The development and productivity of organisms, populations, communities, and ecosystems are naturally constrained by environmental factors. These constraints can be viewed as being environmental stressors (or stressors). For example, an individual plant may be stressed by inadequate nutrition, perhaps because of infertile soil or competition with nearby plants for scarce resources. Less-than-optimal access to nutrients, water, or sunlight results in physiological stress, which causes the plant to be less productive than it is genetically capable of being. One result of this stress–response relationship is that the plant may develop relatively few seeds during its lifetime. Because reproductive (and evolutionary) success is related to the number of progeny an organism produces to carry on its genetic lineage, the realized success of this individual plant is less than its potential.

Similarly, the development and productivity of an animal (including any human) are constrained by the environmental conditions under which it lives. For instance, an individual may have to deal with stresses caused by food shortage or by difficult interactions with other animals through predation, parasitism, or competition for scarce resources.

The most benign (or least stressful) natural environments are characterized by conditions in which factors such as moisture, nutrients, and temperature are not unduly constraining, while disturbances associated with disease, wildfire, windstorm, or other cataclysms are rare. These kinds of relatively benevolent conditions allow the most complex and biodiverse ecosystems to develop, namely old-growth rain forest and coral reefs. Other environments, however, are characterized by conditions that are more stressful, which therefore limits their development to less complex ecosystems, such as prairie, tundra, or desert.

All ecosystems are dynamic, in the sense that they change profoundly, and quite naturally, over time. Many ecosystems are especially dynamic, in that they regularly experience large changes in their species, amounts of biomass, and rates of productivity and nutrient cycling. For example, ecosystems that occur in seasonal climates usually have a discrete growing season, which is followed by a dormant period when little or no growth occurs. To varying degrees, all of the natural ecosystems of Canada are seasonally dynamic: a warm growing season is followed by a cold dormant period when no plant productivity or growth occurs. Animals may survive the hard times of winter by migrating, hibernating, or feeding on plant biomass remaining from the previous growing season.

Ecosystems that have recently been affected by a disturbance (an episode of destruction) are particularly dynamic because they are undergoing a process of ecological recovery known as succession. Succession occurs in response to changes associated with natural disturbances such as a wildfire, windstorm, or insect or disease epidemic. These cataclysmic stressors kill many of the dominant organisms in an ecosystem, creating opportunities for relatively short-lived species, which may dominate the earlier years of the post-disturbance recovery. Succession also occurs after anthropogenic disturbances, such as a deliberately lit wildfire or a clear-cut of mature timber.

Even highly stable ecosystems such as tropical rainforest and communities of deep regions of the oceans change inexorably over time. Although catastrophic disturbances may affect those stable ecosystems, they are rare under natural conditions. Nevertheless, as with all ecosystems, these stable types are influenced by pervasive changes in climate and by other long-term dynamics, such as evolution.

In fact, natural environmental and ecological changes have caused the extinction of almost all of the species that have ever lived on Earth since life began about 3.5 billion years ago. Many of the extinctions occurred because particular species could not cope with the stresses of changes in climate or in biological interactions such as competition, disease, or predation. However, many of the extinctions appear to have occurred synchronously (at about the same time) and were presumably caused by an unpredictable catastrophe, such as a meteorite colliding with the Earth.

Environmental stressors and disturbances have always been an important, natural context for life on Earth. So, too, have been the resulting ecological responses, including changes in species and the dynamics of their communities and ecosystems.