Biology  5/e   Raven/Johnson
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Overpopulation: The environment limits the size of all populations on this planet. Each habitat or ecosystem has a specific carrying capacity for each of the populations living within it. Humans, however, have been able to minimize environmental controls and expand greatly the carrying capacities of their local environments because of technological achievements in areas such as agriculture, medicine, housing, transportation, and industrial development. As a result, the human population has grown explosively and is continuing to grow at an alarming rate. There is a limit to how much carrying capacities can be increased, and no population can survive successfully if it exceeds the carrying capacity.

Population growth characteristics: Every population has the innate capacity for exponential growth. In nature, however, this rapid growth does not usually continue for long periods because of limiting factors imposed by the environment. These factors may be density-independent (such as a blizzard or fire) and cause the populations to crash. Or the factors may be density-dependent (such as competition and limited resources) and cause the growth rate to slow down as the population gets larger and approaches the carrying capacity of the environment. This results in a typical sigmoid growth curve. The actual growth rate of a population is determined by a complex interaction of factors such as the population's size, age distribution, reproductive strategies, emigration and immigration rates, and interactions with the environment and other species. The human population has been able to overcome many environmental limitations and has increased at a great rate.

Cycling of minerals and flow of energy: Ecosystems regulate the flow of energy through themselves and the cycling of mineral nutrients within themselves. Energy flows from the sun to the photosynthesizers and on to the primary and secondary consumers and the decomposers. The transfer of energy is not particularly efficient. Plants capture only about 1% of the solar energy available to them, and each trophic level captures only about 10% of the energy available from the preceding level. Minerals and other nutrients are continually cycled from abiotic reservoirs (such as the air, oceans, rocks, and soil) to the living organisms of the ecosystem, back to the reservoirs, and so on ad infinitum.

Characteristics of biomes: Each biome is a particular assemblage of organisms that occurs over a large geographical region of the earth. The characteristics of a biome are distinct from those of other biomes. These characteristics are both abiotic (e.g., climate – temperatures, precipitation, solar energy) and biotic (e.g., species diversity, types of species present). The eight major biomes on the earth are tropical rainforests, savannas, deserts, temperate grasslands, temperate deciduous forests, temperate evergreen forests, taiga, and tundra.

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