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 COEVOLUTION 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 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
THE NICHE 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 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