A condition is an abiotic environmental factor that influences the functioning of living organisms. For most, we can recognize an optimum level at which an organism performs best. Ultimately, we should define 'performs best' from an evolutionary point of view, but in practice we mostly measure the effect of conditions on some key property like the activity of an enzyme or the rate of reproduction.

The ecological niche is not a place but a summary of an organism's tolerances of conditions and requirements for resources. The can the biota keep up with the pace?

modern concept - Hutchinson's «-dimensional hypervolume - also distinguishes fundamental and realized niches.

Temperature is discussed in detail as a typical, and perhaps the most important, condition. Individuals respond to temperature with impaired function and ultimately death at upper and lower extremes, with a functional range between the extremes, within which there is an optimum, although these responses may be subject to evolutionary adaptation and to more immediate acclimatization.

The rates of biological enzymatic processes often increase exponentially with temperature (often Q10 ~ 2), but for rates of growth and development there are often only slight deviations from linearity: the basis for the day-degree concept. Because development usually increases more rapidly with temperature than does growth, final size tends to decrease with rearing temperature. Attempts to uncover universal rules of temperature dependence remain a matter of controversy.

We explain the differences between endotherms and ecto-therms but also the similarities between them, ultimately, in their responses to a range of temperatures.

We examine variations in temperature on and within the surface of the earth with a variety of causes: latitudinal, altitudinal, continental, seasonal, diurnal and microclimatic effects, and, in soil and water, the effects of depth. Increasingly, the importance of medium-term temporal patterns have become apparent. Notable amongst these are the El Nino-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO).

There are very many examples of plant and animal distributions that are strikingly correlated with some aspect of environmental temperature but these do not prove that temperature directly causes the limits to a species' distribution. The temperatures measured are only rarely those that the organisms experience. For many species, distributions are accounted for not so much by average temperatures as by occasional extremes; and the effects of temperature may be determined largely by the responses of other community members or by interactions with other conditions.

A range of other environmental conditions are also discussed: the pH of soil and water, salinity, conditions at the boundary between sea and land, and the physical forces of winds, waves and currents. Hazards, disasters and catastrophes are distinguished.

A number of environmental conditions are becoming increasingly important due to the accumulation of toxic by-products of human activities. A striking example is the creation of 'acid rain'. Another is the effect of industrial gases on the greenhouse effect and consequent effects on global warming. A projected rise of 3-4°C in the next 100 years seems a reasonable value from which to make projections of ecological effects, though global warming is not evenly distributed over the surface of the earth. This rate is 50-100 times faster than postglacial warming. We must expect latitudinal and altitudinal changes to species' distributions and widespread extinctions of floras and faunas.

Chapter 3

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