CONCEPTUAL LIFE SCIENCE
Ecology
DEFINITION OF ECOLOGY
Ecology is the science that deals in the interrelationships between organisms and
their environment. The environment contains physical factors and living factors.
THE ENVIRONMENT
Physical factors
The physical environment consists of the soil and rocks in the surroundings of the
organism. The light, heat and water in the surroundings are also part of the physical
environment.
Living factors
The living environment consists of all plants and animals nearby which interact
with the organism. This is a community. All of the organisms of the same species within
the community constitute a population.
Energy in the environment
Figure E-1. The pyramid of energy in ecology.
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Food chains
All energy on the surface of the Earth is derived from the Sun. Photosynthesis
occurs in the primary producers that are green plants. Primary consumers eat the plants.
Predators eat the primary consumers. The predators are secondary consumers. This
constitutes a food chain. There are higher levels of a food chain. There may be tertiary
or quaternary consumers. In North America most food chains end with a predatory bird
such as the eagle.
Flow of energy in the food chain
Energy in food chains always begins with the Sun. Sunlight is absorbed by green
plants. The green plant uses photosynthesis to trap solar energy and convert it to glucose.
The glucose molecule contains stored chemical energy.
The primary consumer eats the green plant and gains some of the energy from it.
The primary consumer is a herbivore because it eats the plants. Most of the energy in the
plant is lost when it is eaten.
The secondary consumer eats the primary consumer. It is a carnivore. The
carnivore obtains some of the energy stored in the animal it eats but most of the stored
energy is lost. Thus, energy is lost as the food chain grows longer. In order to obtain the
maximum amount of energy stored by the original producer plant, you have to eat the
plant directly.
Food webs
Food webs are more complicated than food chains because some consumers may
be eaten by several predators. This causes branches in food chains.
Figure E-2. Diagram of a food web.
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Succession of plants
Newly exposed rock will first be colonized by lichens. These are joined by small
plants like mosses and liverworts. After some soil begins to form, ferns and grasses
move in. More soil forms and then shrubs and pine trees begins to grow. The thickness
of the soil continues to increase. The shrubs and pine trees are replaced by oak trees,
beech trees and finally maple trees. In the temperate forests of the United States, the
climax vegetation will be maple trees. The sequence from bare rock to mature forest
takes about 100 years.
Table XIII. Ecological succession.
Early (Pioneer) Plants
1.
Lichens
2.
Liverworts and mosses
3.
Ferns
4.
Grasses and shrubs
Intermediate Plants
5.
Pine trees
6.
Oak trees
7.
Beech trees
Climax Forest Plants
8.
Maple trees
Biomes
A biome is a collection of many communities which is characterized by the same
form of climax vegetation. Terrestrial biomes are listed in Table XIV. `The climax
vegetation is a botanical community. For example, the northeastern United States is in
the biome known as the Temperate Deciduous Forest. “Deciduous” means that the leaves
fall off the trees in the autumn.
The terrestrial biomes depend on climate. The climate changes as you travel
toward the North Pole. It changes in a similar fashion as you go high up into the
mountains. Climates similar to Tundra and Taiga are found on high mountains.
The marine biome includes all of the communities that are found in the ocean.
The types of living things that are present in a given part of the marine biome depend on
the water temperature, availability of food and nutrients for the sea life, and other factors.
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Table XIV. Terrestrial biomes.
Biome
Tundra
Taiga
Temperate Deciduous Forest
Grassland
Desert
Tropical Rain Forest
Climate
Very cold, long winters,
very short growing season,
only the topsoil thaws (permafrost)
Cold winters, moderate growing
season (no permafrost)
Cold winters, hot summers, plentiful
Rainfall, long growing season
Cold winters, hot summers, not
Enough rainfall to support trees
Hot days, cool nights
very little rainfall
Hot days all year long; frequent,
Heavy rain, very humid
Vegetation
Lichens, mosses, grasses,
small flowering plants
Conifers (pine, spruce and
fir trees)
Deciduous trees
many flowering plants
Mainly grasses
Plants adapted to growing
with little water
Plants with large leaves
adapted for heavy rainfall
MATERIAL CYCLES
Nature works in cycles. There are a number of cycles in the ecosystem that serve
to regenerate and renew materials for use by living things.
Figure E-1. The carbon-hydrogen-oxygen cycle.
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The carbon-hydrogen-oxygen cycle
Photosynthesis by green plants releases oxygen into the atmosphere because
oxygen is the waste product of photosynthesis. Living things use oxygen for their
respiration and release water and carbon dioxide into the atmosphere. These materials
are then used by the plants for more photosynthesis.
The nitrogen cycle
Nitrates from the soil are absorbed by the green plants and are used to make plant
proteins. Animals make proteins from amino acids derived from plant proteins. Dead
plant and animal tissue is decomposed by bacterial action. Other bacteria in the soil
transform nitrogen products to produce more soil nitrates for plants.
Figure E-2. The Nitrogen cycle.
RELATIONSHIPS BETWEEN ORGANISMS
Between organisms of the same species there can be sexual contact, cooperation
and social organization, especially in a large population. Between different species, the
interactions are commensalism, mutualism, parasitism and predation. All of these
interactions are forms of symbiosis.
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Symbiosis
“Symbiosis” means living together. The term does not indicate the nature of the
relationship. Life scientists prefer to use one of the other terms that give a more precise
indication of the relationship.
Commensalism
Commensalism is where one organism benefits but does not cause any harm to
the other. An example is Spanish Moss. It hangs from trees in Florida. It is not a
parasite; all it needs is a place to hang. It gets its water from the air.
Mutualism
In mutualism, a mutually beneficial relationship exists where both partners
benefit. An example is the relationship between the legumes (beans, peas, clover, alfalfa)
and the nitrogen-fixing bacteria. The bacteria produce usable nitrogen for the plant and
the plant gives them a place to live.
Termites eat wood that they cannot digest. There are bacteria and protozoa in
their digestive systems that can digest cellulose to produce glucose. The microorganisms
benefit because they have a place to live and a constant supply of cellulose to digest. The
termites get the benefit from the glucose released from the cellulose by the bacteria and
protozoa.
A cow eats grass that it cannot digest. In the multichambered stomach of the cow
are bacteria and protozoa that digest the cellulose for the cow. In return they get a place
to live and a food supply.
Parasitism
In parasitism, the parasite benefits but the host does not. Parasites are generally
internal. In the lower phyla of the animal kingdom, most groups contain one or more
parasites of various kinds. Parasites require the host for reproduction.
Predation
A predator is a free-living and feed on other organisms. Some, like lions, kill and
eat an entire organism. Others, like mosquitoes, take only a small part.