful species, or in which two or more poisonous or
distasteful species resemble each other.
 Parasitism involves one organism feeding on, but not always killing, another. Parasites are
grouped into two general categories—external parasites (ectoparasites) and internal parasites
(endoparasites).
 Competition occurs when two or more species use the same scarce resource.
 Competition may cause competitive exclusion, the extinction of one competitor from the
community. It may also be a strong selective force in the evolution of niche differences
among competitors.
 In mutualism, both interacting species benefit. The relationship between flowering plants
and their pollinators is an example of mutualism.
 In commensalism, one species benefits and the other is not affected.
 Species richness is the number of species in a community.
 Species diversity incorporates both species richness and the abundance of each species.
 As a general rule, species richness is greatest near the equator.
 Larger areas generally support more species. This is called the species-area effect.
 Species interactions such as predation can promote species richness.
 Species richness improves a community’s stability.
 Succession is a change in the species composition of a community. Primary succession is the
assembly of a community on newly created habitat. Secondary succession is the change in
an existing community following a disturbance.
 Pioneer species are the initial colonists of a disturbed area. They are usually small organisms
that grow fast, reproduce quickly, and disperse their seeds well.
 Succession at Glacier Bay proceeded from bare debris left behind by a retreating glacier to
small plants such as mosses, to alders, to spruce and hemlocks.
 Secondary succession occurs in areas where the original ecosystem has been cleared by a
disturbance, such as clearing for agriculture. It proceeds from plants commonly thought of as
weeds to a stable end point called a climax community.
Vocabulary List
Character displacement
Climax community
Commensalism
Competition
Competitive exclusion
Ectoparasite
Endoparasite
Herbivore
Host
Mimicry
Mutualism
Parasite
Parasitism
Pioneer species
Pollinator
Predator
Prey
Primary succession
Resource partitioning
Secondary compound
Secondary succession
Species diversity
Species richness
Species-area effect
Stability
Succession
Symbiosis