chapter 4: species interactions and community ecology
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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 parasitesThey 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 indirectOne 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
Predation: process by which individuals of one species (predators) capture, kill, and consume individuals of another species (prey)
• Exploitation: one member benefits while the other is harmed (+/- interactions)– Predation,
parasitism, herbivory
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: parasitismParasitism: 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,
caterpillarsCoevolution: 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
reproductionDefenses 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 communityCommunity 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 itThere 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 organismsFood webs are greatly simplified and leave out most species
Some organisms play big roles
Keystone species: has a strong or wide-reaching impact Far out of proportion to
its abundanceRemoving a
keystone species has substantial ripple effects Alters the food web
Large-bodied secondary or tertiary consumers
Species can change communitiesTrophic cascade: predators at high trophic
levels indirectly promote populations at low trophic levels By keeping species at intermediate trophic levels in
checkExtermination 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 disturbancesCommunities 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 disturbed community may never return to its original state
Primary successionSuccession: 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 landPioneer species: the first
species to arrive in a primary succession area Lichens: fungi + algae
Secondary successionSecondary 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 stabilityAlien (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 speciesTechniques 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 lightControl and eradication are hard and
expensivePrevention, 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
birds 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 Vegetation
structure
There are about 10 terrestrial biomes
Abiotic factors influence biome locationsThe 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 patternsVarious 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 forestDeciduous trees lose
their broad leaves each fall They remain dormant during
winterMidlatitude forests in
Europe, east China, eastern North America
Even, year-round precipitation
Fertile soilsForests: oak, beech,
maple
Temperate grasslandsMore 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, ground-nesting birds, pronghorn
Temperate rainforestU.S. coastal Pacific
Northwest Heavy rainfallConiferous 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 dampLush vegetationDiverse species
But in low densitiesVery poor, acidic soils
Nutrients are in the plants
Tropical dry forestAlso called tropical
deciduous forest Plants drop leaves
during the dry seasonIndia, Africa, South
America, north Australia
Wet and dry seasonsWarm, but less
rainfallConverted to
agriculture Severe soil erosion
SavannaTropical 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
DesertMinimal precipitationSahara: bare, with sand
dunesSonoran: heavily
vegetatedTemperatures vary
widely Day vs. night, seasonally
Soils (lithosols): high mineral content, low organic matter
Animals: nocturnal, nomadic
Plants: thick skins, spines
TundraRussia, Canada,
ScandinaviaMinimal rain, very cold
wintersPermafrost: permanently
frozen soilResidents: polar bears,
musk oxenMigratory birds, caribouLichens, low vegetation,
no trees Alpine tundra: on
mountaintops
Boreal forest (taiga)Canada, Alaska, Russia,
ScandinaviaA few evergreen tree
speciesCool and dry climate
Long, cold winters Short, cool summers
Nutrient poor, acidic soil
Moose, wolves, bears, lynx, migratory birds
ChaparralOccurs 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
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