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UT BIO 311D - Ecosystems (Part II)
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BIO 311D 1st Edition Lecture 33Outline of Last Lecture I. Laws of ThermodynamicsII. Conservation of MassIII. Energy and other limiting factorsIV. Gross and Net productionV. Primary ProductionVI. light LimitationVII. Nutrient LimitationOutline of Current Lecture I. CompetitionII. Ecological Niches and Natural SelectionIII. Character DisplacementIV. PredationV. Species DiversityVI. Diversity and Community StabilityVII. Tropic StructureVIII. Food WebsCurrent LectureCommunity interactions are classified by whether they help, harm, or have no effect on the species involved• Ecologists call relationships between species in a community interspecific interactions• Examples are competition, predation, herbivory, symbiosis (parasitism, mutualism, and commensalism), and facilitation• Interspecific interactions can affect the survival and reproduction of each species, and the effectscan be summarized as positive (+), negative (–), or no effect (0)Competition• Interspecific competition (–/– interaction) occurs when species compete for a resource in short supplyCompetitive Exclusion• Strong competition can lead to competitive exclusion, local elimination of a competing species• The competitive exclusion principle states that two species competing for the same limiting resources cannot coexist in the same placeEcological Niches and Natural Selection• The total of a species’ use of biotic and abiotic resources is called the species’ ecological niche • An ecological niche can also be thought of as an organism’s ecological role• Ecologically similar species can coexist in a community if there are one or more significant differences in their niches• Resource partitioning is differentiation of ecological niches, enabling similar species to coexist in a community• A species’ fundamental niche is the niche potentially occupied by that species• A species’ realized niche is the niche actually occupied by that species• As a result of competition, a species’ fundamental niche may differ from its realized niche• For example, the presence of one barnacle species limits the realized niche of another species• The common spiny mouse and the golden spiny mouse show temporal partitioning of their niches• Both species are normally nocturnal (active during the night)• Where they coexist, the golden spiny mouse becomes diurnal (active during the day) Character Displacement• Character displacement is a tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species• An example is variation in beak size between populations of two species of Galápagos finches Predation• Predation (+/– interaction) refers to an interaction in which one species, the predator, kills and eats the other, the prey• Some feeding adaptations of predators are claws, teeth, fangs, stingers, and poisonSpecies Diversity• Species diversity of a community is the variety of organisms that make up the community• It has two components: species richness and relative abundance– Species richness is the number of different species in the community– Relative abundance is the proportion each species represents of all individuals in the community• Two communities can have the same species richness but a different relative abundance• Diversity can be compared using a diversity index– Shannon diversity index (H)H = –(pA ln pA + pB ln pB + pC ln pC + …)where A, B, C . . . are the species, p is the relative abundance of each species, and ln is the natural logarithm• Determining the number and abundance of species in a community is difficult, especially for small organisms• Molecular tools can be used to help determine microbial diversityDiversity and Community Stability• Ecologists manipulate diversity in experimental communities to study the potential benefits of diversity– For example, plant diversity has been manipulated at Cedar Creek Natural History Area in Minnesota for two decades• Communities with higher diversity are– More productive and more stable in their productivity– Better able to withstand and recover from environmental stresses– More resistant to invasive species, organisms that become established outside their native rangeTrophic Structure• Trophic structure is the feeding relationships between organisms in a community• It is a key factor in community dynamics• Food chains link trophic levels from producers to top carnivoresFood Webs• A food web is a branching food chain with complex trophic interactions• Species may play a role at more than one trophic level • Food webs can be simplified by • Grouping species with similar trophic relationships into broad functional groups• Isolating a portion of a community that interacts very little with the rest of the


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