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Extinction rates on islands are related to the size of the island. ... Can raise more young if they migrate than if they stayed in the tropics. ...

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top down or bottom up

Page 496

Top Down or Bottom Up?

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 +/-

keystone species

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
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
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
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.
species area curves
Species Area Curves
  • Species area curve predicts that larger islands will have more species than smaller islands.
  • S=cAzwhere
    • 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
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 biogeography9
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
neotropical migratory birds
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
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
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
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
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
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
Seasonal Habitats
  • Wintering Grounds
    • Sufficient food for premigration preparations
  • Migratory Habitat
    • Fat reserves, nutrients, vulnerability to predation
  • Breeding Grounds
    • Reproductive success
habitat variety important
Habitat Variety Important
  • With the diversity of migratory birds, a diversity of habitats is needed in the migratory habitats.
    • Reduces competition