Chapter 53 Community Ecology
Key Vocabulary Terms: community, interspecific interaction vs. intraspecific interaction, coevolution,
predation, parasitism, herbivore, carnivore, omnivore, camouflage, aposematic coloration, mimicry
(Batesian vs. Mullerian), endoparasite vs. exoparasite, interference competition, exploitative
competition, competitive exclusion principle, ecological niche, fundamental niche, realized niche,
symbiosis, commensalisms, mutualism, keystone species, exotic species, introduced species,
ecological succession, primary succession, secondary succession
Chapter Outline
COMMUNITY:
Differ significantly from one ecosystem to another in terms of species
diversity
INTERSPECIFIC INTERACTIONS THAT LIMIT POPULATION SIZE
Competition: General
Interspecific competition
Interaction of individuals of different species
Use the same resource that is in short supply
Greatest between organisms that obtain food in same manner
Most intense between closely similar organisms
Intraspecific competition occurs between individuals of the same species
COEVOLUTION
Organisms Change Relative to One Another Over Time
Flowering plants evolve in relation to pollinators
Pollinators, in turn, utilize flowers for food
Long-Term Mutual Evolutionary Adjustment of Features of One Group to Another
PREDATOR-PREY INTERACTIONS
One Organism Is the Resource of Another
Commonly thought of in terms of animals hunting other animals
Plants also possess physical defenses and produce toxic chemicals
Animals also produce toxins and mimic other poisonous animals
Predator-Prey Interactions
Predation limits size of populations
Predation and parasitism are two ends of the same spectrum
Predator may exterminate prey, having no food source it dies out
With refuges for the prey, predator-prey populations will cycle
Prey populations driven to low but recoverable numbers
Predator numbers subsequently decrease
Prey numbers increase
Predator numbers increase
Such relations are important to biological control
Near eradication of prey may cause extinction of predator
Prey must survive in small numbers for predator to survive
Example: prickly pear cactus in Australia
Became abundant in grazing areas
Introduction of moth for biological control
Cactus rare, moths still exist to keep them in check
Example: American chestnut populations damaged by fungus
Organisms producing disease that kills the host are not "successful"
Eliminate own source of food
Less virulent strains favored by natural selection survive
Example: rabbit viral disease, myxomatosis
Rabbits introduced into Australia, soon overpopulated
areas
Virus introduced, most rabbits died
Most virulent strains died along with their rabbit hosts
Populations of both organisms now in balance
Intricate interactions between predators and prey
Predators control levels of some species, survival of other enhanced
Predators greatly reduce competitive exclusion
Feedback systems control structure of natural communities
Plant Defenses
Attempts to limit being eaten by herbivores
Morphological defenses
Thorns and spines limit activities of browsers
Glandular hairs
Deposition of silica toughens plant parts
Chemical defenses
Restrict amino acids, thus limit nutritional suitability
Produce secondary chemical compounds
Distinguish from primary chemical compounds
Primary compounds normally formed in metabolic pathways
Secondary compounds not formed in metabolic pathways
Examples:
Mustard family produces mustard oils
Potato/tomato family rich in alkaloids and steroids
Milkweed/dogbane families produce milky sap containing
cardiac glycosides
Poison ivy group produces urushiol
Chemicals are toxic, or disturb herbivore metabolism and/or
Development
The Evolution of Herbivores
Some feed on restricted group of plants and frequently produces secondary compounds
Example: zebra swallowtail butterflies and paw paw
Example: monarch butterflies and milkweed
Example: amphipods feed on algae
Evolution of plant/herbivore interaction
Plant evolves secondary compound
Not eaten by herbivores, outcompetes others in area
Herbivores evolve ability to break down compounds
Herbivores lack competition from other herbivores
Both plant and herbivore flourish
Chemical Defenses in Animals
Frequently based on plant secondary compounds
Animals store rather than break down compounds
Example: monarch butterflies
Example: other milkweed herbivores
Such poisonous animals are generally brightly colored
Aposomatic coloration
Advertise distastefulness to protect species
Nonpoisonous animals generally are not brightly colored
Cryptic coloration
Animals blend with habitat, thus hidden from predators
Poisonous animals may obtain defenses from other animals
Nudibranchs eat hydroids with stinging cells
Other nudibranchs eat poisonous algae
Many animals produce own poisonous chemicals
Aposematic Coloration
Technical terminology for warning coloration
Characteristics of animals with extensive defenses
Animals must occur at relatively high densities
Generally live in family groups
Camouflaged animals live singly
Selective advantage to animals with similar appearance
Mimicry
Batesian mimicry
Related but unprotected species resemble protected ones
Must be fewer in number than protected species
If in greater numbers, predators learn that most are edible
Poisonous specimen is the model
Nonpoisonous specimen is the mimic
Example: viceroy butterfly
Muellerian mimicry
Unrelated, but protected species resemble one another
Strengthens the distastefulness and provides a group defense
Examples: wasps and bees
Behavior is imitated in both types as well
Mimics must spend much time in model`s habitat
Competitive exclusion principle
Two species competing for the same resource
One species will use the resource more efficiently
That species will eventually eliminate the other species
Results of laboratory experiments not readily predictable
Example: two species of flour beetle
One species would always become extinct
Extinct species dependent on environmental conditions, genetics
Competition: Examples from Nature
Example: two species of barnacles
fig. 53.13
One species lives in shallower water, other in deeper water
In deeper zone, deep species always outcompeted shallow species
If deep species removed, shallow species inhabited deep regions
Deep species conversely could not survive in shallow waters
Example: five species of warblers
All five initially appeared to be competing for same resources
With closer observation, each feeds in different part of tree
Each species thus eats different subset of insects
Species not truly in competition
THE NICHE
Description of a Niche
Includes space, food, temperature, conditions for mating, moisture
Also takes into account behavior at various seasons or times of day
Niche is not synonymous with habitat
Realized niche
Actual niche of an organism
The role the organism plays in a particular ecosystem
Fundamental niche
Theoretical niche
The role the organism would play in the absence of competitors
Complex ecosystem can support more species, i.e. rainforest
Competition more direct in ecosystem with fewer species, i.e. tundra
Niche of an Organism Can Change Over Time
Niche is wider if organisms reach a new habitat lacking other organisms
Species may become increasingly different as they evolve
Possibility for coexistence
No longer subject to competitive exclusion
Restatement of Gause`s principle of competitive exclusion
No two species can occupy the same niche indefinitely
Coexist while competing for the same resources
One or more features of niche will always differ
Niche is a complex concept involving all environmental facets
Role of competitive exclusion more obvious when resources are
drastically limited
Factors defining the niche are difficult to determine
Organisms Interrelate in Distinct Assemblages: Communities
Certain individuals are dominant in such collections
Example: redwood trees in Oregon
Community generally named after dominant species
Other organisms are characteristic as well
Exist under conditions set by dominant species
Niches of organisms overlap one another
Organisms in communities share historical dimension
Similar Communities Stretch Over Vast Areas
Organisms within them interact in similar manners
Organisms follow set patterns of distribution
SYMBIOSIS
Three Major Kinds of Relationships
Commensalism: one partner benefits, other neither benefits nor is harmed
Mutualism: both participants benefit
Parasitism: one partner benefits, other is harmed
Examples:
Lichens = alga + fungus
Mycorrhizae = fungus + plant root
Legumes = plant root + nitrogen-fixing bacteria
Coral reef = complex system with numerous plants and animals
Flowering plants + pollinators
Commensalism
Individuals of one species physically attached to individuals of another
species
Examples:
Birds nesting in trees
Epiphytic plants growing on other plants
Barnacles attached to marine animals
Sea anemones and clown fishes
Certain birds clean parasites off grazing animals
Difficult to ascertain if second partner benefits or not
Gray boundary between commensalism and mutualism
Mutualism
Example: leaf cutter ants
Cut tropical leaves into pieces
Inoculate pieces with specific fungus
Fungi used as food by ants
Example: ants and aphids
Aphids suck plant juices
Ants protect and herd aphids like cattle
Utilize aphid honeydew as food
Example: acacia tree and acacia ants
Trees inhabited by ants produce food for them
Protein-rich Beltian bodies fig 25.13
Nectar at base of leaves
Ants and larvae protected by thorns of tree
Ants in return:
Attack all other herbivores
Cut away branches of competing plants
Wastes provide source of nitrogenous fertilizer
Parasitism
Special form of symbiosis
Parasite much smaller than prey
Parasite in close association with prey
Some animal examples are readily identifiable, while others are not
Vertebrates have animal or protist parasites
Bacteria and viruses are not considered parasites though
Lice are parasites, mosquitos are not
Some flowering plants are parasitic on other plants
Internal parasites more specialized than external ones
More closely linked to host
Morphology and behavior more greatly modified over time
Bodily structure of parasite quite simplified
Stability and Equilibrium
Most ecosystems have taken years, centuries, millennia to allow all of the species present to
adapt to their own niche and play a role in the community
Introduced species or exotic species
Species not native to the ecosystem
Often have very little competition and occupy a niche
New niche often displaces previously occupied niches
Ecosystem undergoes new arrangement and many native species go extinct
Ecological Succession
Primary Succession
Lifeless at start
Soil needs to develop on bare rock
Organisms enter ecosystem in ordered fashion
Climaxes when existing organisms are replaced by own offspring
Secondary Succession
Major disturbance disrupts or destroys existing ecosystem
Succession starts wherever conditions permit (not from beginning)
Continues and ends much the same as primary succession