Chapter 4: Species Interactions and Community Ecology

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Transcript Chapter 4: Species Interactions and Community Ecology

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
Predation, parasitism,
“+”: 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
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
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
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
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
• 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
Who eats whom?
 Matter and energy move
through the community
 Trophic levels: rank in
the feeding hierarchy
Producers (autotrophs)
Detritivores and
Producers: the first trophic level
 Producers, or autotrophs (“self-feeders”): organisms
capture solar energy for photosynthesis to produce
Green plants
 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
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
 Food chain: a series of
feeding relationships
 Food web: a visual map
of feeding relationships
and energy flow among
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
 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
Beaver dams, prairie dogs, ants
Communities respond to disturbances
 Communities experience many types of disturbance
 Removal of keystone species, natural disturbances (fires, floods,
 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
Until another
disturbance restarts
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
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
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
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
Widely separated regions share similarities
 Biome: major regional complex of similar
communities recognized by:
Plant type
There are about 10
terrestrial biomes
Abiotic factors influence biome locations
 The type of biome depends on temperature,
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
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
 Midlatitude forests in
Europe, east China, eastern
North America
 Even, year-round
 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
 Bison, prairie dogs, groundnesting birds, pronghorn
Temperate rainforest
 U.S. coastal Pacific
 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
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
 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
 Minimal precipitation
 Sahara: bare, with sand
 Sonoran: heavily vegetated
 Temperatures vary widely
Day vs. night, seasonally
 Soils (lithosols): high
mineral content, low
organic matter
 Animals: nocturnal,
 Plants: thick skins, spines
 Russia, Canada, Scandinavia
 Minimal rain, very cold
Permafrost: permanently
frozen soil
Residents: polar bears, musk
Migratory birds, caribou
Lichens, low vegetation, no
Alpine tundra: on
Boreal forest (taiga)
 Canada, Alaska, Russia,
 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
 Occurs in small patches
around the globe
 Mediterranean Sea,
Chile, California, south
 Densely thicketed,
evergreen shrubs
 Highly seasonal biome
Mild, wet winters
Warm, dry summers
 Fire-resistant plants
 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