Soil Ecology

Soil is a complex mixture of inorganic materials (sand, silt, and clay), decaying organic matter, water, air, and a great array of organisms. Because of its abundance of living organisms, soil is discussed here along with other "biological setting" components, even though soil is sometimes incorrectly described as a physical, non-living entity.

Soil has three basic properties which aid in its identification and taxonomy: color, structure, and texture. Soils often vary substantially from place to place within a watershed, and among different watersheds. To describe their differences, soils are classified into soil orders. Knowing the basic differences among types of soils can be useful for understanding why they vary in their suitability for supporting different land uses and ecological communities. Histosols, for instance, are poorly drained with a high organic content. These, essentially, are wetland soils -- soils where a constant presence of water allows for the growth of water-tolerant plants, but breakdown of organic matter is slow due to the anaerobic condition of the soil. Oxisols, on the other hand, occur throughout the tropics -- these soils are low in nutrients because most available nutrients are being utilized. The extremely high turnover rate (of matter and particularly nutrients) in the tropics makes these soils, when taken alone, highly infertile. This is one of the key reasons why deforestation in tropical watersheds creates a serious dilemma: once the vegetative cover is removed, there is little capacity for the site to self-restore.

Food Webs and Trophic Ecology

Terrestrial and aquatic ecosystems have characteristic trophic (feeding) patterns that organize the flow of energy into, through, and out of the watershed ecosystem and support the growth of organisms within the system. Food "chains" are rarely linear, hence the term food web, often used to describe the trophic interactions of organisms within an ecosystem.

Within a food web, organisms interact and, in the process, may directly or indirectly affect other organisms. The example pictured is a simple, aquatic-only food web; when the whole watershed's terrestrial components are also considered, food webs can be very complex with numerous interactions among land-based and water-based species. Food webs also often recognize the different roles species play by terming them producers (organisms that generate food, primarily through photosynthesis), consumers (first-order consumers are vegetarians, second-order consumers feed on first-order, etc.), and decomposers (which feed on dead tissue and return nutrients and energy to other parts of the cycle), among other terms.

Species with especially far-reaching effects on an ecosystem are called keystone species. These species differ from dominant (i.e. abundant) species in that their effects are much larger than would be predicted from their abundance. They have a disproportionate effect on the composition of communities and ecosystem function. A keystone species' presence is often the lone reason for the presence of other organisms and/or the maintenance of unique ecological areas. The effects of keystone species are context-dependent, meaning that a species is not always a dominant controlling agent across its entire range, through all stages of its life cycle, or at all times of the year. The American Alligator is an example: it is not the most common species in southern swamplands and bayous, but it is an important predator that also modifies aquatic habitat structure by creating `gator wallows'.

Indicator species are species whose presence or absence indicates an environmental change. 19th-Century coal miners used to keep a caged canary with them in the mine shaft; the especially-sensitive canary would signal the presence of dangerous, flammable gases by ceasing to sing, and dying. The ever-cognizant miners could evacuate upon noticing the death of the pet bird. This is the concept behind an indicator species: they are modern-day canaries-in-a-coal mine. Watershed canaries in a coal mine include several types of aquatic invertebrates that are labeled "intolerant" of poor water quality, and amphibians such as frogs and salamanders.