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