Biology 1B, Ecology Lecture 7, Page 1 Professor Resh • Spring 2011*Assigned readings, 8th Edition pp. 1198-20147th Edition pp. 1171-1175CommunitiesOutline of Lecture 7A. Views of communitiesB. CoevolutionC. If there are so many herbivores why are there anyplants left?D. Animal defensesE. Mimicry1. Batesian2. MullerianF. Community diveristyG. Are all species equally important to communityfunctioning?H. Succession typesI. Successional processes (three processes)J. Richness changes during successionA. Populations of organisms that livetogether at a given site form a biologicalcommunity.1. One view: a chance assembly ofspecies found in the same areabecause they have similar ecologicalrequirements.2. Another view: organisms are unitedin some way by ecologicalrelationships; they influence eachother’s abundance and distribution.B. Coevolution is the mutual influence thatoccurs in the evolution of two differentspecies that interact with each other andreciprocally influence each other.C. If there are so many herbivores why arethere any plants left?1. Morphological and chemicaldefenses2. Predators and parasitesBiology 1B, Ecology Lecture 7, Page 2 Professor Resh • Spring 2011*Assigned readings, 8th Edition pp. 1198-20147th Edition pp. 1171-1175CommunitiesD. Animal defenses: morphological andchemical, warning (aposomatic)coloration, crypsis.E. Mimicry: the deceptive resemblance ofone species to another1. Batesian: the model is distasteful,the mimic is not.2. Mullerian: several unrelatedspecies, all of whom would beprotected on their own, evolve tolook alike.F. We can consider communities in terms ofthe influence that one species (e.g. aredwood) has on species around it or interms of a common resource, or bysummarized properties.1. Richness (number of taxa), evenness(distribution of individuals among thedifferent species), numbers, diversity indices(which are combinations of richness andevenness), trophic status, and stability (theability of a community to return to itsoriginal composition followingdisturbances) can be used to characterize acommunity.2. Two communities with same richness andnumber of individuals can have verydifferent evenness, and therefore differentdiversity index values.3. Because of the popular use of the term“biodiversity,” the term diversity nowconnotes both species richness and speciesdiversity values (which is confusing).Biology 1B, Ecology Lecture 7, Page 3 Professor Resh • Spring 2011*Assigned readings, 8th Edition pp. 1198-20147th Edition pp. 1171-1175CommunitiesG. Are all species equally important tocommunity functioning?1. Dominant species can be quick inestablishment, be specialists, or begeneralists.2. Keystone species: found in communitieswhen some primary producer or consumer iscapable of monopolizing a basic resourceand thus excludes other species; the resourcemonopolist is itself preferentially consumedor destroyed by the keystone species. Whenthe keystone species is there and controls themonopolist, resources are made available forother species.H. Succession: the orderly and repeatablychange in a community over time.1. Primary succession occurs in areasessentially barren of life because ofthe complete absence of soil.2. Secondary succession occurs whenthe community has been cleared bysome disturbance but some soil isleft intact.3. Climax: “end points” ofcommunities; an equilibrium ofrichness and composition is reached.I. Successional processes1. Faciliation: early species change theenvironment and create conditionsfor late arriving species.2. Tolerance: early species areoutcompeted by late-arriving speciesthat are able to tolerate lower levelsof resources.3. Inhibition: early species preventestablishment of later species.Biology 1B, Ecology Lecture 7, Page 4 Professor Resh • Spring 2011*Assigned readings, 8th Edition pp. 1198-20147th Edition pp. 1171-1175CommunitiesWhat is the Resource Ratio Model of Tilman(1988) and why is it appealing to ecologists(go to the library and look it up)?J. Richness changes during succession1. Species that are more durablecompetitors replace “fugitive” orr-selected species.2. Predation, parasitism, increasedcompetition, and other interactionsincrease richness.K. Intermediate Disturbance Hypothesis:richness is highest when disturbances aremoderate in frequency and severitybecause organisms typical of differentsuccesional stages occur.frequency of disturbancerichnessrare frequenthighlowBiology 1B, Ecology Lecture 7, Page 5 Professor Resh • Spring 2011*Assigned readings, 8th Edition pp. 1198-20147th Edition pp. 1171-1175CommunitiesL. Disturbance; any relatively discrete eventin time (coming from outside thecommunity) that changes substrate orresource availability, and createsopportunities for new colonies orindividuals to be established (may benatural, animal-influenced, human-influenced).M. Why is disturbance of interest toecologists?What is its role in succession?What factors result in the length of timeneeded for a community to recover fromdisturbance (study group
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