Chapter 21
Community Ecology
21.1 Species Interactions
 Communities contain populations that
interact in many ways
 There are five major types of interactions
(symbioses): predation, parasitism,
competition, mutualism, and
commensalism
Predation
 A predator captures,
kills, and consumes
the prey
 Predation determines
where and how a
species lives
 Regulates population
size
Predators and Natural Selection
 Natural selection favors
adaptations that improve
the efficiency of
predators (the better the
hunter, the more food to
survive)
 Examples: predator
adaptations such as coat
color, sharp teeth or
claws, able to find prey
(sense of smell, accurate
vision)
Prey and Natural Selection
 The survival of prey animals depend on
their ability to avoid being eaten
 Examples: camouflage, poison,
behavior, ability to hide, able to outrun
predator
Mimicry
 A harmless species
resembles a
poisonous one
 The mimic is
protected since it is
avoided
Mullerian Mimicry
 When several dangerous/poisonous
species have similar warning coloration
 Example: the black and yellow striped
pattern on bees and wasps
Batesian Mimicry
 When a harmless species mimics the
warning coloration of a dangerous
species
 Example: coral snakes and scarlet king
snakes
Plant-Herbivore Interactions
 Plants have evolved adaptations that
protect them from being eaten by
herbivores
 examples: thorns, spines, stinging hairs,
tough leaves, chemical defenses
Secondary Compounds
 Some chemicals in plants are poisonous,
irritating, or bad-tasting
 Many plant chemicals are used for
medicines
 There are many medicinal cures in
rainforest plants, but they are being
destroyed due to habitat destruction!
Parasitism
 One individual is harmed while the other
benefits
 The parasite feeds on the host
 Ectoparasites (external)
 Endoparasites (internal)
Parasitism and Evolution
 Parasitism has caused an evolution of a
variety of host defenses
 Natural selection favors adaptations that
allow a parasite to exploit its host (very
specialized bodies and lifestyles)
Competition
 The use of the same
limited resource by
two or more species
 Competitive
exclusion: one
species can be
eliminated from a
community because
of competition
Competition in Paramecia
 G.F. Gause studied competition between
two species of paramecia in the lab
Character Displacement
 Competitors may evolve niche
differences or anatomical differences
that lessen the competition
 Example: Darwin’s finches
Resource Partitioning
 When similar species coexist, each
species uses only part of the available
resources
 Example: warblers forage in different
types of trees
Mutualism
 A cooperative relationship in which both
species benefit
 example: pollination (pollinators are
attracted to a food source and transfer
pollen to other flowers while feeding)
Commensalism
 One species benefits while the other is
not affected
 Example: cattle egrets feed on insects
that fly out of the grass when the Cape
buffalo is walking
21.2 Properties of Communities
 Species richness: the
number of species a
community contains
 Species diversity:
relates the number of
species to the
relative abundance
of each species
Patterns of Species Richness
 Varies with latitude
 The closer to the equator, the more
species (greatest in tropical rainforests)
 E.O. Wilson found more ant species in a
single tree in Peru than there are in the
entire British Isles
Tropical Rainforests
 Climate is more stable than a temperate
area
 Plants can photosynthesize year-round
 Tropical rainforests have the highest
biodiversity on Earth
The Species-Area Effect
 Larger areas usually
contain more species
than smaller areas
(more habitats)
 Most often applied to
islands
 Reducing the size of
a habitat reduces the
number of species
Community Stability
 Stability refers to a community’s
resistance to change and is directly
related to species richness
 The more species, the more interactions
and the better a habitat can withstand a
disturbance
21.3 Succession
 A gradual process of change and
replacement of the types of species in a
community
 Ecosystems are constantly changing
Primary Succession
 Occurs on a surface where no
ecosystem existed before
 Example: rocks, volcanic islands, cliffs,
sand dunes
Primary Succession
 Happens very slowly
 Bare rock  lichens  moss 
grasses  shrubs  trees
 Example: Galapagos Islands
Secondary Succession
 Most common type of succession
 Occurs on a surface where an
ecosystem previously existed
 Occurs after an ecosystem is disturbed
by humans or by storms, floods,
earthquakes, or volcanoes
Secondary Succession
 Pioneer species- the first organisms to colonize
any newly available area
 Grasses and weeds  shrubs  shade
intolerant trees  shade tolerant trees 
climax community
Example: eruption of Mt. St. Helens destroyed
44,460 acres of forest
Old-field Succession
 When farmland is abandoned, it will start
to turn back into a forest ecosystem
 Pioneer plants such as grasses and
weeds will start to grow
 After about 100 years, the field will return
to a climax community
Old-field Succession