The
First Organisms
Close to
2.5 billion years ago, the earth's surface and atmosphere
were stable enough to support primitive life. Single-cell
organisms began to develop in the seas that covered the
planet. Most of them were very simple single-cell bacteria
that fed on chemicals in the ocean's waters.
A simple
organism known as blue-green algae appeared and spread
across the seas. Blue-green algae are still alive today. It
was very important to the future of the planet because
blue-green algae used sunlight and water to make food, and
in the process, created oxygen. As the blue-green algae grew
in the earth's seas, they began to fill the atmosphere with
oxygen.
The oxygen
blue-green algae produced made it possible for other types
of organisms to develop. These organisms needed oxygen to
carry out their life processes of growth, feeding,
responding and reproducing. Unlike the blue-green algae,
these organisms could not produce their own food. They
needed oxygen to perform their life processes of growth,
feeding, responding, and reproducing. In return, they
produced CO2, which the algae needed to perform its life
processes. A precise balance between plants and animals was
established.
Classification
In order
to study nature, scientists have classified the life forms
in nature, or put them into groups. Organisms are classified
according to how closely they are related. Large groups are
broken down into smaller and smaller groups. A kingdom is
the largest unit of classification. There are five kingdoms
in nature. One of those kingdoms is Animalia, or the animal
kingdom.
There are
two main groups of animals: vertebrates and invertebrates.
Vertebrates are a subgroup of the Phylum Cordata, or animals
that have a spinal chord. These include fish, amphibians,
reptiles, birds and mammals. You might not think it, but
invertebrates, or animals without a spinal chord, make up
most of the animal kingdom. These include sponges,
jellyfish, worms, arthropods (insects, shrimp, spiders),
mollusks (snails, clams, octopuses), and echinoderms (sea
urchins, sea stars).
Organisms
in the animal kingdom consist of many different species.
Some animals most familiar to us are mammals, birds, fish
and insects.
Animal
cells don't have the rigid cell walls that plant cells have.
Most animal bodies are made up of organized cells that are
specialized to perform a specific task. Other cells are
organized into even more specialized organs. Most animals
are capable of moving relatively fast, unlike plants. Most
animals reproduce sexually.
All told,
around 9 or 10 million species of the kingdom Animalia
inhabit the earth; the exact number isn't known. Most of
them are in the Arthropod phylum, or animals with jointed
legs, like insects and crustaceans. In fact, some scientists
believe that if we were to identify all species in the
tropical rain forests, the ranks of Arthropoda would be over
10 million!
Animal
Ecology
A habitat
is any place where a particular animal or plant species
lives. Examples of a habitat include a lake, a desert, or
forest, or even a drop of water.
All
habitats on Earth are part of the biosphere. Since the Earth
is always changing, habitats are continually changing as
well.
Descriptions
of environments using temperature and rainfall are used to
group habitats together. Habitats of similar climate and
vegetation are called biomes. In different parts of the
world, the same biome may contain different species, but
will contain similar life forms. For example, trees are the
dominant forms of the rain forest, no matter where the
rainforest is located.
Animals,
which live within a same-species group, and occupy an area
at the same time, are part of a population. All members of
the same population have certain traits in common.
Populations of different plants and animals interact with
each other, and together, these populations form
communities. Plants and animals in a particular ecological
community, or biome, must be adapted to the same living
conditions so they can all survive in the same
biome.
Many
populations can live in the same area because each species
fills a specific role in the community. This role is called
a niche. What an animal eats, and where it eats are also
part of its niche. Giraffes can live in the same area as
gazelles because they eat different plants and don't compete
with each other. Dung beetles bury the feces of these
animals and lay their eggs in it. The hatching grubs feed on
the feces. The buried feces also fertilize plants, which in
turn feeds the gazelle and giraffe. Each plant and animal
has its own niche in the ecological community, and is
important in some way to the survival of the
other.
Living
organisms are usually classified as consumers (animals),
producers (plants), or decomposers (fungi), depending on how
they get their food. Consumers are, either herbivores,
carnivores, or omnivores. Herbivores are called primary
consumers because the feed directly on producers. Carnivores
feed on other consumers. Omnivores eat both plants and
animals. However, animals are seldom completely carnivorous
or herbivorous. Some carnivores, such as bears, foxes, and
the family cat or dog, will at times eat plants. Herbivores
will sometimes eat small insects or grubs as
well.
Limiting
factors
There are several limiting
factors in an environment which determine whether an
organism can live in a particular environment. Limiting
factors on land include temperature, water, light,
competition, and soil. Every organism needs certain
requirements for its survival.
Most organisms can survive
if the temperature is within a certain range. The freezing
condition of the tundra has resulted in animals with thick
fur, lots of body fat, and small ears. Animals in a desert
will have large ears, like the elephant, to disperse body
heat.
The amounts of sunlight
and rainfall are also limiting factors for both plants and
animals. Both affect plant growth. Which plants grow in a
biome determines which animals inhabit that biome. For
example, grey squirrels, which feed on nuts, are found in
woodlands, but not in deserts where nuts aren't
found.
Competition results when
two different species try to fill the same niche. This
usually results in one species displacing another species,
or the extinction of one of the species.
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