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.