Stratification and Ecotones

Vertical Distribution Patterns

There is vertical stratification in any ecological community. An ecosystem can be subdivided by the layers of its vegetation. Stratification is determined especially by the sizes and kinds of plants present.

A forest community typically includes a canopy, an understory layer, shrubs, an herb/ground layer, and the forest floor, including roots and soil.

• The canopy is the primary site of energy fixation. It influences the amount of sunlight that penetrates deeper into the forest.
• The understory typically includes tall shrubs and smaller trees. These must be shade-tolerant since they're growing under the canopy trees.
• The shrub layer varies depending on the kind of and location of the forest. For example, there may be differences in the shrub layer of north- vs south-facing slopes within the same forest.
• The types of plants in the herb layer vary depending on the soil humidity and the deepness of shade in an ecosystem.
• The forest floor is mainly a site of decomposition. Fallen leaves and dead plants and animals are returned to the soil.

Stratification also occurs in large bodies of water such as lakes and oceans. In these, the layers are distinguished by light penetration, temperature, amount of dissolved oxygen, etc.

• The epilimnion is the surface water. Light penetrates throughout the epilimnion, so photosynthesis occurs. The epilimnion may be further subdivided into
  • The littoral zone, where the water is shallow and light penetrates to the bottom/floor: this zone is characterized by rooted plants which may include emergent vegetation and some floating plants.
  • The limnetic zone, which is open water with photosynthetic phytoplankton distributed throughout: also common in this area are nekton, organisms such as fish and invertebrates which move freely despite any currents which may be present.
• The compensation level is the depth where light is so low that not much photosynthesis occurs. At this level, cellular respiration and photosynthesis balance each other, while above this level, there is more photosynthesis, and below this level, there is more respiration and decomposition.
• The profundal zone is the area beyond the depth of light penetration. Organisms here depend on settling organic material from above as their source of nutrients and energy. This zone may be divide into:
  • The metalimnion, which has a thermocline (a rapid decrease in temperature as depth increases): since no light penetrates this deep, photosynthesis does not occur.
  • The hypolimnion, which is a deep, cold layer: the water is dense and low in dissolved oxygen, and since no light reaches to this depth, the prevalent organisms are decomposers.
• The benthic zone benthic zone (benthos = the depths of the sea) is the bottom mud/ooze and contains mostly decomposers, collectively referred to as the benthos. Due to their biological activity, little oxygen is present, and most respiration is, therefore, anaerobic.

Horizontal Distribution Patterns

Local areas usually include a variety of communities such as lawns, farmlands, old fields, shrubby areas (brambles and sumac), various woodland with or without undergrowth, etc. The distributions of these communities are tied in with the idea of succession. These distribution patterns are also tied in with the ideas of edge and ecotone. An edge is an area where two (or more) communities meet abruptly. This may result from an abrupt change in soil type or other natural causes (inherent) or as a result of human activities or fire (induced). An ecotone is an area where two (or more) communities meet and intergrade.

Plants are competitive and will grow from the edge as far into each of the communities as each species is able. Some highly-adaptable or opportunistic species (white snakeroot, garlic mustard) can grow in both communities. Some animals must live in an edge/ecotone community. For example, an American Robin nests in trees but forages for food (pulls up worms) in grassy field areas. Edge or ecotone areas generally have a greater variety and density of life than other habitats. Typically, they have species from both of the two adjoining habitats plus a number of specifically edge species. This is known as the edge effect and the greater the contrast between the two communities involved, the richer the species diversity in the edge community.