Biology · Ch 14 — Ecosystems and Energy Flow
Ecosystem: Structure and Function
Ecosystem: Structure and Function
An ecosystem is a self-regulatory and self-sustaining structural and functional unit of nature (the biosphere), containing both biotic and abiotic components that interact continuously with one another. The term 'ecosystem' was coined by Tansley in 1935. Ecosystems vary enormously in scale, from a small pond to a large ocean, or from a small farmland plot to an entire village landscape — the whole biosphere can itself be thought of as one single global ecosystem made up of countless smaller local ecosystems. Because studying the entire earth-ecosystem at once would be too complex, it is conventionally divided into terrestrial ecosystems (forest, grassland, desert) and aquatic ecosystems (lakes, wetlands, rivers, seas, oceans, estuaries). Ecosystems are also classified as natural ecosystems, which are self-sustaining and need no human input, and artificial ecosystems such as a farmland, fish tank, or managed pond, which require a constant input of energy or materials from people to keep functioning.
Within any ecosystem, identifying and counting the plant and animal species present gives its species composition, and how these species are distributed in space produces two recognisable patterns. Stratification is the vertical distribution of different plant and animal species occupying different height layers — in a forest this typically runs from an emergent layer of the very tallest trees, through a canopy, an understorey, down to a shrub layer at ground level; a similar vertical layering exists in the open ocean, divided into epipelagic, meso-pelagic, bathy-pelagic and benthic zones. Zonation, by contrast, is the horizontal distribution of species across a landscape, observed in both aquatic and terrestrial settings but most pronounced where two habitats meet — for instance, the edge of a large lake or a beach typically shows distinct intertidal, littoral and sub-littoral zones as one moves from open water toward dry land.
All the biotic and abiotic components of an ecosystem are linked together to function as a single 'ecosystem unit' through four essential processes that every ecosystem must perform to remain self-sustaining: productivity (autotrophs converting inorganic material into organic matter using solar energy, which is then consumed by heterotrophs), decomposition (the breakdown of dead organic material and its mineralization), nutrient cycling (the storage and repeated transport of minerals between the abiotic and biotic worlds), and energy flow (the unidirectional passage of energy from producers to consumers, ultimately dissipated as heat). A small pond illustrates all four processes neatly and self-sufficiently: its abiotic component is water carrying dissolved substances plus the rich bottom sediment; solar input and seasonal changes in temperature and day-length regulate the whole system's functioning; phytoplankton, algae and aquatic plants act as producers; zooplankton, aquatic insects and fish act as consumers; and fungi and bacteria at the bottom act as decomposers.
What this figure shows. A layered cross-section of a forest showing the vertical distribution of plant life into four zones stacked one above another. At the very top sits the emergent layer, formed by the tallest trees whose crowns rise above the general canopy. Below it is the dense canopy layer made of closely spaced tree crowns that intercept most of the sunlight. Beneath the canopy lies the understorey of smaller trees and saplings tolerant of shade, and at ground level is the shrub layer of low bushes and herbaceous growth. The diagram illustrates how different plant forms occupy distinct height zones within the same habitat, a pattern called stratification, the same principle by which the open sea is divided into epipelagic, meso-pelagic, bathy-pelagic and benthic zones.
14.1: Stratification of Plants in a Forest.
What this figure shows. A cross-section of a coastal wetland running from open water toward dry land, showing horizontal zones that change with distance from the tideline. Nearest the water are subtidal channels that stay flooded and provide fish habitat while allowing drainage of the mudflats. Next are the mudflats, rich in invertebrate life and visited by shorebirds, with algal mats also growing here, alternately exposed at low tide and covered at high tide. Moving landward is the low marsh, dominated by cordgrass and used by herons, egrets and the clapper rail, followed by the high marsh, which supports pickleweed and patches of cordgrass and is a good habitat for the Savannah sparrow. The diagram marks the high-tide and low-tide lines to show how each zone's exposure to water governs which organisms live there, illustrating that horizontal zonation is most pronounced at the boundary between two adjoining habitats.
14.2: Zonation in a Wetland.