An ecosystem is a biotic community together with its physical environment, considered as an integrated unit. Implied within this definition is the concept of a structural and functional whole unified through life processes. An ecosystem may be characterized as a viable unit of community and interactive habitat. Ecosystems are hierarchical and can be viewed as nested sets of open systems in which physical, chemical, and biological processes form interactive subsystems. Some ecosystems are microscopic and the largest comprises the biosphere. Ecosystem restoration can be directed at different-sized ecosystems within the nested set, and many encompass multiple states, more localized watersheds, or a smaller complex of aquatic habitat.
(US Army Corps of Engineers EP 1165-2-502, Supporting Policy Information for Ecosystem Restoration, http://www.usace.army.mil/inet/usace-docs/eng-pamphlets/ep1165-2-502/basdoc.pdf).
Put another way, each organism (including humans) is affected by and interacts with its environment. That environment is formed from a combination of interactive non-living and living elements. The non-living elements include: energy, water, air, substrate, and chemical constituents such as nutrients. The living elements include plants, vertebrate animals, invertebrates, and microorganisms. When we consider both forms of elements and their interactions as a single entity, we have an ecosystem at some level of organization.
Definition and delineation of an ecosystem can become as complicated as one wants to make it. In truth, an ecosystem can be a log in the forest and its associated microorganisms, a small lake and its tributary, or the Gulf of Mexico. From a practical view in the Corps, we usually think of ecosystems as a body of water and associated land, e.g., a stream and riparian corridor, wetland and buffers, coastal pond and mudflats, reservoir and project lands, etc. Our missions and responsibilities are often concerned with water and the land and water interface, i.e., wetlands, riparian, floodplain, and other aquatic systems. This concentration allows us to define ecosystems that are of a workable size and complexity.
What Are the Characteristics of an Ecosystem?
One guaranteed characteristic, no matter how defined, is that an ecosystem contains complex relationships and interactions among its members. These interactions are what make the whole thing work. We are faced with making a complex system simple enough to manage, and then with realizing that managing the system in a simple manner will be inadequate to accommodate the complexities and maintain the system. It's enough to make your head spin. As Likens (1998:247) said, it is a "false assumption that there will be simple, all-inclusive answers to complex, ecological questions."
An additional "wrinkle" is that part of the complexity is interrelated with human activities in an ecosystem, making the practice of ecosystem management much more difficult than traditional resource management.
Another characteristic of ecosystems is that they are scale-dependent, meaning that their properties and characteristics - everything we can describe about them - may be different when we look at them at different scales. No matter how we define an ecosystem, there is always a unit one level larger that must be considered.
A consequence of the characteristics of ecosystems and our agency responsibilities is that we must impose boundaries around areas to study or manage them.