Top Down or Bottom Up?

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Top Down or Bottom Up?
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Bottom Up Control ? resources control community N
? V ? H ? P Top Down Control ? Predators control
the community N ? V ? H ? P 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 /-
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Keystone Species
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• 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

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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.

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Sea Otters

• Key Stone Predator in North Pacific
• Once extremely abundant, reduced to near
  extinction in the early 1900s 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.

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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.

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Species Area Curves

• Species area curve predicts that larger islands
  will have more species than smaller islands.
• ScAz 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

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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

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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

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Island Biogeography
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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 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

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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.

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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.

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When To Migrate?

• Internal clock controls the onset of migration
  and the premigration preparations.
• Environmental factors control this clock
• Certain 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

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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)

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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

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Seasonal Habitats

• Wintering Grounds
• Sufficient food for premigration preparations
• Migratory Habitat
• Fat reserves, nutrients, vulnerability to
  predation
• Breeding Grounds
• Reproductive success

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Habitat Variety Important

• With the diversity of migratory birds, a
  diversity of habitats is needed in the migratory
  habitats.
• Reduces competition