Powerpoint: Feb 15 Food Webs and Energy Flow
Learning Objectives
- Identify the abiotic and biotic components of an ecosystem
- Describe the roles of photosynthesis and cellular respiration within a pyramid of energy
- Compare photosynthesis and cellular respiration in terms of the reactants, products and chemical equations
- Explain the roles of producers, consumers and decomposers in ecosystems
- Explain the process of a trophic cascade
- Explain the process of bioaccumulation
Understanding Ecosystems
Ecology is the study of interactions of organisms with one another and their physical surroundings.
Why is it important for us to study ecology? In order to properly care for, protect and be stewards to the planet, we must first understand how the living world operates. Just like it is not possible to care for a person without a proper understanding of how the human body operates, we must understand how the planet operates. Just as people are a single living organism, so the earth also operates as a single living species.
To understand and appreciate the ecology of the earth, we must take a holistic approach. Instead of considering the components of the ecosystem and biosphere as being independent, like the parts of a puzzle, we are better able to appreciate the interconnectedness of the earth by considering each component in relation to other parts. For example, a deer is an organism, that does not live independent of the grass and plants it eats, that does not live independent of the air it breathes, that does not live independently of the soil it contributes to through defecation, that does not live independently of the wolves that feed on it… A deer is not only a deer; it is a critical link in the ecosystem it exists.
It is very difficult to study systems with a holistic approach. Therefore, in studying ecology, we often artificially separate these components into smaller parts called ecosystems (division of the biosphere including abiotic and biotic factors affecting organisms and their way of life). We also consider the different parts of the ecosystem, called the abiotic and biotic systems.
| Abiotic | Biotic |
| The non-living portions of the ecosystem, or the parts of nature that are derived from living organisms.
· Soil · Air · Atmosphere · Temperature · Sunlight · Water (rain/ponds/etc.) |
The living portions of the ecosystem, or the parts that are derived from living organisms.
· Plants · Birds, small mammals, insects · Microorganisms · Fungi/decomposers · Humans (yes, we are a part of the living world too) |
It is important to recognize that even as we artificially separate the components of the ecosystem, that they do not exist in isolation from each other. For example, the trees and plants will fundamentally change the atmosphere of the ecosystem it exists; animals that defecate or die will contribute to the soil structure, etc. The physical environment will in turn impact the biotic organisms and their way of life.
Energy Flow in the Ecosystem
All life on earth depends on energy in order to function. Energy, unlike nutrients, cannot be recycled. Once it is used, it cannot be used again. For example, plants use about half the energy it obtains from photosynthesis almost immediately. Therefore, energy is described in terms of energy flow. Unlike nutrients, which can be broken down and reused by other organisms when organisms die, energy that is used cannot be recovered.
Therefore, energy must be replenished. In most ecosystems, the ultimate source of energy comes from the sun. Autotrophic organisms (plants, and photosynthetic protists and monerans) capture the energy of the sun through a process called photosynthesis.
6CO2 + 6H2O + (sunlight) — > C6H12O6 + 6O2
The energy from sunlight is captured in the above process and stored in the C6H12O6 that is produced, otherwise known as glucose, a sugar and energy source. Since autotrophic organisms are able to get energy from this non-living source (sunlight), they are known as producers.
Consumers (heterotrophs) feed on other living things in order to survive, they cannot obtain their own food. Since producers are the only ones, which can obtain energy, all consumers obtain energy from the sun directly or indirectly. They take up this energy in a process called cellular respiration.
C6H12O6 + 6O2 –> 6CO2 + 6H2O + Energy
However, half of the sugar produced by the producers is used up in the daily processes and functions of the organism, like a car engine using up fuel as it runs. Therefore, there is theoretically only half the energy left for consumers to obtain. But since consumers cannot possibly consume all the producers on this planet, the amount of energy that is obtained in the next level is even less than half.
Trophic levels
All organisms on this earth are tied together in networks of feeding relationships (Miller and Levine). We can represent this relationship in terms of a food pyramid. Producers, which obtain energy from the sun, make up the, largest, broadest part of the food pyramid. But remember!
- For every level, a large part of the energy is used up immediately.
- Some of the energy is used to produce structures, such as wood, which are not edible to most organisms.
- Not all of the organisms at each level are consumed. Some die without being consumed.
For all these reasons, for each level of the trophic pyramid, the pyramid becomes smaller and smaller. This represents two things: (1) a smaller biomass (2) less energy in the trophic level. As a rule of thumb only about 10% of the energy from each trophic level is retained.
The trophic level which feeds on producers is called the primary consumers, the trophic level that feeds on the primary consumers are called secondary consumers and the trophic level which feeds on tertiary consumers are tertiary consumers and so on.
There is still one very important component missing in the picture though! Plants, animals and microbes alike that die without being consumed, are broken down by the decomposers. Decomposers come in the form of microbes and fungi alike. They are essentially the recyclers and cleaner-uppers of the natural world!
Decomposers feed on all trophic levels.
This is a very simple construct of the natural world. However, it is important to note that this is oversimplified. In the natural world for example, hawks do not only feed on snakes, they may feed on mice as well. Snakes do not only feed on mice, they may feed on other organisms. The feeding relationships of the natural world is better represented as a web than a pyramid:
Trophic Cascade
Trophic cascades happen when predators in a food web suppress or somehow alter the behaviour of their prey, releasing the next trophic level from predation. See trophic cascades in action in the video below!
An example of this is the wolves of yellowstone national park, which were re-introduced for the first time in 70 years in 1995. Wolves preyed on the deer, which lowered in abundance, and avoided valleys and gorges because of the threat of wolves. This allowed vegetation to increase in abundance, benefiting many other species.
Bioaccumulation
Around the 1940’s and 50’s, DDT, an insecticide, was used to kill various crop pests. At around the same time, bald eagles, peregrine falcons and many other birds of prey started to decline significantly. It was soon discovered (though not soon enough) that these two events were linked. The birds of prey had significantly declined, because their eggshells had thinned so much it was unable to protect the chicks inside, and would even break as the parents sat on the egg to warm it. In the bodies of these birds, high concentrations of DDT was found.
As it turns out, the DDT had somehow made it up the food chain to the top predators. And not only that, the concentration of DDT in their bodies was much higher than for example, in the fish they ate. This is the process known as bioaccumulation: accumulation of substances, such as pesticides or other chemicals in an organism.
Why does the concentration of DDT get higher and higher? Suppose there was a little bit of DDT in every insect. The fish would eat many many of these insects, and accumulate the DDT in their bodies. The larger fish would eat many many of these smaller fish and accumulate the DDT in their bodies, and hawks would eat many many of these larger fish and accumulate more and more into its one body. By the time it reaches the top predators, the amount of DDT that has been “bioaccumulated” is extremely high.
