Chapter 4: Species Interactions
and Community Ecology
Central Case Study: Black and White and
Spread All Over
 In 1988, discharged ship
ballast water accidentally
released zebra mussels into
Lake St. Clair
 By 2010, they had spread
to 30 states

No natural predators,
competitors, or parasites
 They cause millions of
dollars of property damage
each year
Species interactions
 Species interactions are the backbone of communities
 Effects of species interactions on the participants:
Type of interaction
Effect on Species 1
Effect on Species 2
Competition
–
–
Predation, parasitism,
herbivory
+
–
Mutualism
+
+
“+”: positive effect
“–”: negative effect
Competition occurs with limited resources
 Competition: multiple organisms seek the same
limited resource

Food, water, space, shelter, mates, sunlight, etc.
 Intraspecific competition: between members of the
same species

High population density: increased competition
 Interspecific competition: between members of
different species


Strongly affects community composition
Leads to competitive exclusion or species coexistence
Results of interspecific competition
 Competition is usually subtle and indirect
 One species may exclude another from using the
resource


Zebra mussels displaced native mussels in the Great Lakes
Quagga mussels are now displacing zebra mussels
 Or, competing species may be able to coexist
 Natural selection favors individuals that use different resources
or shared resources in different ways
Resource partitioning
 Resource
partitioning:
competing species coexist
by specializing


By using different resources
(small vs. large seeds)
Or using shared resources
differently (active during day
vs. night)
An exploitative interaction: predation
• Exploitation: one
member benefits while
the other is harmed
(+/- interactions)
– Predation,
parasitism, herbivory
 Predation: process by which individuals of one
species (predators) capture, kill, and consume
individuals of another species (prey)
Predation affects the community
 Interactions between predators and prey structure
food webs
 The number of predators and prey influences
community composition
 Predators can, themselves, become prey


Zebra mussels eat smaller types of zooplankton
Zebra mussels are prey for North American predators (fish,
ducks, muskrats, crayfish)
Predation can drive population dynamics
 Increased prey populations increase food for predators
 Predators survive and reproduce
 Increased predator populations decrease prey
 Predators starve and their populations decrease
 Decreased predator populations increase prey
populations
Insert Fig. 4.4
Predation has evolutionary ramifications
 Natural selection leads to evolution of adaptations that
make predators better hunters
 Individuals who are better at catching prey:


Live longer, healthier lives
Take better care of offspring
 Prey face strong selection pressures—they are at risk of
immediate death

Prey develop elaborate defenses against being eaten
Prey develop defenses against being eaten
An exploitative interaction: parasitism
 Parasitism: a relationship in which one
organism (parasite) depends on another (host)


For nourishment or some other benefit
The parasite harms, but doesn’t kill, the host
• Some parasites contact hosts infrequently
– Cuckoos, cowbirds
• Some live within the host
– Disease, tapeworms
• Some live on the
hosts’ exterior
– Ticks, sea lampreys
Parasite – host relationships
 Parasitoids: insects that parasitize other insects
 Kill the host
 Example: wasp larvae burrow into, and kill, caterpillars
 Coevolution: hosts and parasites become locked in a
duel of escalating adaptations


Has been called an evolutionary arms race
Each evolves new responses to the other
 It may not be beneficial to the parasite to kill its host
An exploitative interaction: herbivory
 Herbivory: animals feed on the
tissues of plants

Widely seen in insects
 May not kill the plant
 But affects its growth and reproduction
 Defenses against herbivory include:
 Chemicals: toxic or distasteful
 Thorns, spines, or irritating hairs
 Herbivores may overcome these
defenses
Mutualists help one another
 Two or more species benefit from their interactions
 Each partner provides a service the other needs (food, protection,
housing, etc.)
 Symbiosis: a relationship in which the organisms live
in close physical contact (mutualism and parasitism)



Microbes within digestive tracts
Mycorrhizae: plant roots and fungi
Coral and algae (zooxanthellae)
 Pollination: bees, bats, birds, and others transfer
pollen from one flower to another, fertilizing its eggs
Pollination
• In exchange for the plant nectar, the animals pollinate
plants, which allows them to reproduce
Ecological communities
 Community: an assemblage of populations of
organisms living in the same area at the same time


Members interact with each other
Interactions determine the structure, function, and species
composition of the community
 Community ecologists are interested in how:
 Species coexist and interact with one another
 Communities change, and why these patterns exist
Energy passes among trophic levels
 One of the most
important species
interactions

Who eats whom?
 Matter and energy move
through the community
 Trophic levels: rank in
the feeding hierarchy



Producers (autotrophs)
Consumers
Detritivores and
decomposers
Producers: the first trophic level
 Producers, or autotrophs (“self-feeders”): organisms
capture solar energy for photosynthesis to produce
sugars



Green plants
Cyanobacteria
Algae
 They capture solar energy and use photosynthesis to
produce sugars
Consumers: consume producers
• Primary consumers: second trophic level
 Organisms that consume producers
 Herbivorous grazing animals
 Deer, grasshoppers
 Secondary consumers: third trophic level
 Organisms that prey on primary consumers
 Wolves, rodents, birds
 Tertiary consumers: fourth trophic level
 Predators
 Hawks, owls
Detritivores and decomposers
 Organisms that consume nonliving organic matter
 Detritivores: scavenge waste products or dead bodies
 Millipedes, soil insects
 Decomposers: break down leaf litter and other
nonliving material


Fungi, bacteria
Enhance topsoil and recycle nutrients
Energy, biomass, and numbers
 Most energy that organisms use in cellular
respiration is lost as waste heat


Less and less energy is available in each successive trophic
level
Each trophic level contains only 10% of the energy of the
trophic level below it
 There are also far fewer organisms and less biomass
(mass of living matter) at the higher trophic levels
A human vegetarian uses less energy and has a smaller
ecological footprint than a meat eater
Pyramids of energy, biomass, and numbers
Food webs show relationships and energy
flow
 Food chain: a series of
feeding relationships
 Food web: a visual map
of feeding relationships
and energy flow among
organisms
Food webs are greatly
simplified and leave
out most species
Some organisms play big roles
 Keystone species:
has a strong or widereaching impact

Far out of proportion to its
abundance
 Removing a keystone
species has substantial
ripple effects

Alters the food web
 Large-bodied secondary
or tertiary consumers
Species can change communities
 Trophic cascade: predators at high trophic levels
indirectly promote populations at low trophic levels

By keeping species at intermediate trophic levels in check
 Extermination of wolves led to increased deer
populations …


Which overgrazed vegetation …
Which changed forest structure
 Ecosystem engineers: physically modify the
environment

Beaver dams, prairie dogs, ants
Communities respond to disturbances
 Communities experience many types of disturbance
 Removal of keystone species, natural disturbances (fires, floods,
etc.)
 Human impacts cause major community changes
 Resistance: a community resists change and remains
stable despite the disturbance
 Resilience: a community changes in response to a
disturbance, but later returns to its original state
 Or, a
community may never return to its
original state
Primary succession
 Succession: the predictable
series of changes in a community

After a severe disturbance
 Primary succession:
disturbance removes all
vegetation and/or soil life

Glaciers, drying lakes, volcanic lava
covering the land
 Pioneer species: the first
species to arrive in a primary
succession area

Lichens: fungi + algae
Secondary succession
 Secondary succession: a disturbance has
removed much, but not all, of the biotic community

Fires, hurricanes, logging, farming
 Aquatic systems can also undergo succession
 Ponds eventually fill in to become terrestrial systems
 Climax community: remains in place with few
changes

Until another
disturbance restarts
succession
Communities may undergo shifts
 Community changes are more variable and less
predictable than early models of succession suggested



Conditions at one stage may promote another stage
Competition may inhibit progression to another stage
Chance factors also affect changes
 Phase (regime) shift: the overall character of the
community fundamentally changes


Some crucial threshold is passed, a keystone species is lost, or an
exotic species invades
Example: overfishing and depletion of fish and turtles has
allowed algae to dominate coral reef communities
Invasive species threaten stability
 Alien (exotic) species: non-native species from
somewhere else enters a new community
 Invasive species: non-native species that spreads
widely and become dominant in a community




Introduced deliberately or accidentally
Growth-limiting factors (predators, disease, competitors, etc.)
are absent
Major ecological effects
Pigs, goats, and rats have destroyed island species
 But some invasive species (e.g., honeybees) help
people
Invasive mussels modify communities
Controlling invasive species
 Techniques to control invasive species include:
 Removing them manually
 Applying toxic chemicals
 Drying them out, depriving them of oxygen
 Introducing predators or diseases
 Stressing them with heat, sound, electricity, carbon
dioxide, or ultraviolet light
 Control and eradication are hard and expensive
Prevention, rather than control, is the best policy
Altered communities can be restored
 Humans have dramatically changed ecological
systems

Severely degraded systems cease to function
 Restoration ecology: the science of restoring an
area to an earlier (presettlement) condition

Tries to restore the system’s functionality (e.g., filtering of
water by a wetland)
 Ecological restoration: actual efforts to restore an
area

Difficult, time-consuming, and expensive
It is best to protect natural systems from degradation in
the first place
Examples of restoration efforts
 Prairie restoration: replanting native species,
controlling invasive species, controlled fire to mimic
natural fires
 The world’s largest project: Florida Everglades



Flood control and irrigation removed its water
Populations of wading
dropped 90–95%
It will take 30 years
and billions of dollars
to restore natural
water flow
birds
Widely separated regions share similarities
 Biome: major regional complex of similar
communities recognized by:


Plant type
Vegetation
structure
There are about 10
terrestrial biomes
Abiotic factors influence biome locations
 The type of biome depends on temperature,
precipitation

Also air and ocean circulation, soil type
 Climatographs: a climate diagram showing an area’s
mean monthly temperature and precipitation
Similar biomes occupy
similar latitudes
Aquatic systems have biome-like patterns
 Various aquatic systems comprise distinct
communities


Coastlines, continental shelves, open ocean, deep sea
Coral reefs, kelp forests
 Some coastal systems (estuaries, marshes, etc.) have
both aquatic and terrestrial components
 Aquatic systems are shaped by



Water temperature, salinity, dissolved nutrients
Wave action, currents, depth, light levels
Substrate type
 Animals, not plants, delineate marine communities
Temperate deciduous forest
 Deciduous trees lose their
broad leaves each fall

They remain dormant during
winter
 Midlatitude forests in
Europe, east China, eastern
North America
 Even, year-round
precipitation
 Fertile soils
 Forests: oak, beech, maple
Temperate grasslands
 More temperature difference
 Between winter and summer
 Less precipitation supports
grasses, not trees
 Also called steppe or prairie
 Once widespread, but has
been converted to
agriculture
 Bison, prairie dogs, groundnesting birds, pronghorn
Temperate rainforest
 U.S. coastal Pacific
Northwest
 Heavy rainfall
 Coniferous trees: cedar,
spruce, hemlock, fir
 Moisture-loving animals

Banana slug
 Erosion and landslides
affect the fertile soil
 Most old-growth is gone
as a result of logging
Tropical rainforest
 Southeast Asia, west Africa




Central and South America
Year-round rain and warm
temperatures
Dark and damp
Lush vegetation
Diverse species

But in low densities
 Very poor, acidic soils
 Nutrients are in the plants
Tropical dry forest
 Also called tropical
deciduous forest

Plants drop leaves during
the dry season
 India, Africa, South
America, north Australia
 Wet and dry seasons
 Warm, but less rainfall
 Converted to agriculture

Severe soil erosion
Savanna
 Tropical grassland




interspersed with trees
Africa, South America,
Australia, India
Precipitation occurs only
during the rainy season
Animals gather near
water holes
Zebras, gazelles, giraffes,
lions, hyenas
Desert
 Minimal precipitation
 Sahara: bare, with sand
dunes
 Sonoran: heavily vegetated
 Temperatures vary widely

Day vs. night, seasonally
 Soils (lithosols): high
mineral content, low
organic matter
 Animals: nocturnal,
nomadic
 Plants: thick skins, spines
Tundra
 Russia, Canada, Scandinavia
 Minimal rain, very cold





winters
Permafrost: permanently
frozen soil
Residents: polar bears, musk
oxen
Migratory birds, caribou
Lichens, low vegetation, no
trees
Alpine tundra: on
mountaintops
Boreal forest (taiga)
 Canada, Alaska, Russia,
Scandinavia
 A few evergreen tree species
 Cool and dry climate


Long, cold winters
Short, cool summers
 Nutrient poor, acidic soil
 Moose, wolves, bears, lynx,
migratory birds
Chaparral
 Occurs in small patches
around the globe
 Mediterranean Sea,
Chile, California, south
Australia
 Densely thicketed,
evergreen shrubs
 Highly seasonal biome


Mild, wet winters
Warm, dry summers
 Fire-resistant plants
Conclusion
 Species interactions affect communities
 Competition, predation, parasitism, competition, mutualism
 Causing weak and strong, direct and indirect effects
 Feeding relationships are represented by trophic levels
and food webs
 Humans have altered many communities

Partly by introducing non-native species
 Ecological restoration attempts to undo the negative
changes that we have caused