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A population consists of all the individuals of a species that live together in one place at one time. Every population tends to grow because individuals tend to have multiple offspring over their lifetime. But eventually, limited resources in an environment limit the growth of a population. Populations change because of birthrate, death rate, and movement from place to place. Limiting factor: anything that restricts the number of individuals in a population Carrying capacity: the largest number of individuals of one species that an ecosystem can support over time The place where a particular population of a species lives is its habitat. • The physical aspects of a habitat are called abiotic factors. • The organisms in a habitat are called biotic factors. The many different species that live together in a habitat are called a community. When two species use the same resource, they participate in a biological interaction called competition. Resources for which species compete include food, nesting sites, living space, light, mineral nutrients, and water. Competition occurs for resources in short supply. Niche: an organism’s role or “job” in its environment • What it eats • What it does • Where it lives Adaptation: a characteristic or behavior that improves the survival of an organism; often part of the niche Ecology is the study of the interactions of living organisms with one another and with their physical environment (soil, water, climate, and so on). An ecosystem, or ecological system, consists of a community and all the physical aspects of its habitat, such as the soil, water, and weather. Most ecosystems contain a few large animals and some smaller animals. Ecosystems tend to contain more plants than animal life. The most plentiful organisms in an ecosystem are usually microscopic bacteria and protists. Predation is the act of one organism killing another for food. Plants protect themselves from herbivores is with thorns, spines, and prickles. In symbiosis two or more species live together in a close, long-term association. Parasitism is one type of symbiotic relationship that is harmful to the host organism. Mutualism is a symbiotic relationship in which both participating species benefit. A third form of symbiosis is commensalism, a symbiotic relationship in which one species benefits and the other is neither harmed nor helped. The number of species living within an ecosystem is a measure of its biodiversity. Many studies of natural ecosystems have demonstrated that predation reduces the effects of competition. Because predation can reduce competition, it can also promote biodiversity, the variety of living organisms present in a community. The more biodiversity a community has, the more stable the community is. Organisms that first capture solar energy or chemical energy in hydrothermal vents, the producers, include plants, some kinds of bacteria, and algae. Consumers are those organisms that consume plants or other organisms to obtain the energy necessary to build their molecules. Bacteria and fungi are known as decomposers because they cause decay. Decomposition of bodies and wastes releases nutrients back into the environment to be recycled by other organisms. In most ecosystems, energy does not follow simple straight paths because animals often feed at several levels. This creates an interconnected group of food chains called a food web. During every transfer of energy within an ecosystem, energy is lost as heat. Also, energy is used at each level by the organisms for growth, movement (animals), reproduction. This energy is not transferred to the next level. An energy pyramid is a diagram in which each level is represented by a block, and the blocks are stacked on top of one another, with the producers on the bottom. Because the energy stored by the organisms at each is about one-tenth the energy stored by the organisms in the level below, the diagram takes the shape of a pyramid. Most terrestrial ecosystems involve only three or, on rare instances, four levels. Too much energy is lost at each level to allow more levels.