Chapter 7
Community Ecology
Core Case Study:
Why Should We Care about the American Alligator?
•	Hunters wiped out population to the point of near extinction.
•	Alligators have important ecological role.

Core Case Study:
Why Should We Care about the American Alligator?
•	Dig deep depressions (gator holes).
–	Hold water during dry spells, serve as refuges for aquatic life.
•	Build nesting mounds.
–	provide nesting and feeding sites for birds.
–	Keeps areas of open water free of vegetation.
•	Alligators are a keystone species:
–	Help maintain the structure and function of the communities where it is found.
COMMUNITY STRUCTURE AND SPECIES DIVERSITY
•	Biological communities differ in their structure and physical appearance.

Species Diversity and Niche Structure: Different Species Playing Different Roles
•	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.
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.

Animation: Area and Distance Effects
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.
Keystone Species: Major Players
•	Keystone species help determine the types and numbers of other species in a community thereby helping to sustain it.
Foundation Species: 
Other Major Players
•	Expansion of keystone species category.
•	Foundation species can create and enhance habitats that can benefit other species in a community.
–	Elephants push over, break, or uproot trees, creating forest openings promoting grass growth for other species to utilize.
Case Study: 
Why are Amphibians Vanishing?
•	Frogs serve as indicator species because different parts of their life cycles can be easily disturbed.

Case Study: 
Why are Amphibians Vanishing?
•	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.
Video: Frogs Galore
SPECIES INTERACTIONS: 
COMPETITION AND PREDATION
•	Species can interact through competition, predation, parasitism, mutualism, and commensalism.
•	Some species evolve adaptations that allow them to reduce or avoid competition for resources with other species (resource partitioning).
Animation: How Species Interact
Resource Partitioning
•	Each species minimizes competition with the others for food by spending at least half its feeding time in a distinct portion of the spruce tree and by consuming somewhat different insect species.
Niche Specialization
•	Niches become separated to avoid competition for resources.
Animation: Gause’s Competition Experiment

SPECIES INTERACTIONS: 
COMPETITION AND PREDATION
•	Species called predators feed on other species called prey.
•	Organisms use their senses to locate objects and prey and to attract pollinators and mates.
•	Some predators are fast enough to catch their prey, some hide and lie in wait, and some inject chemicals to paralyze their prey.
PREDATION
•	Some prey escape their predators or have outer protection, some are camouflaged, and some use chemicals to repel predators.

SPECIES INTERACTIONS: PARASITISM, MUTUALISM, AND COMMENSALIM
•	Parasitism occurs when one species feeds on part of another organism.
•	In mutualism, two species interact in a way that benefits both.
•	Commensalism is an interaction that benefits one species but has little, if any, effect on the other species.
Parasites: Sponging Off of Others
•	Although parasites can harm their hosts, they can promote community biodiversity.
–	Some parasites live in host (micororganisms, tapeworms).
–	Some parasites live outside host (fleas, ticks, mistletoe plants, sea lampreys).
–	Some have little contact with host (dump-nesting birds like cowbirds, some duck species)
Mutualism: Win-Win Relationship
•	Two species can interact in ways that benefit both of them.
Video: Clownfish with Anemone
Commensalism: Using without Harming
•	Some species interact in a way that helps one species but has little or no effect on the other.
ECOLOGICAL SUCCESSION: COMMUNITIES IN TRANSITION
•	New environmental conditions allow one group of species in a community to replace other groups.
•	Ecological succession: the gradual change in species composition of a given area
–	Primary succession: the gradual establishment of biotic communities in lifeless areas where there is no soil or sediment.
–	Secondary succession: series of communities develop in places containing soil or sediment.
Animation: Succession
Primary Succession: 
Starting from Scratch
•	Primary succession begins with an essentially lifeless are where there is no soil in a terrestrial ecosystem

Secondary Succession: 
Starting Over with Some Help
•	Secondary succession begins in an area where the natural community has been disturbed.

Can We Predict the Path of Succession, and is Nature in Balance?
•	The course of succession cannot be precisely predicted.
•	Previously thought that a stable climax community will always be achieved.
•	Succession involves species competing for enough light, nutrients and space which will influence it’s trajectory.
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.
Video: Bald Eagles
Video: Bees Pollinating
Video: Cliff Nesters
Video: Eagle Fishing
Video: Grizzly Bear Feeding
Video: Migratory Birds
Video: Owl Hunting
Video: Pelican Colony
Video: Saguaros
Video: Wolf Pack