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6th Grade Common Assessment Cycle 2 Study Guide Reading
Ecosystems - Trophic Levels
The organization of communities is based on the use of energy from the Sun within a
given ecosystem. Organisms within a community are dependent on the survival of the other
organisms because energy is passed from one organism to another.The Sun's energy cycles
through ecosystems from producers through consumers and back into the nutrient pool through
decomposers.
A trophic level describes the feeding level that an organism belongs to.Producer, decomposer,
primary consumer, secondary consumer, and tertiary consumer are all trophic levels that can
be used to describe an organism's place in an ecosystem.
Producer
Producers, such as green plants, use photosynthesis to make their own food using energy from
the Sun. Other producers include algae and some kinds of bacteria and protists.
Green plants, such as sunflowers, are producers that use energy from the Sun to make
food. All of the other organisms in an ecosystem depend on producers for energy.
Consumer
Consumers are organisms that gain energy by eating producers and/or other
consumers. Primary consumers are organisms that feed off of producers.Herbivores are
primary consumers. For example, a deer that eats grass is a primary consumer. Secondary
consumers are organisms that eat primary consumers. Carnivores are secondary
consumers. A wolf that kills and eats a deer is a secondary consumer. Next come tertiary
consumers, then quaternary consumers, and so forth until the top carnivore is reached.
Consumers eat other organisms. Deer are primary consumers because they eat grass, which is
a producer. Wolves are secondary consumers because they eat primary consumers like deer.
Decomposer
Decomposers are organisms that consume dead organisms and release nutrients from dead
plants and animals into the soil, water, and atmosphere.The role that decomposers play in an
ecosystem is crucial. Decomposers are important for the water, carbon, nitrogen, and oxygen
cycles. The nutrients that decomposers release into the soil are also used by producers to make
food.Fungi, such as mushrooms, are examples of decomposers. Some kinds of bacteria are
also decomposers.
Decomposers, such as mushrooms, are vital to ecosystems because they decompose dead
organisms and recycle nutrients back into ecosystems.
Matter & Energy in Ecosystems
Food chains, food webs, and energy pyramids are models that can show the flow of energy and
matter through ecosystems.Matter and energy are conserved as they are transferred and
transformed.
Food Chains
A food chain describes the feeding relationships and energy flow between species within an
ecosystem.
Producers receive energy from the Sun and make food. Producers are the beginning of a food
chain because all of the other organisms in the food chain depend on the food energy that is
made by producers. The next organisms in the food chain are primary consumers, which eat
producers. Next comesecondary consumers, then tertiary consumers, and so forth until the
top consumer is reached. All organisms in the food chain are decomposed bydecomposers.
This food chain shows the flow of energy from a producer, algae, to the consumers in the
ecosystem. Minnows are primary consumers, salmon are secondary consumers, and bears are
tertiary consumers.
The arrows in a food chain represent the direction of energy flow. An arrow points from the
organism that is being consumed to the organism that is receiving energy. For example, in the
food chain above, an arrow points from the algae to the minnow. This means that the minnow
is consuming the algae and receiving energy.
Food Webs
A food web is a group of interconnected food chains. Food webs are commonly used to show
the flow of energy through an ecosystem. However, they are also useful tools for following the
flow of matter. As in food chains, the arrows in a food web indicate the direction the energy
and organic matter flow within an ecosystem.
Organisms within a food web can belong to more than one trophic level, or feeding level. For
example, in the food web below, krill are primary, secondary, and tertiary consumers. Krill are
primary consumers because they eat phytoplankton, which are producers. Krill are also
secondary consumers because they eat herbivorous zooplankton, which are primary
consumers. Krill are tertiary consumers because they eat carnivorous zooplankton, which are
secondary consumers.
An Antarctic food web is shown in the picture above. Organisms in food webs can belong to
more than one feeding level.
Another food web is shown below.
In one of the feeding relationships shown in this food web, organic matter from the grass—in
the form of the sugars, proteins, and water that make up the grass's leaves—becomes
incorporated into the grasshopper. The organic material in the grasshopper is transferred to the
frog when the frog eats the grasshopper and then to the copperhead when the copperhead eats
the frog.When the copperhead dies, its matter will be broken down by decomposers into
smaller molecules which can be released into the atmosphere or become part of the
soil. Ultimately, the atoms in the released molecules will be used by other organisms to build
body structures.
Energy Pyramid
An energy pyramid is a diagram that shows the relative amounts of energy located within
each trophic level. The trophic levels are stacked one on top of another, with the producers on
the bottom. Each level in an energy pyramid has less energy available to it than the level
below.
Most of the stored energy in an ecosystem is in plants and other producers.This is because
most of the energy in an energy pyramid is used or lost as heat energy as it moves up the
pyramid. In fact, only about 10% of the energy produced at each level is available to the one
above it. This is the reason that consumers in an ecosystem cannot outnumber producers, and
predators cannot outnumber prey.
The size of each level of the energy pyramid is determined by the amount of energy stored in
the organisms at that trophic level.
An average of only 10% of the energy from the previous level moves to the next level. The
rest is used up or lost as heat energy.
Conservation of Matter and Energy in Ecosystems
Ecosystems are generally not considered closed systems in terms of matter.Organisms in an
ecosystem give off gases, water, and other wastes that may find their way into the atmosphere
or other ecosystems. Likewise, when producers take in carbon dioxide and produce sugars
with them, they are bringing new matter into the ecosystem from the surroundings.
However, matter is conserved as it moves from one step in a food chain or web to another. For
example, if a bird swallows a grasshopper, all of the matter that made up the grasshopper's
body is inside the bird's body. Some of the matter will be used to make new building and
energy-producing materials for the bird's body. The rest of the matter will be converted into
waste materials that are eliminated from the bird's body and deposited back into the
environment as urine, feces, and waste gases.
Decomposers break down animal wastes and the remains of dead plants and animals. Through
this process, matter is converted back to water and nutrients, which are then returned to the
soil and the atmosphere. This allows the cycle to continue.
Matter and energy are transferred from one organism to another in ecosystems.
Energy is also transformed as it flows through an ecosystem. The total amount of energy in the
universe remains constant, but, unlike matter, energy is able to move freely into and out of the
Earth system. Almost all of the energy entering ecosystems comes from the Sun. Producers are
able to capture some of the Sun's energy during photosynthesis and convert it to chemical
energy.
When consumers feed on producers, some of the producer's energy is transferred to the
consumer. Energy is transferred in a similar fashion each time any consumer or decomposer
feeds on any other organism.
Organisms also use energy to perform life functions. For example, animals use energy to
contract muscles during movement. This process transforms some energy into heat, which is
transferred to the environment. Therefore, when that animal is later consumed, only some of
its energy is transferred to the organism consuming it.
All of the energy that is captured by producers is eventually released and returned to the
environment as heat.
Basic Needs & Organism Growth
All living organisms have basic needs. Organisms must be able to acquire the things that they
need from their habitats in order to grow and survive.
Basic Needs of Animals
The basic needs of animals include shelter, food, water, air, space, waste removal, and a
particular range of temperatures.

Shelter—Animals need shelter because shelter provides protection from the elements and
from predators. A bird's nest is an example of shelter.
 Food—Animals need food because it provides the energy and nutrients necessary to
perform life functions. Some animals eat plants. Some animals eat other animals. Some
animals eat both plants and other animals.
 Water—Animals need water because it transports nutrients and wastes in the animals'
bodies, and it helps keep body temperatures constant.
 Air—Animals need air because oxygen is needed for the chemical process that releases
energy from food.
 Space—Animals need space because overcrowded areas can lead to starvation and
disease.
 Waste removal—Animals need to be able to remove wastes because waste is produced as
animals use food, water, and air for life processes. Animals must be able to get rid of this
waste to survive.
 Temperature range—Animals are best suited to survive in certain temperature ranges.
Animals meet their needs in the habitat, or environment, in which they live.The environment
is everything that surrounds and affects an animal. This includes the plants and other animals
in the area, rocks, soil, air, and water.Both living and non-living things are part of an
environment.
Basic Needs of Plants
The basic needs of plants include sunlight, water, air, soil, and space.


Sunlight—Plants get energy from the Sun. Chlorophyll found in plant leaves uses sunlight
to change water, minerals, and carbon dioxide into food. This process is called
photosynthesis.
Water—Plants must have water because it helps carry nutrients from the soil to the
plant's roots. Water is part of plant cells and is necessary for photosynthesis.

Minerals & Nutrients From Soil—Plants need soil because they use it to get water,
nutrients, and minerals.
 Air—Plants also get nutrients from the air. During photosynthesis, plants take in carbon
dioxide from the air and release oxygen.
 Space—Plants need space to grow. If there are too many plants for the amount of sunlight,
soil, minerals, or space, some of the plants will not survive.
Basic Needs & Organism Growth
While the growth of an organism is partially controlled by the organism's genetic code,
availability of resources also plays a role. Organisms need access to enough resources to meet
their basic needs. For example, if a plant receives very little water for an extended period of
time, the plant might grow much slower than normal. If it is deprived of water for long
enough, the plant may never reach the size that it would have if it had received enough water
during its growth. Likewise, if a plant does not receive enough sunlight or if its environment is
too hot or too cold, then the plant may not grow as large as its potential, and it may eventually
die.
Animal growth is also impacted by the availability of resources to meet the animal's basic
needs. For example, the growth of a koi fish is dependent on the space available to it in its
environment. A koi fish will grow much larger in a large environment than it will in a small
environment. This is due to differences in food availability, as well as water quality.
A koi fish that grows up in a small pond like this one may never get as large as if it had grown
up in a larger pond.
If an animal does not receive enough food or nutrients when it is young, it may grow up to be
smaller than it would have otherwise.
Biotic & Abiotic Factors in Ecosystems
Ecosystems can be characterized by their biotic and abiotic factors. The biotic factors in an
ecosystem are things that are alive. The abiotic factors are things that are not alive.
Biotic Factors
The biotic factors in an environment are the living parts. These include plants, animals, and
other organisms that live there. For example, deer, maple trees, and mushrooms are some of
the living things that might be found in a forest.
Some ecosystems are characterized mainly by the types of plants living in them. An example
is a hardwood forest, which is characterized by the hardwood trees that grow there.
Plants and fungi are both examples of the living parts of an ecosystem.
Abiotic Factors
The abiotic factors in an environment are not alive. These include rainfall, temperature,
sunlight, water, soil, rocks, and air. For example, very low temperatures and very little
precipitation are some of the nonliving factors that are found in the tundra.
Sunlight is a nonliving part of an ecosystem.
Rainfall and temperature are important because they define many ecosystems.For example, a
tropical rainforest is defined by its high rainfall and high temperature throughout the year. A
desert is defined by its low annual rainfall of less than 25 cm.
The abiotic parts of an ecosystem can include many other factors. For example, the abiotic
parts a river ecosystem might include speed of flow, depth of the water, and the amount of
minerals in the water. The abiotic parts of a grassland area might include the nutrients in the
soil, wind, and weather patterns such as dew forming in the morning.
The nonliving parts of this savannah ecosystem determine what kinds of plants and animals
can live there.
Link Between Biotic & Abiotic Factors
Biotic and abiotic factors exist together in an ecosystem and impact each other.
Abiotic factors often determine what living things can survive in the ecosystem.For example,
animals that can survive in the Arctic have to be adapted to cold climates, while animals that
can survive in deserts need to be adapted to dry climates. Little or no plant life can survive in
the Arctic, while a wide range of plants live in rainforests.
The abiotic factors in an ecosystem also provide the things plants and animals need to
survive. These include sunlight, water, and soil for plants. For animals, suitable temperature
ranges, water, and oxygen in the air are critical for life.
Biotic factors can also impact the abiotic parts of an ecosystem. For example, plants add
oxygen to the atmosphere, worms living in the ground aerate the soil, and algae growing in
lakes can reduce the sunlight reaching the bottom of the lake.
Resource Availability & Populations
The growth of a population in an ecosystem is limited by the resources available. Populations
can only grow to a certain point before they begin to run out of resources that the organisms
need in order to survive.
Population Growth
Populations of organisms do not grow linearly. That is, graphs of their populations do not
form a straight line. Instead, populations grow exponentiallybecause the more organisms there
are, the faster the population grows.
For example, if two dogs produce six puppies, and later, those six puppies each produce six
puppies of their own, within two generations, 36 puppies will have been produced from the
original two dogs!
This model, however, shows a very small growth ratio. In reality, it is more likely that the
original two dogs had a set of six puppies every year for several years. Their puppies also
probably produced multiple sets of puppies. This means that the dog population would be
much greater than 36.
Carrying Capacity
As a growing population reaches the limits of the available resources in an environment, its
size levels off to "match" the amount of resources available. In doing so, the population has
reached its carrying capacity. When populations fall below the carrying capacity, the birth
rate tends to go up because there are more resources available. When
populations overshoot the carrying capacity, on the other hand, they generally experience a
substantial decline, or crash, before stabilizing again.
The carrying capacity of a population can be determined by analyzing graphs that track
population size over long periods of time. For example, the carrying capacity of a population
is represented by the red line in the graph below. This is the equilibrium point between small
increases and decreases in the population size.
This pattern continues with only slight changes during periods in which the environment stays
roughly the same. In cases of drastic change, the carrying capacity could change. For example,
if the major food source of a population dies out, the carrying capacity for that population will
drop. Conversely, an increase in available food can cause the carrying capacity for an
organism to increase. For instance, the Agricultural Revolution was responsible for raising the
carrying capacity for humans by increasing the resources available to support the human
population.
Limiting Factors
Limiting factors limit a population's growth. These factors can be resources that organisms
need in order to live and that are present in limited quantities.Limiting factors can be biotic or
abiotic. They include food, water, space, and shelter.
For a plant population, a limiting factor might be how much light there is, or it might be the
availability of good soil. For a hyena population, a limiting factor might be competition for
food with other predators. The number of plants or hyenas in a specific ecosystem will be
limited by the resource that runs out first. This will be the limiting factor. Different
ecosystems can have different limiting factors, even for similar populations.
The limiting factor for a population of hyenas could be many different things, including
competition with other predators.
Limiting factors are sometimes created by humans. If there is a chemical spill that kills plant
and animal life in an area, this could limit how successful the surviving populations can
be. Humans also destroy large amounts of plant and animal habitats. So space is often a
limiting factor for many wild plant and animal populations. All stable populations are subject
to at least one limiting factor.
Patterns of Population Growth
The pattern of growth for a population can change as environmental conditions cause resource
availability to change. These changes could include:
 a change in the food supply

a change in the water supply

a change in temperature

crowding and competition
Competition
Since there are limited amounts of resources in an ecosystem, if one organism gets a particular
resource, another does not. This leads to competition as two organisms try to use the same
resources. Food, water, sunlight, and space are examples of resources that organisms compete
for.
Plants and animals of the same species may compete for resources such as food, water, shelter,
and space. Populations of different species will also compete with one other if their needs are
the same and they share an ecosystem.
For example, trees in a forest compete for sunlight. As one tree grows taller, the shorter trees
are shaded by it, and they receive less sunlight. The shorter trees may die as a result.
The tall tree and the shorter trees in this picture are competing for sunlight.
Population Dynamics
Populations in an ecosystem affect one another. A change in the number of one type of
organism will cause a change in the number of other types of organisms. These changes can be
studied by looking at the population dynamics of an ecosystem.
The food web above represents four populations within a savanna ecosystem.Antelopes
depend on leaves from trees as their major food source. African wild dogs and lions compete
with each other for antelopes as a food source.
The number of organisms within these populations will always be changing as some
organisms are born and other organisms die. The population size of one species will also
change if the population size of another species changes.
For example, if a drought in the ecosystem caused many of the trees to die, there would not be
enough food for all of the antelope to survive. The antelope population would decrease. If the
antelope population decreased, the number of African wild dogs and lions would also likely
decrease because they would be losing a major food source.
Another example would be if humans hunted and killed many of the African wild dogs and
lions. This would cause an increase in the antelope population because they would not have as
many predators. More antelope means they would eat more leaves from trees. This would
decrease the amount of leaves, which is a resource.
All populations within an ecosystem are related in some way. This means that a change in
even one population in an ecosystem can have drastic effects on the ecosystem as a whole.
Organism Interactions
Organisms interact with others within the same population and with organisms from
populations of different species. These interactions can improve or reduce the chance of survival
for some organisms.
Competition
Resources in any environment are limited. There is only so much access to sunlight, food,
water, shelter, and space. Therefore, plants and animals may have to compete, or struggle, to
get resources for themselves. Many animals mark their territories with scents (smells) to try to
keep other animals away.This helps make sure that the food, water, shelter, and possible mates
in the animal's territory do not get taken by another animal. If one animal trespasses on the
territory of another, a fight may start.
Animals compete for food, territory, and mates.
Trees in a forest compete for sunlight. As one tree grows taller, the shorter trees are shaded by
it, and they receive less sunlight. The shorter trees may die as a result.
Cooperation
Cooperation is a type of interaction in which organisms of the same species work
together. Many species show cooperative behavior, including horses, dolphins, lions, and
ants. Animals that exhibit cooperative behavior often live, travel, and/or hunt in herds or
groups. Living in these groups can provide protection for the animals and a higher success rate
during hunts.
Groups of organisms that live together cooperatively are usually part of ahierarchy of
leadership. Some members of the group have a higher status than other members of the
group. Dominant members determine what the group will do and subordinate members follow
their lead. This helps to eliminate aggression between group members and allows the group to
work together for the benefit of all.
Dominant females lead elephant herds.
Fish travel in schools for protection.
An example of an animal that lives in a cooperative group is the wolf. Wolves live in packs
that usually include six or seven members. There are two leaders within the group, the alpha
male and the alpha female, and these two pack members determine when the pack hunts,
moves location, or stays in an area.The leaders of the pack are usually the pack members with
the most experience in hunting and defending territory. The other pack members have roles
within this pack to help the pack work effectively.
Mutualism
Mutualism is a kind of organism interaction that is mutually beneficial to both organisms
involved. Unlike cooperation, mutualism specifically occurs between members of different
species.
Flowers and their pollinators are one of the most common examples of mutualism because
many kinds of plants depend on insects, such as moths, bees, wasps, and beetles, to perform
pollination in order to reproduce. Plants that rely on pollinators attract the pollinator by the
shape, color, or smell of their flowers. As the pollinator feeds on the nectar or pollen from the
flower, some of the pollen sticks to its legs and body. When the pollinator visits a second plant
of the same species, the pollen from the first plant is transferred to the reproductive organs of
the second plant, and pollination occurs. Both organisms receive a benefit from this
interaction. The pollinator receives access to a food source and the plant is able to reproduce
because of their relationship.
Bees are pollinators that receive nectar or pollen from flowering plants. They also aid in the
pollination of the plant, which makes the relationship mutualistic.
Predation
Predation takes place when an organism captures and consumes another organism as a food
source. The organism that hunts is called the predator and the organism that is consumed is
called the prey.
An example of predation is a bird catching, killing, and eating a frog.
In this example, the bird is the predator and the frog is the prey.
Populations of predators and prey are closely linked. There are always more prey than there
are predators in a stable ecosystem. As the population of one changes, the population of the
other will also change. For example, if a prey population decreases, predators will have less
food available, and the predator population will also decrease.
Predators are very important for ecosystem stability because they keep the population of prey
species from getting too large. Without predators, prey species would overpopulate areas and
use up all of the resources, causing the food supply of the prey to decrease.