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.