Transcript Document
Community Succession • Universal process of directional change in vegetation during ecological time. – Recognized by a progressive change in the species composition of the community • When an area is stripped of its vegetation by fire, flood, glaciation, or volcanic activity, it doesn’t take long for new vegetation to grow. • Three stages – Primary succession, secondary succession, climax community. • Primary Succession - Species are usually ‘fugitve’ or opportunistic species. – High dispersal rates – Rapid growth • Secondary Succession – Species are longer lived than those found in the primary succession • Climax – Stable community. – Species are long lived Species Area Curves • Species area curve predicts that larger islands will have more species than smaller islands. • S=cAz where – S = number of species – c = a constant measuring the number of species per unit area – A = area of island (in square units) – z = a constant measuring the slope of the line relating to S and A Island Biogeography • Number 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 Island Biogeography • Immigration rates on islands are related to the distance from the mainland. – Close islands have greater immigration rates than far islands • Extinction rates on islands are related to the size of the island. – Extinction rates are greater for small islands than for large islands Island Biogeography Neotropical Migratory Birds • Neotropical = ‘New Tropics’ – New World vs old world – Western Hemisphere • Neotropical birds breed in Canada and the United States during the northern hemisphere’s 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 Migration Distance • Varies 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 America – One way mileage = 10,000 • Arctic Tern – – – – Nests as far north as land extends Overwinters near the south pole Sees more daylight than any other species Round trip covers about 22,000 miles. 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. When To Migrate? • Internal clock controls the onset of migration and the premigration preparations. • Environmental factors control this clock – Certain changes in a bird’s environment stimulate the production of certain hormones, which leads to changes in behavior and physiology. – Change in day length for example How To Get There? • Short migraters and waterfowl generally learn breeding and wintering locations from older more experienced birds – Often family members • Most long distance migraters are genetically programmed to make the trip. – First migration is completely under genetic control – Subsequent trips may incorporate previous experiences (return each year to good reproductive grounds) Migration Routes • Follow land through Mexico into the United States • Cross the Gulf of Mexico – First/last encountered land important to survival – Rest and refueling Seasonal Habitats • Wintering Grounds – Sufficient food for premigration preparations • Migratory Habitat – Fat reserves, nutrients, vulnerability to predation • Breeding Grounds – Reproductive success Habitat Variety Important • With the diversity of migratory birds, a diversity of habitats is needed in the migratory habitats. – Reduces competition Page 496 Top Down or Bottom Up? Bottom Up Control resources control community NVHP Top Down Control Predators control the community NVHP Top down control = Trophic Cascade Model Freshwater Pond For Example: Phytoplankton Zooplankton Small Fish Large Fish Remove large fish then small fish increase, zooplankton decreases and phytoplankton increases. Effects will be propagated up and down food chain as a +/- Page 471 Keystone Species • A species that occupies a specific niche that is extremely important in determining community structure. – When that species is removed, the community dramatically changes – Not typically the most common species in a community 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 predation – This 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. Sea Otters • Key Stone Predator in North Pacific – Once extremely abundant, reduced to near extinction in the early 1900’s by the fur trade – Feed heavily on sea urchins and thus can control their populations • Sea urchins feed heavily on macroalgae (kelp) and where sea urchin abundance is high, kelp is basically nonexistent • Where sea urchin abundance is low, kelp is common along with all of the other species associated with it. Case Study • Sea 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.