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

• 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

• Have more births than deaths?– Population increases

• Have more deaths than births?– Population decreases

• Have equal amounts of births and deaths?– Population remains constant

What happens to the population when we….

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 y-

axis 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…

Factors Affecting Population Growth

Factors Affecting Population Growth

• Density-dependent Factors – Competition

– Predation

– Disease

– Parasitism

– Crowding and Stress

• Density-Independent Factors – Weather /Natural

disasters

– Human activities

– Fires

– deforestation

-Involves more than one population-Rapid increase and decrease in populations