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Ecology Notes – Part
4:
Populations
Biology
Spring 2012
Populations
•
Population: all members of a species in a particular area
•
Ways to determine population size
•
Direct Observation: counting all organisms in an area
•
Indirect Observation: counting signs of life and estimate
•
Quadrants: count a number of organisms in a small area,
then multiply by the total area
•
Mark and recapture: organisms are caught, marked,
released, then caught again to count number marked and
number unmarked
Population Changes
•
Populations change in
size when new members
join or when members
leave
•
Ways populations
change size:
•
Births (+) and
Deaths (-)
•
Immigration (+) and
Emigration (-)
Fundamental idea
Population change =
incoming
-
outgoing
(births + immigration) - (deaths + emigration)
Immigration: the number of a species that
moves into the community.
Emigration: number moving out of the
community
Populations
•
Population Density: the
number of individuals from
one population in a
particular area
•
Population density = #
of individuals / unit
area
•
Ex: There are 20
butterflies counted in 10
m2
•
20 butterflies/10m2= 2
butterflies/m2
Beyond density
How a population is spread out within the
ecosystem is also important.
Dispersion: the pattern of spacing within a
population.
Uniform: evenly spaced.
Clumped: areas of high density separated by
lower.
Random: well, random.
So Far...
Energy flow, material cycling
Interactions: feeding, symbiotic, etc.
Populations: change and density
Which graph from simulation?
A
C
B
Any similarities?
Population Changes
When a population is getting started,
there are few limitations on its growth.
Plenty of:
Food
Space
Water
Safety from predators
Exponential Growth
Rapidly expanding population
Few limitations on the population
(births+immigration) > (deaths+emigration)
This is an ideal situation
Most often seen with rebound or invasion
Case Study: Scotch Broom
Native to (originated) in the British Isles.
Imported to Britsh Columbia in late 1800s
as a landscaping plant.
Has been able to spread...
WADOT
Ecology British Columbia
USDA
Invasion!
Scotch broom considered an invasive
species: a species that immigrates into an
ecosystem and is able to outcompete other
species.
Exhibits exponential growth because it is
unusually successful in its new niche.
Strategies
No longer found in nurseries
Public service messages
Removal (difficult)
Integrated Pest Management (seed-eating
beetle)
Nothing lasts forever...
Exponential growth can’t continue
unabated.
What would limit it?
Resources: food, water, habitat
Relationships: consumer/predation
Disease
Simulation limitation?
Back to reality
It’s more realistic for populations to be
more stable at the carrying capacity of the
ecosystem.
Is the largest population an area can
support due to scarcity of resources or
other factors.
At carrying capacity change in
populations are minimized.
What is
happening
with our
formula?
From your prior experience...
What happens when…
Population goes above carrying capacity?
Populations goes below carrying capacity?
Big Idea:
Population size is regulated
by a complex interaction of
biotic and abiotic influences
Limiting Factors
Environmental factors that cause a
population to decrease or to stop growing.
Types of Limiting Factors:
Density Dependent: depends on
population size
Density Independent: population size
doesn’t matter
Examples
Food and water: an environment has a
limited amount of resources (DD)
Space: the amount of area a population
lives in (DD)
Disease: spreads more easily in higher
density.(DD)
Weather: temperatures, rainfall,
hurricanes, and floods can change the
environment and not allow survival (DI)
Populations
•
Carrying Capacity:
the largest population
an area can support
due to limiting factors
•
What happens when…
•
Population goes
above?
•
Populations stays
below?
Populations
•
Kaibab Deer Investigation
Lynx vs. Hare
Populations
•
http://www.youtube.com/watch?v=Ot_
KmOTYfRA
Two more stories of
population influence...
Example: Scotland
Glen Affric,
Caledonia (near
Loch Ness)
Small forests of
pines (Pinus
sylvestris)
Reduced by logging,
sheep herding and
the red deer
(actually an elk).
Caledonian Forest Chain
Wolf
Red Deer
Pine Tree
•
Example: Alaska
•
Sea Otter “Harvesting”
•
Populations lower closer to mainland.
Food chain when otters
were low...
Close to mainland there
were few otters.
Sea urchins, an otter
“favorite” were high.
Urchins are herbivores of
kelp
“Ocean desert”
Food chain when otters
were high...
Farther out the island chain.
Sea urchins were lower.
Kelp forest thriving
Much greater diversity
Adding to the chain...
Scientists observe new orca
pod.
Then sea otter skeletons...
What happened to the sea
urchins?
What happened to the kelp?
What happened to
diversity?
Take-home ideas
Exponential growth when no limiting
factors.
Most populations show some sort of
stability around carrying capacity.
Carrying capacity determined by limiting
factors & other relationships.
Changes in one population (especially top
consumers) has an effect across the food
web.
Populations & Biodiversity
The Habitable Planet:
http://www.learner.org/courses/envsci/i
nteractives/ecology/index.php