Environmental Biology I: Ecology as an Interdisciplinary Science, the Origin of Life and Speciation, Human Ecology, Ecosystem Structure and Function, Energy Flow, Succession, Ecotones, Niches, Stability and Services, the Types of Ecosystem and the Biomes

An ecosystem, a term introduced by Tansley in 1935, has abiotic components (energy, climate, water, inorganic substances and organic compounds such as humus) and biotic components: producers (autotrophs), consumers (herbivores, carnivores, omnivores) and decomposers (bacteria and fungi, with detritivores). Its two basic functions are the one-way flow of energy and the cycling of matter. Gross primary productivity (GPP) is the total rate of photosynthesis; net primary productivity (NPP) is what remains after the producers' own respiration, NPP = GPP − R, and it is the energy actually available to consumers. Tropical rainforests, estuaries, coral reefs and swamps have the highest NPP per unit area, and the open ocean and deserts the lowest, though the ocean's great area makes it a large total producer.

Odum's energy flow models show the same principle at increasing detail. The single-channel model traces energy from producers through herbivores to carnivores, with losses to respiration at every step. The Y-shaped model splits the flow into a grazing food chain (plants to herbivores to carnivores) and a detritus food chain (dead organic matter to decomposers and detritivores to their predators); in most forests and many aquatic systems far more energy passes through the detritus chain. The universal model applies to any living component, showing ingested energy divided into assimilation and egestion, and assimilation into respiration and production. Food chains interlock into food webs, which give consumers alternative prey and so stabilise the community. Because only about 10% of energy passes from one level to the next (Lindeman, 1942), if producers fix 20,000 kJ, herbivores receive about 2,000 kJ and carnivores about 200 kJ, which is why food chains rarely exceed four or five links.

Matter, unlike energy, cycles. Biogeochemical cycles are of two broad types: gaseous cycles, such as those of carbon, nitrogen and oxygen, have their main reservoir in the atmosphere or ocean and are relatively well buffered; sedimentary cycles, such as that of phosphorus, have their reservoir in rocks and sediments, turn over slowly and are easily disrupted, which is why phosphorus lost to the sea is effectively lost to the land for geological time. Each cycle has a large, slow reservoir pool and a smaller, active exchange pool between organisms and their surroundings.

Succession is the orderly, directional change in the species composition of a community over time. Primary succession begins on a surface never before colonised, such as bare rock, a lava flow or a new sand dune, and is slow because soil must first form; secondary succession follows disturbance of an existing community, such as an abandoned field or a burnt forest, where soil remains, and is faster. On bare rock (a lithosere, a kind of xerosere) the sequence runs from crustose lichens, which begin weathering, to foliose lichens, mosses, herbs, shrubs and finally forest; in water (a hydrosere) from phytoplankton to submerged plants, floating plants, reed swamp, sedge meadow, woodland and forest, as sediment and organic matter fill the basin. Changes caused by the organisms themselves are autogenic; those caused by outside forces such as siltation or climate are allogenic.

The end point is the climax community, in equilibrium with its environment. Clements's monoclimax theory held that each climatic region has a single climax; Tansley's polyclimax theory allowed several climaxes set by soil, topography and fire; and Whittaker's climax-pattern hypothesis sees a continuous pattern of climax communities tracking environmental gradients. Connell and Slatyer (1977) proposed three mechanisms by which early species affect later ones: facilitation, tolerance and inhibition. Odum (1969) summarised the trends of a developing ecosystem: biomass and organic matter increase, species diversity and structural complexity increase, food chains become web-like and detritus-based, nutrient cycles become more closed, K-selected species replace r-selected ones, and the ratio of gross production to community respiration (P/R) moves towards one.

An ecotone is the transition zone between two communities, such as a forest edge meeting grassland, or a mangrove between land and sea. It often holds species of both communities plus some found only there, and the resulting tendency for greater diversity and density at the junction is the edge effect; species that prefer such zones are edge species. Fragmentation multiplies edges, which benefits edge species but harms interior species that need large blocks of undisturbed habitat. The habitat is where an organism lives, its address; the niche is its role, its profession. Grinnell (1917) used niche mainly in the spatial sense, Elton (1927) for an organism's functional place in the community, and Hutchinson (1957) defined it as an n-dimensional hypervolume of all the conditions and resources in which a species can persist. The fundamental niche is that hypervolume without competitors; the realised niche is the narrower part actually occupied in their presence. By Gause's competitive exclusion principle, two species with identical niches cannot coexist indefinitely.

Stability has two components: resistance, the ability to withstand a disturbance without changing, and resilience, the ability to return to the original state after one. The diversity-stability hypothesis of MacArthur (1955) and Elton (1958) holds that more diverse communities, with more pathways for energy, are more stable. Robert May (1973) showed mathematically that randomly assembled complex systems tend to be less stable, and the modern view is that real communities are stabilised not by complexity as such but by particular structures, such as many weak interactions, functional redundancy among species and the insurance that different species respond differently to change. Other factors affecting stability are the presence of keystone species, the frequency and intensity of disturbance, and the connectivity of habitat.

Costanza and colleagues (1997) estimated the value of the world's ecosystem services at about 33 trillion US dollars a year, more than the global economic output of the time, a figure intended less as a price than as a demonstration that services treated as free are not.

Ecosystems are classified by habitat (terrestrial or aquatic, and within aquatic, freshwater, estuarine or marine), by origin (natural or human-made, such as croplands, plantations and reservoirs), by the dominant vegetation or physiognomy, and, in Odum's energetic scheme, by the source of energy: unsubsidised natural solar-powered systems such as the open ocean, naturally subsidised ones such as estuaries enriched by tides, human-subsidised solar-powered ones such as farms, and fuel-powered urban-industrial systems.

A biome is a large regional community of plants and animals with a characteristic physiognomy, determined above all by climate: Whittaker's diagram places the world's biomes on axes of mean annual temperature and precipitation. Biomes grade into each other along latitude, and a similar zonation repeats with altitude on high mountains. The syllabus spells chaparral as "Chapparal"; the correct spelling has one p.