"HOW ECOSYSTEMS WORK: ENERGY FLOW and
NUTRIENT CYCLES"
NARRATION FOR "HOW ECOSYSTEMS WORK: ENERGY FLOW and
NUTRIENT CYCLES"
All organisms living in an ecosystem such as the African savanna require energy and
nutrients in order to carry out essential life activities such as growth, respiration and
reproduction. On the savanna as in nearly every ecosystem on Earth sunlight provides the
energy that powers life, while nutrients and the other critical building blocks of life such
as carbon, nitrogen, phosphorous, and water enter the ecosystem from the non-living
environment, the atmosphere and the earth or sea, from which the ecosystem arises.
Solar energy continuously bombards the Earth day after day, year after year
providing an essentially limitless source of energy for the Earth’s ecosystems. That
energy flows through ecosystems, starting with the plants that convert the light energy to
chemical energy via photosynthesis. Energy captured by plants is then transferred to the
herbivores that graze on the plants. From there part of the energy is transferred to
predators as they eat the herbivores. Scavengers also obtain part of the energy that
flooded into the ecosystem as sunlight as they pick at the remains left by predators.
Finally decomposers breaking down what’s left to the molecular level obtain the last
remaining bits of energy from what started as beams of light. It is fortunate for life on
Earth that the flow of energy from the sun is essentially limitless, because as energy
flows from plant to decomposer each organism along the way releases part of the energy
they capture as heat to the atmosphere, energy that is forever lost for biological purposes
within the ecosystem. Thus, for the flow of energy and for life itself to continue in a
ecosystem, an outside source must constantly be injecting it with new energy.
Unlike solar energy the nutrients essential to life are limited. The Earth only has
so much carbon or phosphorus or nitrogen available for living organisms. For example,
each year photosynthesis in plants and other organisms captures approximately 1/7th of
the carbon available in the atmosphere, if biological processes such as cellular respiration
didn’t return most of this captured carbon back to the atmosphere, life as we know it on
Earth would rapidly end, as there would be no carbon in the atmosphere for plants and
other photosynthetic organisms to carry out life processes such as capturing the energy in
sunlight. So in order for life on Earth to continue nutrients must be constantly recycled by
the Earth’s ecosystems.
Thus, energy flows through ecosystems while nutrients cycle through them. The
processes by which energy flows through and nutrients are cycled within an ecosystem
play a major role in shaping the complex interactions that occur between populations
within the Earth’s great living communities.
Let’s now take a look at how the energy and nutrients crucial to life travel through
ecosystems and the interactions their travels generate.
How Ecosystems Works: Energy Flows Through Ecosystems- Primary Producers
Capture Outside Energy
Ninety-three million miles away, as the sun fuses hydrogen into helium,
tremendous quantities of energy are released, in the form of light and other
electromagnetic waves. The solar energy reaching the Earth each day is roughly 100
million times greater than the energy released by a nuclear bomb. Upon reaching the
Earth, clouds and dust in the atmosphere and the Earth’s surface reflect much of this
energy back into space. Most of the remaining energy is absorbed as heat by the Earth
and atmosphere. After reflection and absorption about 1 percent of the solar energy
reaching Earth is left to power nearly all life on Earth. Of this 1 %, plants and other
producers capture less than 3%. Thus the teeming life on Earth is supported by less than
0.03% of the energy reaching it from the sun.
The process that converts this light energy into the chemical energy that powers
nearly all ecosystems is photosynthesis. In photosynthesis, pigments such as chlorophyll
contained in plants, plantlike protists, and cyanobacteria absorb certain wavelengths of
light energy and use it to combine carbon dioxide and water into sugar, a compound that
stores energy in its chemical bonds. Some of the energy stored in the sugar is used to
power other chemical reactions, that convert sugars into starches, cellulose, fats,
vitamins, pigments, and proteins. Photosynthetic organisms are called autotrophs, or
producers, since they produce food not only for themselves but for nearly all other life as
well. The organisms that rely on the high-energy molecules made by autotrophs are
called heterotrophs, or consumers.
How Ecosystems Works: Energy Flows Through Ecosystems- Measuring Primary
Productivity
The amount of life, or biomass, an ecosystem can support is determined by the
amount of chemical energy generated by producers. The total energy that photosynthetic
organisms make available to other members of the community is called net primary
productivity. Net primary productivity can be measured in units of energy (calories), or
as the dry weight of organic material per unit area per year. The productivity of an
ecosystem is influenced by environmental variables, such as temperature range and the
availability of sunlight, water, and nutrients. In the arctic tundra, for example,
temperature and sunlight limit productivity, while in deserts it is water and nutrients that
limit productivity. In temperate wetlands and tropical rain forests where energy and
nutrients are abundant productivity is high. Overall the primary producers in all the
Earth’s ecosystems use solar energy and nutrients to produce an impressive170 billion
tons of organic material per year.
How Ecosystems Works: How Energy Flows Through Ecosystems- Trophic Levels
Ecologists categorize living things according to their role in the flow of energy
through an ecosystem. The flow of energy through an ecosystem begins with
photosynthetic producers and continues on through several levels of consumers. Each
category organism forms a trophic level. Photosynthetic producers, from marsh grasses to
thorny acacia trees form the first trophic level. Herbivores from caterpillars and
grasshoppers to giraffes and elephants, that feed exclusively on producers are considered
primary consumers, and form the second trophic level. Flesh-eating spiders, birds,
hyenas, lions and other insectivores and carnivores that feed primarily on herbivores; are
called secondary consumers and form the third trophic level. Some carnivores such as
eagles and bears occasionally eat other carnivores or insectivores, and when doing so
they form the fourth trophic level: tertiary consumers
How Ecosystems Works: Energy Flows Through Ecosystems- Food Webs are Complex
The feeding relationships of an ecosystem, are often illustrated by food chains in
which representatives of a trophic level are shown eating a representative of the trophic
level below them. In a prairie community, for example, grasses the major producers are
eaten by grasshoppers the primary consumers. Grasshoppers in turn are eaten by field
mice, secondary consumers in the prairie food chain. Field mice are then eaten by
rattlesnakes-tertiary consumers on the food chain, and finally rattlesnakes are eaten by
hawks the quaternary consumers at the top of our hypothetical prairie food chain. But
though food chains illustrate the general flow of energy through a community. The
relationships they depict are largely simplifications; rattlesnakes don’t just eat mice and
hawks don’t just eat rattlesnakes, both for example eat prairie dogs and other members of
the prairie community such as prairie chickens and jackrabbits.
The actual feeding relationships within a given community are much more
accurately by food webs. For example, bald eagles, are secondary consumers when they
eat herbivores like a snowshoe rabbit, tertiary consumers when eating salmon and
secondary, tertiary, or quaternary consumers when eating ptarrmigan depending on
whether the ptarmigan had been eating vegetation, herbivorous insects, or insect eating
spiders. Brown bears which are omnivores complicate matters even further. Acting as
primary consumers when they eat grasses and berries, secondary consumers when eat
herbivores like arctic ground squirrels, and as tertiary and quaternary consumers when
they eat salmon and spider eating ptarmigans respectively. Though complicated and
incomplete the process of diagramming a food web enables one to begin to understand
the complex relationships that exist in living communities.
For simplicities sake, however, some of the most important members of a living
community - decomposers and detritivores - often aren’t represented on food webs.
Decomposers and detritivores are nevertheless critical to the functioning of living
communities as every member of a community upon its death is consumed at least in part
by these often unnoticed inhabitants of living communities.
How Ecosystems Works: Energy Flows Through Ecosystems- Decomposers and
Detritivores
Decomposers and detritivores are among the most important strands in actual
food webs. By liberating nutrients for reuse, they form a vital link in the nutrient cycles
of ecosystems. Decomposers are primarily fungi and bacteria, which digest food outside
their bodies, absorb the nutrients they need, and release the remaining nutrients into the
soil or water. Most detritivores are small, often unnoticed organisms that live on the
refuse of life: molted exoskeletons, fallen leaves, wastes, and dead bodies. Detritivores
form an extremely complex network of organisms that include earthworms, nematode
worms, millipedes, certain insects, protists, and crustaceans, and even a few large
vertebrates such as vultures. These organisms consume dead organic matter, extract some
of the energy stored within it, and excrete it in a still further decomposed state. Their
excretory products serve as food for other detritus feeders and decomposers, until most of
the stored energy has been utilized.
Once-living organisms are reduced to simple molecules such as carbon dioxide
and water that are returned to the atmosphere, and minerals and organic acids that are
returned to the soil. In some ecosystems, such as deciduous forests, more energy passes
through the detritus feeders and decomposers than through the primary, secondary, and
tertiary consumers combined. If the detritus feeders and decomposers were to disappear
suddenly, communities would gradually be smothered by accumulated wastes and dead
bodies. The nutrients stored in these bodies would be unavailable and the soil would
become poorer and poorer until plant life could no longer be sustained. As a result,
energy would cease to enter the community and the higher trophic levels dependent on
the energy captured by plants would disappear as well.
How Ecosystems Works: Energy Flows Through Ecosystems-Energy Flows Between
Trophic Levels
A basic law of thermodynamics states that the use of energy is never completely
efficient. Light bulbs, computers, televisions, cars, in fact anything that consumes energy
gives off part of that energy as heat. For example, as cars convert the chemical energy
stored in gasoline to the energy of movement, approximately 75% of the energy is
immediately lost as heat. So too in living systems; has we humans exercise vigorously,
the energy of muscular contraction produces large amounts of heat as a by-product. The
firing of neurons in the brain; the germination of seeds; the thrashing of the flagella of a
protist; in fact nearly all biological processes give off heat as a by product.
Just as the use of energy by living organisms is inefficient so is the transfer of
energy from one trophic level to the next. When a primary consumer such as a caterpillar
feeds on a producer like a shrub, only a portion of the solar energy originally trapped by
the scrub is available to the caterpillar. This is because some energy originally captured
by the scrub was lost to the atmosphere as heat as the scrub carried out various metabolic
processes. Further, some of these metabolic processes involved building of molecules
such as cellulose, which the caterpillar cannot breakdown and obtain energy from.
Therefore, only a fraction of the energy captured by the scrub at the first trophic level is
available to caterpillar at the second trophic level. In turn, part of that energy that is
obtained by the caterpillar is released as heat to the environment; or used to power
crawling and the gnashing of mouthparts; or to construct the caterpillars indigestible
chitinous exoskeleton. All this energy is unavailable to the bird in the third trophic level
that eats the caterpillar. The bird, in turn, uses much of the energy captured from the
caterpillar to maintain body temperature, power flight, and build indigestible feathers,
beak, and bone. Energy unavailable to the eagle at the fourth trophic level that consumes
the bird.
How Ecosystems Works: Energy Flows Through Ecosystems- The 10% Law
According to the “10% Law” derived from studies of a variety of ecosystems, the
energy transfer between trophic levels in most ecosystems is roughly 10% efficient. This
means that the energy stored in primary consumers (herbivores) is only about 10% of the
energy stored in the bodies of producers. The bodies of secondary consumers, in turn,
possess roughly 10% of the energy stored in primary consumers. In other words, for
every 100 calories of solar energy captured by grass, only about 10 calories are converted
into herbivores, and only 1 calorie into carnivores. Energy pyramids like this one
illustrate the distribution of energy between trophic levels in ecosystems graphically.
The unequal distribution of energy between trophic levels in ecosystems is
reflected in species populations on the African savanna. Here one notices that the
predominant organisms are plants; which have the most energy available to them because
they can trap it directly from sunlight. The most abundant animals are herbivores feeding
on plants, while carnivores like lions are relatively rare. The inefficient transfer of energy
between trophic levels also has important implications for human food production. The
lower the trophic level utilized, the more food energy available to human populations; in
other words far more people can be fed on grain than on the meat produced by grain fed
cattle.
How Ecosystems Works: Nutrients Cycle Through Ecosystems
In contrast to solar energy which continuously flows into ecosystems from the
sun, nutrients are, as mentioned earlier, scarce and so must be recycled. For practical
purposes, the same pool of nutrients has been supporting life for over 3 billion years.
Nutrients are the chemical building blocks of life. Macro-nutrients, such as water, carbon,
hydrogen, oxygen, nitrogen, phosphorus, and calcium are required by organisms in large
qualities. Micro-nutrients, such as zinc, molybdenum, iron, selenium, and iodine, are
required only in trace quantities. Nutrient cycles describe the pathways these substances
follow as they move from the living to the nonliving portions of ecosystems and back
again.
The major source, or reservoir, of important nutrients is generally in the nonliving
environment. For example, the atmosphere is the major reservoir for carbon and nitrogen
while rock is the reservoir for phosphorus. Let’s look briefly at how carbon, nitrogen,
phosphorus and water cycle through ecosystems.
How Ecosystems Works: Nutrients Cycle Through Ecosystems-Carbon Cycles Through
Atmosphere
All organic molecules are formed out of a framework of carbon atoms. Carbon
enters food webs through producers, which trap CO2 from the atmosphere during
photosynthesis. CO2 makes up 0.033% of the gas in the atmosphere. Some of the CO2
captured by producers is returned to the atmosphere through cellular respiration, while
some is incorporated into their bodies and passed on to herbivores. Herbivores, in turn,
return some of the carbon back to the atmosphere as CO2 through cellular respiration and
while incorporating some of it into their body tissues. The transfer of carbon continues in
much the same way through predators, detritus feeders, and decomposers, until ultimately
most the carbon captured by producers is returned to the atmosphere as CO2.
However, some carbon cycles through the environment more slowly. For
example, mollusks extract carbon dioxide dissolved in water and combine it with
calcium to form calcium carbonate CaCO3, from which they construct their shells. The
shells of dead mollusks often collect in deposits like these cockles shells on the shores of
Shark Bay in western Australia. Deposits like these are often transformed into limestone
rich formations like those in the Pinnacles Desert south of Shark Bay. As these limestone
formations dissolve gradually overtime much of much of the carbon rich calcium
carbonate CaCO3 finds its way back into the ocean or other bodies of water where
chemical reactions transform it into molecular forms that can be used as a carbon source
by different aquatic organisms.
Another long-term carbon cycle produces fossil fuels as the carbon found in the
organic molecules of ancient plant and animal remains is transmuted by temperature,
pressure and vast expanses of time into coal, oil, or natural gas. Thus the energy of
prehistoric sunlight is trapped in fossil fuels. This energy is released by combustion. So is
carbon in the form of CO2. Human activities, including burning fossil fuels and cutting
and burning the Earth's great forests where much carbon is stored, thus increase the
amount of carbon dioxide in the atmosphere which as we will see later can have serious
consequences for the biosphere.
How Ecosystems Works: Nutrients Cycle Through Ecosystems- Nitrogen Also Cycles
Through Atmosphere
Nitrogen is a critical component of the amino acids, proteins, nucleic acids
essential to all living organisms. As the Earth’s atmosphere is about 79% nitrogen gas
(N2) on the surface it would appear easy for living organisms to obtain, but neither plants
nor animals can use nitrogen gas directly. Instead, plants require ammonia (NH3) or
nitrates (NO3-). Plants and the rest of living world depend on nitrogen fixing bacteria,
which combine atmospheric nitrogen with hydrogen to produce the ammonia required by
plants.
Some of these bacteria live independently in water and soil, while others exist in
symbiotic associations with plants called legumes (a group including soybeans, clover,
and peas) where they live in special swellings on the roots. Decomposer bacteria can also
produce ammonia from amino acids and urea found in dead bodies and wastes. Much the
ammonia produced by these bacteria is used by other bacteria as an energy source and
converted into nitrates in the process. Nitrates are also produced by electrical storms and
by other forms of combustion that cause nitrogen to react with atmospheric oxygen. In
human-dominated ecosystems such as farm fields, gardens, and suburban lawns,
ammonia and nitrates are supplied by chemical fertilizers; which are produced by using
the energy in fossil fuels to artificially "fix" atmospheric nitrogen.
The nitrogen incorporated into the amino acids, proteins, nucleic acids, and
vitamins of plants eventually finds its way into the bodies of either primary consumers,
detritus feeders, or decomposers. As it is passed through the food web, most of the
nitrogen is liberated in wastes and dead bodies, which decomposer bacteria convert back
to nitrates and ammonia that recycle though the soil and water. However, a small amount
of nitrogen gas is returned back to the atmosphere by denitrifying bacteria found in
anaerobic conditions in mud, bogs, and estuaries. These denitrifying bacteria break down
nitrates to obtain their oxygen molecules and in the process release the nitrogen
molecules back to the atmosphere.
How Ecosystems Works: Nutrients Cycle Through Ecosystems- Phosphorus Cycle a
Sedimentary Cycle
Phosphorus is a crucial component of biological molecules, including the energy
transfer molecules (ATP) and (NADP), nucleic acids, and the phospholipids of cell
membranes. It is a major component of the teeth and bones of vertebrates. The reservoir
of phosphorus for most ecosystems is crystalline rock, where it is found as phosphate
(PO43-). Since phosphorus does not enter the atmosphere, the phosphorus cycle is referred
to as a sedimentary cycle. As phosphate-rich rocks are exposed and eroded, phosphate
ions are dissolved in rainwater.
Dissolved phosphate is readily absorbed through the roots of plants, and by other
autotrophs such as photosynthetic protists and cyanobacteria. From these producers,
phosphorus it passes through a food web. At each level, organisms excrete excess
phosphate. Ultimately, decomposers return the phosphorus remaining in dead bodies back
to the soil and water in the form of phosphate. Here it may be reabsorbed by autotrophs,
or it may become bound to sediment and eventually reincorporated into rock. Some of the
phosphate dissolved in fresh water is carried to the oceans. Although much of this
phosphate ends up in marine sediments, some is absorbed by marine producers and
eventually incorporated into the bodies of invertebrates and fish. Some of these, in turn,
are consumed by seabirds, who excrete large quantities of phosphorus back onto the land.
Guano deposited by seabirds along the western coast of South America, along with
phosphate-rich rock, are the major sources for the phosphates incorporated into
agricultural fertilizers.
In some ecosystems, especially freshwater ones like lakes and rivers, overall
productivity is limited by the availability of phosphates. Run-off from agriculture fields
and stockyards, municipal sewage discharges, and other sources can create imbalances in
these aquatic ecosystems by carrying large quantities of phosphates them. The imbalance
caused by the increased phosphate levels often leads to what ecologists refer to as
accelerated eutrophication. Huge blooms of algae and other photosynthetic organisms
form, increasing the demand for dissolved oxygen in the body of water. Eventually
anaerobic conditions develop that result in the massive die off of animal life. The smell of
decay rises from these bodies of water as anaerobic bacteria release methane and other
gases as they decompose the dead organisms that litter the body of water. As result of the
problems caused by accelerated eutrophication many states have banned phosphate
detergents and taken other measures to insure excessive levels of phosphates don’t enter
aquatic ecosystems.
A comparable situation exists in marine environments such as estuaries where
nitrates are often the limiting factor in primary productivity. Run-off from agricultural
fields, livestock feedlots, and other nitrogen rich sources can flow into rivers and streams
feeding into estuaries. The high levels of nitrates again set off a process of accelerated
eutrophication like that caused by phosphates in freshwater. The results are comparable.
High nitrate levels are a problem in San Francisco Bay, the Mississippi Delta near New
Orleans, and other estuaries throughout the world that receive large amounts of
agricultural run-off.
How Ecosystems Works: Nutrients Cycle Through Ecosystems- The Water Cycle
The water or hydrologic cycle, differs from most other nutrient cycles in that
water usually remains chemically unchanged throughout the cycle. The major reservoirs
for water are the oceans, which cover about three quarters of the Earth's surface and
contain over 97% of the available water. The hydrologic cycle is driven by solar energy,
which causes water to evaporate into the atmosphere, and by gravity, which draws the
water back to earth in the form of precipitation (rain, snow, sleet, dew). Most of the
evaporation of water occurs over the oceans, and much of this water simply returns to the
oceans as precipitation. Water that does fall on the land takes a variety of paths. A
percentage returns to the atmosphere as the result of evaporation from lakes and streams
and the land itself. Much runs off the land back to the oceans via river networks, while a
small amount enters underground reservoirs called aquifers. The rest is taken up by living
organisms, most of which are about 70% water.
Much of the water absorbed by plants, is returned to the atmosphere as a result of
evaporation from their leaves. But a small amount of the absorbed water is combined
with carbon dioxide during photosynthesis to produce high-energy molecules like
glucose. Eventually these high energy molecules are broken down during cellular
respiration and the water and CO2 they contain released back into the atmosphere.
Herbivores, carnivores, and other heterotrophs acquire water from their food and by
drinking it where they can find it. Like plants, heterotrophs return much of the water they
acquire back to the atmosphere via cellular respiration and evaporation and additionally
through excretion.
Although the hydrologic cycle would continue in the absence of life on Earth, the
distribution of life and the composition of biological communities depends on and, to a
great extent is determined by, the patterns of precipitation and evaporation that exist on
our planet. For example, the hydrologic cycle differs considerably in deserts and
rainforests and this is reflected in the composition of their respective living communities.
In deserts a lack of water limits biological productivity, while in rainforests where water
is abundant biological productivity is much higher and any limits placed on biological
productivity are the result of a scarcity of nutrients.
How Ecosystems Works: Imbalances in Nutrient Cycles- CO2 and the Greenhouse
Effect
As we’ve just seen the cycling of nutrients can have a major impact on
ecosystems. However, the cycling of carbon dioxide doesn’t have a major impact just on
individual ecosystems but on the entire biosphere. Carbon dioxide carries out two
essential roles in the biosphere. As we’ve already seen it is the major source of carbon for
the Earth’s living organisms. But atmospheric CO2 also acts something like the glass in a
greenhouse. It allows light energy to enter the atmosphere, but absorbing and holding that
energy once it has been converted to heat. This is critical in retaining heat in the
atmosphere and maintaining temperatures appropriate for carrying out life processes.
However, large rapid increases in CO2 levels accompanied by proportionate increases in
global temperatures could have a dramatic impact on life on Earth.
Between 345 and 280 million years ago, under the unique conditions of the
Carboniferous period, huge quantities of carbon were diverted from the carbon cycle
when the bodies of plants and animals in large tracts of swamp and forest were buried in
sediments and thus escaped the usual decomposition process. The result was that huge
amounts of carbon were taken out of the carbon cycle. Over time heat and pressure
converted the dead organic matter into coal, oil, and natural gas that lay buried beneath
the ground largely inaccessible to the living world. Over the of the millions of years since
the Carboniferous period a new equilibrium was established in the carbon cycle.
However, in the last two centuries human activities have rapidly increased the level of
carbon dioxide in the atmosphere and begun to shift the equilibrium point.
Since the Industrial Revolution, modem cultures have increasingly relied on the
energy stored in fossil fuels. As fossil fuels are burned in power plants, factories, and
cars, CO2 is released into the atmosphere. Without human intervention, carbon buried
during the Carboniferous period would have slowly been returned to the atmosphere
through natural processes over the course of millions of years, but industrialized
societies are currently burning fossil fuels at a rate that is returning buried carbon to the
atmosphere as CO2 at least 100 times faster than would have occurred under natural
conditions.
In addition deforestation, resulting from the cutting down of tens of millions of
acres of forests each year is also increasing CO2 levels in the atmosphere. Deforestation
is occurring principally in the tropics, where rain forests are being eliminated to make
room for agricultural land. After the massive trees that make up these forests are cut the
carbon stored in them is returned to the atmosphere either through burning or
decomposition. Compounding the problem is the fact that these large trees are no longer
alive and taking up CO2. A result of the burning of fossil fuels and the destruction of
forest land the CO2 content of the atmosphere has increased by 25% in the past 200 years.
Many scientists believe that this increase in atmospheric CO2 is leading to the
phenomenon of global warming. But controversy surrounds the issue of global warming
for a number of reasons. First, long term increases in mean global temperatures are
difficult to establish because of the Earth’s vastness, natural fluctuations in temperature,
and limits to technology. But even if it is established that global warming is occurring, it
is also difficult to determine how much of the warming is due to humankind’s release of
CO2 into the environment and how much is due to cyclical climate changes like those that
have occurred throughout the Earth’s history. The final part of the controversy centers
around the possible ramifications of global warming. Would global warming create world
havoc by flooding coastal cities, negatively disrupting global agriculture and the Earth’s
ecosystems or are some of these claims overstated? Might there actually be a net positive
effect for life on Earth in terms of increased primary productivity in ecosystems and on
agricultural land due to increases temperature and precipitation?
No matter what ones position on global warming it is obvious that more study
and research is warranted, not just of global warming but of all the various ways we
humans affect the flow of energy and the cycling of nutrients through the Earth’s
ecosystems. This so that we humans as stewards of the Earth can make better decisions
not only for ourselves but for all organisms with which we share planet Earth.
Summary
In this program we’ve seen that the flow of energy through an ecosystem usually
starts with the conversion of light energy to chemical energy by plants and other
photosynthetic organisms ecologists refer to as producers. The energy then flows through
numerous trophic levels starting with primary consumers such as herbivores, then on to
carnivores such as eagles which may act as secondary, tertiary or quaternary consumers
depending the feeding habits of their prey. As displayed by energy pyramids the amount
of energy available to any given trophic level is approximately only 10% of that available
to the level before. The feeding relationships that transfer chemical energy from one level
to another are illustrated by food chains and more precisely by food webs. The bodies of
these once living organisms and the last bits of energy they contain are then consumed by
decomposers and detritus feeders.
Unlike energy which is basically endless and flows through ecosystems, limited
nutrient resources such as carbon, nitrogen, phosphorus, and water must cycle through
ecosystems so that they can be recycled and used again by other living organisms. The
cycling of nutrients usually includes cycling through parts of the non-living environment
such as the atmosphere or through rocks and sediments. Human activities can create
changes in the cycling of nutrients that may have a dramatic impact on an ecosystem such
as a lake or estuary or in the case of the carbon cycle the entire global biosphere.
Fortunately, ecologists and others are constantly working on ways to better monitor the
flow of energy and the cycling of nutrients through ecosystems and the entire biosphere.