LESSON 4. ECOSYSTEM STRUCTURE
The ecosystem has both structure and function.
Ecosystem structure refers to the organizational set-up of
the system specifically its species composition (kinds and
numbers) and their patterns of distribution in time and
space (vertical and horizontal arrangement of species).
You can therefore divide the ecosystem structure into two:
the physical and the biological structure.
Physical Structure
The physical structure of the ecosystem is described as
follows:
1.
Stratification is the separation of organisms in
space or time. An ecosystem can be stratified in
space either vertically (layers) or horizontally
in concentric rings. In a terrestrial ecosystem
spatial stratification is largely determined by
the plant forms present. In aquatic ecosystems,
spatial stratification is usually determined by
the depth, light penetration, and temperature of
the water (Sutton & Harmon, 1973).
Let us look at the various ways
ecosystem is structurally subdivided.
a.
that
an
Vertical stratification is the distribution
of different life of plants – their size,
branching and leaves. These are influenced
by the vertical gradient of light. Several
layers of vegetation, provide habitat for
animal life in the forest (Smith, 1990).
The upper stratum of a terrestrial
ecosystem, such as the forest, can be
divided into various layers according to
the various heights of its vegetation
(Sutton & Harmon, 1973).
A tropical rain
forest, for instance, has five main layers
(Figure 7).
1.
2.
3.
4.
5.
The tallest trees (overstory) make up
the canopy and receive the full
sunlight.
The
shorter
trees
(understory)
contain
some
of
the
younger
individuals of the canopy species
that do not reach canopy height. The
trees that thrive in this layer
prefer some shade.
The shrubs receive only about 10
percent of the sunlight after it has
filtered down through the overstory
and understory.
The young trees, herbs, and ferns
need very little sunlight of about 1
to 5 percent to exist.
The mosses (ground layer) receive
only 1 percent of sunlight.
1
2
3
4
5
Figure 7.
b.
Stratification in the tropical rain forest
(Sutton and Harmon, 1973)
Horizontal stratification is the study of
vegetation in concentric rings from the
outer boundary of the ecosystem toward the
center (Sutton and Harmon, 1973).
Horizontal heterogeneity results from
an array of environmental and biological
influences (Figure 8.) Soil structure,
soil fertility, moisture conditions and
aspects influence the microdistribution of
plants. Patterns of light and shade shape
the development of understory vegetation.
Run off and small variations in topography
and
microclimate
produce
patterns of plant growth.
well-defined
Grazing
animals
have
subtle
but
important effects on the spatial patterning
of vegetation, similar to the abiotic
disturbances like wind and fire.
Likewise
the
mode
of
plant
reproduction
and
availability of propagules over time affect
vegetation
patterns.
Plants
with
wind
dispersed and animal dispersed seeds have a
wider distribution across the landscape
than plants
with heavy seeds. Vegetative
or clonal reproduction produces distinctive
clumps
or
patches
of
certain
plants.
Allelopathic effects and shading lead to
the suppression of some plant species and
to the development and growth of others
(Smith, 1990).
ABIOTIC DISTURBANCE
Fire Wind
VEGETATION PATCHINESS
(Phenology, Structure, Species Diversity)
CLIMATE
EFFECTS
Microclimate
Run off
VEGETATION
EFFECTS
SOIL
EFFECTS
Nutrients,
etc.
Texture
Topography
Figure 8.
CONSUMER EFFECTS
Grazing preferences
Seed Dispersal
Caching
Nutrient Deposition
Transpling
Allelopathy
Shading,etc.
Reproductive
patterns
General relationship of some of the
major influences that govern vegetational
patchiness in terrestrial environment
(Wiens, 1976 and Smith, 1990)
2.
Edges and Ecotone – These two terms are often
used synonymously but they are different.
An
edge is where two or more different vegetational
communities meet.
An ecotone is where two or
more
communities
not
only
meet
but
also
integrate or blend (Smith, 1990). Figure 9 shows
examples of edge and ecotones.
X
Y
(a)
Edge
Figure 9.
A.
X
X2 Y2 Y
(b)
Ecotone
Edge and Ecotone showing an abrupt, narrow
edge with no development of an ecotone (a)
and an ideal ecotone development in which
plants from both communities invade each
others create a wide ecotone X2Y2 (b).
(Smith, 1990)
Biological Structure
The ecosystem is influenced not only by the physical or
abiotic conditions, but also by biological conditions. The
biological structures of the ecosystem are as follows:
1.
Dominant species – These are single species or group
of species that biologically control an ecosystem or
modify
the
environment
of
that
ecosystem.
The
dominants may be the most numerous, possess the
highest biomass, preempt the most space, make the
largest contribution to energy flow or mineral cycling
or by some other means control or influence the rest
of the ecosystem, (Smith, 1990).
2.
Species richness refers to the number
occupying a specific area (Smith, 1990).
of
species
3.
Species evenness refers to the relative abundance of
individuals among the species (Smith, 1990) or degree
of equitability in the distribution of individuals
among a group of species.
4.
Species diversity refers to the number of different
species and the relative abundance of individual
species in a given area.
This implies both the
richness and evenness of individuals among the species
(Smith, 1990).
5.
Species abundance is associated with species diversity.
It
is
the
relationship
between
abundance
of
individuals within a species and the number of species
having similar abundances.
Activity 4.
Let us review some important concepts we have learned
from our previous lessons.
Answer the question before
going to the next lesson.
1. Enumerate the physical structure of the ecosystem.
a.
b.
c.
d.
2.
Give an example of a vertical stratification
aquatic ecosystem. Draw a diagram below:
3.
Describe the
ecosystem.
a.
b.
c.
d.
e.
different
biological
structures
in
of
an
the