Communities and Ecosystems

Their Interactions and Influences

Australasian gannet (Morus serrator) breeding colony known as the Plateau colony, Cape Kidnappers, Hawkes Bay, New Zealand, November.
Brent Stephenson / naturepl.com / Getty Images

Biologists have a system for not only distinguishing the animals, plants, and environments (habitats, communities) that make up the natural world but also for describing the complex interactions and relationships between them. The classification is hierarchical: Individuals belong to populations, which together form species, which exist within communities, which in turn thrive within specific ecosystems. Energy flows from one organism to another through these relationships, and the presence of one population influences the environment of another population.

All in the Family

A "community" is defined biologically as a set of interacting populations. It is often used to describe the dominant species within a certain area, say, the community of salamanders that lives along the banks of a mountain stream. A community is also the physical environment in which those salamanders thrive—commonly known as a habitat—in this case, a riparian community. Additional examples would be a desert community, a pond community, or a deciduous forest community.

Just as organisms have specific properties that make them unique, such as size, weight, age, sex, and so forth, so too do communities. When conducting their studies, biologists and other scientists make note of the following characteristics:

  • Diversity, or the number of species in the community. A community can also be described as being either densely or sparsely populated as a whole.
  • Relative abundance, which refers to the abundance—or lack thereof—of a species within a community with respect to the abundance of all the other species living in that community.
  • Stability, or how much a community changes or remains static over time. These changes can occur because of internal or external factors, or a combination of both. The members of the community can remain stable and thrive in spite of the changes impacting their environment, or they can be ultra-sensitive to even the slightest changes.

Community Relations

The relationships between populations in a community are varied and may include both positive, negative, and mutually beneficial interactions. Examples of community-level relationships include competition (for food, nesting habitat, or environmental resources), parasitism (organisms that survive by feeding off a host organism), and herbivory (species who depend upon consuming local plant life to survive). These relationships often lead to changes in the genetic makeup of the population. For example, one or another genotype may be more successful due to certain community processes.

The System as a Whole

An ecosystem can be defined as all the interacting components of the physical and biological world. Thus, an ecosystem can encompass multiple communities. Keep in mind that drawing a line around a community or an ecosystem is not a clear matter either. Communities blend together, and there are gradients throughout nature, from one habitat to another—for instance, the oases that exist within desert environments, or the forests that line ocean shorelines in the Pacific Northwest, Alaska, and Scandinavia. We can at best use the concepts of communities and ecosystems to organize our study and understanding of the natural world but are far from being able to assign exact boundaries to these concepts.