Ecology is the study of interactions between organisms and their environment. The environment can include physical attributes such as temperature, water and wind called the abiotic environment, and the influences of other organisms on another organism called the biotic environment. It is important to remember that ecosystems consists of not only natural systems but also human-maintained systems such as wheat field and city parks.

The study of ecology encompasses many objects to study from an individual cell or molecule to the ecosphere. However, we can think of ecological studies in four basic subdivisions:

At the individual level we are concerned with the interactions of a single organism to its biotic or abiotic environment. For example, we may want to know how an individual tree’s height growth will respond to a fertilizer treatment.

As we move from looking at an individual organism into looking at a population of organisms we begin to think about how the population is affected by the environment. Maybe we would like to know how a population of an exotic grass will respond to a severe wildfire.

When we begin to study ecological communities we are generally discussing multiple species of organisms interacting with each other and the environment at a given spatial scale. When we study plant communities, we are often looking at changes in the composition and structure and the causes of those changes (when those changes take place through time they are often called succession). For example, we may study the effects of fire exclusion on grassland community structure and composition.

There are typically two broad approaches to studying ecology: the vertical approach and horizontal approach.

In the vertical approach to ecology, we generally study how ecosystems function. For example, determining how and why carbon, water and nitrogen cycles change in response to fire is a vertical approach to studying ecology.

The horizontal approach to ecology consists of studying the spatial arrangement and interactions between and among ecosystems. Through this approach, we can identify, map and compare ecosystems across landscapes (Barnes et al. 1998). For example, we may be interested in the effects of the spatial arrangement of fuels treatment projects across several ecosystems on landscape-level fire behavior.

Disturbance regimes, vegetation, and ecological processes occur at many different spatial scales. We can generally define these spatial scales into three groups.

The macro-level ecosystems are major units and are typically composed of seas or continents (Barnes et al. 1998). For example we could study carbon release rates across North America. Within the continents, we have the meso-level ecosystems. These ecosystems are typically identified as mountain ranges or large plains, for example the Rocky Mountains (Barnes et al. 1998). The meso-level ecosystems are further broken down into the micro level. Micro-level ecosystems are local areas such a specific mountain or valley (Barnes et al. 1998).

As one moves from the macro-level ecosystems to the micro-level ecosystems we reduce the amount of complexity. That is to say that the micro-level ecosystems are more homogenous than the macro- and meso-level ecosystems.

Although there are many ways to classify ecosystems, we will only cover a few common approaches here. One of the most common ways to classify ecosystems is to use a climatic classification such as ecoclimate zones (Bailey 1988). This method of classification is widely used and defines patterns of world climate. Examples of eco regions include Temperate semi-arid and temperate continental regions.

Another common approach to classifying ecosystems is to use vegetation types or biomes (Tallis 1991). This approach works well at the macro- and meso-level. Included in this type of classification would be life zones and plant community types. Examples of biomes include coniferous forests and tundra.

Another common approach to classifying ecosystems is using physiography. Physiography is the configuration of the earth’s surface and the associated substance or parent material. The term physiography is short for physical geography. Physiography is a useful classification for ecosystems at any scale.

Additional Information:
Ecoregions of the United States
Biomes of the world
Physiography, and a map of Earth's Regions