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NICHOLLS BIOL 404 - LECTURE NOTES

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Factors Limiting Distribution: Dispersal – Chapter 4DispersalExample: Zebra MusselExample: Gypsy MothExample: Chestnut BlightExample: California Sea OtterThree Modes of Dispersal:Jump and Diffusion vs. SecularSimple Diffusion Mathematically:Oak Tree Dispersal Following Last Ice Age:Measuring Tree Seed DispersalDispersal Can Be Affected by BarriersIs Dispersal The Only Limit To Distribution?Terms for Introduced Nonnative SpeciesHow Successful Are Introductions? -actually, they are usually failuresStatistical Generalizations: Tens RuleLocal Scale DispersalColonization and Extinction: KrakatauGroups Colonizing KrakatauContinental Drift – Disjunct DistributionsExplanation Of Disjunct DistributionsWhy Disperse?Examples of Dispersal AbandonmentFugitive SpeciesSummaryFactors LimitingDistribution: Dispersal – Chapter 4Dispersal•The transport of animals to geographical areas not currently inhabited by that species.–Simplest explanation as to why a particular species may not be located in a geographical are.•If colonization is successful, dispersal will result in gene flow and thus affect the genetic structure of a population.–Also result in a founder effectExample: Zebra MusselProbably introduced from ship ballast water.Spread throughout the majority of the Mississippi drainage within 10 years.Example: Gypsy MothAccidentally introduced by a French astronomer in 1868.Control programs ceased around 1900.Began to spread again, and accidentally transported to Michigan.Spread 21 km per year between 1966 and 1989.Example: Chestnut BlightFungus that is lethal to Chestnuts.First noticed in 1900, and apparently introduced on nursery stocks from Asia.Search began in 1927 to find blight resistant trees.Oak-chestnut forests have been replaced with oak or oak-hickory forests.Example: California Sea OtterThought to be extinct in 1911.Small population found in 1914 – was now protected.Northern range spread about 1.4 km/year and southern range spread about 3.1 km/year.Southern otters move more as individuals and northern suffering higher mortality?Three Modes of Dispersal:•Diffusion – Gradual movement of a population across a hospitable terrain for a period of several generations.•Jump Dispersal – Movement of individual organisms across large distances of inhospitable habitat followed by the successful establishment of a population in the new area.•Secular Dispersal – Diffusion occurring in evolutionary time.Jump and Diffusion vs. Secular•Most colonization's involve Jump dispersal first, followed by Diffusion.•Secular diffusion occurs over geologic time. Although the geographic range is expanding, natural selection is causing migrants to diverge from the ancestral population.–Not of immediate interest for ecologists working in ecological timeSimple Diffusion Mathematically:Distance moved = DnlogeR0Where:D = average dispersal distancen = number of generationsR0 = reproductive rate per generationCan this explain the spread of oak trees since the last ice age?Oak Tree Dispersal Following Last Ice Age:Distance moved = DnlogeR0If a tree produces 107 (R0)seeds per generation for 300 (n) generations and seeds disperse 30 m(D) with each generation then:Dispersal distance = (30m)(300yr)(loge(107seeds/yr) = 36 kmHowever, actual migration distance since the last ice age is about 1,000 km – known as Reid’s paradox.Measuring Tree Seed DispersalSeed traps can be use to determine seed dispersalDistances are too small to account for dispersal of trees after the ice age.Answer to Reid’s paradox seems to lie in haphazard, long range dispersal of seeds.Colonization not driven by mean seed dispersal, but extreme dispersal events.Wind, animals, Johnny Appleseed!Dispersal Can Be Affected by Barriers•Freshwater organisms are prevented from dispersing by land and saltwater–Local populations strongly affected by jump dispersal•Water can be a barrier to some terrestrial animals–Ruffed Grouse found only on three Michigan islands of the great lakes, all within 800 m of the mainland–Palmer (1962) showed that these birds could not fly for more than 800 m – can’t colonize far islands by jump dispersal–Artificial stockings have been successful–Isn’t it ironic that immobile trees colonized offshore islands that flight capable birds didn’t!Is Dispersal The Only Limit To Distribution?•Humans have moved many species around the globe – often with disastrous consequences (think locally: nutria, hydrilla and water hyacinth).–Humans have allowed several species to bypass traditional geographic barriers•However, many times it is not just inaccessibility that determines whether or not a species is found in a particular habitat.Terms for Introduced Nonnative SpeciesStates Transition DefinitionImportedEscapingBrought into the CountryTransition from imported to introducedIntroducedEstablishingFound in the wild; feralTransition from introduced to establishedEstablishedBecoming a PestHas a self-sustaining populationTransition from established to pestPest Has a negative economic (ecological?) impactHow Successful Are Introductions?-actually, they are usually failuresOverall, continental bird introductions are successful about 10 – 30% of the time.Statistical Generalizations: Tens Rule•Williams and Fitter (1996) predicted that –1 species in 10 imported to a country becomes introduced–1 in 10 of introduced species becomes established–1 in 10 of the established species becomes a pest•Of course exceptions occur–Much of Hawaii has been cleared, so habitats have become unsuitable to native birds but not introduced onesLocal Scale Dispersal•Transport is rarely a limiting factor in plant dispersal–Seeds/spores carried primarily by wind or animals–Rumex crispus var. littoreus not limited by dispersion or seed-germination–Wind dispersed seeds and spores usually colonize disturbed areas first•Small animal are often dispersed by wind–Spiders–Mosquitos•Salt marsh mosquitoes from LA found 74-106 km offshore!Colonization and Extinction: Krakatau•August 26, 1883 – Krakatau exploded (25 km3)•Two islands a few kilometers away were completely covered in ash•The nearest island not destroyed by the explosion was 40 kilometers awayTime Since ExplosionSpecies found9 months Single Spider3 years Blue-green algae; 11 fern species; 15 flowering plant species10 years Coconut trees growing25 years 263 species of animals; dense forestGroups Colonizing


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