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EcologyStudying Populations
Levels of Organization
Levels of Organization
• Species- A group of organisms that can mate with each other and produce fertile offspring
• Population-a group of the same kind of species living in an area
• Community- all the different populations that live together in an area
• Ecosystem- a community plus abiotic factors
Studying Populations• What is an estimate? It is an approximation of
number, based on reasonable assumptions
• Direct Observation– Counting each member of a population– Example: counting all the crabs in a tide pool.
• Indirect Observation– Counting evidence or traces of a population– Example: Look at the mud nests built by cliff swallows, by
counting the entrance holes, you can determine the number of swallows per nest in this area.
• In many cases, it is not possible to count signs of every member of a population. The population may be to large or spread over a wide area. In this case scientists make an estimate.
• Sampling– Counting the number of a population in a small area to
estimate the number in the whole habitat
• Mark-and-Recapture– Catching some, tagging/marking them and releasing,
recapture and count how many tagged/marked
Studying Populations
• What method am I using if …– I dig up and count each worm in the football
field?– I count how many worms are in 1 square yard
and use that information to estimate the total number of worms in the entire football field?
– I look for birds nests?– Capture birds, put a tag on them, and then
come back later to catch them again and see how many are tagged?
Changes in Population Size
• What is the main way new organisms join a population?
• What is the main way they leave?
• Moving In Moving Out/Exit
• Immigration Emigration
• Birth rate is the number of births in a population in a certain amount of time.– Example: a population of 100 rabbits produces
600 young a year. The birth rate in this population would be 600 young per year.
• Death rate is the number of deaths in a population in a certain amount of time. -Example: If 400 rabbits die in a year the death rate would be 400 rabbits per year.
Population Statement
• If birth rate > death rate, population size increases.
• If death rate> birth rate, population size decrease.
Population Density
• How many members of a population are in an area at one time.
# of individuals
• Population density = -----------------------
unit area
For example: You count 20 butterflies in a garden measuring 10 square meters. The population density would be 20 butterflies per 10 square meters, or 2 monarchs per square meter.
Limiting Factors
• Carrying Capacity– The largest population an
area can support
• Causes a population to stop growing or get smaller
• What are some limiting factors?– Food, water, space, and
weather conditions
Limiting Factors
• Living space is another limiting factor.Example: Sunflowers growing in a field.
The field has reached its carrying capacity for sunflowers when no more can grow in the field.
• Weather conditions such as temperature and the amount of rainfall can also limit population growth.