Energy Flow through Food Chains

Energy Flow through Food Chains

December 15, 2019 0 By William Morgan


In the previous video clip, we mentioned
the Everglades food web. Food chains and food webs are simply models that show
how energy flows through an ecosystem. Notably this is different than how
matter, such as water carbon and nitrogen, cycles within an ecosystem. Since energy
flows through ecosystem, ecosystems must have a consistent energy source. For most
ecosystems that energy source is the Sun, which then can be used by the first
level of the food chain… photosynthetic plants and algae, the producers. Before we
dive into the Everglades food web, let’s cover some basic physics related to
energy. First, of course, we always seem to start with definitions. Energy is the
ability to do work. Some common units that we use to measure energy
include joules, kilojoules, calories, kilocalories, watt hours, kilowatt hours,
etc… If you’ve taken a physics class, you likely know that there are seven
different forms of energy. Kinetic energy is the energy of movement, mathematically
it can be expressed as 1/2 mv squared, where m is mass, and v is
velocity. Potential energy the energy of position, which mathematically can be
expressed as mgh. m being mass, g the acceleration due to gravity, and h is the
height. Radiant energy, which is the energy in the form of electromagnetic
waves. Chemical energy… energy stored in chemical bonds. Thermal energy (heat) …
energy that flows from objects with higher temperatures to objects with
lower temperatures. Electrical energy… the energy of moving electrons, and
nuclear energy… the energy found within the atomic nuclei. So all types of
energy can be classified as one of these seven forms. Thermodynamics is the
science that studies energy and energy transformations, and two of the laws of
thermodynamics will be important as we study energy flow through ecosystems.
First the first law of thermodynamics… energy is neither created nor destroyed,
but it can change form. This is important. Energy can change from one form to
another. For example, plants change the radiant energy from the Sun into
chemical energy that they store within their body. We can change the chemical
energy that we get when we eat that plant into kinetic energy as we move
around or heat. Further, a car changes chemical energy within gas into
kinetic energy. A toaster changes electrical energy into thermal energy
and some radiant energy, and a lightbulb changes electrical energy into radiant
energy.. and from thermal energy. So energy can change form. Now interestingly, when
energy does change form, some of the useful energy is dissipated as heat
(thermal energy), and this is the second law of thermodynamics. As energy is
transformed from one form to another, some useful energy is dissipated as heat
and released to the cooler surroundings. So no energy transformation is ever 100%
efficient. There’s always some energy that is dissipated as heat and unusable. A result of this is that organism always
need a constant input of energy, and another result is that food chains are
limited in length because as energy is changing form from one step of the food
chain to another, some of this energy is “lost”. So let’s look at a food chain.
Energy flows through ecosystems through food chains. Each step along a food chain
is called a trophic level, and the first trophic level is called the “producers”
These are organisms who get their energy directly from the Sun through the
process of photosynthesis. As you can see from the general equation for
photosynthesis shown at the bottom of this slide,
plants use carbon dioxide water and sunlight to create sugar and waste
products water and oxygen. The sugar is essentially stored energy that the
plants can use later…. or animals can eat. The second trophic level is called the
“primary consumers”. These are the organisms that get their energy from
eating plants. Sometimes we call them herbivores. Thus, the solar energy that the plant stored as sugar is now used by animals. Notably when animals or plants use that stored energy, the process is called
respiration… which you can see is simply the reverse of photosynthesis… and thus
it yields energy. Animals like us need sugar, oxygen, and water to provide us
with energy, and water and CO2 areour waste products. The third trophic level is the
secondary consumers. Predictably these organisms eat the primary consumers. The
fourth trophic level is the tertiary consumers, who eat the secondary
consumers… and it continues in this manner. The secondary consumers and
beyond are all carnivores. Check out this simple grassland
ecosystem food chain, paying attention to the terms used. Also note that the Sun is
included as the initial source of energy. The diagram also shows an another important player in any food chain…. the decomposers. These organisms, for example
bacteria and fungi, break down organic matter to its simplest constituent, such
that plants can once again take the matter (nutrients) back up with water
through their roots, and the matter can continue to cycle within the ecosystem.
Another interesting and very important concept that we can glean from the
diagram is that energy as energy is transferred from one trophic level to
another, some useful energy is dissipated as heat energy. This essentially follows
the laws of thermodynamics. We remember that the second law of thermodynamics
says that as energy is transformed from one form to another, some useful energy
is dissipated as heat. Thus there is less and less energy available for each
trophic level along a food chain. In fact, a general estimate is that only about
10% of the energy is transformed up from one trophic level to the next. Wow!… that’s
not much! Some of this energy has dissipated as heat, as it’s transformed,
or some of it’s converted to other forms of energy, for example kinetic energy. But
importantly there’s another reason for the low rate of energy transfer as well. The entire embodied energy of one trophic level is NOT consumed by the
next trophic level. For example, bones and hair are rarely completely consumed by
the next trophic level. Further, feces still have a significant amount of
stored chemical energy, and the next trophic level is not eating this either. That’s not to say that this energy goes to waste, some of is used by decomposers. But it does help explain why such a small
percent of the energy of each trophic level is transferred along the food
chain, and it helps explain why food chains are generally quite short. They
would need an enormous base to make a long food chain. Food webs are simply
slightly more complex models that show all the different feeding relationships
that exist within the community. It is rare that an organism only has one food
source, as a food chain suggests. Thus a food web is a more robust picture of who eats whom…and how the energy moves through the community. Of course you can dissect a
food web and come up with several different food chain within that food
web, if you so desire. Here’s the Everglades food web that we’ve already
looked at several times. Again it’s simply a more robust model of the energy
transfer through an ecosystem. However you can dissect it and find specific
food chains as well. Yet another model for showing the energy transfer through
a community is called an ecological pyramid. The ecological pyramid, sometimes
called the food pyramid, gives us a visual representation of the energy loss
at each trophic level. The first trophic level, the producers, has to be very large
in order to support a long food chain. Again remember the second law of
thermodynamics, and remember that the entire trophic level is NOT eaten by the
next trophic level. Indeed the energy pyramid helps us visualize why food
chains generally are not longer than five or six trophic levels. There simply
isn’t enough energy! In order to have a food chain of six trophic levels or
more, there would have to be a very large base of producers. Finally, here’s an
ecological pyramid of the Everglades. Notice the large base…and then it gets
narrower as it goes up, because at each look at each step along the way
only about 10% of the energy from one trophic level is being transferred up to
the next trophic level. On this particular ecological pyramid, they’ve
placed many different species from each trophic level on the pyramid, so you can
get a feeling for the food web as well. Here you can compare the food web and
ecological pyramid model simultaneously.. and see how they’re similar. See their
strengths and their weaknesses. Notably, several questions on this week’s
assignment pertain to the concept of food chains and food webs. In particular,
you’ll be watching a video on Mono Lake and answering question about the Mono
Lake food web.