Ecology

Ecology is the study of relationships between living things and their environment. The
term ‘environment’ refers to the conditions that surround an organism. These include both
abiotic (non-living) components and biotic (living) components. Ecology therefore incorporates
most aspects of biology with elements of physics, chemistry, geography and geology. In other
terms, it is the study of the life-supporting layer of land, air and water that surrounds the earth,
called the biosphere.
Biome - A biome is a geographically extensive type of ecosystem. A particular biome occurs
wherever environmental conditions are suitable for its development, anywhere in the world.
Biomes are characterized by the life forms of their dominant organisms, but not necessarily by
their particular species.
Ecosystem – is a stable, settled unit of nature consisting of a community of organisms,
interacting with each other and with their surrounding physical and chemical environment. They
are more or less self-contained functional units. Within an ecosystem there are two major
processes to consider:
i. The flow of energy through the system.
ii. The cycling of nutrients within the system.
Example: a fresh water pond has its own community of plants to collect the necessary sunlight
energy to supply the organisms within it. Nutrients such as nitrates and phosphates are recycled
within the pond, with little or no loss or gain between it and other ecosystems.
Differences between a biome and an ecosystem:
i. Biomes and ecosystems both contain plants, animals, and environmental factors;
however, biomes are much larger areas that can include several ecosystems.
ii. Biomes are determined by climatic factors such as temperature, precipitation, and
latitude.
iii. Ecosystems are defined by the interaction of organisms in trophic interactions, rather than
by the climatic factors of an area.
Community – consists of the total of all the populations that are living and interacting in a
habitat. Within a pond the community would include the populations of rooted, floating and
submerged plants; the population of bottom-living animals; the populations of fish and
non-vertebrates of the open water; and the populations of surface-living organisms.
Population – consists of all the living things of the same species in a habitat at any one time. The
members of a population have the chance of interbreeding, assuming the species concerned
reproduces sexually. The boundaries of populations are often hard to define, except those of
aquatic organisms occurring in a small pond (as they are clearly limited to the boundary of the
pond).
Example: all mature jaguars can breed with one another and so form a single population.
However, the woodlice on a decaying lice can, in theory, breed with those on a log a kilometre or
more away. In reality, the distance can make interbreeding unlikely and therefore they can be
considered different populations. Where the boundary lies between these two populations is
unclear.
Habitat – is the locality in which an organism occurs; it is where the organism is normally found.
For example, the leafy canopy of trees in a rainforest may be a habitat for macaws. If the area is
extremely small, we call it a microhabitat. The insects that inhabit the crevices in the bark of a
tree are in their own microhabitat. Conditions in a microhabitat are likely to be very different
from conditions in the surrounding habitat.
Ecological niche – this is all of the ranges of environmental conditions and resources required for
an organism to survive, reproduce and maintain a viable population. It is also sometimes referred
to as the ecological role of a species within its community. Some of these species may appear
very similar, but their nesting habits or other aspects of their behaviour will be different, or they
may show different level of tolerance to, e.g., a pollutant or a shortage of oxygen or nitrates. Any
differences in niche, however small, limit competition between species. No two species occupy
exactly the same niche.
Competition – resources of every sort are mostly in limited supply, and so organisms must
compete for them. For example, plants may compete for space, light and mineral ions. Animals
may compete for food, shelter and mates. When a resource is in short supply and preventing
unlimited growth, it is known as a limited factor.

Food chains and food webs

The organisms found in a tropical rainforest, or any other ecosystem, rely on a source of
energy to carry out all their activities. The ultimate source of this energy is sunlight, which is
converted to chemical energy by plants (producers) and then passed as food from one animal
(consumer) to another.
Producers
Producers are photosynthetic organisms that manufacture organic substances using light energy,
water and carbon dioxide (photosynthesis). They are classified as autotrophic. The rate at which
they produce this organic food is referred to as their productivity.
i. Gross primary productivity (GPP) is the total production of organic food in a given
area and in a given time. It depends on the types of plant growing there, their density
and the climate.
ii. Net primary productivity (NPP) is the rate of production of organic food after
allowing for that lost vis respiration by the plant – in other words, the production of
material that might be eaten by consumers.
Consumers
Some organisms’ nutrition is dependent upon plant nutrition directly or indirectly to survive.
Their nutrition is classified as heterotrophic and they are a diverse group. Those that eat
producers are called herbivores; Carnivores feed on animals; and Omnivores eat both plants and
animals.
Decomposers
When producers and consumers die, some energy is ‘locked up’ in the complex organic
molecules of which they are made. This energy is used by a group of organisms that break down
these complex materials into simple components again. In doing so, they release valuable
minerals and elements in a form that can be absorbed by lants and so contribute to nutrient
recycling. Organisms that feed on dead plant and animals and on the waste matter of animals are
known as detritivores. Decomposers, such as bacteria and fungi, cause the decay (breakdown) if
dead material, releasing the organic nutrients.
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The feeding relationship in which a carnivore eats a herbivore, which has been eating
plants, is an example of a food chain. At each step in a food chain energy containing materials
are transferred. The steps of steps of the food chain are recognized as feeding levels or trophic
levels. A trophic level is a stage in a food chain at which organisms obtain their food in the same
general manner; for example, all the herbivores are at the same tropic level of ‘primary
consumers’.
Food chains are of two types. If the chain is based on living plants it is known as a
grazing chain; if it is based on dead plant material it is a decomposer chain or a detrital chain.
The shortest food chain has three levels:
Grass –> sheep –> human
And the longest usually no more than four or five:
Banana tree –> herbivorous insect –> spider –> tree frog –> jaguar
The arrows on food chain diagrams represent the direction of energy flow.
Most food chains connect with other chains, since most organisms are the prey of more
than one predator. Food chains are linked together to form a food web.
 Energy Transfer
Only about 1% of the sun’s light energy is captured by green plants and made available to
successive organisms in the food chain. These in turn pass on only a fraction of the available
energy at each stage.
Energy losses in food chains
Plants normally convert between 1% and 3% of the sun’s energy available to them into organic
matter. Losses occur in a number of ways:
i. Over 90% of solar energy is reflected back into space by clouds and dust or absorbed
by the atmosphere and re-radiated.
ii. Not all wavelengths of light can be absorbed and used for photosynthesis.
iii. Light may not fall on a chlorophyll molecule.
iv. Low carbon dioxide levels may limit the rate of photosynthesis.
Plants then lose 20% - 50% of their gross primary production (GPP) via respiration, leaving little
to be stored as potential food for herbivores. Even then, only about 10% of the net primary
production (NPP) of plants is used by herbivores for growth. This low percentage is as a result of
the following:
i. Some of the plant is not eaten.
ii. Some parts are eaten but cannot be digested (e.g. they are lost in faeces).
iii. Some of the energy is lost in excretory materials (e.g. urine)
iv. Some energy losses occur in respiration and heat loss to the environment. These
losses are high in mammals and birds because of their constant body temperature.
 Much energy is needed to maintain their body temperature when heat is constantly
being lost to the environment.
It is the relative insufficiency of energy transfer between trophic levels that explains why:
i. Most food chains have only four or five trophic levels because insufficient energy is
available to support a breeding population at trophic levels higher than these.
ii. The biomass of organisms is less at higher trophic levels.
iii. The total amount of energy stored is less at each level as one moves up a food chain.
It is possible to construct ecological pyramids representing the numbers, biomass or stored
energy of organisms at different trophic levels in a food chain.
Pyramid of Number – this pyramid focuses on the total number of an organism at a specific
trophic level. Usually, the number of organisms at lower trophic levels are greater than the
numbers at higher levels.

Pyramid of Biomass – this is the total mass of the plants and/or animals in a particular place. It is
normally measured over a fixed period of time. A more reliable, quantitative description of a
food chain is provided when, instead of counting the organisms at each level, their biomass is
measured.

Pyramid of Energy – collecting data for pyramids of energy can be difficult and complex, but the
result is a true representation of the energy flow through a food web, with no anomalies. Data are
collected in a given area for a set period of time, usually a year.
Note Briefly
● In both pyramids of numbers and biomass, only the organisms present at a particular time
are shown; seasonal differences are not taken apparent.
● The pyramid of numbers has certain draw backs as certain things are not reflected.
● The results of a pyramid of energy are much more reliable than those for biomass,
because two organisms of the same dry mass may store different amounts of energy.