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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
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 +/-
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.