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Community EcologyWhat is a community?Pattern vs. ProcessSlide 4Emergent Properties of a CommunitySlide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Example ProblemAnswer:Clicker QuestionSuccession, Disturbance and ChangeSlide 18Slide 19Some Agents of DisturbanceSlide 21Disturbance, Invasion, SuccessionSlide 23SuccessionSlide 25Slide 26Slide 27Slide 28Slide 29Are Climax Communities Real?Slide 31Are biological communities real functional units?The Kiddie Pool ExperimentSlide 34Slide 35A Simple Food WebSlide 37Slide 38Slide 39Examples of Keystone SpeciesTrophic CascadesTop down vs. Bottom upSlide 43Types of Interspecific InteractionsNeutralismCompetition“Styles” of CompetitionSome specific types of competitionExample of Interference CompetitionOutcomes of CompetitionThe Competitive Exclusion PrincipleEvidence for Competitive Exclusion.Slide 53Other experiments...Resource PartitioningSlide 56Slide 57Character DisplacementExample of Character DisplacementCompetition and the NicheTwo organisms cannot occupy exactly the same niche.AmensalismSlide 63Example: Allelopathy in the California ChaparralCommensalismSlide 66MutualismExamples of MutualismMutualistic SymbiosisFacultative vs. Obligate MutualismsOther Examples of MutualismsPredation, Parasitism, HerbivorySlide 73Slide 74Slide 75Slide 76Slide 77Slide 78Predator-Prey and Parasite-Host CoevolutionSlide 80Example of Parasite-Host CoevolutionPredator-Prey Population DynamicsSlide 83Example of Regulation of Host Population by a HerbivoreSlide 85Slide 86The time-lag effect may lead to predator-prey oscillations.Cycles in the population dynamics of the snowshoe hare and its predator the Canadian lynx (redrawn from MacLulich 1937). Note that percent mortality is an elusive measure, it may, or may not, be useful since mortality varies with environment and time.Slide 89Slide 90Community EcologyReading: Freeman, Chapter 50, 53What is a community?•A community is an assemblage of plant and animal populations that live in a particular area or habitat.–Populations of the various species in a community interact and form a system with its own emergent properties.Pattern vs. Process•Pattern is what we can easily observe directly - vegetation zonation, species lists, seasonal distribution of activity, and association of certain species.•Process gives rise to the pattern- herbivory, competition, predation risk, nutrient availability, patterns of disturbance, energy flow, history, and evolution.•Community ecology seeks to explain the underlying mechanisms that create, maintain, and determine the fate of biological communities. Typically, patterns are documented by observation, and used to generate hypotheses about processes, which are tested.–Not all science is experimental. Hypotheses tests can involve special observations, or experiments.Emergent Properties of a Community•Scale•Spatial and Temporal Structure•Species Richness•Species Diversity•Trophic structure•Succession and Disturbance•Scale is the size of a community. •Provided that the area or habitat is well defined, a community can be a system of almost any size, from a drop of water, to a rotting log, to a forest, to the surface of the Pacific Ocean.•Spatial Structure is the way species are distributed relative to each other.•Some species provide a framework that creates habitats for other species. These species, in turn create habitats for others, etc.•Example: Trees in a rainforest are stratified into several different levels, including a canopy, several understories, a ground level, and roots. Each level is the habitat of a distinct collection of species. Some places, such as the pools of water that collect at the base of tree branches, may harbor entire communities of their own.•Temporal structure is the timing of the appearance and activity of species. Some communities, i.e., arctic tundra and the decay of a corpse, have pronounced temporal species, other communities have less. •Example: Many desert plants and animals are dormant most of the year. They emerge, or germinate, in response to seasonal rains. Other plants stick around year round, having evolved adaptations to resist drought.•Species Richness - is the number of species in a community. Clearly, the number of species we can observe is function of the area of the sample. It also is a function of who is looking. Thus, species richness is sensitive to sampling procedure•Diversity is the number of species in the community, and their relative abundances.•Species are not equally abundant, some species occur in large percentage of samples, others are poorly represented.•Some communities, such as tropical rainforests, are much more diverse than others, such as the great basin desert.•Species Diversity is often expressed using Simpson’s diversity index: D=1-piExample Problem•A community contains the following species: • Number of Individuals•Species A 104•Species B 71•Species C 19•Species D 5•Species E 3•What is the Simpson index value for this community?Answer:•Total Individuals= (104+19+71+5+3)=0•PA=104/0=.51 PB=19/0=.09•PC=71/0=.35 PD=5/0=.03PE=3/0=.0•D=1-{(.51)+(.09)+(.35)+(.03)+(.0)}•D=1-.40=.60Clicker QuestionIn the example above, what was the species richness?A. .60B. 0 individualsC. 5 speciesD. .40E. None of the aboveSuccession, Disturbance and Change•In terms of species and physical structure, communities change with time.–Ecological succession, the predictable change in species over time, as each new set of species modifies the environment to enable the establishment of other species, is virtually ubiquitous.•Example; a sphagnum bog community may persist for only a few decades before the process of ecological succession changes transform it into the surrounding Black Spruce Forest.•A forest fire may destroy a large area of trees, clearing the way for a meadow. Eventually, the trees take over and the meadow is replaced.•Disturbances are events such as floods, fire, droughts, overgrazing, and human activity that damage communities, remove organisms from them, and alter resource availability.Some Agents of Disturbance•Fire•Floods•Drought•Large Herbivores•Storms•Volcanoes•Human Activity•Disturbance creates opportunities for new species to


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UIC BIOS 101 - Community Ecology

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