Community Succession Universal process of directional change in vegetation during ecological time. –Recognized by a progressive change in the species composition

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<ul><li><p>Community SuccessionUniversal process of directional change in vegetation during ecological time.Recognized by a progressive change in the species composition of the communityWhen an area is stripped of its vegetation by fire, flood, glaciation, or volcanic activity, it doesnt take long for new vegetation to grow.Three stages Primary succession, secondary succession, climax community.</p></li><li><p>Primary Succession - Species are usually fugitve or opportunistic species.High dispersal ratesRapid growthSecondary Succession Species are longer lived than those found in the primary successionClimax Stable community.Species are long lived</p></li><li><p>Species Area CurvesSpecies area curve predicts that larger islands will have more species than smaller islands.S=cAz whereS = number of speciesc = a constant measuring the number of species per unit areaA = area of island (in square units)z = a constant measuring the slope of the line relating to S and A</p></li><li><p>Island BiogeographyNumber of species (on a plot or island) is a balance between immigration and extinction.If immigration exceeds extinction, then the number of species will increase.Number of species usually at equilibrium</p></li><li><p>Island BiogeographyImmigration rates on islands are related to the distance from the mainland.Close islands have greater immigration rates than far islandsExtinction rates on islands are related to the size of the island.Extinction rates are greater for small islands than for large islands</p></li><li><p>Island Biogeography</p></li><li><p>Neotropical Migratory BirdsNeotropical = New TropicsNew World vs old worldWestern Hemisphere</p><p>Neotropical birds breed in Canada and the United States during the northern hemispheres summer and spends the rest of the year in the tropics.Defined as a species in which the majority of individuals breed north of the Tropic of Cancer (latitude = 23 degrees north)About 200 species</p></li><li><p>Migration DistanceVaries across species and within species Shortest (a few hundred miles) are those birds that breed in the southern US and overwinter in Mexico. Some of the longest are birds that breed in the arctic tundra in northernmost Canada and winter as far south as the southermost tip of South AmericaOne way mileage = 10,000Arctic Tern Nests as far north as land extendsOverwinters near the south poleSees more daylight than any other speciesRound trip covers about 22,000 miles.</p></li><li><p>Why Migrate?They can take advantage of seasonally abundant food supply and avoid times and places that food supply is low.Flying insects, caterpillars, fruits and nectar are abundant during our spring and summer, but not winter.Ultimate reason is breeding success.Can raise more young if they migrate than if they stayed in the tropics.Abundant protein-rich food, longer daylight hours, more room, possibly fewer predators.</p></li><li><p>When To Migrate?Internal clock controls the onset of migration and the premigration preparations.Environmental factors control this clockCertain changes in a birds environment stimulate the production of certain hormones, which leads to changes in behavior and physiology.Change in day length for example</p></li><li><p>How To Get There?Short migraters and waterfowl generally learn breeding and wintering locations from older more experienced birds Often family membersMost long distance migraters are genetically programmed to make the trip.First migration is completely under genetic controlSubsequent trips may incorporate previous experiences (return each year to good reproductive grounds)</p></li><li><p>Migration RoutesFollow land through Mexico into the United StatesCross the Gulf of MexicoFirst/last encountered land important to survivalRest and refueling</p></li><li><p>Seasonal HabitatsWintering GroundsSufficient food for premigration preparations </p><p>Migratory HabitatFat reserves, nutrients, vulnerability to predation</p><p>Breeding GroundsReproductive success</p></li><li><p>Habitat Variety ImportantWith the diversity of migratory birds, a diversity of habitats is needed in the migratory habitats.Reduces competition</p></li><li><p>Top Down or Bottom Up?Bottom Up Control resources control communityN V H PTop Down Control Predators control the communityN V H PTop down control = Trophic Cascade ModelFreshwater Pond For Example: Phytoplankton Zooplankton Small Fish Large FishRemove large fish then small fish increase, zooplankton decreases and phytoplankton increases.Effects will be propagated up and down food chain as a +/-Page 496</p></li><li><p>Keystone SpeciesA species that occupies a specific niche that is extremely important in determining community structure.When that species is removed, the community dramatically changesNot typically the most common species in a communityPage 471</p></li><li><p>Pisaster ochraceous (a starfish)Keystone species in the rocky intertidal communities of western North America.Is a strong predator for a mussel (Mytilus californianus)The starfish can not eat large mussels, so the mussels have a size-related refuge from predationThis mussel can out-compete other invertebrates for space, but the starfish takes away that competitive edge.When the starfish were removed, mussel numbers increased and excluded other invertebrates and algae from attachment sites.</p></li><li><p>Sea OttersKey Stone Predator in North PacificOnce extremely abundant, reduced to near extinction in the early 1900s by the fur tradeFeed heavily on sea urchins and thus can control their populationsSea urchins feed heavily on macroalgae (kelp) and where sea urchin abundance is high, kelp is basically nonexistentWhere sea urchin abundance is low, kelp is common along with all of the other species associated with it.</p></li><li><p>Case StudySea otters have declined (sometimes 25% per year) in Alaska since about 1990, and the kelp beds have begun to disappear as sea urchins increased.Killer whales are suspected because their prey base (seals, sea-lions) has declined, and their predation on sea otters has increased.Seals and Sea-lion population declines have been attributed to a decline in their food base (fish).Fish declines have been attributed to overharvesting in the North Pacific.So, overharvesting of fish may have led to a cascade of events that were unexpected.</p></li></ul>