Components of an Ecosystem

Different actors, components and compartments make up an ecosystem. These partial systems or system components interact with each other, with strong or weak interactions, with linear or nonlinear interactions, with direct or indirect interactions, and with positive or negative interactions. Depending on the research question asked, the following criteria to separate different components of an ecosystem are the ones most frequently used:

- Above-ground versus below-ground: This approach roughly separates autotrophic from heterotrophic processes (exception: belowground root (i.e. autotrophic) respiration). In addition, it is possible to separate the ecosystem into soil versus vegetation compartments

- Trophic levels: Separation of organisms present in an ecosystem into producers, consumers and decomposers describes the energy (and matter) flux within an ecosystem

- Functional groups: Many species that have similar characteristics are often considered a functional group or functional type (e.g. nitrogen-fixing plants, trees, invasive species, phloem-sucking insects and parasites; Chap. 20, Sect. 20.2). Species can belong to many different functional groups—for example, Robinia pseudoacacia, a nitrogen-fixing tree species, is also considered invasive in Europe

- Overstorey and understorey: The vertical structure of vegetation is important for the coupling of ecosystems to the atmosphere, particularly for energy, trace gas fluxes, atmospheric deposition or rain interception. In forests, the separation into trees, shrubs, lianas/vines and herbaceous plants (understorey vegetation) is often used. For grasslands, but also forests and woodlands, separation into the upper and lower canopy, as well as in understorey or suppressed plant species is frequent

Often, multiple components and actors, as well as their interactions, are investigated in the same study; then a so-called systems approach is used, which also provides the conceptual framework to connect all components and actors mechanistically—for example, in a mathematical model. In contrast, studies focusing on single species—for example, dominant species (dominating in abundance) or keystone species (being a major driver of change or of certain processes)—or on species interactions within a plant community, are often carried out within the context of evolutionary biology, community ecology and biodiversity research.

These different components of an ecosystem are not equally distributed in space. They often form a mosaic of different components (e.g. in a riparian zone); they might create patterns and thus patchiness (e.g. the hummocks and hollows in a bog). These patterns might be vertical (facilitated by different height growth, resulting in a canopy structure) and/or horizontal (facilitated by plant density and species composition). The origins of these patterns are manifold. Patterns might be the consequence of soil conditions, topography, disturbance, competition or management. In any case, research in such a patchy ecosystem must pay special care to capture this spatial variability in order to represent the entire ecosystem and not only one or several components.