Scope, Sequence, and Coordination
A Framework for High School Science Education
Based on the National Science Education Standards
Limiting Factors for Populations
Organisms, Ecosystems, and Population Growth: Interrelationsips and Interdependencies
Living organisms have the capacity to produce populations of infinite size, but environments and resources are finite. This fundamental tension has profound effects on the interactions between organisms.
Human beings live within the world’s ecosystems. Increasingly, humans modify ecosystems as a result of population growth, technology, and consumption. Human destruction of habitats through direct harvesting, pollution, atmospheric changes, and other factors is threatening current global stability, and if not addressed, ecosystems will be irreversibly damaged.
The organization of ecosystems is based upon populations interacting with each other and with abiotic factors of the environment. The interaction of populations sets up a community. Populations may interact in positive or negative ways. An example of a positive interaction is seen in the pollinating activities of flowering plants. In this symbiosis, the flower is fertilized while the pollinator collects its food.
Predator-prey relationships show a positive as well as negative association. Competition for resources can also cause negative interaction. In this case the population most affected by the competition is eliminated from a niche. Because of these interactions, numerous adaptations have evolved that prevent elimination of populations from a selected ecosystem. Species have adapted to be able to coexist with each other by sharing resources, reducing competition, and entering into positive symbioses.
In any community, populations will tend to replace each other in an orderly process. This is due to the fact that habitat populations change. Use of nutrients and other "abiotic" factors by resident populations causes habitats to change, resulting in a replacement process, or succession. This process of community change results in a series of transitory communities until a final or mature community is established. Given sufficient time and stability of biotic and abiotic factors, a climax will be reached. The climax community can last for hundreds of years uninterrupted.
All populations have an inherent tendency to increase in size. This potential increase is extremely high for most species. This type of exponential growth begins slowly and then continues on a rapid incline as more reproductive individuals are produced each generation. Control of population growth is based upon limiting factors and population interactions in each ecosystem. Resources such as food, water, oxygen, and space availability, as well as predation, competition, and parasitism, place environmental limits on population growth. These limits set the carrying capacity of the ecosystem. Population size will oscillate around this carrying capacity. When a population exceeds carrying capacity, a strain upon resources could result in a sharp decline in the population.
Human civilization has brought about dramatic changes in the ecosystems of the world. These changes have resulted in major environmental problems, which in turn directly affect the survival of all species on Earth. Because of agricultural practices, technological advances, and medical triumphs, world population growth has reached a size well over 5 billion. This "population explosion," coupled with a lack of understanding of ecological principles, has resulted in massive pollution of land and water, destruction of habitats and loss of bio-diversity, possible climate changes that could result in global warming, and penetration of the protective ozone layer shielding all life from harmful UV rays. Policy decisions facing planet Earth should focus on population control, recycling of human waste, and development of alternative energy sources, and should develop a better understanding of the human impact on ecosystem balance.
Population, community, niche, habitat, competition, ecosystem, limiting factors, exponential growth mortality
Succession, serial stages, ecotone, pioneer species, climax communities, associations, community
Symbiosis, predator-prey, mutualism, amensalism, commensalism, parasitism, succession, biotic potential, agriculture, technology, human population growth form, biodiversity, layers of the atmosphere, waste disposal, fossil fuels, recycling, pesticides, pollutants, endangered species, abiotic factors
Density-dependent and density-independent factors, carrying capacity, acid rain, ozone depletion, succession, climax concept
Interdependence, population levels of species fluctuate with environmental conditions
Competitive exclusion principle, equilibrium hypothesis of island biogeography, climax communities, facilitation hypothesis, inhibition hypothesis, J-shaped and S-shaped growth form, age distribution pyramids, r and K strategies, global warming, succession