Understanding POPULATIONS

Chapter 20

Definition• A population is a group of

organisms that belong to the same species & live in a particular place at a particular time, using same resources.

Examples: • All of the bass in a pond• All of the blue jays in Washington County• All of the red maples in Western PA

Examples of populations:

All the bacteria of one All the gorillas of 1

kind on this agar plate species on this mountain

Population ecology

• is concerned with fluctuations in population size and the factors that regulate populations.

Properties of Populations

1. Size2. Density3. Dispersion4. Population dynamics;

A. Growth rateB. Age structureC. Survivorship.

May differ from properties of the individual

1. Population Size• Is the # of individuals• Can be difficult to measure directly

• How is counting a tree species different than counting a fish population?

• Ways to find size:

–Counting individuals (might be impossible for some large, mobile, widespread populations)

–Estimate

2. Population Density

- Measures how crowded a population is.

• Expressed as number of individuals per unit or volume.

• Example:

pop. density of humans in USA is 30people/ sq km.

Estimating Density

• 1. Count all individuals in a sample plot. (Estimates become more accurate as sample plots increase in size.)

• 2. Estimate by indirect indicators (such as number of nests or droppings or tracks.)

• 3. Mark-recapture method. (animals trapped in boundaries, marked & later retrapped. )

To Estimate Population Size

• Instead of laboriously counting everyone, just count organisms in 1 area & multiply

Example- If 25 oaks in 1 Km2, then estimate 250 in 10 km2.

(This assumes even distribution, or there will be an error in #)

3. Population Dispersion

• Clumped-most frequent pattern of distribution. Individuals clustered together in groups in response to uneven distribution of resources.

• Even - members maintain a minimum distance from one another, generally indicates strong competition

• Random - spacing is the least common pattern.

4. Population Dynamics• All populations change in size

and composition over time.

A. Growth Rate includes:

• Birth rate = # births

• Death (mortality) rate = #deaths

• Life expectancy = average life span

B. Age Structure• The distribution of ages in a

population.

• Important to know ages of individuals.

• Example- older populations do not reproduce but a young population may have rapid growth.

Age structure represented by graphs.

users.rcn.com/.../ P/Populations.html

C. Patterns of Mortality

•Survivorship curves- Show the likelihood of survival at different ages for an organism.

• Data tends to fall into 3 types

3 types of Survivorship curves

–Type I - likelihood of dying is

small until old age (humans, elephants)

–Type II - likelihood of dying is even over lifetime. (birds)

–Type III - likelihood of surviving youth is small (many die young) but if live- then may live to be old (insects, fish)

Charles Darwin

Calculated that a single pair of elephants could increase to a population of 19 million individuals in 750 years.

Since the earth is NOT overrun with elephants, this is evidence that factors in the environment limit population growth

Charles Darwin (1809-1882)His Life and Work

*Born to a wealthy family*Studied medicine - quit.*Then studied to enter the

clergy.*Spent a lot of time collecting

biological specimens. * Quit religion studies to

become a NATURALIST.

Voyage on the H.M.S. Beagle

• studied many things on the islands passed by boat.

• In part because he was so seasick he wanted to get off the boat.

• Basis for his life work.

Darwin’s Evolutionary Theory(which he liked to call “natural selection”

not “evolution”)

States that:

-over many generations, natural selection causes characteristics of populations to change.

-natural selection is responsible for evolution.

What is survival of the fittest?

• It is NOT- “King of the Jungle”- with the meanest, biggest, most muscular animal killing all the smaller ones.

• Example- 2 dogs- One is the perfect specimen- shiny fur, wins shows, smart, etc. & the other is small, ugly, mangy, living in the alley.

The ugly dog visits every female dog in the county and reproduces many times, but the perfect dog never has puppies-

• which dog will pass on his traits?

Measuring Populations

1. Growth rate

2. Exponential Model

3. Logistic Model

4. Population Regulation

1. Growth Rate

• Definition:–The amount by which a population’s size changes over time.

• 4 Processes that affect growth:

–Birth

–Death–Immigration (individuals moving in)

–Emigration (individuals moving out)

• Usually assume Immigration & Emigration are zero

Growth Rate

• Describe & graph growth rate mathematically

• Divide large populations into groups of 1000.

• Growth = Birth rate – Death rate

Example population

• If 32 Births & 10 Deaths per 1000 per year of individuals, & total population is 40,000.– Birth rate is 0.032– Death rate is 0.010–Growth rate is 0.022

• To find the # of new individuals, multiply growth rate by total population:–40,000 X 0.022 = 880 - increase in 1 year

2. Exponential Model• Describes a population that increases

rapidly after only a few generations.• The larger it gets, the faster it grows.• Assumes birth/death rate are constant• Assumes unlimited resources (like food).

http://www.agen.ufl.edu/~chyn/age2062/lect/lect_27/39_05.GIF

Limiting factors• In reality, populations cannot

grow indefinitely because– Resources become depleted

– Waste accumulates

• As Competition increases –Birth rate declines

–Mortality increases

3. Logistic Model• Builds on Exponential Model• Accounts for limiting factors

(environmental resistance)

•Carrying Capacity–The number of individuals the

environment can support over a long period of time.

http://www.stockton.edu/~cromartw/populationbio/logistic.JPG

Population Regulation• Density–independent factors

–Weather, fire, etc–Reduces population regardless of size

• Density–dependent factors–Resources shortages- like food, nesting

sites- triggered by increase in population

Populations may fluctuate in response to predator- prey relationships

Small Populations• Decreased genetic variations due to

small gene pool.• Inbreeding (mating with close relatives)• May reduce species ability to adapt to

changes in the environment • May be due to human activities• Examples: in wild

– Less than 200 Siberian tigers left– Only 9 California condors in wild 1980s

Human Population History• For most of Homo sapiens existence,

population grew very slowly• Small Nomadic Hunter gather groups• Agricultural Revolution- about 10,000 years

ago- Human population grew moderately.• Population Explosion – Middle ages• Times of decrease from disease, wars.• Population grew from 3 to 5 billion people

between 1960 & 1987.

Growth today• Developed Countries

–Leveling off or decreasing

• Developing Countries–Still growing rapidly

Demographic Transition Model

Questions:1. Which of the following is a population?

A. all the fish in a pondB. all the birds in New York CityC. all the members of a family of humansD. all the fish of the same species in a lake

2. Which of the following refers to the population size that can be sustained by an environment over time?A. bell curveB. allele frequencyC. carrying capacityD. exponential growth

d. c.

More questions:3. Which of the following is true in the exponential model of

population growth?a. Population growth continues indefinitely.b. Population growth stops at the carrying capacity.c. Population growth increases and then decreases.d. The immigration rate falls with increasing population size.

4. Which of the following is a density-dependent factor for a population of deer in a forest?a. a droughtb. a landslidec. a period of freezing weatherd. the number of cougars in the forest

a. d.