Environmental Engineering Reference
In-Depth Information
ECOSYSTEM CHANGES THROUGH SUCCESSION
A number of important changes occur in ecological communities during the course of
succession. Through succession there is a progressive increase in the height of the
dominant plants, increasing diversity of growth form, and increasing differentiation of
communities into strata. Species diversity increases during the development stages of the
succession, but this trend is reversed in the later stages, and the climax does not have the
highest species diversity. Progressive soil development shows itself in increasing depth of
soil, a larger organic matter content and sharper differentiation into soil horizons. The
nutrient content of the soil also increases, as colloidal content is increased by mineral
weathering and by humification. The stock of nutrients held in the standing crop (i.e. the
biomass) also increases as the biomass itself gets larger towards climax. The US ecologist
E. P. Odum studied the overall changes in community attributes during the course of
succession. His findings are summarized in Table 21.2. Pioneer communities are
characterized by extreme and variable physical conditions. Pioneer communities are
composed of few species, but their densities and primary productivities are high, owing
to their efficient seed dispersal and wide ecological tolerance. The pioneer community is
fast-growing but of low stability. Mature communities have greater complexity of
structure and species show increased specialization. The intricate organization gives a
higher level of homeostatic regulation in mature communities. This means that internal
regulation and symbiosis are well developed, so that external effects have declining
importance and impact.
Energy exchanges are a vital part of ecosystems (see Chapter 22). During succession
there are some important changes in energy attributes, as listed in Table 21.3. The P/R
ratio (the ratio of gross production to respiration) indicates that the communities tend to
achieve a balance of these in the later stages of succession; thus all solar energy fixed by
the community is used in respiration. The P/B
Table 21.2 Community changes through
succession
Attribute
Early
Late
Organic matter
Small
Large
Nutrients
External
Internal
Nutrient cycles
Open
Closed
Role of detritus
Small
Large
Diversity
Low
High
Nutrient conservation
Poor
Good
Niches
Wide
Narrow
Size of organisms
Small
Large
Life cycles
Simple
Complex
