UNIT 2 NOTES - ECOLOGY
Ecology is the scientific study of interactions among organisms and between organisms
and their environments. The part of the Earth that can sustain life is known as the
biosphere, which includes the land, air, and water. The biosphere extends about 5 miles
into the atmosphere and about 7 miles below the surface of the ocean. Ecology teaches us
that organisms interact with each other and interact with their environments; those
interactions are necessary to maintain life.
***Remember the biological levels of organization: atoms – molecules – cells – tissues –
organs – organ systems - organisms – populations – communities – ecosystems – biome biosphere***
AUTOTROPHS
Sunlight is the main energy source for life on Earth. All energy originates from the sun.
The sun’s energy is transferred to plants and converted to food in the process of
photosynthesis. That food, which now contains large amounts of energy, is made available
to other organisms that consume the plants. Plants (and some algae and some bacteria)
that can convert the sun’s energy into food are called autotrophs. Another word for
autotroph is producer.
Photosynthesis is a process that converts light energy into a food source called
carbohydrates. Those carbohydrates include simple sugars as well as starches.
Photosynthesis requires four things to occur: sunlight, water, carbon dioxide, and
chlorophyll. Chlorophyll is the green pigment found in plants that actually traps the sun’s
energy. The chemical formula for photosynthesis is:
SUNLIGHT + 6CO2
+ 6H2O
6O2 + C6H12O6
Some other autotrophs are capable of making food through a process called
chemosynthesis. Chemosynthetic organisms (mostly bacteria) produce food by
converting the energy in chemical bonds of inorganic molecules into carbohydrates.
HETEROTROPHS
Many organisms cannot produce their own food and therefore must acquire energy from
other organisms. These are called heterotrophs, or consumers. There are many different
types of heterotrophs: (1) herbivores – only eat plants; (2) carnivores – only eat animals;
(3) omnivores – eat both plants and animals; (4) detritivores – feed on plant and animal
remains and other dead matter; and (5) decomposers – break down organic matter and
recycle it. Decomposers are primarily bacteria and fungi.
The process of breaking down food into a usable form of energy is called cellular
respiration. Sometimes this is done in the presence of oxygen, and sometimes this is
done without oxygen being present. Below is the formula for cellular respiration (with
oxygen) called aerobic respiration:
C6H12O6
+ 6O2
6CO2
+ 6H2O
+ ENERGY (ATP)
FEEDING RELATIONSHIPS AND ENERGY FLOW
Energy flows through an ecosystem in one direction only: from the sun or inorganic
compounds to autotrophs (producers), and then from the autotrophs to various heterotrophs
(consumers). This flow of energy is shown in a food chain.
A food chain shows how energy flows along a straight line. For example, a food chain
might look like this: sun  algae  minnow  bass  human  bacteria.
The direction of the arrows shows the direction energy is flowing as it travels through
the food chain. Not all energy present at one level passes to the next level, however.
Only about 10% of the energy at one level passes to the next level. What happens to the
other 90% of the energy that was present? Most of it was used up by the organism being
eaten (to carry out daily life activities), and a small percentage is released into the
atmosphere as heat.
A food web is more complex than a food chain. A food web is several food chains
interlinked due to organisms in one food chain also feeding on other food chains, therefore
connecting them. Each step in a food chain or a food web is called a trophic level.
Producers always make up the first trophic level. Consumers make up the second, third, or
higher trophic levels. Primary consumers are those that feed directly on the autotrophs in
a food chain. Secondary consumers are those that feed directly on the primary
consumers in a food chain. Tertiary consumers are those found anywhere above the
secondary consumers on a food chain (4th trophic level or higher). It is important to know
that an organism can occupy more than one spot in a food web; an organism might be a
secondary consumer and also be a tertiary consumer, depending on what all they are
feeding on in the food chain.
CYLCES OF MATTER
As important as energy is to an ecosystem, organisms need more than energy to survive.
They also need water, minerals, and other life-sustaining compounds. In most organisms,
more than 95% percent of the body is made up of just four elements: oxygen, carbon,
hydrogen, and nitrogen. These elements are very common on Earth, but they must be
recycled in order for them to be in a chemical form that cells can put to use.
Matter is recycled within and between ecosystems in biogeochemical cycles. This means
that the cycles connect biological, geological, and chemical aspects of the biosphere.
The water cycle – the process by which water changes from liquid form to atmospheric
gas is called evaporation. Groundwater evaporates and enters the atmosphere in this
process. Water from plants’ leaves evaporates as well, but we call that process
transpiration.
As the sun heats the atmosphere during the day, warm moist air rises. As it rises the water
begins to cool and condenses, forming tiny droplets that form clouds. When the droplets
are large enough, the water returns to Earth in the form of precipitation (rain, sleet, snow,
hail, etc.).
The carbon cycle – Carbon is a key ingredient of living tissue. Carbon is recycled in four
main types of processes: (1) biological processes, such as photosynthesis, respiration, and
decomposition, take up and release carbon and oxygen; (2) geochemical processes, such as
erosion and volcanic activity, release carbon dioxide to the atmosphere and oceans; (3)
mixed biogeochemical processes, such as the burial and decomposition of dead organisms
and their conversion under pressure into coal and petroleum (fossil fuels), store carbon
underground; (4) human activities, such as mining, cutting and burning forests, and
burning fossil fuels, release carbon dioxide into the atmosphere.
The burning of plant matter (biomass) as a source of fuel is one way to reduce the
greenhouse effect. Plants (biomass) take in carbon dioxide first and use it during
photosynthesis. When they are burned as fuel, the carbon dioxide they release is not as big
of a factor since it is carbon dioxide that was already present. Burning of coal or other
fossil fuels adds carbon dioxide to the atmosphere that was not present before, thus adding
to the greenhouse effect.
The nitrogen cycle – all organisms require nitrogen to make amino acids, which in turn
are used to build proteins. Many different forms of nitrogen occur naturally in the
biosphere: nitrogen gas makes up 78% of Earth’s atmosphere; ammonia, nitrate ions, and
nitrite ions are found in waste materials of many organisms and in dead, decaying matter;
nitrogen is in large bodies of water, and in nitrate (a major component of plant fertilizers).
Even though nitrogen gas is the most abundant gas in the atmosphere, only certain types of
bacteria can use it in its natural form. Those bacteria live primarily in soil and on the roots
of plants, and they convert nitrogen gas into ammonia in a process called nitrogen
fixation. Other bacteria then convert the ammonia into nitrates and nitrites. Only after the
ammonia is converted to nitrates and nitrites can producers (plants) use them to make
proteins. Consumers then can eat the producers and reuses the nitrogen to make their own
proteins. When organisms die, decomposers return nitrogen to the soil as ammonia. The
ammonia may again be taken up by producers, but other soil bacteria convert nitrates into
nitrogen gas in a process called denitrification. Denitrification then releases nitrogen into
the atmosphere once again.
The phosphorus cycle – phosphorus is essential to living organisms because it forms part
of important life-sustaining molecules as DNA and RNA. Phosphorus is not very common
in the biosphere, and it does not enter the atmosphere like other gases. Phosphorus
remains mostly on land in rock and soil minerals, and in ocean sediments. Once the rocks
and sediments erode, some of the phosphate washes into rivers and streams, and makes it
way to oceans where it is used by marine organisms. Other phosphorus stays on land and
cycles between organisms and the soil. Plants absorb phosphate from the soil or from
water and bind it into organic compounds; then consumers can obtain it and be passed
through food chains and food webs.
FACTORS THAT AFFECT ECOSYSTEMS
There are two types of factors that can affect an ecosystem: biotic factors which are any
living organisms, and abiotic factors which are nonliving things in an ecosystem. Abiotic
factors can be things like water, temperature, weather, light, soil, rocks, wind, natural
disasters, etc.
The place an organism lives and carries out its life activities is known as its habitat. The
role of that organism, or what it does in an ecosystem, is known as its niche. Niches might
include being a producer, a consumer, or a decomposer.
SYMBIOTIC RELATIONSHIPS
Symbiosis is defined as a relationship in which two organisms live closely together. There
are several types of symbiosis: mutualism-two organisms live closely together and both
benefit from each other; parasitism-two organisms live closely together and one benefits,
but one is harmed; commensalism-two organisms live closely together and one benefits,
and one is not affected at all.
Predator/Prey relationship- a predator hunts an animal, kills it, and eats it. The animal
the predator hunts is known as the prey.
POPULATIONS- there are three important characteristics of a population: (1) its
geographic distribution-describes the area (size range) inhabited by a population (2) its
density-describes the number of individuals per unit area and (3) its growth ratedescribes the increase or decrease in total number of individuals in the population.
A population’s size can be affected by three factors as well: the number of births, the
number of deaths, and the number of individuals that enter or leave the population.
If the birth rate is greater than the death rate, the population size increases.
If the death rate is greater than the birth rate, the population size decreases.
Immigration describes the movement of individuals into an area.
Emigration describes the movement of individuals out of an area.
Factors that might affect a population by causing its growth to decrease or to stagnate are
called limiting factors. There are two types of limiting factors: (1) density-dependent
factors – these factors depend directly on the population’s size. Factors that are densitydependent operate most strongly when a population is large and dense. They do not affect
small, scattered populations as much. Density-dependent limiting factors include
competition (for things like food, shelter, water, mates, etc.), predation, parasitism, and
disease. (2) density-independent factors – these factors affect all populations in similar
way, regardless of the population size. Density-independent limiting factors include
natural disasters (earthquakes, volcanic eruptions, wildfires, etc.), seasonal cycles (wet/dry
seasons, hot/cold seasons), unusual weather (droughts, floods, etc.), and certain human
activities (damming rivers, cutting down forests, etc.).
ECOLOGICAL SUCCESSION
Ecosystems are constantly changing in response to natural and human disturbances. As an
ecosystem changes, older inhabitants gradually die out and new organisms move in,
causing further changes in the community. This series of predictable changes that occurs
in a community over time is called ecological succession.
Succession that occurs on surfaces where no soil exists is called primary succession.
This might occur in areas where volcanic eruptions lead to the formation of new islands or
cover land with lava rock; it can also occur when glaciers melt and leave bare rock
exposed for the first time.
In primary succession the first organisms to occupy and populate the area are called
pioneer species. This is usually something like a lichen, which is a symbiotic
combination of a fungus and an alga. The lichen can grow on bare rock, and as they grow
they help break up the rocks to begin the process of making soil. When lichens die they
add organic material to help form the soil and allow plants to grow. Once plants begin to
populate an area (producers) other organisms will come to that area to feed (consumers).
When a disturbance of some kind changes an existing community, without removing the
soil, then secondary succession can follow. Examples of this could include clearing land
and plowing it for farming, or wildfires that burns some type of woodland or grassland.
Plants in these areas have roots/seeds that will sprout from underground and begin to
repopulate the area.
The process of succession proceeds in a series of predictable stages and ends up with a
mature, stable community known as a climax community. These climax communities
might look like they will not undergo further succession and become permanent, but they
will also undergo changes. Things like long-term climate changes and introduction of
nonnative species can greatly affect these communities.