Download Chapter 21 Community Ecology

Chapter 21 Community Ecology
This chapter introduces the five major types of close interactions, or symbioses, among
species – predation, parasitism, competition, mutualism, and commensalisms.
Interactions Among Species
- Interactions among various species comes from a long history of evolution in
which the species adjust to one another
o Coevolution: the back-and-fourth evolutionary adjustments between
interacting members of an ecosystem
Predators and Prey Coevolve
- Predation: the act of one animal feeding on another
o Predator – captures, kills, and consumes another individual.
o Prey – the thing that will be consumed.
 Predators often develop adaptations to catching their prey.
1. example: spider web, heat-sensitive pits of snakes, sharp
teeth, camouflage.
 Prey often use mimicry.
1. Mimicry – a harmless species, looks like a poisonous one.
Plant Defenses Against Herbivores
- Prey species have evolved to escape, avoid or fight off predators
o Plants use thorns, spines and prickles for protection
o Plants also use chemical compounds for protection which are
sometimes their primary defense against herbivores
 Secondary compounds: defensive chemical compound
produced by a plant
How Herbivores Overcome Plant Defenses
- Certain herbivores are able to feed on plants that are protected by defensive
chemicals
o For example, some herbivores have evolved to be able to break down
the defensive chemical
Parasitism
Parasitism: a form of predation in which one organism feeds on and usually lives on or in
another
o Typically, parasites do not kill their prey, also called “host”
 Hosts provide food and a place to live
 Hosts provide a means of transportation for a parasites
offspring to another host

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Ectoparasites – external parasites. They live on their host but
do not enter.
1. ex: fleas, ticks, lice, leeches, etc.
 Endoparasites – internal parasites. They live inside their host’s
body.
1. ex: protists such as malaria parasites and tapeworm.
because parasites have the potential to cause problems, hosts have developed
lots of ways to keep them out.
o Skin, chemicals in eyes, tears, saliva, etc.
parasites have learned to adapt to their hosts.
o Tapeworms don’t have a digestive system because they absorb
nutrients directly through their skin (because they are located in the
digestive system of their host).
Competition
Common Use of Scarce Resources Leads to Competition
- Competition: ecological interaction between two or more species that use the
same scarce resource such as food, light and water
o Most competition does not involve fighting
 Some competing species never encounter one another: they
interact only by means of their effect on the resource in
question
- Niche: functional role of a species in an ecosystem
o A niche is the functional role of an organism in its ecosystem: how an
organism lives day to day
o Niches can be described by the space an organism uses, the food an
organisms consumes, temperature ranges an organism requires, etc.
o A niche is not the same thing as a habitat
 A habitat is a location
 A niche is a pattern of living
o A niche is often described in terms of how the energy flow is effected
by the organism within the ecosystem
Dividing Resources Among Species
- It is possible for different species to occupy only a certain portion of a
fundamental niche
o Realized niche: the part of its fundamental niche that a species actually
occupies
 Realized niches reduces competition among different species
 MacArthur suggested natural selection favored a range of
preferences and behaviors among species
 Character Displacement – the phenomenon which shows
that competitors may evolve niche differences or

anatomical differences to lessen the intensity of
competition.
o Ex: Darwin’s finches and their different beak sizes.
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.
Competition Can Limit How Species Use Resources
- Competition can limit how species use resources
o For example, Chthamalus stellatus and Semibalanus balanoides
 The realized niche of Chthamalus is smaller than its fundamental
niche because of competition from the faster growing Semibalanus
Competition Without Division of Resources Leads to Extinction
- Competition is most acute between species who are similar to each other
because they tend to utilize the same resources
o The species that is able to better utilize the resource will eventually
eliminate the other
o Competitive exclusion: local extinction of one species due to competition
 Ex. P. caudatum vs. P. aurelia
When Can Competitors Coexist
- Competitors are able to coexist due to the effects of fundamental niches and
realized niches
Symbiotic Species Are Shaped By Long-Term Relationships
- Symbiosis: two or more species living together in a close, long-term
association
o Symbiotic relationships can benefit both organisms or just one leaving
the other unharmed
o Parasitism is a symbiotic relationship which detrimental to the host
organism
Mutualism
- Mutualism: a symbiotic relationship in which both participating species
benefit
o An example of mutualism is that of an ant and an aphid or the
relationship between plants and pollinators.
Commensalism
- Commensalism: a form of symbiotic relationship in which one species
benefits while the other is neither harmed nor helped
o An example of commensalisms is the relationship between small
tropical fish and sea anemones
Predation Can Lessen Competition
- Predation can lessen competition
o For example, when sea stars are removed from an ecosystem the diversity
of its prey species decreases
 This gave mussels, the superior competitor, the opportunity to
“drive out” the other prey species of the ecosystem
 By preying on mussels, sea stars keep the mussel populations too
low to drive out other species
o Biodiversity: the variety of species living within an ecosystem
 Biodiversity is promoted by predation
 Biodiversity is a measure of both the number of different species in
a community and the relative numbers of each of all the species
Properties of Communities
Species Richness and Diversity
- Species Richness – the number of species a community contains.
- Species Diversity – the number of species in the community is compared to
the relative abundance of each species.
o Species richness varies latitude. As a rule, the closer a community is to
the equator, the more species it will contain.
 Could be due to the tropics not being disturbed during the ice age.
- Species-area effect – larger areas generally contain more species than smaller
areas.
 Usually applied to tropical island areas.
Community Stability
- Stability – indicates the resistance to change of the community.
o Usually, communities with more species are the most stable.
- Tilman’s plot experiments showed two key concepts:
o Increased biodiversity leads to greater productivity
o Biologically diverse areas are more stable than those areas with fewer
species
Succession
- The gradual, sequential regrowth of species in an area.
- Primary succession – the development of life in an area that has not supported
life previously.
o Done by pioneer species (the first species to inhabit an area).
 They are fast growing, small, and fast reproducing.
- Secondary succession – the sequential replacement of species that follows
disruption of an existing community.
- Climax community – once growth has proceeded through a predictable series
of stages, it reaches a stable end point. This is the climax community.