The
environment includes both living and nonliving parts. The study of the
interrelationships between the living and non-living portions of the
environment is the subject matter of ecology.
Ecologists study the interactions between living organisms and between
organisms and their nonliving environment. Understanding these
interrelationships and interactions can aid in the development of useful
conservation measures.
ORGANISMS AND THEIR
ENVIRONMENTS
You're already aware of some
relationships among organisms and their environments. For example, you know
pollution can harm environments and therefore, organisms. You also know about
relationships between certain organisms. You know bacteria and fungi are
organisms that cause diseases in other living things. Organisms and their
environments interact in many ways. It has been suggested that the phrase,
"Everything is connected to everything else," describes the
relationships between organisms and their environments. Look for examples of
these connections as you study this unit.
The
Biosphere
The portion of Earth that supports
life is called the biosphere. The biosphere extends
from several kilometers up in the atmosphere to the deepest parts of the
oceans. It includes the solid portion of the land where life is found.
Sometimes the biosphere is called the ecosphere because the three regions of
the biosphere--air, water, and land--are connected. If chemicals, such as pesticides,
are sprayed into the air, they may eventually pass into water systems or may
cover the land. Fertilizers spread on the surface of the land may get into the
water or air. Care must be taken to protect all parts of the biosphere so that
organisms in each part survive.
Every organism in the biosphere
depends on its environment for survival. The environment supplies organisms
with energy and materials for growth and repair. Plants use sunlight, water,
carbon dioxide, and inorganic nutrients for photosynthesis. Animals use plants
and other organisms for their supply of energy and organic matter. Fungi and
bacteria get their energy and materials by decomposing dead matter and wastes.
Organisms depend on nonliving and
living factors in the environment.
Nonliving factors in the environment are called abiotic factors.
Abiotic factors include water, soil, temperature, light, air, and minerals.
Living factors in the environment are called biotic factors. All the living organisms in a
pond represent the biotic factors. Plants, bacteria, fish, worms, and crayfish
are typical pond organisms.
Populations and Communities
Just as cells are grouped into
tissues and tissues into organs and then systems, organisms also can be put
within groups. A population is a group of organisms of the same species that
live in one area during some specific time. A species is considered to be a
group of organisms that are capable of breeding with each other under natural
conditions and producing fertile offspring. For example, mosquitoes on the surface
of a pond in the spring and maple trees in a Vermont forest in the fall make up
two populations.
Populations can be grouped together.
All the populations of different species that interact with each other within
an area make up a community. All the protists, plants, and animals that interact
on a coral reef make up a reef community.
Within a community, each organism is
found in a specific location. The habitat is the environment of a
particular type of organism. For example, ferns are found in a moist, shady
floor habitat of a forest community. The habitat of some snails is the leaf
litter on the forest floor. In a pond community, a frog's habitat is near the
edge of the water and includes both water and land. A trout in the same
community has its habitat in the deeper, cooler portion of the pond.
All of the biological, chemical, and
physical factors of a species environment are part of its niche. The niche includes what a species needs to survive and reproduce in its
environment. What organisms eat, how they get food, how they attract mates,
where they live and what they do in their environment make up the niche. The
habitat is part of an organism's niche. A habitat is sometimes considered a
species address. The niche is the lifestyle or occupation of a species.
Habitats often overlap and different
organisms can be found in the same location. However, no two species can occupy
exactly the same niche at the same time for very long. If they do, they begin
to compete for the same basic and essential requirements. You may think that
birds within a tree have the same niche. Careful observation will reveal
differences -- some birds eat insects while others eat seeds; some feed beneath
the tree while others feed in the tree. Some birds even get their food away
from the tree. The birds may also have different methods of reproduction. They
may have different mating behaviors, and they may nest in different spots.
Ecosystems
The biotic community and its abiotic
environment interact and function as a system. This interactive system is
called an ecosystem. The interaction involves the transfer of energy and
materials among the organisms. Ecosystems have no size limitations. They may be
as large as a desert or as small as the drops of water on a plant leaf. Plants,
soil bacteria, soil nutrients, air spaces, and light and temperature are part
of the interactive system within a garden.
An ecosystem is self-sustaining when
three conditions are met. First, it must have a relatively constant source of
energy. Sunlight supplies the energy to most ecosystems. Second, energy must be
converted by a living system into chemical bond energy in organic molecules.
Plants, algae, and certain groups of bacteria accomplish this through the
process of photosynthesis. Third, organic matter and inorganic nutrients must
be recycled for reuse. In most ecosystems, this recycling is carried out by decomposers.
An ecosystem becomes unstable when
any of these three conditions is affected.
For example, if the flow of energy from the sun is disrupted,
photosynthesis if affected. Without the food of plants, other organisms and the
plants themselves would die off. If essential nutrients are unavailable or if
certain species die off, the ecosystem could lose its ability to sustain
itself. To remain stable, an ecosystem needs to maintain a dynamic balance
between its biotic and abiotic factors.
BIOTIC RELATIONSHIPS
Suppose you are going to run in an
important race tomorrow. You have heard that "carbohydrate loading"
tonight will provide additional energy. Therefore, you carefully select high
carbohydrate foods. Like other living things, you get energy from your foods.
The foods that you and other animals consume are byproducts of other organisms.
Not only do you consume other
organisms, but you also compete with other organisms for food. Every summer
insects take a large share of home gardens. As much as you might not like to
think about, your body is the home of many parasites. Guess who is the source
of their food?
Feeding Relationships
Within an ecosystem, organisms that
make food by photosynthesis are called producers.
Recall that plants, certain protists and some monerans use energy from the sun
in this process. Producers become the food and energy source for consumers. Consumers are organisms that feed on
other living things. They include animals, fungi, bacteria, and some protists.
Consumers that feed directly on the
producers are called primary consumers. Primary consumers are the food for the
secondary consumers. Animals that derive nearly all of their food resources
from plant matter are called herbivores.
Secondary and higher level consumers that get most of their food from eating
the flesh of other animals are known as carnivores.
Omnivores eat both plants and
animals.
Decomposers are consumers that break
down plant and animal remains and wastes. They decay the organic matter, making
its parts available for reuse. The most common decomposers are bacteria and
fungi. Scavengers are animals
that feed on the dead bodies of other animals. Saprobes are organisms that
obtain their nutrition from plant and animal remains.
Energy flows through an ecosystem
when organisms feed. Organisms are consumers. Higher-level consumers are not
required for an ecosystem to be self-sustaining.
Producers are called autotrophs,
which means "self-feeders" since they "feed themselves" by
making food in the process of photosynthesis. Autotrophs, such as plants,
convert inorganic sources of energy into organic forms. Consumers are called heterotrophs
which means
"other-feeders" since they feed on other organisms. Heterotrophs
require organic molecules to carry out their life functions.
Food Chains and Food Webs
In a self-sustaining ecosystem very
little is wasted. Herbivores, like the grasshopper, eat the leaves of plants.
In turn, carnivores like snakes or bullfrogs eat the grasshoppers. Other
animals such as hawks may eat these carnivores. When any of these organisms
die, decomposers in turn, consume them. After the organisms are decomposed,
their nutrients are eventually taken up and used by green plants. In this way,
matter is transferred through the ecosystem. Nutrients are transferred from
producers to consumers in a feeding relationship known as a food chain. Each organism that eats or
decomposes another is thus a link in that chain.
Food chains often are often unstable
because a change in the population size of any species may affect the chain in
either direction. For example, if a primary consumer depends on one plant
species for its food, the loss of that species could result in the death of the
consumer. As an example, giant pandas have nearly become extinct because they
eat bamboo shoots almost exclusively. The supply of these plants is growing
smaller because the pandas' habitat is being destroyed by humans.
Simple food chains of this type are
rare in nature. Food chains are often seen in ecosystems that are attempting to
re-establish themselves after volcanic activity or fires. Food chains are also
seen in newly formed areas such as new islands. The various feeding levels of producers and
consumers in a food chain are called trophic
levels. Producers belong to the first trophic level, primary consumers
the second, secondary consumers the third. In nature, most organisms rely on
many different sources of food for their nutritional needs. Animals may feed on
several different types of food at the same or different trophic level.
Depending on the availability of the specific foods, foxes may eat mice,
rabbits, berries, or insects. Sea otters eat clams, sea urchins, mussels, and
abalone. Bears eat plant parts as well as fish.
Omnivores are both primary and
secondary consumers dependent upon whether they are eating plant or animal
matter. Complex interrelationships begin to develop involving different trophic
levels. Food chains interconnect into a more complex feeding sequence known as
a food web. Food webs represent
a more diversified feeding sequence and provide greater stability to the
ecosystem.
Energy Flow through an
Ecosystem
While matter passes from part to
part within an ecosystem, energy flows through it. Energy from the sun is
collected, concentrated and converted into chemical bond energy by
producers. As organisms at each
succeeding trophic level break down their food, chemical bond energy in the
food is released as chemical energy and heat. This process of energy transfer
and conversion is not 100 percent efficient and is governed by the Second Law
of Thermodynamics or Energetics.
Green plants convert only a small
percentage of the sun's energy into chemical bond energy of organic molecules.
Producers, consumers, and decomposers use some of the energy to maintain their
life functions. Consumers are unable to digest some of the materials they take
in. The energy in those undigestible foods is released as metabolic wastes. The
energy in the waste material is not available to the consumer that produced it.
The waste may be used as an energy source by organisms like the dung beetles.
Each time that energy is converted
from one form to another, some of the energy is given off as heat. This energy
is no longer available to the organism for work or growth. Ecosystems cannot
sustain themselves without a constant input of energy. Organisms within the
ecosystem require a constant supply of food to serve as fuel in maintaining
their life processes. In both cases, a constant source of energy is needed
because of the inefficiency of energy conversion.
As energy flows through the
ecosystem, only a small amount is used to produce new growth materials for the
organisms within each level. Actual percentages vary with each trophic level
and type of organism, but most scientists have agreed to the ten percent rule.
This rule suggests that only about 10 percent of the energy in the organic
matter at each trophic level is converted to usable materials for growth and
development at the next trophic level. The remaining 90 percent is unavailable
for work or growth because it has been given off as heat or is bound in waste
materials. If 1000 units of energy are available to a first level consumer, the
second level consumer that eats it has about 100 units of energy available to
it.
Scientists use models to show the transfer
of usable and unusable forms of energy in food chains. Pyramid models of this
type are only representations of the natural world. They can be used to
illustrate basic ideas and aid in predicting possible outcomes.
The pyramid of energy shows that as
the number of links in the food chain increases, the amount of usable energy
available to the next trophic level decreases. The pyramid of numbers shows
that fewer organisms are supported at higher levels of a food chain. Biomass is
a measure of the amount of living matter. The pyramid of biomass is a model that
shows that less living matter can be supported at higher trophic levels.
Energy pyramids are always pyramid
shaped, but this is not always true for the pyramids of numbers and biomass.
The pyramid of numbers, for example, is affected the size of the organisms. One
tree might provide food for thousands of insects. A single tuna provides tuna
fish sandwiches for a larger number of humans. The pyramid of biomass might
also be distorted. For example, the biomass of a whale is several times greater
than the total biomass of the plankton it consumes. The whale survives because
the plankton reproduce very rapidly and thus maintain their population size.
The pyramid models emphasize four
important ideas. 1) All food chains begin with producers. 2) Consumers depend,
directly or indirectly, on producers for their energy. 3) The amount of usable
energy available to each trophic level is directly related to the number of
links in a food chain. 4) Solar energy is required at the producer level to
begin the flow of energy through the ecosystem. These principles reinforce the
basic idea of interconnectedness of biotic and abiotic environmental factors.
Factors that Affect Population Size
Interactions among organisms affect
the sizes of populations and the way these populations are distributed. These
interactions can affect energy flow in food chains and webs. The interactions
also help maintain the size and vitality of populations and prevent the
depletion of needed resources like food and water. The end result of these
interactions is the maintenance of a dynamic balance or homeostasis within the
ecosystem.
Organisms may compete for food,
water, light, space, mates, and nutrients. Organisms compete with each other to
obtain the same essential resources. Competition between organisms of the same
species is intraspecific competition.
Two sand crabs may compete for food; two robins may compete for mates. Interspecific competition occurs
between organisms of different species. For example, kelp and red algae may
compete for space, nutrients, and light in the ocean. If organisms that are
competing modify their requirements, the competition is reduced.
If competition is severe enough, it
may cause organisms to migrate in search of resources. It can even cause some
organisms to die. Forests and prairies have been destroyed in favor of
farmlands and housing development sites. Humans compete with other organisms
within these environments for food and shelter and in the process cause the migration
or death of many organisms. For example, the California condor is a species
that is nearly extinct due to humans competing for its habitat. The nesting
areas of these birds have been destroyed. Now humans are trying to save the
condors by breeding them in captivity. Many other species, both plants and
animals, are being threatened in the same way by competition with humans.
Population sizes within communities
are affected not just by competition, but also by the kinds of food each
organism consumes. An organism that captures and eats another is a predator. The organism that a predator
captures is the prey. Predators
and prey regulate the population size of each other. When the number of prey
increases, an increase in the number of predators results. With the increase in
predators, more prey are captured and eaten. The result is that the prey population
decreases. With a decrease in the prey population, less food is available for
the predator population. Therefore, a decrease in the predator population
follows.
This type of relationship is not
always obvious since most organisms rely on several food sources in a food web.
Owls may eat mice, rats, rabbits, or other prey, depending on their availability.
Usually the classic pattern of increase and decrease in predator-prey
relationships is seen when the species are in a confined or restricted area
with few additional food sources. Other factors such as sickness, over-hunting,
and earthquakes can cause short-term imbalances in either a predator or a prey
population.
ABIOTIC FACTORS
Not many people live in Death Valley,
especially in the summer. The sun scorches the land. Daytime temperatures top
50oC. Rainfall in Death Valley is usually less than 40 mm per year.
Physical factors determine the types of organisms that live in this environment.
Light, temperature, and moisture play major roles in the distribution of plant
and animal communities.
Light, Temperature, and
Water
You may have tried to grow
houseplants outdoors and found that the sun burned the leaves. Perhaps you have
forgotten to water your garden plants and found that they had wilted or died in
the summer heat. Different plant species have different light requirements.
Ferns on a forest floor require shade or diffuse sunlight. Other plants, like
the desert cacti, require bright light. The intensity and duration of light
affects plant growth and distribution. At the equator, plants get 12 h of light
each day. In Alaska, plants may get 22 h of light each day in the middle of
summer and about 2 h of light each day in the middle of winter.
Radiant energy of the sun is changed
to heat as it is absorbed by air, land, and water. This energy conversion helps
maintain the earth's temperature and affects the movement of water through the
biosphere. The movement of water affects the movement of other essential
chemical elements.
Temperature affects the rate of
metabolic processes, reproduction, and survival of plants. Differences in air
temperature create air movements that carry moisture toward or away from
plants. Air temperature determines the amount of water vapor and other gases
that the air may hold. Soil temperature determines the rate of water absorption
by the roots of plants and the rate of root growth.
Species distribution depends on
moisture. Some organisms are at home in rain forests where it rains every day.
Others are adapted to life in deserts where water is in short supply.
Well-aerated soil is filled with air passages allowing the circulation of gases
such as oxygen, carbon dioxide, and nitrogen. Moisture clings to the surfaces
of soil particles creating conditions that support bacteria, fungi, and
protists. These soil microbes make chemical nutrients available to plants. Some
microbes use up the nutrients, thus retarding plant growth.
In an ecosystem, the biotic communities
interact with the nonliving environment. Abiotic environmental factors control
the distribution, size, reproduction, nutrition, and overall metabolism of the
living communities.
Chemical Cycling in
Ecosystems
Organisms require various chemical elements
for growth and maintenance. Elements required in large amounts are known as
macronutrients. Carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur are
macronutrients that makeup about 95 percent of the mass of all living things.
Other elements such as copper, iodine, and manganese are required in very small
amounts. Elements required in small amounts are known as micronutrients or
trace elements. Earth contains only a fixed amount of these chemical elements.
It is important that they be recycled quickly and efficiently.
Elements follow a circular path from
the abiotic environment to organisms and back again to the environment. This
two-way exchange between the living and nonliving components within the
ecosystem is called a cycle. Chemicals are continuously removed from the
atmosphere, water, and land. They are used by living organisms and then
returned in some form to the nonliving environment. A biogeochemical cycle is the cyclic movement of chemicals
between living and nonliving components of the environment. Biogeochemical
cycling is also called nutrient cycling.
Elements such as carbon, oxygen, and
nitrogen are found in large quantities in the oceans and atmosphere. These
elements are often found combined with each other in these areas. Biogeochemical
cycles are often classified by the storage site or reservoir of the element. Carbon, oxygen, and nitrogen participate
in gaseous cycles because of their atmospheric reservoir and the fact that
these elements are often found in gaseous form.
The elements phosphorus, sulfur,
calcium, magnesium, and copper are found bound into the solid matter of earth's
crust. These elements are involved in sedimentary
cycles for they are usually found in solid form in rock. The hydrologic cycle is the movement of
water from sea to the land and back again to the sea. Movement of elements
within and between the air, land, and water reservoirs is slower than movement
of these elements between organisms.
Many elements are found combined in
nature. Thus, biogeohemical cycles are often interconnected. Oxygen makes up
about 20 percent of the atmospheric gases. Carbon, in the form of carbon
dioxide, makes up about 0.03 percent of those gases. Oxygen is part of the
water molecule. Both carbon dioxide and oxygen dissolve in water. In
photosynthesis, carbon dioxide and water combine to form organic compounds.
During the process, oxygen is released. Organisms use oxygen in aerobic
respiration, releasing carbon dioxide and water.
The Carbon Cycle
Carbon dioxide moves from the
atmosphere to producers who use it in photosynthesis. Consumers and decomposers
eat the producers and each other. Carbon is passed through the food chain.
During respiration, these organisms release carbon dioxide back into the
atmosphere or waters. Carbon dioxide also enters the atmosphere when fossil
fuels and wood are burned. Volcanic activity and the weathering of
carbon-bearing rocks also add carbon dioxide.
Large amounts of carbon are found in
ocean waters. It is dissolved as carbon dioxide or stored as calcium carbonate
in rocks and animal shells. Carbon dioxide diffuses from the water to the
atmosphere. It returns to the waters by precipitation. The remains of plants
and animals may become compacted into carbonate rock. Limestone is a typical
example.
The Nitrogen Cycle
Nitrogen, like carbon, is an
essential element for living organisms. Nitrogen gas (N2) makes up nearly 78%
of the atmosphere. Unfortunately, plants, animals, fungi, and protists cannot
use nitrogen in this form. Most nitrogen gas is converted into usable form by
bacteria living in the root cells of certain plants. A small amount of nitrogen
is converted to usable form by lightning.
Bacteria and lightning convert
nitrogen gas into nitrate or nitrite ions, ammonia gas, or ammonium ions.
Nitrates dissolve in the soil water. They are taken up by the roots of plants
and used to produce proteins and other organic nitrogen molecules. These nitrogen-containing
molecules pass through the food chain. Animal wastes are converted to ammonia
or ammonium ions by decomposers. Ammonium ions are converted to nitrites or
nitrates and used by bacteria for energy. Other bacteria may convert the
ammonia, nitrates, or nitrites back to nitrogen gas.
The Phosphorus Cycle
The phosphorus cycle consists of two
interconnecting cycles. Phosphates in the rock and soil are taken up by plants.
The plants are eaten by herbivores and phosphorus passes through the food
chain. The phosphates reenter the soil in the form of animal wastes. This
portion of the cycle is relatively rapid and localized.
Some phosphates enter water systems
and eventually find their way to the sea. Phosphates are used by algae and the
algae are eaten by fish. In turn, the fish are eaten by birds. Bird waste, rich
in phosphorus, is deposited on islands. Some of the phosphorus is washed into
the oceans. The ocean sediments attract and bind phosphorus very tightly. Over
long periods of time, phosphorus is returned to the land as mountains or
islands rise from the sea bed. As the phosphorus is weathered or eroded, it
returns to the oceans or is passed through the food chain. This portion of the
cycle may take as long as a million years.
The Water Cycle
Water is the most abundant substance
on earth's surface. Water moves from the atmosphere to the earth's surface in
the form of rain, snow, or dew. Several routes are available to water that
falls on land surfaces. It may runoff into rivers, lakes, streams, or the
oceans. It may soak into the ground. There it may be taken up by the roots of
plants. Water often seeps into the ground and becomes part of the groundwater supply. Groundwater may be
stored in underground streams or lake-like areas called aquifers. This
underground water supply moves by gravity and may come to the surface as a spring.
Water returns to the atmosphere by evaporation or transpiration. Evaporation occurs when liquid water is converted
to its gaseous form and moves from a surface. This movement may be from the
surface of a lake, the soil, or an animal. Evaporation helps moderate the
temperature in an area and concentrates minerals in the soil.
Transpiration is a special case of
evaporation. Transpiration refers to
water lost from the surface of a plant, usually its leaves. This water had
carried nutrients up from the roots and through the plant. The connection
between the water cycle and other biogeochemical cycles again becomes evident.
Not all the materials that cycle
through an ecosystem are helpful to living things. Lead, mercury, cadmium, and
radioactive strontium-90 are just a few examples. The proper functioning of the
biosphere depends on the maintenance of its ecosystems. Ecosystems depend on
the maintenance of dynamic balance and interconnections between their biotic
and abiotic components. As you can see, everything is connected to everything else.