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Ch. 5: Ecosystems and Living Organisms
Objectives:
 Describe the factors that contribute to an organism’s ecological niche.
 Define competition, and relate the concepts of competitive exclusion and
resource partitioning.
 Define symbiosis, and distinguish among mutualism, commensalism, and
parasitism.
 Define predation, and describe the effects of natural selection on predatorprey relationships.
 Define keystone species, and discuss the sea otter as a keystone species.
Community Ecology
 Community: an association of different populations of organisms that live and
interact in the same place at the same time
o the organisms in a community play a variety of roles and interdependent
on one another in many ways
 Ecological niche: an organism’s role within the structure and function of an
ecosystem
o takes into account all aspects of the organism’s existence  the jobs and
relationships it has, all the things it needs to survive, its habitat, etc.
 if there was no competition, an organism’s niche could be very large
(fundamental niche), but because competition does exist, organisms
are limited in the lifestyles they can actually live and the resources
they can actually use (realized niche)
 example: mangrove trees versus other tree species
o a species’ adaptations help determine its niche  determine the species’
tolerance for different environmental conditions (organisms cannot just
live any old place!)
 limiting resource: any resource at a suboptimal level relative to an
organism’s need for it or at a level in excess of an organism’s
tolerance for it
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 3 main types of interactions that occur among species in a community:
Competition
 can be either intra- or inter-specific
 competitive exclusion: no two species can indefinitely occupy the
same niche in the same community
 one species will eventually outcompete the other and exclude
the losing species from a portion of the niche
 competition is always negative for all species involved (takes E to
compete)
 natural selection favors individuals of each species that avoid
or attempt to reduce competition  resource partitioning
o may include timing of feeding, location of feeding, nest
sites, etc.
Symbiosis
– when individuals of one species usually live in or on the individuals of
another species
– at least one of the species will use the other’s resources and benefit
– the result of coevolution (the evolution of one species directly
affects the evolution of another, and vice versa)
• e.g. flowers + animal pollinators
– flowers attract animals with scents, colors, foods
– animals have body parts and behaviors that are perfect
for carrying pollen
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Mutualism
 +, +
o e.g. flowers + animal pollinators!
o e.g. mycorrhizae
o mutualistic associations between fungi and about 80% of all
plants
o fungus absorbs nutrients and provides them to the plant
o the plant provides food for the fungus
o plants grow more vigorously and are better able to tolerate
stressors
Commensalism
 +, o
o e.g. trees + epiphytes
o small plants that live attached to the bark of tree branches
o the epiphyte has better access to water and light
Parasitism
 +, o the parasite lives on (exo-) or in (endo-) the host, and
weakens it, but rarely kills it
o e.g. dogs + ticks
o e.g. humans + tapeworms
Predation
 includes animals eating animals AND animals eating plants
 coevolution of predator and prey strategies
Predator Strategies
 pursuit
 requires traits that increase hunting efficiency (speed,
intelligence)
o e.g. orcas
 ambush
 make use of camouflage and lures
o e.g. goldenrod spider
o e.g. anglerfish
Prey Strategies
 plant defenses  since plants can’t move, they have to have
adaptations that protect them
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 hard to eat = spines, thorns, tough leaves, wax
 unpalatable or toxic = protective chemicals (e.g. marijuana,
opium poppy, tobacco, peyote)
 example of coevolution  milkweed and monarch
butterflies
 animal defenses  the #1 defense used is RUNNING
 mechanical defenses = barbed quills, shells
 social behavior = dilution effect
 chemical defenses
o e.g. South American poison arrow frog
 warning coloration
 camouflage
o e.g. caterpillar “twigs,” pipefish “eelgrass,” pygmy
seahorse “coral”
 Keystone Species
 species vital in determining the nature and structure of the entire
ecosystem
 usually not the most abundant, but if you take them out of
the ecosystem …
 e.g. gray wolf, sea otter