Announcements September 8, 2006. Population Biology Lecture Objectives: 1.Learn the population...

Announcements

September 8, 2006

Population Biology

Lecture Objectives:

1. Learn the population characteristics that determine population growth rate

2. Understand why the number of individuals in a population may change over time.

3. Understand the different types of population growth curves

4. Understand the difference between K-selected and r-selected species

Population Characteristics

Population – All organisms of the same kind found within a specific geographic region

Populations can differ in:

*Birthrate

*Death rate

*Sex ratio

*Age distribution

*Density

*Population growth rate

Birthrate (Natality)

Birthrate (b)—Number of individuals added through reproduction over a particular time period.

- In many species, birthrate is influenced by the amount of food available

- Sexually reproducing species must also find mates

- In humans, expressed as # of babies/1000 individuals/year

Asexual ReproductionFemales that reproduce asexually do not need to find a mate

New Mexico Whiptail Daphnia

Mortality

Survivorship curve – a graph showing the proportion of individuals likely to survive to each age

Three main types:Type IType IIType III

Type I survivorship curveSurvival is high until old age

Examples:• Dall sheep• Humans in

developed countries

Type II survivorship curve

Mortality is spread evenly among all age groups

Example:•Many species of birds

Type III survivorship curve

Very high mortality among the young

Examples:• Plants• Animals

that produce many offspring

Survivorship curves (continued)

One major factor determining the shape of the survivorship curve is the degree of parental care

How long do parents care for the offspring after they are born?

Population growth rate

Population growth rate (r) – the rate at which the size of the population changes

r = b – d

Assumes no immigration or emigration—when this happens population growth rate is then more generally, (birth + immigration) – (death + emigration)


If r is positive, birthrate is higher than the death rate and the population size increases

If r is zero, birthrate equals death rate and the population size stays the same

If r is negative, birthrate is less than the death rate and the population size gets smaller

r > 0(+)

r = 0

r < 0(-)

Example for a population of algae

Dispersal: Movement of individuals

Emigration: leaving a population, often from crowded areas or in response to environmental change

Immigration: emigrating individuals become immigrants in a new population (joining a population)

Other factors influencing population growth rate

Dispersal


Sex Ratio

Sex Ratio—Relative number of males and females in a population.

In many sexually reproducing species, with separate sexes, sex ratio is 1:1

In other species (e.g., asexuals, social insects) can have far more females than males


Age Distribution (Age Structure)

Age Distribution—Number of individuals of each age in the population.

Greatly influences reproductive rate of a population.

Summary of factors influencing population growth rate

Birthrate Death rate

Sex Ratio Age distribution

Immigration Emigration

How many individuals are in a population at any given time?

How many individuals will be in the population one generation later?

Which of these age distributions has a Type III survivorship curve?

A B C

Population Density (N)—Number of individuals per unit area.

High population density may lead to increased competition for resources, increased emigration, increased death rate

Population Density

Population 1

N = 4 flowers/m2 N = 12 flowers/m2

Population 2

Population Growth Curve Biotic Potential—Inherent reproductive

capacity. Generally, biotic potential is much above

replacement level. Natural tendency for increase.

When not limited, populations tend to grow exponentially

How to combine number of individuals (N) and population growth rate (r)?

Exponential Growth

Note: This equation is provided only to help. You do not need to memorize it. You will not be asked about the equation on a

test.

Change in number over time

rNdtdN =


Number of individuals

The number of individuals in each generation is a multiple of the previous generation

Example of exponential growth

Populations do not grow exponentially forever

Population Growth Curve

Typical Growth Curve Lag Phase—First portion of the curve; slow

population growth. Exponential Growth Phase—More organisms

reproducing causing accelerated growth; continues as long as birth rate exceeds death rate.

Stable Equilibrium Phase—Death rate and birth rate equilibrate; population stops growing.

Carrying Capacity —Number of individuals of a species that can be indefinitely sustained in a given area.

Environmental Resistance

Environmental Resistance—Any factor (limiting factor) in the environment influencing carrying capacity. Four main factors:Raw material availabilityEnergy availabilityWaste accumulation and disposalOrganism interaction

Logistic Equation

⎟⎠⎞

⎜⎝⎛ −=

KN

rNdtdN

1

Note: This equation is provided only to help. You do not need to memorize it. You will not be asked about the

equation on a test.

Causes of population crashes

Population size can sometimes overshoot carrying capacity

Environmental resistance factors can change, lowering carry capacity

When population > carrying capacity,death rates > birth rates

Population crashes!!

Example 1: bacterial population crash due to toxic waste accumulation

Example 2: Population cycling due to organism interaction

Hare population:birth rate limited by food availabilitydeath rate determined by predators, food

Lynx population:birth and death rate determined by food availability (hares)

Example 2: Population cycling due to organism interaction

Start: both have low population density Hares – high food, low predators = pop increase

over next generations Lynx – as hares increase, more food = pop

increase Hares – when high pop density, increased

competition for food and increased predation = low birth rate & high death rate = sharp pop decrease

Lynx – when high pop density and few hares, low food = low birth rate & high death rate = sharp pop decrease

Back to start

Population Cycles

Remember: Several factors influence population growth rate:

Birthrate Death rate

Sex Ratio Age Distribution

Immigration Emigration

But, there are also additional things about the individuals that influence how the population grows

Life history traits – characteristics of an individual that influence survival and reproduction

Age at maturity

11 - 20 years 3-6 years

Atlantic SalmonAfrican elephant

2 months

House Mouse


Atlantic SalmonAfrican elephantHouse Mouse

1 calf every 3-8 years

1,500 to 8,000 eggs once

5-8 young every month

Number of offspring produced



Number of reproductive events

~3 - 10 1~6-12



Lifespan

60 - 70 years 3-6 years~2 years

How do the following life history traits affect the rate of reproduction?

Age at maturity

#offspring

# reprod. events

life span

2 mo many one long

2 yr few many short

2 mo many many long

2 yr many many long

K-strategists:

Remember that “K” is Carrying Capacity

•long lived•produce few offspring•parental care

Reproductive strategies and population fluctuations

–controlled by density-dependent limiting factors

factors more severe as population increases

example: food

K-strategists:•stable populations at carrying capacity (K)•examples: humans, deer, lions


Gypsy moth caterpillars

Remember that “r” is population growth rate

r = b - d

r-strategists:•small•short life•many offspring



r-strategists:•rarely reaches K•little or no parental care



–good at using temporarily abundant resources

r-strategists:•controlled by density-independent limiting factors:

weather, pond drying


Points to know:

Understand the three types of survivorship curves.

What factors influence population growth rates?

What are some life history traits?

Know the difference between K- and r- strategists and what kind of limiting factors each has.

Announcements September 8, 2006. Population Biology Lecture Objectives: 1.Learn the population...

Documents

Transcript of Announcements September 8, 2006. Population Biology Lecture Objectives: 1.Learn the population...