POPULATIONS AND THEIR SIZE Definition of a population: All ... · Population Change Population...

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Ecologists

work at

scales

Ecologists

work at

scales

Populations

• Groups of organisms of

the same species that live

within a given area

ristics

We can characterize individual

populations in terms of …

Variation

Patterns of Dispersion

Demographics

Size and Density

Limits on population growth

Human population growth

• Members of a population show similarities

because they belong to the same species.

Members of a population also show variation.

Most variation falls in a “normal” distribution (bell-shaped frequency).

ristics

Variation

Demographics

Size and Density

There are three common patterns of

population distribution:

Uniform

ristics

Variation

Demographics

Size and Density

Demographics

• Characteristics of a population

• Examples:

– Average age

– Average size

– Average number of offspring

• http://quickfacts.census.gov/qfd/states/53000.html

ristics

Variation

Demographics

Size and Density

Four Factors Influence the

Size of a Population:

• Natality: Birth Rate (offspring

produced and added to population)

Mortality: Death Rate

(individuals that die)

• Immigration: Movement of members of the

species into the area

• Emigration: Movement of members of the

species out of area to live elsewhere.

Population Change

Population Change =

(natality + immigration) –

(mortality + emigration)

Exponential Curve

Unlimited Growth

(J-Shaped)

Populations tend to

grow in size for the

simple reason that

individuals tend to

have more than one

offspring.

Can this happen in nature?

• Yes: Bacteria

–If there are no

factors limiting

the growth, their

number will

increase rapidly.

Exponential growth of bacteria

Can this happen in nature?

• Yes: Invasive species

– Uniform habitat

– No predators

– No disease

– Unlimited area

– Abundant food

• Pythons

Logistic Curve

Limited Growth (S-Shaped)

3 Phases:

1. Exponential growth Phase

2. Transitional Phase

3. Plateau Phase

Exponential Growth Phase

• Population increases

exponentially.

• Resources are abundant.

• Predators and disease are

Transitional Phase

• As a result of intra-specific

competition

– for food, shelter, nesting

space, etc.,

– and the build up of waste.

• The growth rate slows

– Birth rates decline and

death rate increases

Plateau Phase

• Natality and mortality are equal so population size is

constant.

• When the number of individuals in the population

have reached the maximum which can be supported

by the environment.

The number is called the

CARRYING CAPACITY

Growth of a population of fur seals

Population size oscillates around

the carrying capacity (K)

overshoot

oscillations

ristics

Variation

Demographics

Size and Density

Limits on Population Growth

• Density Dependent Limits

– Food

– Water

– Shelter

– Disease

• Density Independent Limits

– Natural Disasters

– Humans (logging, mining, farming)

Water and shelter are

critical limiting factors in

the desert.

Fire is an example of a

Density independent

Limiting factor.

ristics

Variation

Demographics

Size and Density

World Population Events

Time unit Births Deaths Growth

-------------------------------------------------

Year 131,571,719 55,001,289 76,570,430

Month 10,964,310 4,583,441 6,380,869

Day 360,470 150,688 209,782

Hour 15,020 6,279 8,741

Minute 250 105 146

Second 4.2 1.7 2.4

-------------------------------------------------

What’s Behind Population Growth

• Three Factors

– Fertility

– Infant Mortality

– Longevity

• Animal Domestication

and Agriculture

– Provided for a few to

feed many

• Industrial Revolution

– Growth of Cities and

Infrastructure

• Water

• Energy

• Transportation

– Increased Productivity

– Nutrition

– Sanitation

– Medicine

Age structures of three nations

Are these growing, shrinking or stable populations?

Shrinking Growing Stable

Exponential growth of the human population

does not currently show

density effects that typically

characterize natural populations.

Limited resources eventually will

cause human population

growth to slow,

but global human

carrying capacity is not known.

Population Predictions

• Most

predictions:

9-12B by 2050

10-15B by 2100

• Large

uncertainties

Resource Limits

• Land

– Deforesting to acquire more arable land

– Would run out in next century at current yields

• Water

– In 1950 people used half of accessible water

– Are now dependent on dams

– Pollution loses 33% of potential water

– Getting close to limits

• Energy

– growth very high last fifty years

– Mostly hydrocarbon fuels

– Nonrenewable resource consumption

– Climate change issues

Question:

Why are humans

destroying the earth?

destructamundo

Question:

Why do locusts destroy crops?

nymph aggregating nymphs adults feeding

swarming destrutamundo

Limits: Locust Freedom Without Responsibility

= Destroyed Crops (destruction of environment)

It’s a

free country!

I’ve got my rights!

Who’s going to stop me?

Question:

Why do some microbes make us sick?

©Phage et al.

Limits: Pathogen Freedom Without Responsibility

= Disease! (destruction of the body environment)

It’s a free country Who’s going

to stop me?

Bacterial

pathogens

Question:

Why are humans

destroying the earth?

destructamundo

Human Freedom Without Responsibility

= Destructamundo! (destruction of environment)

It’s a free country

Who’s going to stop me?

urban sprawl

global warming

deforestation

desertification overpopulation

air polution

water polution

loss of

habitat

overconsumption

conspicuous consumption

loss of farmland

overfishing

greenhouse effect

ozone hole

mass extinction

loss of wetlands

NIMBY = “not in my

backyard”

lack of cooperation

out-of-control

materialism

lack of cooperation special interests

destruction TEOTWAWKI = “the end of the world as we know it”

radical anti-environmentalism

might makes right

short-term thinking

fish kills

toxic algal

blooms

erosion

loss of topsoil

bigger is better monoculture

pesticides

the bottom line

Humans need to be better ecologists.

POPULATIONS AND THEIR SIZE Definition of a population: All ... · Population Change Population...

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