Science of Water : Concepts and Applications

170 The Science of Water: Concepts and Applications
the carrying capacity is the maximum number of species that can be supported in a bioregion.
A pond may be able to support only a dozen frogs depending on the food resources for the frogs
in the pond. If there were 30 frogs in the same pond, at least half of them would probably die because
the pond environment would not have enough food for them to live. Carrying capacity is based on
the quantity of food supplies, the physical space available, the degree of predation, and several other
environmental factors (environmental resistance).
The carrying capacity is of two types: ultimate and environmental. Ultimate carrying capacity
is the theoretical maximum density, that is, it is the maximum number of individuals of a species in
a place that can support itself without rendering the place uninhabitable. The environmental carrying
capacity is the actual maximum population density that a species maintains in an area. Ultimate
carrying capacity is always higher than environmental carrying capacity. Ecologists have concluded
that a major factor that affects population stability or persistence is species diversity. Species diversity
is a measure of the number of species and their relative abundance.
If the stress on an ecosystem is small, the ecosystem can usually adapt quite easily. Moreover,
even when severe stress occurs, ecosystems have a way of adapting. Severe environmental change
to an ecosystem can result from such natural occurrences as fi res, earthquakes, and fl oods, and
from people-induced changes such as land clearing, surface mining, and pollution. One of the most
important applications of species diversity is in the evaluation of pollution. Stress of any kind will
reduce the species diversity of an ecosystem to a signifi cant degree. In the case of domestic sewage
pollution, for example, the stress is caused by a lack of DO for aquatic organisms.
Ecosystems can and do change. For example, if a fi re devastates a forest, it will grow back, eventually,
because of ecological succession. Ecological succession is the observed process of change
(a normal occurrence in nature) in the species structure of an ecological community over time.
Succession usually occurs in an orderly, predictable manner. It involves the entire system. The science
of ecology has developed to such a point that ecologists are now able to predict several years
in advance what will occur in a given ecosystem. For example, scientists know that if a burned-out
forest region receives light, water, nutrients, and an infl ux or immigration of animals and seeds,
it will eventually develop into another forest through a sequence of steps or stages. Ecologists
recognize two types of ecological succession: primary and secondary. The particular type that
takes place depends on the condition at a particular site at the beginning of the process. Primary
succession, sometimes called bare-rock succession, occurs on surfaces such as hardened volcanic
lava, bare rock, and sand dunes, where no soil exists, and where nothing has ever grown before (see
Figure 6.13). Obviously, to grow, plants need soil. Thus, soil must form on the bare rock before
succession can begin. Usually this soil formation process results from weathering. Atmospheric
exposure—weathering, wind, rain, and frost—forms tiny cracks and holes in rock surfaces. Water
collects in the rock fi ssures and slowly dissolves the minerals out of the rock’s surface. A pioneer
soil layer is formed from the dissolved minerals and supports such plants as lichens. Lichens
gradually cover the rock surface and secrete carbonic acid, which dissolves additional minerals
from the rock. Eventually, mosses replace the lichens. Organisms called decomposers move in and
feed on dead lichen and moss. A few small animals such as mites and spiders arrive next. The result
is what is known as a pioneer community. The pioneer community is defi ned as the fi rst successful
integration of plants, animals, and decomposers into a bare-rock community.
After several years, the pioneer community builds up enough organic matter in its soil to be
able to support rooted plants like herbs and shrubs. Eventually, the pioneer community is crowded
out and is replaced by a different environment. This, in turn, works to thicken the upper soil layers.
The progression continues through several other stages until a mature or climax ecosystem is
developed, several decades later. In bare-rock succession, each stage in the complex succession
pattern dooms the stage that existed before it. Secondary succession is the most common type of
succession. Secondary succession occurs in an area where the natural vegetation has been removed
or destroyed but the soil is not destroyed. For example, succession that occurs in abandoned farm
fi elds, known as old-fi eld succession, illustrates secondary succession. An example of secondary