Tolerance LimitsEach environmental factor (temperature, humidity, food supply, etc.) has both minimum and maximum levels beyond which a species cannot survive or is unable to reproduce: lower & upper limits of tolerance. Tolerance range is the range between these 2 limits. An environmental gradient is a stepwise increase or decrease in an environmental factor – for example, pH, rainfall, temperature, and so on. Species can tolerate different environmental gradients. For instance, humans have a relatively narrow range of temperatures at which they can survive without clothing and shelter. Similarly, humans can live on relatively few types of unprepared food. Becoming very efficient at living in a narrow environmental range is a type of specialization. Generally, the more highly organisms are adapted to a narrow environmental range, the less adapted they are to changes in the environment. Why would this be true? 3. Abiotic and biotic interactions Some physical factors affect organisms directly: tº, wind, pH, and salinity; Others act as resources: nutrients, light and water. Abiotic and biotic factors in ecosystems also interact with each other: climate and soil conditions determine which plants will live in a certain area. Plant lite in turn affects the distribution of animals. Climate + Soil → Plants → Animals Living organisms also affect abiotic factors: plants replenish oxygen in the atmosphere by the process of photosynthesis. H2O + CO2 + Sun light energy → Glucose + O2 Anthropogenic factor - the influence of humans on abiotic variables may be substantial. Increasing CO: output from the burning of fuels and increasing deforestation may cause global CO2 levels to increase so much that the world’s climate might change slightly (so-called global warming). Other anthropogenic changes include increased SO2 output from fossil fuel burning (coal, oil, natural gas), leading to acid rain, acid lakes, and acid streams.
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