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every living thing or organism needs energy .
Energy Flow Through Ecosystems
Energy in a Food Web
All living things require energy in one form or another. Energy is required by most complex metabolic pathways (often in the form of adenosine triphosphate, ATP), especially those responsible for building large molecules from smaller compounds, and life itself is an energy-driven process Living organisms would not be able to assemble macromolecules (proteins, lipids, nucleic acids, and complex carbohydrates) from their monomeric subunits without a constant energy input.
It is important to understand how organisms acquire energy and how that energy is passed from one organism to another through food webs and their constituent food chains. Food webs illustrate how energy flows directionally through ecosystems, including how efficiently organisms acquire it, use it, and how much remains for use by other organisms of the food web.
Energy is acquired by living things in three ways:
and the consumption and digestion of other living or previously living organisms:
Photosynthetic and chemosynthetic organisms are both grouped into a category known as autotrophs: organisms capable of synthesizing their own food (more specifically, capable of using inorganic carbon as a carbon source).
Photosynthetic autotrophs (photoautotrophs) use sunlight as an energy source, whereas chemosynthetic autotrophs (chemoautotrophs) use inorganic molecules as an energy source.
Autotrophs are critical for all ecosystems. Without these organisms, energy would not be available to other living organisms and life itself would not be possible.
Photoautotrophs, such as plants, algae, and photosynthetic bacteria, serve as the energy source for a majority of the world’s ecosystems. These ecosystems are often described by grazing food webs.
Photoautotrophs harness the solar energy of the sun by converting it to chemical energy in the form of ATP. The energy stored in ATP is used to synthesize complex organic molecules, such as glucose.
Chemoautotrophs are primarily bacteria that are found in rare ecosystems where sunlight is not available, such as in those associated with dark caves or hydrothermal vents at the bottom of the ocean.
Many chemoautotrophs in hydrothermal vents use hydrogen sulfide, which is released from the vents as a source of chemical energy. This allows chemoautotrophs to synthesize complex organic molecules, such as glucose, for their own energy and in turn supplies energy to the rest of the ecosystem.
Productivity within Trophic Levels
Productivity within an ecosystem can be defined as the percentage of energy entering the ecosystem incorporated into biomass in a particular trophic level. Biomass is the total mass, in a unit area at the time of measurement, of living or previously living organisms within a paticular trophic level.
Ecosystems have characteristic amounts of biomass at each trophic level. For example, in the English Channel ecosystem the primary producers account for a biomass of 4 g/m (grams per meter squared), while the primary consumers exhibit a biomass of 21 g/m. The productivity of the primary producers is especially important because these organisms bring energy to other organisms by photoautotrophy or chemoautotrophy.
Because all organisms need to use some of this energy for their own functions scientists often refer to the net primary productivity of an ecosystem. Net primary productivity is the energy that remains in the primary producers after accounting for the organisms’ respiration and heat loss. The net productivity is then available to the primary consumers at the next trophic level. In the above example, 13,187 of the 20,810 kcal/m /yr were used for respiration or were lost as heat, leaving 7,632 kcal/m /yr of energy for use by the primary consumers.
Transfer of Energy between Trophic Levels
Large amounts of energy are lost from the ecosystem from one trophic level to the next level as energy flows from the primary producers through the various trophic levels of consumers and decomposers. The main reason for this loss is the second law of thermodynamics, which states that whenever energy is converted from one form to another, there is a tendency toward disorder (entropy) in the system. In biologic systems, this means a great deal of energy is lost as metabolic heat when the organisms from one trophic level consume the next level.
In the ecosystem example, we see that the primary consumers produced 1103 kcal/m /yr from the 7618 kcal/m /yr of energy available to them from the primary producers. The measurement of energy transfer efficiency between two successive trophic levels is termed the trophic level transfer efficiency (TLTE) and is defined by the formula:
TLTE = production at present trophic level X 100
production at previous trophic level
In the TLTE example, the efficiency between the first two trophic levels was approximately 14.8 percent. The low efficiency of energy transfer between trophic levels is usually the major factor that limits the length of food chains observed in a food web. The fact is, after four to six energy transfers, there is not enough energy left to support another trophic level. In the current example, only three energy transfers occurred between the primary producer, (green algae), and the apex consumer (Chinook salmon).
Ecologists have many different methods of measuring energy transfers within ecosystems. Some transfers are easier or more difficult to measure depending on the complexity of the ecosystem and how much access scientists have to observe the ecosystem. In other words, some ecosystems are more difficult to study than others, and sometimes the quantification of energy transfers has to be estimated.
Another main parameter that is important in characterizing energy flow within an ecosystem is the net production efficiency. Net production efficiency (NPE) allows ecologists to quantify how efficiently organisms of a particular trophic level incorporate the energy they receive into biomass; it is calculated using the following formula:
NPE = net consumer productivity × 100
Net consumer productivity is the energy content available to the organisms of the next trophic level.
Assimilation is the biomass (energy content generated per unit area) of the present trophic level after accounting for the energy lost due to incomplete ingestion of food, energy used for respiration, and energy lost as waste. Incomplete ingestion refers to the fact that some consumers eat only a part of their food.
For Example, When a lion kills an antelope, it will eat everything except the hide and bones. The lion is missing the energy-rich bone marrow inside the bone, so the lion does not make use of all the calories its prey could provide.
The inefficiency of energy use by warm-blooded animals has broad implications for the world's food supply. It is widely accepted that the meat industry uses large amounts of crops to feed livestock, and because the NPE is low, much of the energy from animal feed is then lost. There has been a growing movement worldwide to promote the consumption of non-meat and non-dairy foods so that less energy is wasted feeding animals for the meat industry.
For example, it costs about 1￠to produce 1000 dietary calories (kcal) of corn or soybeans, but approximately $0.19 to produce a similar number of calories growing cattle for beef consumption. The same energy content of milk from cattle is also costly, at approximately $0.16 per 1000 kcal. Much of this difference is due to the low NPE of cattle.
NPE measures how efficiently each trophic level uses and incorporates the energy from its food into biomass.In general, cold-blooded animals (ectotherms), such as invertebrates, fish, amphibians, and reptiles, use less of the energy they obtain for respiration and heat than warm-blooded animals (endotherms), such as birds and mammals. The extra heat generated in endotherms, although an advantage in terms of the activity of these organisms in colder environments, is a major disadvantage in terms of NPE.
Many endotherms have to eat more often than ectotherms to get the energy they need for survival. In general, NPE for ectotherms is an order of magnitude (10x) higher than for endotherms. For example, the NPE for a caterpillar eating leaves has been measured at 18 percent, whereas the NPE for a squirrel eating acorns may be as low as 1.6 percent.
What law of chemistry determines how much energy can be transferred when it is converted from one form to another?
a. the first law of thermodynamics
b. the second law of thermodynamics
c. the conservation of matter
d. the conservation of energy
Answer B) Large amounts of energy are lost from the ecosystem from one trophic level to the next level. The main reason for this loss is the second law of thermodynamics, which states that whenever energy is converted from one form to another, there is a tendency toward disorder (entropy) in the system.
What are organisms called that can make their own food using inorganic molecules?
a. autotrophs, b. heterotrophs, c. photoautotrophs, d. chemoautotrophs
Answer D) Organisms capable of synthesizing their own food (more specifically, capable of using inorganic carbon as a carbon source) are known as autotrophs.
assimilation biomass consumed and assimilated from the previous trophic level after accounting for the energy lost due to incomplete ingestion of food, energy used for respiration, and energy lost as waste
biomass total weight, at the time of measurement, of living or previously living organisms in a unit area within a trophic level
chemoautotroph organism capable of synthesizing its own food using energy from inorganic molecules
ecosystem dynamics study of the changes in ecosystem structure caused by changes in the environment or internal forces
food chain linear representation of a chain of primary producers, primary consumers, and higher- level consumers used to describe ecosystem structure and dynamics
food web graphic representation of a holistic, non-linear web of primary producers, primary consumers, and higher-level consumers used to describe ecosystem structure and dynamics
gross primary productivity rate at which photosynthetic primary producers incorporate energy from the sun
net consumer productivity energy content available to the organisms of the next trophic level
net primary productivity energy that remains in the primary producers after accounting for the organisms’ respiration and heat loss
net production efficiency (NPE) measure of the ability of a trophic level to convert the energy it receives from the previous trophic level into biomass
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