Energy
The Currency of Nature |
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Ecology is essentially the study of
the economy of nature. The currency of this economy is energy. Energy
allows organisms to run their metabolism, to grow, to produce offspring
and to obtain more energy. Ecologists study the ways that species
“make a living”: how they extract resources, maximize
profits, minimize costs and compete with others. It is important
to remember that species do not exist independently of other species.
Ecosystems are made up of interacting populations of organisms and
their physical environment; populations are linked by energy flow.
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Trophic levels - feeding relationships
and flows of energy |
| Ultimately, all of the energy used by
all organisms comes from the sun.
Some organisms (such as plants, algae) use energy from sunlight
to convert carbon dioxide and water into carbohydrates; that is,
they make their food[#](Taiz & Zeiger 2002).
For this reason, they are referred to as producersOrganism that synthesizes its own food, rather than consuming
the tissues of other organisms.
Other organisms use the energy stored in the tissues of producers.
HerbivoresOrganism that eats plants or algae, getting its
energy from the 1st trophic level. (also known as primary consumers) consume plants or parts
of plants. Some organisms feed on and break down particles of
organic matter, such as the remains or wastes of organisms. Predators
get their energy by killing and eating other animals. And parasites
can infest any of these, extracting energy while (usually) keeping
the host alive.
The specifics of these feeding relationships can be described
as a food web[#](Molles 2004).
More generally, organisms in a food web can be grouped
into trophic (feeding) levels based on where they get their food
(see Figure 1).
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Producers are responsible for all of the energy that enters a
food web, so they are grouped as the first trophic level. Trophic
levels 2 and higher contain consumersOrganism which gets its energy from the tissues of other
organisms., those organisms that feed on others.
If producers can support enough herbivores, another trophic level
may be sustained - one made up of predators, which get their energy
by eating animals from trophic level 2 or higher. Some ecosystems
are productive enough (that is, they have enough energy in them)
to support a fourth trophic level of top predators that eat from
levels 2 and 3.
Two other groups take advantage of the energy sequestered by
plants: decomposersOrganism that eats dead or decaying organisms and parasites.
The energy available in an ecosystem is limited to that which
is photosynthesized by producers (often ~1% of solar input of
energy)[#](Molles 2004).
In fact, it is limited more than you might expect. About
96% of the biomass of producers becomes detritus, unavailable
to consumers. What’s more, as energy moves among trophic
groups, about 90% of the energy present in one trophic level never
makes it to the next trophic level. Energy is used to fuel metabolism,
respiration and heat production. Energy is stored or used for
growth, and some is lost to parasites. These losses of energy
limit the number of trophic levels in an ecosystem[#](Molles 2004).
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| Why is this important? |
| Remember that, within any population, more offspring
are born than can possibly survive. Some die before or shortly after birth, hatching,
or germination. Those that survive have another problem: They need
energy. Energy is an absolute necessity. All life depends on it.
At a minimum, energy is needed simply to survive and grow. Energy used for these purposes can
be called 'operating costs'.
For example, an acorn (which contains a small ‘packet’
of energy from the parent tree to fuel germination) sprouts and
grows roots, stems, and leaves. As it grows, the tree needs energy
to produce more wood, roots, stems, and leaves each year. Photosynthesis
uses energy to create and transport sugars, and roots need energy
to take in and transport nutrients and water[#](Taiz & Zeiger 2002).
Any energy intake beyond that needed to cover operating costs
can be stored or used for reproduction. We can call this 'profit'.
Typically, organisms have enough energy ‘profits’
for both reproduction and storage. Short-lived species, such as
annual plants, have no need to store energy; all profits can be used for reproduction.
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| Maximizing Profits |
| Any traits that reduce operating costs will also
increase profits. Reducing wasteful spending and increasing efficiency
are two ways to reduce operating costs.
For example, many animals that spend their lives in total darkness,
such as in caves or underground, do not have functioning eyes. Eyes require
energy to build and maintain. If an individual has a mutation
that disrupts the construction of eyes and
if eyes are of no use, the energy that would have been used to
maintain eyes can be used to increase reproductive success[#](Culver, et al. 1995).
Over time, the increased
reproductive success of eyeless-types would increase their proportion
in the population. Eventually, there would be no individuals with
eyes, because eyes are a waste of energy.
A mutation that improves
metabolic efficiency would similarly become abundant if the extra
profits increase reproductive success for those with the mutation. A
good example of increased efficiency comes from plants in hot,
dry climates. Plant leaves have small openings (stomatesSmall openings (primarily) on the underside of leaves. Their functionis to allow CO2 into the leaf and the waste products of photosynthesis out,and to regulate passage of water
vapor out of the leaf.)
through which carbon dioxide enters. Photosynthesis uses
energy from sunlight to convert carbon dioxide to carbohydrates,
in a process called the Calvin Cycle. Photosynthetic reactions
take place inside structures called chloroplastsA chlorophyll-containing structure found in algae and plant
cells.Photosynthesis takes place here.
(in tissues called mesophyll; see Figure 2), and in most plants, carbon
dioxide enters the Calvin Cycle directly.
When stomates are open, carbon dioxide can enter and oxygen can
exit. But there's a tradeoff: water
can also escape. In hot and dry conditions, plants close their
stomates to prevent water loss. As a result, levels of oxygen
(produced by photosynthetic reactions) increase.
The Calvin Cycle runs most efficiently using carbon dioxide,
but can also use oxygen. In fact, a key enzyme is more likely
to bind to oxygen. With stomates closed, the Calvin Cycle will
run out of carbon dioxide, and begin to use oxygen. When this
happens, the cell uses more energy in making sugars, often causing
a net loss of energy[#](Taiz & Zeiger 2002).
A relatively recent (in evolutionary time) adaptation has arisen
in some plants - particularly grasses - from warmer regions, especially
areas with low atmospheric carbon dioxide concentrations. In these
plants, carbon is sequestered in mesophyll and moved into adjacent
cells (called bundle-sheath cells). Segregated from the oxygen
in mesophyll, the Calvin Cycle proceeds more efficiently because
it uses only carbon dioxide[#](Taiz & Zeiger 2002).
This newer type of carbon fixation/photosynthesis (known as C4) has apparently
evolved independently numerous times from species with the more primitive C3 carbon
fixation[#](Osborne & Beerling 2006).
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Individuals can also possess traits that improve their ability
to sequester energy. Examples among animals include stronger claws
for digging or improved teeth for grinding or cutting. Changes
in the digestive system also can save energy, either by reducing
the energy needed to digest food or by increasing the amount of
energy that can be extracted from each item of food. For example,
mammals have trouble digesting plants because mammals lack the enzymes
needed to break down cellulose, a sugar that makes up plant cell walls.
buffalo and deer have a modified stomach in which plant material
is processed multiple times (see
Figure 3). This process increases both energy-use efficiency
and energy intake[#](Vaughan, et al. 2000).
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Profits can be saved, for example, for use during
times when food is scarce or during periods of hibernation. The
oak tree stores energy over winter to begin growing leaves the
following spring (until they can begin photosynthesis).
Profits are spent on many aspects of reproduction. Plants spend
energy on flowers, nectar, pollen and ovules, and then produce
fruits and seeds. Animals spend energy on sperm, or eggs
and embryos. They usually have to find mates, and perhaps compete
with others in order to mate. Many species feed and protect
their offspring.
Individuals with higher profits have an advantage over competitors.
Some species use the higher profits to produce more offspring.
Others devote more energy to fuel each offspring's growth.
If genetic changes make more energy available for reproduction
and thereby improve reproductive success, those genetic changes
will become increasingly common. The population evolves
and adapts to its environment.
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