Populations: Extinctions and Explosions

Chapter 28

What happens when people build on natural habitats?

• Habitats shrink in size

• Small habitats support fewer individuals

• Fewer individuals means more inbreeding

• Inbreeding decreases genetic variation and increases genetic defects

• Population spirals downward toward extinction

The Extinction Vortex

Factors that cause extinctions

• Deterministic events – predictable events such as habitat destruction and hunting

• Stochastic events – unpredictable and infrequent events such as unusual storms

Deterministic Factors that lead to extinction

• Destroying ALL the local populations

• Over hunting

• Habitat destruction

• Habitat fragmentation

• Edge effects

Stochastic factors

• A freak storm could kill most of the healthy females in a small population.

• The next year most or all of the offspring could be males.

• Alleles could be lost through genetic drift.

• These effects would not be likely in a larger population

How can we preserve species?

• Some studies have predicted that about half of all the world’s species will go extinct in the next decade or two unless something is done to prevent it.

• We must set aside a large percentage of our land.

• The preserves need to be large enough to minimize edge effects

• Corridors that allow individuals to travel from one population to another

How do populations grow?

• Which would you rather have– $10 a day for two weeks– Or starting with one penny, twice the amount

you got the day before for two weeks?

• This is an example of exponential growth – the bigger the population, the faster it grows

• The growth rate of a population is the difference between the average birth rate and the average death rate = r

• Rmax – the intrinsic rate of natural increase

• Exponential growth is always temporary because the environment cannot support that many individuals.

• The number of individuals that the environment can support indefinitely is called the carrying capacity or K

• When K is exceeded, more individuals die off.

• When the birth rate is equal to the death rate there is zero population growth.Immigration and emigration can also affect population growth.

• K is determined by some limiting factor.

Density-dependent factors

• Competition

• Predation

• Parasitism

• Disease

• All limit growth in proportion to the density of the population

Density- independent factors

• Hurricanes

• Tornadoes

• Drought

• Fires

• Volcanic eruptions

The goal of every species is to have as many descendents as possible survive

to adulthood.

1. r-selected species have as many offspring as possible

1. Often pioneer species

2. K-selected species reproduce in smaller numbers, and spend more time and energy raising their young.

Age structure of a population

• Cohort – individuals that enter the population at the same time

• Depends on surviorship and birth rate

• Growing populations have a graph like a pyramid

• Stable populations have a rectangular shape

Do we need to worry about the growth of the human population?

• For many years people have been concerned about increasing population

• Growth has slowed because people and countries have done something about it.

Questions remain:

• Will epidemics like AIDS or wars reduce the world’s population growth?

• What is the carrying capacity of the world for humans?

• For humans and other creatures?

How has the human population sidestepped limiting factors?

• People spread into more habitats because they could build shelters, use fire, and communicate knowledge.

• When we were hunter-gatherers the population was probably stable.

• Farming increased the availability of food, allowing the human population to grow

• Closed sewers, antibiotics, chlorinated water, refrigeration and vaccines caused the death rate to drop dramatically in the 19th and 20th centuries.

• We subsidize our cities and agriculture – bringing in additional energy from other places.

• Ecological footprint – the area of land needed to produce all of the resources a single person uses.

• Average American – 5.1 hectares • (hectare = 100 acres)

• Person from India – 0.4 hectares

• U.S. = 290 million people X 5.1 hectares =

1.48 billion hectares

Area of U.S. = 0.91 billion hectares

India = 1.05 billion people X 0.4 =

0.42 billion hectares

How can we feed the world?• We supply developing countries with high yield

crops– These crops need chemical fertilizers and irrigation– Decreased genetic diversity– Monoculture – growing only one crop on a farm or

in an area• Easier to pick• Natural disasters can wipe out entire economies• More inviting to pests and parasites

• Crop yields have leveled off, but population is still growing.

Grain production has leveled off, but the human population continues to grow.

We are using up limited resources and polluting renewable resources.

• Power plants produce thermal pollution

• Increasing erosion by logging and agriculture

• We dump raw sewage and wastes into water

• Poisons, heavy metals, radioactive substances, pesticides and chemicals are hard to get rid of.

• Biomagnification – toxins increase in animals higher up the food chain

• Burning hydrocarbons increases carbon dioxide in the air – green house gas

• Sulfur and nitrogen in fuels cause acid rain and smog

• Chlorofluorocarbons – spray cans, air conditioners, refrigerators and cleaners – destroy the ozone layer– Increase cancer– Extinction of species– Birth defects

What can we do?

• Reduce

• Reuse

• Recycle

• Use environmentally friendly products

• Vote!!