BOT108 2ND EXAM

• Disjunction


  Discontinuous range of a taxon in which at least two closely related populations are separated by a wide geographic distance
• Possible explanations for disjunction

  • Dispersal
  • Geological events
• Dispersalists


  • Believe dispersal explains disjunct distributions
• Extensionists


  • Believe geological events explain disjunct distributions
• Alfred Wegener's Theory of Continental Drift


  Provided new theory critical to understanding species' distributions
• Vicariance


  1. Geographical range of a taxa is split into parts by the formation of a barrier

  2. After vicariance events, evolutionary divergence can give rise to different (but related) species in the disjunctions
• Dispersal and vicariance events


  Both are important in determining species distributions
• Ecological dispersal


  Movement of individuals away from an existing population or parent organism to reduce intra-specific competition, but habitat similarity decreases with distance
• Darwin: '"Neither the similarity or dissimilarity of the inhabitants of various regions can be wholly accounted for by climatic and other physical conditions."'
• Biogeographic dispersal


  Species shift their ranges by moving over long distances across large barriers, infrequent but very important, mostly historic examples
• Why disperse?


  • Reduce intraspecific competition
  • Find more suitable habitats
• Darwin: '"Barriers of any kind…are related in a close and important manner to the differences between the productions [organisms] of various regions."'
• Successful range expansion


  1. Travel to new area

  2. Tolerate conditions of new habitat

  3. Reproduce in new habitat
• Biogeography


  The study of how species are articulated on the landscape in space and time

  
• Mechanisms of dispersal


  • Active (movement by organism's own means)
  • Passive (movement by stronger force like water, wind, or another organism)
• Active dispersal - water

  • Aquatic organisms
• Island Biogeography


  • A subdivision of biogeography that relates the manner in which species distributions are influenced and restricted by "islands"
  • The "island" is any area of habitat surrounded by an inhospitable matrix to the species occurring on that island

  
• Active dispersal - plants


  • Seed dispersal by plants

  
• Passive dispersal - water


  • Floating seeds, fruits, spores
• General Island Types


  • Oceanic Islands - Never connected to a mainland source
  • Continental/Land bridge Islands - Connected to mainland during glaciations or prior to flooding
  • Virtual Islands - Isolated communities separated via some sort of barrier
• Passive dispersal - floating objects


  • Seeds, fruits, spores dispersed on floating objects
• Island Size


  Species richness depends on island size and distance from mainland

  
• Diaspore


  Plant dispersal unit consisting of seed plus additional tissues adapted for dispersal

  
• Small islands have less habitat, smaller populations, and higher rates of extinction (intra,inter-specific competition)
• Wind dispersal of large propagules

  • Pappus
  • Wings
  • Tumbleweed
• Distant islands have lower rates of colonization, however this depends on dispersal mechanism of the species
• Exozoochory


  • Seeds dispersed on outside of animal
• For conifers and flowering plants in the Pacific, diversity is much lower in the more isolated island groups of the central and eastern Pacific
• Endozoochory


  • Seeds dispersed on inside of animal
• Dispersal Hypothesis


  Species originated in one area and dispersed to other areas
• Jump dispersal


  Colonization of new areas over long distance, explains wide and often discontinuous distributions, accounts for similarities/differences among biotas in different geographic areas
• Vicariance Hypothesis


  Areas were formerly contiguous, and were occupied by a common ancestor. Speciation occurred once barriers arose
• Diffusion


  Slow dispersal of individuals spreading out from the margins of the species' range, often follows jump dispersal
• MacArthur and Wilson (1967): '"Theories, like islands, are often reached by stepping stones…"'
• Secular migration


  Very slow dispersal (e.g. hundreds of generations) that commonly involves evolutionary changes in the dispersing populations
• Theory of Island Biogeography

  The number of species of a given taxon that become established on an island represents a dynamic equilibrium controlled by the rate of immigration of new species and the rate of extinction of previously established species

  
• Secular migration - camels


  • Slow dispersal over hundreds of generations
• Biogeography Theory in a Nutshell

  • Equilibrium number of species, but constant turnover
  • Migration slows as richness increases
  • Extinction increases with richness
  • Equilibrium = rate of extinction, rate of colonization intersect
  • Colonization balances extinction

  
• Barriers


  Abiotic or biotic features that restrict movement of genes or individuals from one place to another, species-specific, organisms in fluctuating environments more tolerant
• Theory of Island Biogeography

  • Immigration varies with the distance of the island from the mainland (or the pool of potentially colonizing species)
  • The rate of colonization declines as species richness increases, because there are fewer potential colonists and fewer unexploited niches