Changes in Populations Science Foundations

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

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

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.

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.

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

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!

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

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.

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.

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.

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.

Examples Reflexes: A person’s 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.

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.

Geotropism Plants respond to gravity by sending roots “down.” It is called geotropism.

Phototropism Phototropism adjusts plants exposure to light.

Thigmotropism Thigmotropism is a plant’s response to touch.

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.

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.

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.

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.

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.