Changes in Populations

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Changes in Populations

• Science Foundations

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Ecological Succession

• Populations of species found in certain habitats
• change over time. This concept, called
• ecological succession or environmental
• variation.

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Primary Succession

• Primary succession consists of a sequence of
• communities developing in a newly exposed
• habitat devoid of life.
• Examples
• Succession on a bare rock
• Newly deposited sand
• New land from cooled lava

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Secondary Succession

• Secondary succession consists of
• communities taking place on sites that have
• already supported life.
• Examples
• Old field succession
• Clear cut forests
• Burned areas
• Difference between primary and secondary
  succession is that secondary starts with soil.

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Pioneer Community

• The original plant forms that colonize an area
• are called the pioneer community. These must
• be a strong group, able to withstand the forces
• of nature.
• Bare rock would be an unwelcome habitat for
  most plants, but lichens are up to the challenge.
  Blown there by the wind, lichens attach to the
  rock and begin the process of breaking it down as
  it carries on its life functions. As soil
  accumulates, a bare field forms. This eventually
  becomes the home of grasses and flowers.

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The Progression
1. Rock
2. Lichen
3. Soil
4. Grasses
8. Hardwood Forest
5. Flowers
6. Shrubs
7. Pine Forest
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Climax Community

• A hardwood forest of trees like oak, beech, and
• maple trees, grows in the final stage of
• development. This final stage is called the
• climax community. The climax community
• does not change much, as long as no disasters
• strike, like wild fires or tornadoes!

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Animals

• The animals that live in the habitat change
• as the plants change.
• Small animals like mice and rabbits, prefer
• the open field.

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Ponds

• Pioneer communities may sometimes be ponds
• that develop underwater plants, then cattails. As
  
• the pond fills with sediment, grasses may begin
• to grow and over many years, the succession
• eventually results in a deciduous forest for that
  
• area.

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Survival of the Fittest

• Survival of the fittest is a situation where
• organisms compete for the available resources
• in their environments food, shelter, or mates.
• Those that are more fit will be able to win this
• competition. An animal that outperforms others,
• such as running faster, locating food better, or
• surviving more fights, will be more likely to
• reach maturity and eventually produce
• offspring. These traits will then be passed on to
  
• succeeding generations.

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Geographic Isolation

• Changes in population can also result from
• geographic isolation. Geographic isolation
• occurs when a population become separated by
• changes in the environment.
• For a species to exist in a changing
• environment, members must adapt in order to
• survive and reproduce. If this does not occur,
• the species will become extinct.

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Responses and Behavior

• One very important life function is that of
• response. Response characteristics of cells
• enable organisms to react to stimuli in their
• environment.
• A group of complex responses is called a
• behavior.
• A reflex is an involuntary response to
• environmental stimuli.
• An instinct is a genetically controlled behavior.

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Examples

• Reflexes
• A persons eye blinks as an object flies past it.
  
• A dog sneezes when it inhales pepper.
• Instincts
• Birds respond to changes in season by migrating.
• A bees developing into drones.

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Tropism

• Tropisms are examples of responses to
• environmental change.
• A tropism is an involuntary response of an
• organism involving orientation toward or away
• from external stimuli.

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Geotropism

• Plants respond to gravity by sending roots
• down. It is called geotropism.

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Phototropism

• Phototropism adjusts plants exposure to light.

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Thigmotropism

• Thigmotropism is a plants response to touch.

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Adaptation

• Organisms which do not, or cannot respond to
• changes in the environment reduce their
• chances for surviving. This may mean death for
• the individual organism and, in extreme cases,
• extinction of the species if too many die before
• they are able to reproduce.
• Response to the environment is, therefore,
• closely related to adaptation.

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Adaptations

• Adaptations allow one kind of organism to live
• in conditions where other organisms cannot.
• Examples
• A cactus has special adaptations which allow it
  to live in a dry, hot environment.
• Polar bears have adaptations which allow them to
  live in very cold environments.
• 1. The polar bear could not survive in the
  desert and the cactus could not live in the
  Arctic.

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Homeostasis

• All organisms must have life functions which
• work together to produce and maintain a stable
• internal environment. This is the principle of
• homeostasis.
• Adaptations are made to produce homeostasis.

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Adaptation Example

• Certain rabbits turn from brown to white in the
• winter so they can be camouflaged with the
• snow. Since rabbits with dark coats were more
• easily seen, they were more likely eaten.
• Gradually, the proportion of winter-white rabbits
  
• to winter-dark rabbits would increase until there
  
• were no dark rabbits left.

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Human Example

• In areas where there is a higher incidence of
• malaria, there is also a higher incidence of
• sickle cell anemia. This is because the sickle
• shaped cells cannot be infected with malaria.
• Over time, the people in the area with sickle
• shaped red blood cells survived because they
• were not catching malaria and people with
• normally shaped red blood cells were dying.