Research in Ecology - Energy Flow and Food Webs

Energy Flow and Food Webs in Ecosystems

Energy (E) is defined in physics as the capacity to do work. Energy can exist in different forms, such as heat, light, chemical energy, and electrical energy. The study of energy is called Thermodynamics.

Kinetic Energy - the energy of movement (example: )
Potential Energy - stored energy (example: )

Energy is often measured in units called Joules (J).

One J = about 1/4 calorie
one calorie is the amount of energy needed to raise the temperature of 1 gram of water by 1^o C (Celsius)
one dietary Calorie is equal to 1000 calories, and is correctly called one kilocalorie

The First Law of Thermodynamics states that Energy cannot be created or destroyed. It can only be changed from one form to another. This means that the total amount of energy and matter in the Universe remains constant, merely changing from one form to another.

The Second Law of Thermodynamics states that in all energy exchanges, if no energy enters or leaves the system, the potential energy of the system will always be less than that of the initial system.
The "missing" energy will have been converted to entropy, defined as energy unavailable to do work. All systems in the universe tend to go from a state of organized order to a state of chaos (entropy).

Energy in Biological Systems

It all begins with The Sun, which provides all the earth's energy in the form of electromagnetic radiation.

The smallest unit of light energy is known as a quantum, which has properties of both a particle (it can be deflected by solid matter) and a wave (it travels through space in an up and down pattern at a specific wavelength).

Not all quanta are the same. Although they all travel through space at the speed of light (299,792,458 meters per second), they may do so at different wavelengths...

...and different frequencies.

Different wavelengths of electromagnetic energy correspond to different physical entitles which react with matter in different ways.

Visible Light: Quanta You Can See

The quanta that we can perceive as light are called photons, and photons of different wavelengths comprise the visible spectrum.

Humans can see photons ranging in wavelength from about 380 nm (violet) to about 700 nm (red), and a photon will be perceived by your brain as a certain color depending on its wavelength when it hits the color-sensing photoreceptors of your retina.
(Note: A nanometer is a unit of distance equal to 10^-9 meters, or 0.000000001 meters. Very tiny!)

The shorter the wavelength, the higher the frequency, and the higher the energy.

The highest energy photons are in the violet region; the lowest energy ones are in the red region

The Wonders of Photosynthesis

Energy flow on earth begins with certain types of organisms absorbing the light energy of the sun and converting it into chemical energy.

An autotroph (auto = "self" and troph = "feeding") is an organism that captures energy and stores it in the chemical bonds of organic molecules that it manufactures from inorganic molecules. They are also known as producers or primary producers.

A heterotroph (hetero = "other" and troph = "feeding") is an organism that eats other organisms to obtain energy. They are also known as consumers.

The most common means by which autotrophs make organic molecules (sugar) is via PHOTOSYNTHESIS.

(Autotrophs that capture light energy are called photoautotrophs, though there are other kinds of autotrophs.)

PLANTS are photoautotrophs that absorb photons only in a specific region of the spectrum. They have special pigments (a "pigment" is any substance that absorbs light) called chlorophylls and carotenoids which absorb light, capturing its energy to be packaged as sugars.

This happens inside a special organelle called a chloroplast.

Photons interact with matter--including plants--in one of three ways. A photon striking matter (liquid, gas or solid) can be

transmitted (it passes through the matter)
reflected (it bounces off the matter and changes direction)
absorbed (its energy is converted into the energy of the molecule it hits)

Only absorbed photons have biological activity.

Plant pigments absorb photons in the violet/blue region and in the red region.

Thus, only violet, blue and red light will drive photosynthesis.

All other wavelengths are reflected, which is why plants look green. They are reflecting or transmitting the green light, not using it to make sugar.

Overall, the chemical reaction of photosynthesis is as follows:

which means that it takes

six molecules of carbon dioxide plus
12 molecules of water

in the presence of light and the proper enzymes in the cell, to make

one molecule of glucose
6 molecules of oxygen
6 molecules of water

The sugar (glucose) is the storage form for energy in plants, and it's often converted into long chains for long-term storage as carbohydrate, which forms the body of the plant.
The oxygen and water are side products that are not used by the plant in this reaction.

Why Photosynthesis?

What does the plant do with the sugar molecules, once it has them?

body structure (it links sugars to make carbohydrates, such as cellulose)
energy storage (for all the work the plant does)

In fact, any living organism burns organic molecules to release the stored energy and use it to drive its own chemical reactions. The process living organisms use to release the energy stored in sugar is called cellular respiration, and its chemical equation is exactly the opposite of photosynthesis:

which means that

one molecule of glucose in the presence of
six molecules of oxygen and
six molecules of water

can be "burned" to release stored energy as well as the "waste" products of

6 molecules of carbon dioxide and
12 molecules of water

Do Plants Make that Sugar for YOU?

NO. Plants make sugar for their own use. As they collect solar energy and store it as sugar, they are also burning that sugar to release its stored energy. This is used to run the plant's chemical reactions that allow it to function as a plant.

Fortunately for the consumers (heterotrophs), plants usually have some energy left over after photosynthesis. This is what becomes the biomass (dry weight) of the plant, and it's what the heterotrophs eat, stealing the plant's hard-won energy.

Community Interactions Drive Energy Flow

Most everyone has seen a food web.

The food web is made up of organisms at different levels of feeding, known as trophic levels

primary (1^o) producers - organisms that can perform photosynthesis.
primary (1^o) consumers - organisms that eat primary producers.
secondary (2^o) consumers - organisms that eat primary consumers.
tertiary (3^o) consumers - organisms that eat secondary consumers.
quaternary (4^o) consumers - organisms that eat tertiary consumers

...and so on.

Decomposers are a special type of heterotroph/consumer that can eat dead, organic matter (detritus, carrion) and convert it back into its inorganic components. The only organisms that can decompose organic matter are bacteria and fungi.

We also can categorize animals on the basis of the exact type of food they eat. Everyone knows...

carnivore - animal that eats meat
herbivore - animal that eats plant matter
omnivore - animal that eats a variety of things (plant and animal)

But there are also...

detritivore - eats dead, organic matter (detritus), but does not decompose it
insectivore - eats insects
frugivore - eats fruits

Insert your own favorite "vore" here!

The Food Web reflects the flow of energy and nutrients through ecosystems, which we'll cover in more detail later. But for now, we're going to create our own Food Web in a Florida Ecosystem.

Woohoo!