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

–Birthrate, death rate, individuals entering/leaving

***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

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

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

Two types of growth

• Exponential growth– Individuals reproduce at a constant rate– Population multiplies by constant FACTOR

over constant time interval• Logistic growth

– Occurs when a populations growth slows or stops after a period of exponential growth

– As population approaches carrying capacity, BR may decrease, DR may increase or both, until equal

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• No net increase or decrease in population• 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…

Limiting Factor

• Condition that can restrict a population’s growth

• Could be:– Space– Disease– Availability of food

Factors Affecting Population Growth

Factors Affecting Population Growth

• Density-dependent Factors • Factor that limits

population as population density increases

– Competition – Predation – Disease– Parasitism – Crowding and

Stress

• Density-Independent Factors • Factors that affect

population but are unrelated to population density; affect population regardless of size

• Insects vulnerable to this

– Weather /Natural disasters

– Human activities– Fires– deforestation

Boom and Bust Growth Cycles

• Increase rapidly for a period of time followed by a sharp decrease in population for a brief period of time

• Still not completely understood• Hypothesis:

– Changes in food supply– Stress from overcrowding– Other organisms influence on population

Boom-and Bust Population Cycles

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