Although ecologists tend to regard ecosystems as basic structural units, it can be difficult (if not impossible) to formally define the boundaries of a given ecosystem. As such, ecosystems are better thought of as conceptual rather than actual geographical locations. Rarely are ecosystems isolated from one another; rather, they should be considered parts of a larger functioning whole that together comprise the biosphere ("the place on Earth's surface where life dwells").
Despite the fact that clear boundaries between ecosystems may be difficult to identify, the myriad interactions that take place within an ecological community can often be observed and defined. These interactions may be best described by detailing feeding connections (what eats what) among biota in an ecosystem, thereby linking the ecosystem into a unified system of exchange.
All life forms in an ecosystem can be broadly grouped into one of two categories (called trophic levels):
- Autotrophs, which produce organic matter (food) from inorganic substances; and
- Heterotrophs, which must feed on other organisms in order to obtain organic matter.
In general, trophic levels are used to describe the way in which a particular organism within an ecosystem gets its food. Using this description, we can restate and reorganize the categories above to define the three basic ways organisms acquire their food:
- Producers (autotrophs) do not usually eat other organisms but pull nutrients from the soil or the ocean and manufacture their own food using photosynthesis. In this way, it is the energy from the sun that usually powers the base of the food chain.
- Consumers (heterotrophs) cannot manufacture their own food and need to consume other organisms.
- Decomposers break down dead plant and animal material and wastes and release them into the ecosystem as energy and nutrients for recycling.
Within ecosystems, the biotic factors that comprise the categories above can be organized into a food chain in which autotrophic producers use materials and nutrients recycled by decomposers to make their own food; the producers are in turn eaten by heterotrophic consumers. In real world ecosystems, there are multiple food chains for most organisms (since most organisms eat more than one kind of food or are eaten by more than one type of predator). Additionally, the movement of mineral nutrients in the food chain is cyclic rather than linear. As a consequence, the intricate network of intersecting and overlapping food chains for an ecosystem is more commonly represented as a food web. A food web depicts a collection of heterotrophic consumers that network and cycle the flow of energy and nutrients from a productive base of self-feeding autotrophs .
A simplified food web
This image shows a simplified food web model of energy and mineral nutrient movement in an ecosystem. Energy flow is unidirectional (noncyclic) and mineral nutrient movement is cyclic.
Microorganisms play a vital role in every ecological community by serving both as producers and as decomposers. Although plants are the most common primary producers, autotrophic photosynthetic microbes (such as cyanobacteria and algae) can harness light energy to generate organic matter. Additionally, in zones where light cannot penetrate (and thus photosynthesis cannot be the basic means to produce energy), chemosynthetic microbes provide energy and carbon to the other organisms in the ecosystem. Other microbes are decomposers, with the ability to recycle nutrients from dead organic matter and other organisms' waste products. Decomposition is critical as most of the carbon and energy incorporated into plant tissues during photosynthesis remains uneaten when the plant tissue dies (and therefore must be broken down before it can be made available for recycling).