A tabular model for ecosystem development
Wednesday, November 6, 2013
10:33 PM
Labels: a tabular model for ecosystem development , Ecosystem development and evolution , 0 comments
Labels: a tabular model for ecosystem development , Ecosystem development and evolution , 0 comments
A more general and complete summary of important changes in
community structure and function in the sere, as revealed by the study of the
large, open systems of nature, is showing this post....
Expected trends in the gradient from youth to maturity are
grouped under several headings. Although ecologists have studied succession in
many parts of the world most of the emphasis to date has been on the
descriptive aspects such as the qualitative changes in species structure. Only
recently have the functional aspects of succession also been considered.
Consequently, some of the items listed in this post must be considered
hypothetical in the sense that they are based on good experimental or
theoretical evidence, but have not been verified by adequate data from the
field. Five aspects seem most significant require a bit more explanation as
follows.
The kinds of plants and animals that change continuously
with succession. Those species that are important in the pioneer stages are not
likely to be important in the climax. When the density of species in a sere is
plotted against time, a characteristic stair – step graph is obtained, as
illustrated in this post. Such a pattern usually is apparent whether we are
considering a specific taxonomic group, such as birds, or a trophic group, such
as herbivores or producers. Typically, some species in the gradient have wider
tolerances or niche preferences than others and, therefore, persist over a
longer period of time. Thus, in the terrestrial succession pictured in thispost pine trees and cardinals persist throush longer periods of time than do
most of the other species. In general, the more species in the group (whether
taxonomic or ecological) that are geographically available for colonization,
the more restricted will be the occurrence of each species in the time
sequence. This kind of regulatory adjustment is the result of competition
coexistence interactions discussed in the next post.
Biomass and the standing crop of organic matter increase
with succession. In both aquatic and terrestrial environments the total amount
of living matter (biomass) and decomposing organic materials tend to increase
with time. Also, many soluble substances accumulate; these include sugars,
amino acids, and many organic products of microbial decomposition. These liquid
products that leak out from the bodies of organisms are often collectively
known as extra metabolites. Some of these substances provide food for
microorganisms, and perhaps also for microorganisms. Other substances
are equally important in that they may act as inhibitors (antibiotics) or as
growth promoters (as, for example, vitamins), sub stances produced by one
organism may inhibit the further growth of that species (thus providing
population self regulation, or they may act on completely different species.
This was dramatically drought to our attention by the discovery of penicillin
and other bacterial antibiotics produced by fungi. In other cases, increasing
organic matter stimulates the growth of bacteria that manufacture vitamin B12,
a necessary growth promoter for many animals (many are unable to
manufacture this and other vitamins themselves). Where extra metabolites do
prove to be regulatory, we would be justified in calling these substances
environmental hormones since by definition a hormone is a ‘’chemical
regulator.’’ Chemical regulation is one way of achieving community stability as
the climax is approached, because the physical as well as the chemical perturbations
(as, for example, light and water relations) are buffered by a large organic
structure. There is no question that the increase in amount of and the change
in organic structure are two of the main factors bringing about the change in
species during ecological development.
The diversity of species tends to increase with succession.
Initially this is the case, although it is not clear from the present data that
the change in variety of taxa follows
the same pattern in all ecosystems. Increase in diversity of heterotrophs is
especially striking; the variety of microorganisms and heterotrophic plants and
animals is likely to be much greater in the later stages of succession than in
the early stages. Maximum diversity of autotrophs in many ecosystems seems to
be reached earlier in succession. The interplay of opposite trend makes it
difficult to generalize in regard to diversity. The increase in size of
individual organisms and the increase in competition tend to reduce diversity,
while the increase in organic structure and variety of niches tends to increase
it. As we have already pointed in the discussion of diversity in this post
there may be an optimum level of diversity for a given energy flow pattern. We
can state that, in general, rapid growth seral stages will tend to have a low diversity on the order
of 0.1 or 0.2 on the scale used in this post while mature stages will tend to
have a higher level on the order of 0.7 or 0.8 unless there is a large energy
subsidy that counteracts this patter.
A decrease in net community
production and a corresponding increase in community respiration are two of the
most striking and important trends in succession. These changes in community
metabolisms are shown graphically in this post which compares ecosystem development
in a small laboratory microcosm and in a large natural forest. Total production
(PG ) increases faster than energy expenditure (P) at first, so a
large net production (PN ) results in a rapid increase in biomass
(B). gradually, equilibrium is established, in about 100 days in the microcosm
and 100 or more years in the forest. Perhaps the best way to picture this
overall trend is as follows; species, biomass, and the P/R ratio continue to
change long after the maximum gross primary production possible for the site
has been achieved. As one evidence for this we may cite the situation in regard
to leaves in a terrestrial broadleaved succession. Agricultural scientists have
repeatedly found that maximum productivity of broad leaved crops occurs when
the leaf surface area exposed to the incoming light from above is about 4 or 5
times the surface area of the ground. Any increase in leaves beyond this level
does not increase the photosynthetic rate per square mater, since increased
shading cancels any advantage that might accrue from increased photosynthetic
tissue. In fact, the increased respiration of the extra leaves that do not
receive adequate light may reduce the net production of the crop. In a forest
the leaf area apparently continues to increase far beyond that limit
experimentally shown to increase gross production, since leaf area per ground
is often 10 or more in an old forest. Since forests are among, the most
successful of ecosystems with a long geological history of survival, we may
well consider the possibility that the extra leaves have other important
functions in the ecosystem in addition to production of food. The undoubtedly
help moderature and moisture and provide reserves that are important during
periods of climatic stress or insect of climatic stress or insect or disease
attack.