Summary

This chapter examined the ecosystem as the basic functional unit of ecology, in which a community of living organisms interacts continuously with its physical and chemical surroundings. Ecosystems were classified along two patterns -- terrestrial versus aquatic, and natural versus artificial -- and shown to occur at every scale from a small puddle to an entire ocean.

Every ecosystem is built from abiotic components (physical factors such as sunlight and temperature, and chemical factors such as inorganic nutrients and organic humic substances) and biotic components, classified functionally as producers (autotrophs that manufacture their own food, chiefly by photosynthesis), consumers (herbivores, and secondary and tertiary carnivores, that depend directly or indirectly on producers), and decomposers (chiefly bacteria and fungi that break down dead organic matter and release its nutrients back into the environment).

Productivity, the rate at which producers generate new biomass, was distinguished into gross primary productivity (GPP, the total rate of photosynthetic energy capture) and net primary productivity (NPP, what remains after subtracting the producers' own respiratory loss, R), and was further distinguished from the related but different snapshot measures of standing crop (biomass present at an instant) and standing state (nutrient concentration present at an instant). Decomposition was examined as the sequence -- fragmentation, leaching, catabolism, humification and mineralization -- by which decomposers break detritus down into simple inorganic nutrients, closing the nutrient cycle.

Energy flow was shown to be strictly unidirectional through an ecosystem, governed by the ten per cent law of energy transfer between trophic levels, and this steep, compounding loss of usable energy at every step was identified as the reason food chains rarely exceed four or five trophic levels. Grazing food chains (beginning with living plants) and detritus food chains (beginning with dead organic matter) were distinguished, and shown typically to interconnect into a larger, more stable food web.

Ecological pyramids -- of numbers, of biomass, and of energy -- were introduced as ways of graphically comparing trophic structure across ecosystems; the pyramids of numbers and biomass can, in specific cases such as a single-tree food chain or an aquatic ecosystem, appear inverted, but the pyramid of energy is always upright as a direct consequence of the ten per cent law. …