Download Chapter 52

52
Community Ecology
Lecture Outline
I.
Community structure and functioning
In a community, populations interact in a variety of ways
Within a community, no species exists independently of other species
The niche is a species role in the community
The niche describes all aspects of an organism’s existence
The niche describes not only the habitat, but much more
The fundamental niche is the niche that an organism would occupy in the absence of
competition
The realized niche is the niche that an organism occupies in the presence of competition
Limiting resources restrict the ecological niche of a species
Limiting factors as varied as soil composition, climatic extremes, or population
interactions may influence the niche
Any environmental resource that is scarce or unfavorable is called a limiting resource
Biotic and abiotic factors may influence a species' ecological niche
Barnacles in the intertidal zone is an example
Competition is intraspecific or interspecific
Intraspecific competition exists between members of the same species
Interspecific competition exists between members of different species
Competition between two species with overlapping niches may lead to competitive
exclusion
Competitive exclusion results in the exclusion of one species due to interspecific
competition
Coexistence occurs when overlap between niches is reduced
Gause performed the first experiments that showed the effect of competition on
fundamental and realized niches
Character displacement reduces interspecific competition
Darwin’s finches exemplify character displacement
Competing species evolve slightly different niches to avoid competition
Natural selection shapes the body forms and behaviors of both predators and prey
Predation is the consumption of one species, the prey, by another, the predator
Pursuit and ambush are two predator strategies
Predators that chase their prey are pursuit predators
Some predators are camouflaged to avoid notice by their prey; others may attract their
prey like the anglerfish
Chemical protection is an effective plant defense against herbivores
Plants use spines, tough leaves, and chemicals to detract herbivores
Milkweeds produce chemicals that are poisonous to all but a few insects
Monarch caterpillars graze on milkweeds and accumulate chemicals in
their bodies that make them unpalatable to birds and other
predators
Animals possess a variety of defensive adaptations to avoid predators
Animals may flee, hide, live in groups, or have mechanical defenses to avoid being
preyed upon
Warning coloration results in avoidance by experienced predators
Batesian mimicry is the resemblance of a harmless organism to a harmful or unpalatable
organism
Müllerian mimicry is the similar morphology of a group of harmful or unpalatable
organisms
Monarch and viceroy butterflies are examples of Müllerian mimicry
Symbiosis involves close associations between species
Symbiosis is any intimate relationship or association between members of two or more
species
In mutualism, benefits are shared
Nitrogen-fixing bacteria and legumes are mutualists
Zooxanthellae and reef-building coral are mutualists
Mycorrhizae facilitate mutualistic relationships between fungi and the roots of a wide
variety of plants
Commensalism is taking without harming
Epiphytes living on tropical trees benefit from the habitat of the host, but the host is not
harmed or benefited
Parasitism is taking at another's expense
The parasite benefits, while the host is harmed
A well-adapted parasite does not kill the host
A parasite that causes the death of the host is a pathogen
Keystone species affect the character of the community
Keystone species have a great effect on other species in the community
Keystone species are commonly top predators
Dominant species influence a community as a result of their greater size or abundance
Trees are the dominant species of forests because they change the local environment;
similarly, cordgrass in salt marshes, prairie grasses in prairies, and kelp in kelp beds
Animals can also be dominant species, such as coral in coral reefs and cattle in
overgrazed rangelands
II.
Community biodiversity
Species richness is the number of species within a community; species diversity is
a measure of the relative importance of each species
Ecologists seek to explain why some communities have more species than others
Isolated communities are typically less diverse than continental communities
Species diversity typically increases in less stressful habitats
High latitude communities have lower species diversity
Ecotonal zones have high species diversity, known as the edge effect
Geologically older habitats have higher species diversity
Species richness probably causes community stability
The traditional view stated that stability was a result of community complexity
Newer mathematical models suggest that complexity does not lead to stability
III.
Community development
A. Succession is the process of community development over time, with one
species being replaced by another
Primary succession occurs when a community develops in a "lifeless" environment
During primary succession on bare rock, the rock is eventually transformed into soil
Primary succession occurs on newly formed volcanic lava and recently glaciated rock
Secondary succession occurs when a community develops where a previous community
existed
Secondary succession occurs where soil already exists, such as areas that have been
denuded or modified by fire or agriculture
Abandoned farmland has been extensively studied
Disturbance influences succession and species richness
The idea of a climax community has fallen out of favor; rather, communities exist in a
state of continual disturbance
The intermediate disturbance hypothesis says that species richness is greatest at
moderate levels of disturbance
Ecologists continue to study community structure
Clements developed ideas about communities, now called the organismic model, which
emphasizes the interdependence of species within the community
Gleason developed ideas, called the individualistic model, which emphasizes species
individuality
Most research supports the individualistic model
Research and Discussion Topics

How is diversity related to successional stages? Is diversity greatest at early, middle,
or climax stages?

Investigate the wide variety of secondary compounds found in plants. Look at
plants that produce chemicals used as drugs by humans, plants that produce
chemicals that we use for their taste (e.g. horseradish and various spices), and plants
that produce noxious chemicals, such as poison ivy and poison oak.

Interestingly, although it was long believed that persons indigenous to hot climates
used more spices than persons living in cold climates because the spices would mask
the foul taste of food that had "gone bad," recent research shows that many spices,
herbs, and condiments like garlic and onion, actually act to keep bacterial numbers
low. The components of the Asian five-spice powder, or the combination
traditionally used to spice a salsa (garlic, salt, pepper, chilies, and cilantro) have been
shown to greatly prevent the growth of potentially harmful bacteria in food.