As Harry Callahan said in the movie Magnum Force, "A man's got to know his limitations." Organisms all around the world may not know, but they can often sense, their tolerance -- the limits on their ability to withstand changes in an environment or ecosystem. An organism's ability to tolerate changes can affect both its ability to survive and reproduce in an existing ecosystem and its ability to move to new ecosystems.
All organisms have adaptations that enable them to survive under certain conditions in their environments or ecosystems. Some of the abiotic, or nonliving, factors that affect organisms' ability to survive include temperature, light availability, soil type, water, salinity levels of soil or water, oxygen, acidity/alkalinity (pH levels) of soil or water, inorganic nutrient levels, other chemicals, radiation, seasonal temperature and weather changes, wind, air or water pressure, ocean waves, topographical features and altitude. In the ocean, hydrostatic pressure becomes a factor in limiting what kinds of creatures can survive at great depths. In mountainous areas, oxygen availability in the atmosphere can be reduced, which is reflected in the physiology of organisms that live there.
Biotic, or living, factors can also affect an organism's ability to survive in an ecosystem. They include food availability, competition with other organisms, plant cover availability, predation, disease, parasitism, crowding, habitat fragmentation and the presence of human populations. A lack of trees can affect populations of birds or other arboreal organisms, which may rely on tree cover for nesting and hiding from predators. Some biotic factors affect abiotic factors as well, such as plants that grow taller than competing plants and block sunlight, or a lack of decomposing organic matter or nitrogen-fixing bacteria that leads to low nitrogen levels in the soil.
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The range of variation under which a species can function and reproduce is called its tolerance range. Some organisms possess a wide range of tolerance for some environmental conditions, but most survive best within a narrower range called their optimum range. As conditions in an environment move further from the optimum range for a species, populations of that species tend to dwindle. Species that tolerate a narrow range of conditions for a specific environmental factor may be given a name with the prefix "steno-," such as stenohalines, which can tolerate only a narrow range of salinity. Those organisms that tolerate a wide range of conditions have the prefix "eury-," such as eurytopics, which can thrive in a wide range of environments. Fish in estuaries, where salt levels in the water can vary, are euryhalines. Introduced species that out-compete native species might possess the advantage of a wider range of tolerance than the native species. When human activities -- including habitat destruction, the burning of fossil fuels and pollution -- alter an environment, it may test some species' tolerance limits beyond their ability to survive; death or even extinction of species may result.
Some organisms, called extremophiles, have adapted to survive in environments that the vast majority of other organisms on Earth could not tolerate. Acidophiles live at very low pH levels, endoliths inside rocks or in the pores between mineral grains, halophiles in extremely high salinity, anaerobes in the presence of no oxygen, psychrophiles in temperatures at 15 degrees Celsius or below, barophiles at extremely high hydrostatic pressure and xerophiles in places with almost no water. Oddly enough, extremophiles may have a narrow range of tolerance. For example, obligate anaerobes cannot grow in an environment when oxygen is present, and some will even die.