NEW AIM: How do scientists describe population growth?

NEW AIM: How do scientists describe population growth?

Population Ecology- The study of changes in population size and the environmental factors that regulate it.

Population- group of individuals of a single species that occupy the same general area

1. Rely on same resources

2. Influenced by the same biotic and abiotic factors

3. High likelihood of interbreeding


Population Density- Number of individuals per unit area or volume

1. Count all individuals (not always possible)

2. Count individuals in several random spots and then extrapolate

3. mark-recapture:

How can we determine this?

- Capture some animals and mark (left)

- Release back to population

- Let animals mix back for days/weeks

- Randomly catch animals again

- Now you get mix of marked and unmarked

Great, but what does that do for me?


Population Density- Number of individuals per unit area or volume

1. Count all individuals (not always possible)

2. Count individuals in several random spots and then extrapolate

3. mark-recapture:

How can we determine this?

- Capture some animals and mark (left)

- Release back to population

- Let animals mix back for days/weeks

- Randomly catch animals again

- Now you get mix of marked and unmarked

- The ratio of marked to unmarked gives population estimate


Population Dynamics- Pop. Density is obviously not going to be static

1. Birth/Death

2. Immigration / Emigration

Additions/subtractions occur through

AIM: What are some of the factors that limit population growth?

Dispersion- Pattern of spacing among individuals in a population

1. Clumped

What types of patterns are observed?

- Most common (we do this of course)

- Can be associated with:

1. Resource location like food or water

2. mating

3. Improve odds of predators

4. Improve odds of prey

- Pack of wolves more likely than single wolf to capture a prey animal

- Herding behavior = use genetically unrelated individuals as shield against predators

- Improve odds of finding a mate



2. Uniform



1. Direct interaction of population members

- Penguins to right will aggressively defend their space from other penguins

- Territoriality – defending a space against encroachment by other members of population

- Some plants secrete chemicals that inhibit germination of seeds



3. Random



1. Absence of strong attraction/repulsion amongst individuals

2. Key biotic/abiotic factors are homogenous (equally distributed) throughout area – food, water, etc…


Demography- Study of statistics of populations and how they change over time.

What influences density and dispersion of populations?

1. Ecological needs of a species (food, water, sunlight, humiditiy, etc…)

2. Structure of the environment (are resources evenly distributed, are they found in certain areas, are there predators, etc…)

3. Interactions between individuals of population

These factors will be the cause of vital statistics like:

1. Death Rates (Survivorship)


Demographers construct Life TablesLife tables

- Age specific summaries of the survival patterns of a population that allow one to estimate how long a person of a given age will live – great for life insurance companies

Cohort

- Group of individuals the same age that demographers follow from birth to death for each age group.

Let’s take this life table data and look at it graphical in what are called survivorship curves…


Demographers construct Life TablesSurvivorship curves

- Graphical representations of life tables

Survivorship curves for male and female Belding’s ground squirrels. Be aware that Y-axis is logarithmic.

Survivorship curves can be classified into three general types as shown on the next slide…


Demographers construct Life TablesSurvivorship curves

Three general types:Type I

Type III

- Flat at start meaning low death rate at early and middle life and then drops steeply

- Ex. Humans, many other large mammals with high parental care

- Drops steeply reflecting high death rate in young individuals, but then flattens out for those few individuals that make it to a critical age.

- Ex. Organisms producing many offspring, but little parental care like fish, marine invertebrates like oysters, which release millions of eggs of which most are eaten by predators.

Type II

- Between type I and type III with a fairly constant death rate over a lifetime - Ex. Annual plants, some rodents, some lizards, etc…


Demography- Study of statistics of populations and how they change over time.

What influences density and dispersion of populations?

1. Ecological needs of a species (food, water, sunlight, humiditiy, etc…)

2. Structure of the environment (are resources evenly distributed, are they found in certain areas, are there predators, etc…)

3. Interactions between individuals of population

These factors will be the cause of vital statistics like:

1. Death Rates (Survivorship)

2. Reproductive Rates


Demographers construct Reproductive TablesReproductive tables

- Fertility schedule, age-specific summary of reproductive rates in a population

Constructed by measuring reproductive output of a cohort from birth to death – the number of offspring a group has in a lifetime.


Review Demography


Life History TraitsLife History

Traits of a population that affect reproduction and survival

1. Age at which reproduction begins

2. How often organism reproduces

3. Number of offspring produced during each reproductive episode

What are the three general traits affecting life history?

***Excluding humans, organisms do not choose consciously when to reproduce or how many offspring to have…




- “one-shot” pattern of reproduction

- Ex. Pacific salmon will produce thousands of eggs one time and then die.

1. Semelparity (big-bang reproduction)

Under what environmental conditions would this be a good strategy to evolve?

- When survival rate of offspring is low due to highly variable / unpredictable environment.




2. Iteroparity (repeated reproduction)

- We obviously display iteroparity along with birds, most reptiles, all mammals, and most fish.

- Individuals will have smaller numbers or offspring, but consistently over a lifetime

Under what environmental conditions would this be a good strategy to evolve?

- When environment is more dependable and competition against other individuals is critical

- A few, relatively large, well-provisioned individuals will stand a better chance of survival and reproduction




Why are there not species that combine the two and have tons of offspring regularly?

Evolutionary trade-offs, there is just not enough time, energy and nutrients available to do this.

AIM: How do scientists describe population growth?

Let’s look at population growth of bacteria starting from a single bacterium

- binary fission every 20 minutes under ideal conditions

Q: How many will there be after 36 hours?


Let’s look at population growth of bacteria starting from a single bacterium

- binary fission every 20 minutes under ideal conditions

Graph it:

Q: How many will there be after 36 hours?

A: 2108 (enough to cover the planet one foot deep)

2n Where n = # of divisions


1. Exponential Growth Model

(J-shaped curve)

What does the rate of growth depend on in this model?

The number of individuals ONLY

So why is Earth not covered in bacteria?


2. Population limiting factors- environmental factors that restrict population growth

Ex. Fur seals on St. Paul Island



Ex. Fur seals on St. Paul Island

1. Uncontrolled hunting until 1925

2. Population jumped until 1935

- Almost exponential3. Population stabilized

- Some hunting- limited breeding grounds



3. Logistic Growth Model

(S - shaped curve)

Q: How does the logistic model differ from the exponential model?




(S - shaped curve)

Q: How does the logistic model differ from the exponential model?

A: It accounts for limiting factors in the environment




(S - shaped curve)

Carrying capacity (K)- max. population the environment can support without degrading the habitat

Determined by:

The species itself and the environment (resources available, predation, etc…)

**Carrying capacity if obviously not fixed. It depends on abundance of limiting resources at a particular space and time. Figure out way to make or get more food…carrying capacity increases.


- environmental factors that restrict population growth

When is growth rate the lowest?

When is it the highest?

NO POPULATION FITS EITHER MODEL PERFECTLY

1. When population is small or large

Limited number of individuals and then limited resources

1. Population at intermediate level relative to carrying capacity.

2. Plenty of resources

2. Population limiting factors



Population limiting factors- environmental factors that restrict population growth

Human Population

We have a major problem here. What is our future? The number one problem on this planet for humans and many, many other species is our own overpopulation.What is the solution?


Population ecology

Exponential growth model

Logistic Growth Model- S-shaped

- NO limiting factors

Review

- Growth rate determined by population size and type of organism

- J-shaped

- Population limiting factors

FEW POPULATIONS FIT EITHER MODEL PERFECTLY AS YOU WILL SEE ON THE NEXT SLIDE…


FEW POPULATIONS FIT EITHER MODEL PERFECTLY:


K-selection (quality) vs r-selection (quantity)

K-selection- Population selected for fewer offspring and higher parental care.

- Typically found when environments are stable resulting in high population density and therefore intense competition – fighting other individuals, not the environment

- iteroparity

- Traits typically large body size, long life expectancy, fewer offspring, higher parental care.

r-selection

- Population selected for having many offspring with low parental care.

- Typically found when environments are unstable/unpredictable resulting in lower population densities – have many and hope something makes it – fighting the environment , not other individuals.

- semelparity

- Traits typically large body size, long life expectancy, fewer offspring, higher parental care.


Let’s look at some specific population limiting factors and examples.

NEW AIM: What are some of the factors that limit population growth?

Population limiting factors

1. Biotic factors

a. Competition among members (intraspecific)

Song Sparrow Population Experiment(small island in British Columbia)

Fig. 35.4A

- add food

- Clutch size increased

- food supply, water, space, energy (oil), mates, etc…



1. Biotic factors

a. Competition among members (intraspecific)- food supply, water, space, energy (oil), mates, etc…

Competition for space…



1. Biotic factors

a. Competition among members (intraspecific)i. food supplyii. space (territory)

A. Dispersion patterns1. clumped

2. uniform

3. random


Dispersion patterns



1. Biotic factors

b. Health- increased spread of disease with increased pop. density- accumulation of waste



1. Biotic factors

c. Predation

Fig. 35.5

“boom-and-bust” cycles

As the hare population increases, so does the Lynx population, which will cause the hare population to decrease and in turn the lynx population decreases, etc…



1. Biotic factors

d. Physiological factors

White-footed mouse experiment

1. Enclose in a small field

2. Reproduce quickly to 30 or 40

3. Reproduction declines and pop. stabilizes around 404. Add more food and water

- no change

RESULT: high population induces stress syndrome (hormonal changes)

- sexual maturity delayed, reproductive organs shrink, depressed immune system



2. Abiotic factors

a. Climate and weather

Ex. Aphids

- insects that feed on phloem sap of plants

1. Exponential growth in spring

2. Rapid die out in hot, dry summer

3. A few individuals survive and reproduce when conditions improve

What happens to mosquitoes and grasshoppers during winter?

- die out completely, leaving only fertilized eggs

Fig. 35.4B



2. Abiotic factors

b. Fire, flood, etc…



1. Biotic factorsa. Competition among members

- food supply- space

b. Healthc. Predationd. Physiological factors

2. Abiotic factors

a. Climate and weatherb. Natural disasters

Almost all populations are regulated by a mixture of these and other factors

(25-year study of the song sparrow)

Fig. 35.4C


Age Structure of HUMAN populations

Age structure of a given population can indicate if the population is growing or declining as well as current social structures…

Indicate if the above human populations are growing or declining.


Age Structure of HUMAN populations

What do these allow us to predict about social structure?Ex. Afghanistan is going to have issues with opportunities for jobs and education with the huge influx of young people. In the US, a smaller population of working people will be supporting a larger population of retired people…


Ecological footprintThe amount of land required to sustain an individual or a population/nation in terms of food/water and absorbing generated waste in hectares (ha; 1 hectare = 100m x 100m)

Calculate your footprint online…

Ecological capacityHow much land a country actually has per person (US has 6.2 ha/person, but uses 8.4 ha/person on average)


1. One species of forest bird is highly territorial, while the second lives in flocks. What is each species’ likely pattern of dispersion? Explain.

2. Each female of a particular fish species produces millions of eggs per year. What is its likely survivorship pattern? Explain.

3. Consider two rivers: One is spring fed and is constant in water volume and temperature year-round; the other drains a desert landscape and floods and dries out at unpredictable intervals. Which is more likely to support many species of iteroparous animals and why? Would you guess this to be an r-selection or K-selection species and why?

4. Where is exponential growth by a plant population more likely – on a newly formed island or in a mature, undisturbed rain forest?

5. Identify three factors that limit population size.

“Milestone QUESTIONS”


“Milestone QUESTIONS” on next slide…study, study…

NEW AIM: How do scientists describe population growth?

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