Essay on Correlation of Environmental Gradients and Community Gradients !
Environmental units which afford uniform environmental conditions to the inhabiting species are called biotopes. The physical features of a biotope maintain the populations within the communities. In drastic environmental conditions the flora and fauna undergo abrupt changes and only those that are adapted to withstand the rapid variations are preferred by the environment.
For example, in communities of littoral zone or of deserts, only those organisms that are typically adapted to withstand the environmental hazards can survive. Thus, different environmental factors (e.g., temperature, rainfall, salinity, etc.) have a profound control over the populations so that habitats and communities vary along different environmental gradients or there is intergrading among them.
ADVERTISEMENTS:
For example, the gradual slope of land into the sea exhibits various grades of environmental factors and animal communities also correspondingly show a graduation from the shore through the intertidal zone to the deep sea region. Characteristic of an environmental gradient is a distinct zonation, each zone harboring its own assemblage of species adapted to one another and giving way at a sharp boundary to another assemblage of species adapted to one another.
For example the elevation gradient includes decreasing mean temperatures, decreasing lengths of growing seasons, increasing rainfall, increasing wind speeds, and so on, toward high elevation.
All these factors act together on plants and animals. The assemblage of environmental factors that change together through the space along which a community gradient occurs and that influence its population may be termed as a complex gradient (see Whittaker, 1970).
ADVERTISEMENTS:
The environment gradient (complex-gradient) and community gradient or coenocline, go hand in hand and both gradients collectively form the ecocline, the gradient of ecosystem. Thus, species shift in abundance and dominance due to change in altitude, moisture, temperature, and other physical conditions. One species may be dominant in one group, an associated species in another. This sequence of communities showing a gradual change in composition is called a gradient or continuum (Curtis, 1959).
However, there are some communities such as ponds, tidal beaches, grassy balds, island of spruce and fir within a hardwood forest, all of which have sharply defined boundaries. Here the vegetational pattern is discontinuous. Gradient analysis of different ecoclines have revealed following trends and patterns:
1. Along a gradient from a “favourable” environment to an “extreme” environment there is normally a decrease in the productivity and massiveness of communities. The decrease in amount of organic matter per unit area is expressed in decrease of height of dominant organisms and percentage of the ground surface covered.
ADVERTISEMENTS:
2. Towards increasingly unfavourable environments there is a stepping-down of community structure and a reduction of strata differentiation, with generally smaller numbers of forms, (The different classes or kinds of forms of plants such as trees lianas, shrubs, epiphytes, herbs and thallophytic are referred to as growth-forms) arranged in fewer and lower strata.
3. Some kinds of species-diversities of certain communities decreases from favourable to extreme environments, whether the latter are extremes of drought, or of cold, or of adverse soil chemistry or (for the sea cost) of tidal exposure.
4. Each growth-form has its characteristic place of maximum importance along the ecoclines—the rosette trees in some tropical forests, semi shrubs in desert and adjacent semiarid communities, and so on. Some growth forms, for example, grasses and grass-like plants may have more than one area of importance along the major ecoclines.
5. Some growth-forms may be dominant in similar environments in widely different parts of the world. Thus, similar environments on different continents tend to have communities of similar physiognomy. This adaptive convergence at the level of the community is one of the major generalizations about the geography of life.