Transcript Studying Populations

Studying Populations
Studying Populations
• When studying a bald eagle
population, you might ask yourself
the following questions?
– How has the population changed over
time?
– Is the number of bald eagles more,
less, or the same as it was 50 years
ago?
• To answer these questions you
must first determine the present
size of the bald eagle population.
Population Density
• Population Density
– Number of individuals in a specific area.
Population Density 
Number of individual s
Unit area
• Ex. You counted 50 monarch butterflies in a
garden measuring 10 square meters
– Population Density = 50 butterflies/10 square meters
– Population Density = 5 butterflies per square meter
Determining Population Size
• Direct Observation
– Ex. Count all the bald eagles living in an area.
• Indirect Observation
– Count number of organisms by observing their
tracks or other signs.
– Ex. Count entrance holes for number of swallow
families. Average is 4 swallows per nest.
So…multiply number of holes times 4
• Sampling
– Estimate the numbers of organisms in a large
area
– Ex. Count number of organisms in a small area
and then multiply to find the number in a large
area.
Determining Population Size
• Mark & Recapture
– Some animals captured, marked, and released into the
environment.
– Another group is captured, ecologists count the marked
animals in this group
– Using mathematical formula, the ecologists can
estimate the total population of those organisms in the
area
– Ex. Catching field mice
Changes in Population
• Populations can change in size when new
members enter the population or when
members leave the population.
– Ex. Births and deaths
• Birth Rate
– Number of births in a population in a certain amount of
time.
– Ex. 1,000 snow geese produces 1,400 goslings per
year
Changes in Population Size
• Death Rate
– Number of deaths in a population in a certain amount of
time.
– Ex. 500 geese die per year
• If birth rate > death rate, population size increases
• If death rate > birth rate, population size decreases
Changes in Population Size
• Immigration
– Moving into a
population.
• Emigration
– Leaving a
population
• Graphing Changes
in Populations
Factors that Affect Populations
• Limiting Factor
– An factor that limits how many organisms can live in an
environment.
• Carrying Capacity
– The largest population that an environment can support.
• Food
– Suppose a giraffe needs to eat 10 kg of leaves each day. Trees in
an area can provide 100 kg of leaves each day.
– Area could support 5 giraffes but not 15 giraffes
– No matter how much shelter, water, and other resources there
might be, the population will not grow much higher than 10 giraffes
Limiting Factors
• Space
– Ex. Plants: young
plants can’t grow
under big trees
• Weather
– Ex. Severe
weather,
temperature,
amount of rainfall
Summary Questions
• List four ways of determining population size.
• How are birth and death rates related to population size?
• List three limiting factors for populations. Choose one and
explain how this factor can limit population growth.
• Explain why it is often necessary for ecologists to estimate
the size of a population.
• A field measures 50 meters by 90 meters. In 1 m2, you
count 3 grasshoppers. Estimate the total population of
grasshoppers in the field. What method did you use to
make your estimate.