Ecosystem productivity

In an ecosystem, the connections between species are generally related to their role in the food chain. There are three categories of organisms:
 
The leaf is the primary site of photosynthesis in plants.
Producers or Autotrophs -- Usually plants or cyanobacteria that are capable of photosynthesis but could be other organisms such as the bacteria near ocean vents that are capable of chemosynthesis.
Consumers or Heterotrophs -- Animals, which can be primary consumers (herbivorous), or secondary or tertiary consumers (carnivorous and omnivores).
Decomposers or Detritivores -- Bacteria, fungi, and insects which degrade organic matter of all types and restore nutrients to the environment. The producers will then consume the nutrients, completing the cycle.

These relations form sequences, in which each individual consumes the preceding one and is consumed by the one following, in what are called food chains or food networks. In a food network, there will be fewer organisms at each level as one follows the links of the network up the chain, forming a pyramid.

These concepts lead to the idea of biomass (the total living matter in an ecosystem), primary productivity (the increase in organic compounds), and secondary productivity (the living matter produced by consumers and the decomposers in a given time).
 
An ecological pyramid

Tertiary consumers

Secondary consumers

Primary consumers

Primary producers

These last two ideas are key, since they make it possible to evaluate the carrying capacity -- the number of organisms that can be supported by a given ecosystem. In any food network, the energy contained in the level of the producers is not completely transferred to the consumers. The higher up the chain, the more energy and resources are lost. Thus, from a purely energy and nutrient point of view, it is more efficient for humans to be primary consumers (to subsist from vegetables, grains, legumes, fruit, etc.) than to be secondary consumers (consuming herbivores, omnivores, or their products) and still more so than as a tertiary consumer (consuming carnivores, omnivores, or their products). An ecosystem is unstable when the carrying capacity is overrun.

The total productivity of ecosystems is sometimes estimated by comparing three types of land-based ecosystems and the total of aquatic ecosystems. Slightly over half of primary production is estimated to occur on land, and the rest in the ocean.
The forests (1/3 of the Earth's land area) contain dense biomasses and are very productive.
Savannas, meadows, and marshes (1/3 of the Earth's land area) contain less dense biomasses, but are productive. These ecosystems represent the major part of what humans depend on for food.
Extreme ecosystems in the areas with more extreme climates -- deserts and semi-deserts, tundra, alpine meadows, and steppes -- (1/3 of the Earth's land area) have very sparse biomasses and low productivity
Finally, the marine and fresh water ecosystems (3/4 of Earth's surface) contain very sparse biomasses (apart from the coastal zones).

Ecosystems differ in biomass (grams carbon per square meter) and productivity (grams carbon per square meter per day), and direct comparisons of biomass and productivity may not be valid. An ecosystem such as that found in taiga may be high in biomass, but slow growing and thus low in productivity. Ecosystems are often compared on the basis of their turnover (production ratio) or turnover time which is the reciprocal of turnover.

Humanity's actions over the last few centuries have seriously reduced the amount of the Earth covered by forests (deforestation), and have increased agro-ecosystems. In recent decades, an increase in the areas occupied by extreme ecosystems has occurred, such as desertification.

Ecological crisis

Generally, an ecological crisis occurs with the loss of adaptive capacity when the resilience of an environment or of a species or a population evolves in a way unfavourable to coping with perturbations that interfere with that ecosystem, landscape or species survival (Note: The concept of resilience is not universally accepted in ecology, and moreso represents a contingent within the field that take a holist view of the environment. There are also many ecologists that take a reductionistic perspective and that believe that the environment, at base, is indeterministic). It may be that the environment quality degrades compared to the species needs, after a change in an abiotic ecological factor (for example, an increase of temperature, less significant rainfalls)[citation needed]. It may be that the environment becomes unfavourable for the survival of a species (or a population) due to an increased pressure of predation (for example overfishing). Lastly, it may be that the situation becomes unfavourable to the quality of life of the species (or the population) due to a rise in the number of individuals (overpopulation).

Ecological crises vary in length and severity, occurring within a few months or taking as long as a few million years. They can also be of natural or anthropic origin. They may relate to one unique species or to many species, as in an Extinction event. Lastly, an ecological crisis may be local (as an oil spill) or global (a rise in the sea level due to global warming).

According to its degree of endemism, a local crisis will have more or less significant consequences, from the death of many individuals to the total extinction of a species. Whatever its origin, disappearance of one or several species often will involve a rupture in the food chain, further impacting the survival of other species.

In the case of a global crisis, the consequences can be much more significant; some extinction events showed the disappearance of more than 90% of existing species at that time. However, it should be noted that the disappearance of certain species, such as the dinosaurs, by freeing an ecological niche, allowed the development and the diversification of the mammals. An ecological crisis thus paradoxically favoured biodiversity.

Sometimes, an ecological crisis can be a specific and reversible phenomenon at the ecosystem scale. But more generally, the crises impact will last. Indeed, it rather is a connected series of events, that occur till a final point. From this stage, no return to the previous stable state is possible, and a new stable state will be set up gradually (see homeorhesy).

Lastly, if an ecological crisis can cause extinction, it can also more simply reduce the quality of life of the remaining individuals. Thus, even if the diversity of the human population is sometimes considered threatened (see in particular indigenous people), few people envision human disappearance at short span. However, epidemic diseases, famines, impact on health of reduction of air quality, food crises, reduction of living space, accumulation of toxic or non degradable wastes, threats on keystone species (great apes, panda, whales) are also factors influencing the well-being of people.

Due to the increases in technology and a rapidly increasing population, humans have more influence on their own environment than any other ecosystem engineer.