Within its habitat an organism is influenced by various environmental factors to which, as a rule, it is quite well adapted, even under significant fluctuations. This adaptation of the organism to the abiotic conditions in its range, that is, the chemical and physical conditions, has taken place over thousands or even millions of years, over evolutionary time scales. Stress is produced when these environmental variables lie, either temporarily or continuously, outside the range to which the biological system (a cell or species) is adapted. These kinds of stress situations can be generated in various ways:

• when temporary fluctuations in the abiotic conditions occur (for example, seasonal or weather anomalies);
• when pelagic organisms (i.e. free-swimming in the water column), such as planktonic larvae, drift away from the centre of their habitat and colonize the margins of the distribution area where the environmental conditions are not ideal;
• when climate zones shift more rapidly than species can evolutionarily adapt to the new conditions.

Organisms are not always defenceless or at the mercy of change. They are quite capable of adapting to new conditions and responding to stress. Adaptation is possible in three ways. The quickest of these, phenotypic plasticity, can be observed within days or weeks: individuals adjust to new conditions in their habitat by changing their growth form, metabolism, or diet. Of course this is only possible to a limited extent. Relatively rapid adaptations over a few generations are also possible through selective processes. When genotypes exist within a population, i.e. individuals have certain traits that are not immediately apparent but are contained in their genetic makeup, and these individuals are better able to cope with the new conditions than others of their species, then they will assert themselves fairly quickly. The productivity and survival of the population is thereby assured. Evolutionary processes in the classical sense, such as the random occurrence of a mutation that allows survival in the changing environment, are generally too slow in most species with long-lived generations to keep up with the habitat changes caused by climate change.

5.1 > Different environmental variables have different effects on organisms. Many plants or animals thrive best at moderate temperatures – the intermediate optimum (left). If it becomes warmer or colder the situation deteriorates. The productivity of the organism declines. The “extreme optimum” follows a different pattern: in a clean environment the organism functions optimally. If toxins spread in the habitat, productivity declines. In both cases the animal or plant experiences stress that can lead to death. If the stress persists for an extended period, then species in the affected area can even become completely extinct. © maribus (after M. Wahl)