Like a walk on sand, snow or (as in the case of the famous Laetoli footprints) volcanic ash, life in general leaves traces in the environment. The ecological footprint is a concept used in ecology to quantify such traces for a given biological unit, typically a species or ecosystem. It has been developed by reversing the idea behind carrying capacity (which indicates how much biomass can be sustainably supported by a given patch of biologically productive land) and describes, more specifically, how large a biologically productive area is needed to sustain a given number of individuals in the long run. In the process, all ecological interactions for which data exist are converted into corresponding units of such areas, and the sum of all these items can then be normalized by the arable surface actually available. Since the 1990s, variants of the ecological footprint have been widely used to estimate the environmental impact of humanity as a whole or specific subsets thereof (e.g. a nation, town, company, school, household or individual) on our planet.
One of the characteristics of life is metabolism, i.e. the conversion of external energy (e.g. sunlight or food) into forms that can support the specific structures (e.g. leaves or bones) and activities (blossoming, mobility or thoughts) of an organism. One side product of such conversions is heat, others typically include water, salts, organic compounds, and oxygen or carbon dioxide and, more rarely, wit and ideas. However, what is waste for one kind of organism (or remaining of it once it is dead) may be a source of nutrients for another, thereby providing the basis for food webs and other ecological interactions between individuals and populations. Consequently, large portions of the biomass on Earth are constantly being biodegraded and serve as renewable sources of energy, thereby somewhat alleviating the limits to growth of an individual or population, as imposed by the principally limited availability of any kind of natural resource. In the remaining cases, the waste products were usually deposited in places where important prerequisites for biodegradation are lacking, e.g. suitable amounts of water, oxygen, or pressure.
The biomass cycle sketched out so far exhibits additional complexities in cases that involve further interactions between individual, species, generations, or the living and non-living world, e.g. symbioses (in which organisms of different species act as a metabolic unit), biomineralisation (due to which some of the products of an organism may exist much longer than the producer itself), fossil fuels (i.e. sources of energy whose age exceeds the average generation time of their users by several orders of magnitude), or the manufacturing and use of tools (that provide new context for structures and activities).