THE
ENVIRONMENT
The word
environment refers to the various conditions that surround and affect the form
and function of plants and animals. The conditions are referred to as
environmental or ecological factors.
All the
environmental factors fall into two major groups namely:
(i)
Abiotic factors –are the
non-living or physical and chemical factors
(ii)
Biotic factors – include all
the living organisms.
The physical
factors can be divided into three groups namely:
(i)
Climatic factors – which relate
to the aerial environment of the plant.
(ii)
Topographic factors – which
relate to the form and behaviour of the earths surface. Ecology is therefore
the
(iii)
Edaphic or soil factors- like
physical and chemical nature of the soil which operate within smaller and
localised limits.
Therefore the
environment can basically be divided into two: the abiotic and biotic
environments.
These environmental factors do not exist individually in nature or
are not found in isolation; many of the environmental factors are found
interacting with each other. The biological, physical and chemical environment
interact i.e. the factors are in nature found interacting to form a very
complex system. The physical chemical environment interacts, and the biotic environment
also interacts and all these then interact all at the same time or at different
times. This system of interaction of the environmental factors both biotic and
abiotic to form a complex system constitutes or forms what is known as an Ecosystem or ecological system. These
ecosystems are real; they exist from small to large, terrestrial to fresh water
to marine, field to laboratory. In the laboratory, you can find an aquarium, or
a pond in the field, a lake, (small to large, e.g. lake Bogoria to L. Victoria)
to seas and to large oceans or terrestrial like grasslands, woodlands, forests
etc. These ecosystems are many and come in different sizes, shapes etc. They
are unique, especially in their combination of biotic and abiotic factors. They
also have same attributes in common which are recognisable, analysable and
predictable.
The study of these ecosystems forms the science referred to today as
Ecology. A word coined or suggested
by Ernest Haecke (German) about 100 years ago. It is a Greek word where oikos =
house and logos (study of) (study of house) – the study of earth. Ecology is therefore the study of the
interrelationship between plants, animals, micro-organisms in relation to each other
and the physical and chemical environment in which they naturally occur;
whereas an ecosystem is an assemblage or collection of plant, animal and
microbial species in a particular place which interact with each other and with
their physical, chemical environment in such a way as to form a self sustaining
(maintaining) and self-regulating system e.g
hv |
The interaction of all
these factors forms an ecosystem.
The components of an ecosystem are the components of our
environments and can be studied, collected, measured, analysed and interpreted.
The various environmental factors include the following:
1. Abiotic or physical
factors
(a) Climatic factors
- Light
- Temperature
- Precipitation
- Atmospheric humidity
- Wind
(b) Topographic factors
- Altitude
- Slope
- Exposure
(c) Edaphic factors
- Soil mineral component
- Soil organic component
- Soil water and solutes
- Soil air
- Soil organisms
- Soil reaction
2. Biotic factors
- Plants
- Animals
- Man
1. CLIMATIC FACTORS
The chief factors of the climate are light, temperature,
precipitation, atmospheric humidity and wind. Consequently, these are the ones
that determine the types of vegetation
over large areas. All plant communities are subject to their influence.
Different combinations of these factors account for different types of climates
and are responsible for different types of vegetation all over the earth.
(a) Light
-
Provides the energy needed by
green plants for photosynthesis. It is also an essential factor for the primary
production of plant material upon which all other living organisms depend
directly or indirectly.
-
The most important role of
light is in photosynthesis where chlorophyllous tissues use light energy to
build energy rich complex organic compounds from simple low energy inorganic
substances.
-
Light is also important factor
in transpiration which lowers the temperature of plants.
-
In nature the effect of light
is uniform over wide areas and therefore it is not likely to affect the general
features of plant communities.
(i)
Light intensity
Different species of plants vary in their demands for
light. Majority of plants grow in direct sunlight and they are referred to as heliophytes or sun-loving plants. Small
plant communities grow in deeply shaded areas and are called Sciophytes or shade-loving plants e.g.
fir, spruce. However, most plants are not very strict (rigid) in their
requirement of light intensity. Some heliophytes are capable of growing in
shade though not so well and they are referred to as facultative sciophytes.
Those which fail to grow in shade are obligate
heliophytes. Facultative heliophytes
– are sciophytes which can grow in sunlight. Obligate sciophytes - are sciophytes which cannot grow in sunlight.
(ii)
Light duration (Photoperiodism)
Photoperiodism referrers to the number of hours in a day
during which a plant is exposed to sunlight. Different species have different
critical photoperiods which usually range between 11 to 14 hours of day length
or 10 to 13 hours of dark phase. Plants have been divided depending upon the
day length required to induce flowering.
o
Short-day plants
They flower only within a range of relatively short photoperiods.
Blooming may occur less profusely within a range of photoperiods somewhat
longer the most favourable ones. Under still longer photoperiods or continuous
illumination, short-day plants do not bloom but remain in the vegetative state
indefinitely.
o
Long-day plants
They flower easily only under a range of relatively long
photoperiods, up to and including continuous illumination. Blooming may occur
less profusely under shorter photoperiods. Under still shorter photoperiods,
the plants remain in the vegetative state indefinitely.
o
Indeterminate (Indifferent
Plants)
They flower easily over a wide range of day lengths from relatively
short photoperiods to continuous illumination.
The knowledge of photoperiodism is of great importance in selection
of species and their seasons of cultivation depending upon the plant parts
which are economically important. For
example in the sugar cane (short-day plant): the vegetative growth is more
important for sugar. If the continuity of the long dark period is broken by a
few minutes of illumination, the flowering is checked. This results in greater
cane growth and higher sugar yield.
(b) Temperature
- Is
a measurement of the degree of hotness. It is one of the most critical factors
of the environment and it regulates all
the chemical processes of plant metabolism and also many physical properties.
- Plant
growth takes place within a particular temperature range: a minimum temperature
below which no growth occurs, a maximum temperature above which growth ceases
and an optimum temperature for maximum growth. These three parameters are the cardinal temperatures of a species and
they vary widely from species to species.
- Each
species has its own minimum and maximum beyond which its life activity ceases.
The optimum temperature at which a plant thrives best also varies with
different species. The optimal temperature range for a species to complete its
life cycle imposes the greatest restriction in its distribution.
- Different
species have different temperature requirements and therefore temperature is a
major influence on the natural distribution of plants on a continental,
regional and local scale. Temperature varies with latitude, altitude and also
daily and seasonally.
(c) Precipitation
- The
principal forms of precipitation are rainfall, hail, snow and dew. Of these,
rainfall is the most important as most plants absorb water from the soil. Water
is essential to plants for their life processes such as seed germination and
transport of nutrients absorbed in solution from the soil.
- Water
is also needed for cell protoplasm and metabolic reactions including
photosynthesis. Thus plant productivity is closely related to water supply.
- The
quantity and pattern of precipitation determines the main vegetation areas of
the world. Great contrasts in the natural vegetation are produced by
differences in the amount and seasonal distribution of precipitation. Generally
3 distinct kinds of vegetation are developed as a result of rainfall:
Grasslands
– lighter rainfall
Forests
– heavy rainfall throughout the year
Deserts
– erratic rainfall
- Water availability
also influences the life-from and structure of plants.
Hydrophytes – are plants that live
partly or entirely submerged
in water; Mesophytes – plants of habitats where water supply is neither
excessive nor deficient; Xerophytes
– plants of dry habitats
(d) Wind
- Wind
has several important effects. It increases evaporation and therefore
transpiration; it causes mechanical damage; circulates oxygen, carbon dioxide
and water vapour; aids in dispersal of seeds and fruits, and gives rise to wind
pruning. Wind pruning takes place when buds on the windward side of trees and
shrubs are killed by drying and freezing, which are accelerated by the wind.
- Strong
winds also cause permanent curvatures in plants on exposed places where the
trees usually have an asymmetrical appearance with few branches and leaves on
the windward side.
- In
desert areas wind aids in formation of sand dunes, and stable sand dunes may become
habitats for growth of desert plants.
- Trees
in areas experiencing strong winds may have stunted and distorted growth.
(e) Atmospheric
humidity
- Refers
to the amount of water vapour present in the atmosphere. The quantity of water
vapour present in the atmosphere at a particular place depends on several
factors e.g. temperature, pressure, wind, vegetation, and soil water content.
- Increase
in temperature increases the capacity of the atmosphere for holding water
vapour. Conversely, decrease in temperature decreases the capacity of air fro
holding water vapour and some of the vapour condenses to form dew.
- When
the atmosphere contains the maximum possible amount of water vapour, it is said
to be saturated at the particular
temperature and pressure. In unsaturated
condition, the water vapour content of the air is usually expressed as the relative humidity (R.H), which is the
ratio between the actual humidity present (P) and the saturation humidity
possible at that temperature R.
Where R = total amount of water vapour required to completely
saturate the air at a
particular temperature.
P = amount of vapour actually present i.e. its absolute humidity.
Humidity affects the rate at which water evaporates from the surface
of organisms which in turn influences their distribution.
2. TOPOGRAPHIC FACTORS
-
They include: altitude, slope,
exposure, direction of mountain chains and valleys etc.
-
Topography influences plants
mainly through its effect upon edaphic, drainage and climatic conditions. With
increased altitude, temperatures decrease but precipitation and exposure to
wind increases i.e. wind action becomes greater. The vegetation responds by
framing zones of different communities. Therefore with an increase in altitude,
the vegetation also changes.
-
Slope affects the amount of
water in the soil and its surface, and determines the character of the soil. On
steep slopes, water flows down before it has time to soak into the soil. In
spite of heavy rainfall a sloping soil is usually dry. Humus and newly formed
soils are carried down by the rapidly flowing rain water and deposited in the
valleys. On soil that is level or almost so, new soil is formed by weathering
and humus also accumulates. Valleys and depressions are often subjected to
accumulation of cold, stagnant air which may damage frost sensitive plants.
Soils in valley bottoms are waterlogged.
-
Slope also affects the position
of the water table. In general, the water table is nearer the ground level in
valleys than in hills.
-
Exposure of slopes largely
determines the type of vegetation. A slope exposed to sun and wind bears
vegetation entirely different from that which is less exposed to either.
NB: Taken in isolation, topography may not appear to influence the
distribution of organisms, but in conjunction with factors such as temperature,
light, humidity, wind and surface run-off, the effect is quite significant. For
example, for every 1000m ascent, temperature drops by 6oC which also affects
the distribution of organisms according to their temperature tolerance.
3. EDAPHIC (SOIL) FACTORS
Soil is the top cover of the earth in which plants can grow. It
consists of weathered rock material mixed with decomposed organic matter
derived from the remains of plants and animals. Nearly all the higher plants
are anchored in the soil and obtain their mineral nutrition and water supply
from it.
The best way to study soil is to make a soil profile. A soil profile
is a vertical section of soil from the top mature soil to the underlying parent
rock. The layers differ in the size of their constituent particles and they may
also be distinguished due to their colour difference. In the tropics the soil
might be so deep that it is not possible to get the rock (100m deep). Rock is
the parent material from which soil is formed.
The morphology of a soil profile has a significant influence on the
development of vegetation. A vertical section of the soil reveals it to have
four horizons or layers that are distinguished into horizons A, B, C, and D
respectively. Some of the horizons are subdivided.
A-horizon: top soil in
which crops grow. It is dark brown in colour
B-horizon: subsoil, beneath the top soil. The subsoil contains iron,
aluminium compounds in addition to clay and humus; light brown in colour.
C-horizon: weathered parent material
D-horizon: Parent rock material
There are four main components of soil:
(i)
Mineral (inorganic) materials
derived from parent rock.
(ii)
Organic matter derived from
dead organisms.
(iii)
Soil solution (water)
(iv)
Soil atmosphere (air)
The pore space (occupied by air and water) is about 50%. This is
generally very variable. The solid state is made of 45% mineral and 5% organic
matter. The minerals are extremely variable in size composed mainly of rock
fragments and inorganic compounds of various sizes.
Soils differ in the relative abundance of the components and only
plants adapted to such extremes grow there. Sometimes there are variations in
the soil characteristics at very short distances and this leads to the
appearance of distinct vegetation types in a small area.
(a) The Mineral or inorganic constituent of the
soil
This is normally composed of small rock fragments and minerals of
various kinds. The rock pieces are remnants of massive rocks from which soil
has been formed by weathering. The minerals provide anchorage for plants, pore
space for storage of water and air and nutrients on exchange basis.
Weathering of rocks
Weathering of rocks can be by mechanical or physical, and chemical
processes.
(a) Mechanical or
Physical
(i) Temperature – differential expansion of minerals, frost action
and exfoliation. (Heating and cooling) differential coefficients of thermal
expansion.
(ii) Erosion and deposition- water ice and wind
(iii) Plant and animal influence
(b) Chemical
(Decomposition)
(i) Hydrolysis
(ii) Hydration
(iii) Carbonation and related acidic processes
(iv) Oxidation
(v) Solution
(b) Soil organic matter
Organic matter refers to the plant and animal tissues or residues in
various stages of decay or decomposition. Organic matter enhances the water
storage, cycling of nutrients, improves soil structure, pore space yet forms
only 3 to 5% by weight of the mineral top soil. Organic matter is the main
source of phosphorus, sulphur and nitrogen. It also provides energy for soil
micro-organisms without which biochemical activity would practically come to a
stand still. Plant structures like stems, leaves and roots and animal remains
are converted by the soil micro-organisms into a dark-coloured amorphous
substance in which the parent plant and animal structures are no longer
recognisable. The term humus is applied to this part of soil organic matter.
During the formation of humus, the calcium, nitrogen, potassium and other
minerals bound in the dead plant and animal remains are changed into soluble
compounds which are absorbed by the living plants again. Humus is colloidal in
nature and its capacity to hold water and nutrient ions is greater than that of
clay.
The non-solid or pore spaces in the soil are occupied by either
water or air. The proportion of these two components in a given soil is
interrelated; as one increases, the other decreases and vice versa. An optimum
balance between water and air must be maintained for best plant growth.
(c) Soil water
Soil water is the solvent medium for nutrients needed by the growing
plants, maintaining equilibrium among the cations and anions adsorbed on soil
particles, the living plants and soil water (soil solution) itself.
Water is held within the soil pores with varying degrees of tenacity
(holding firmly) depending on the amount of water present. The dissolved salts
form major source of nutrient supply to growing plants. Not all water that is
present in the soil is available to the plants. The water that can be absorbed
by the plants is termed available water.
The water which the soil does not yield to the plant is termed non-available water. This is tightly
held as thin films surrounding the soil particles and in association with the
soil colloids (hygroscopic water). A soil which contains no more water than the
hygroscopic coefficient will not yield water to plants.
(d) Soil air
Soil that is not water logged contains air. Soil air contains oxygen
necessary for the break down of insoluble minerals into soluble salts and also
in the process of humus formation in which the nutrient material locked up in
plant and animal remains is changed into soluble compounds. Oxygen is also
essential for bacterial nitrification and nitrogen fixation. Soil air also
contains carbon dioxide and atmospheric nitrogen-fixing soil organisms and
other gases. The soil air is not continuous, being located in the soil pores
separated by soil solids. There is a lot of variations in soil air composition
from place to place. Soil air has higher moisture and the concentration of CO2
is higher and that of O2 lower than found in the atmosphere.
(e) Soil Organisms
There are numerous species of bacteria, fungi and protozoa that
inhabit the superficial layers of well aerated soils that are rich in humus.
The organic matter contained in dead plants is necessary for their nutrition.
The microorganisms decompose the organic matter and convert it into humus from
which other plants derive their nutrition.
SOIL PROPERTIES
(i) Soil texture
Refers to the relative proportion of soil particles of various
sizes. This determines the water holding capacity and aeration of the soil.
Texture is determined by the size of the mineral particles that compose the
soil. Soils can be classified according to particle size from coarse stones to
fine clays.
Four major Size class of
inorganic particles and their General Properties.
Particle size Classification Dominant composition
>7.5cm Stones Rock
fragments &visible
2mm-7.5cm Gravel Sandy and
visible
0.2mm – 2mm coarse
sand Primary
minerals
0.02mm 0.2mm Fine
sand
0.002mm – 0.02 silt microscopic/secondary
>0.002mm clay electron
microscope and
Minerals
Soils contain all
these types of particles in various proportions and the sizes of the
predominating particles determine the type of soil of a particular site: clay, loam,
sandy, sandy clay, silt clay, silt etc.
The soil particle size is seldom used to draw a soil texture triangle showing the relationship between particle
size, distribution and its class name.
(ii)
Soil pH
It is a measure of hydrogen ion concentration in the
soil. The acidity or alkalinity of the soil affects plant growth. The solubility
of nutrient ions depend on pH which therefore affects their availability to
plants and hence vegetation distribution. For example calcium phosphates are
less soluble as the pH increases and this leads to a decrease in the
availability of calcium ions. Iron and aluminium phosphates are less soluble as
the pH drops and as such there is a decrease in their availability to plants. A
near neutral pH, between 6.5 and 7.5 is best for phosphorus availability as
well as for the availability of most nutrients needed for plant growth.
However, some species will only grow in acid soils and others only in alkaline
soils. Other species will tolerate a wide range of pH. PH also affects the
biological activities of the soil. Neutral and alkaline soils of good nutrient status
usually contain large numbers of organisms like bacteria, fungi and earthworms.
Plant litter is rapidly decomposed and incorporated into the structure of the
soil. Earthworms are rare in acid and nutrient-deficient soils, and the range
of bacteria present is limited. Consequently the litter decomposes very slowly
and available nitrogen is reduced as nitrifying bacteria are repressed. A
highly alkaline soil may reduce the availability of certain nutrients,
especially the trace elements like manganese and boron.
Soil pH ranges
pH
4.0 – 5.5 strongly
acid
5.5 – 6.5 moderately
acid
6.5 – 7.5 neutral
7.5 – 8.5 moderately
alkaline
8.5 -10 strongly
alkaline.
(iii)
Soil structure
The primary particles of sand silt and clay aggregate in certain
fashion to form large blocks of soil referred to as soil structure. These
aggregation patterns influence the physical properties of soil with respect to
aeration, percolation, water retention etc. Other soil properties include
permeability, cohesion, plasticity among etc.
4. BIOTIC FACTORS - PLANT INTERACTIONS
They include living things and their actions and reactions which
organisms impose on each other and the habitat. In nature, populations of
plants, animals and microorganisms live together in a close association. The
organisms influence each other’s life directly or indirectly as well as the
physicochemical conditions of the environment. There are continuous
interactions between plants and plants and between plants and animals. Plants
interact in various ways and these include the following:
(a). Commensalism
It is an unbalanced relationship between tow different species in
which one is benefited by the other. The latter is neither harmed nor
benefitted by the other. The organism which benefits from the association is
called commensal.
(i)
Lianas
They are woody climbers commonly found in tropical
rainforests. Lianas climb the trees to display their leaves, flowers and fruits
for photosynthesis, pollination and dispersal of fruits and seeds. The tree on
which lianas grow is neither benefitted nor harmed by the liana e.g. Tinospora, Bauhinia.
(ii)
Epiphytes
They are plants growing on others for physical support
only e.g. orchids, ferns, bryophytes, mosses and lichens. They are not anchored to the soil like the
lianas and they do not gain nourishment from the host plant; they are
autotrophs. The plants are provided with special type of epiphytic roots that
readily absorbs moisture, rainwater and dew drops. They absorb mineral
nutrients from the cracks and crevices in the bark of trees. The supporting
tree is neither harmed nor benefitted by epiphytes.
(b). Symbiosis and Mutualism
Symbiosis is a condition where two or more different organisms live
together in close association and they all benefit from the association. The associating organisms are known as
symbionts and they are either permanently or temporarily in close contact.
Symbiosis develops between autotrophs and heterotrophs; heterotrophs depend on
the autotrophs for nourishment and protection; autotrophs depend on
heterotrophs for protection, mineral cycling and other vital functions.
(i)
Lichens
It is an association between fungus and algae. The
mycobiont (fungus) envelopes the phycobiont (algae). The fungal filaments
absorb water and minerals from the substratum and supply them to the algae. The
fungal filaments also retain water that protects the algal partner from
desiccation. Thus the algal component of the lichens are provided with shelter
and protected from intense light and drought by the fungal filaments. In turn
the phycobiont provides food to the mycobiont.
(ii)
Bacterial nodules
Roots of leguminous plants like Casuarina and Podocarpus
are associated with the Rhizobium
bacteria which stimulates the formation of nodules. The bacteria absorb
atmospheric nitrogen and make it available for absorption by the plant. The bacteria
in turn get shelter and food within the root nodules.
(c). Parasitism.
Parasites are organisms which derive their nutrition from the host
organism. They are smaller in size than the host and sometimes host specific.
They are either endo- or ecto-parasites. The parasite benefits from the host,
but the host is in turn harmed by the parasite. For example Pythium and Polyporus are facultative fungal parasites. Polyporus is an endo-parasite on tree trunks. The mycelium draws
its nourishment from the living cells of the host. However, when the tree is
cut, the fungus switches over from parasitic mode to saprophytic one.
Cascuta is a leafless plant devoid of chlorophyll. It is an obligate
or total parasite and depends completely on the host.
(d). Amensalism
This is an interaction that depresses one organism while not
affecting the other i.e. depresses organism A and not B, which remains stable.
Normally, large plants suppress smaller plants (or inhibit some animals) by
shading or any other kind of habitat modification. Some plants release chemical
substances in the surrounding that inhibit or depress the growth of other
plants but the plant remains stable (not
affected by the chemical substances). Penicillium
found in soil produce antibiotic substances that check the growth of a wide
variety of bacteria.
(e). Competition
Competition involves organism generally through utilizing the same
resource. Plants can therefore compete for scarce resources found in their
environment. These can be light, minerals, space, water etc. Competition
usually occurs when plants pre-empt some or all of the suppliers of a
particular substance thereby affecting the functions of the plants like growth,
flowery fruiting etc. Plants especially vascular plants occupy similar
environments with shoots, roots in close proximity if not in contact. The
plants cannot avoid this since they are not able to move away. This means
therefore that plants must compete for the available resources. The weaker
individuals or species are eliminated or suppressed. Competition is either
intra-specific i.e. amongst individuals of the same population or
inter-specific i.e. between two or more different species which occur together.