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 +/-

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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

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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.

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.

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