Transcript What is population?

What is population?
• Group of individuals of the
same species that live in the
same area
Does population change?
• Sea otters, kelp, and sea urchins
•Kelp forests offer a
habitat for sea otters
•Sea urchins feed on
kelp
•Sea otters feed on sea
urchins
• What do you think happens when sea otters are
hunted?
– Sea urchins increase or decrease?
– Kelp forest increase or decrease?
• Sea otters are then placed on the endangered
species list
• So now what happens to the population of sea
otter?
– Starts to increase
• How does this affect the kelp and the sea
urchins?
– Sea urchins start to get eaten again=decrease in #
– Kelp increases b/c less sea urchins to eat them
But now we have a new hunter….
What happens to the sea otter,
kelp, and sea urchins?
What does this tell us about
population?
• Population changes
• There are many factors that influence a
population
– Natural
– Unnatural
• Population density has a great impact on
ecosystems
3 important characteristics of
Population
• Geographic Distribution
• Density
• Growth Rate
***Population Age structure is also
an important characteristic
Geographic
Distribution
• AKA Range
• Describes an area inhabited by a
population
• Can vary
–Few cubic centimeters
–Kilometers of the ocean
Density
• # of individuals per unit area
• Low density
–Cactus in desert
• High density
–Other desert plants and
succulents
Math Time
• Formula for calculating population density
• Population density=Number of individuals
units area
• Problem: Suppose there are 150 bullfrogs
living in a pond that covers an area of 3
square kilometers. What is the density of
the bullfrog population?
• 50 bullfrogs per square kilometer
Growth Rate
• Many factors affect growth rate
Sampling Techniques
• How would we measure the population of
a species?
• Impractical to count each and every one
• Variety of sampling techniques
– Quadrants
– Indirect counting
– Mark-Recapture
Quadrants
• Involves marking off specific area, boundary
• Count specific species within the boundary
• Repeat in several locations within desired
ecosystem
• Average the results to determine population
density
• More quadrants sampled=more accurate
Indirect Counting
• Used for species that are too difficult to see or
move around too quickly
• Does not involve counting organisms
themselves
– Count nests, burrows, tracks
Mark-Recapture
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Most common
Choose a study area
Trap/capture animals
Mark the captured animals and release back into
habitat
– Markings are not to disturb organism
• After a period of time, recapture animals in the
same study area
• Count marked and unmarked organisms
How to Estimate Population from
Mark-Recapture Method
Total population=
(# in first capture) x (# in second capture)
number of marked animals RECAPTURED
Limits to Accuracy
• Involve making assumptions about populations
– Assumptions not valid=estimate not accurate
• Quadrant
– Assumption:
• Organisms distributed evenly in study area
– Problems
• “Clumps”
• Quadrant with clump vs quadrant without clump
– Minimize problem
• Analyze how study population is distributed in order to choose appropriate quadrant
size
• Mark-Recapture
– Assumption:
• Both marked and unmarked animals have same chance of surviving and being
recaptured in second trial
– Problem
• After being captured once, how do you think animals will behave?
• Leads to overestimating population size
– Minimize problem
• Minimize effects of trapping on organisms
3 Factors that affect population
size
• # of births
• # of deaths
• # of individuals that enter or leave
population
Population will increase or
decrease depending on # of
individuals added or removed
What happens to the population
when we….
• Have more births than deaths?
– Population increases
• Have more deaths than births?
– Population decreases
• Have equal amounts of births and deaths?
– Population remains constant
Immigration
• “im”= in
• Migrate= to move from one place to
another
• Immigration is the individual movement
into an area
• Animals in search of mates and food in
new areas
Emigration
• “E” means ‘out’
• Migrate means to move from one place
to another
• Emigrate means individuals moving out of
one place and into another
• Young wolves and bears leaving as they
mature
• Shortage of food
Two types of growth
• Exponential growth
– Individuals reproduce at a constant rate
• Logistic growth
– Occurs when a populations growth slows or
stops after a period of exponential growth
Exponential Growth
• Occurs under ideal conditions with
unlimited resources
• Think about exponents in math….
• Starts slowly then sky rockets to infinity
• Our graph will look like a J
• Bacteria
Lets look at bacteria…
• Bacteria reproduce by splitting in half
• Bacteria have a doubling time of 30 minutes
• If you start will one bacterium, how many bacteria will
there be after the first 30 minutes?
– 2
• After an hour?
– 4
• After an hour and a half?
– 8
• After two hours?
– 16
• After 15 hours?
– Over a billion
Logistic Growth
• As resources become less available, the
growth of the population slows or stops
• S-shape curve
• What we usually see in nature
Carrying Capacity
• The largest number of individuals that a given
environment can support
• The part of the logistic graph after the
exponential growth…the flattening out
• The point at which this flat line reaches the yaxis is the size of the population when the
growth rate reaches zero
• This doesn’t mean the population stops growing
• Many factors slow the growth of plants and
animals…