Food chains and food webs explore the particular trophic relationships between species within an ecosystem--specifically, just who is eating whom. But another very interesting aspect of the structure of an ecosystem is just how the biomass and energy are distributed among the different trophic levels. Note that this is a very different kind of information from that provided by food chains and webs. Here, we ignore individual species altogether and turn our focus on the overall trophic levels.
The descriptive device used to explore this facet of the trophic structure of an ecosystem is called a trophic pyramid. The purpose of a trophic pyramid is to graphically represent the distribution of biomass or energy among the different trophic levels of the ecosystem.
For example, in a biomass pyramid, you would estimate the entire biomass of all producers in an ecosystem, and this total mass would form the "Producer" level of your pyramid. The entire mass of all primary consumers in the ecosystem would form the "primary consumer" level of the pyramid, and so on for all levels represented in the ecosystem. The top level of your pyramid will probably be either tertiary or quaternary consumers. Note that there's a little detail problem here. You'd have to try to examine your omnivore and carnivores carefully to determine what percentage of their food came from each trophic level.
An energy trophic pyramid would be created the same way, except that the levels would represent the amount of energy contained in all of the producers, then primary consumers, then secondary consumers, etc. This is a bit harder and more technical to figure, but good techniques have been developed to estimate the amount of energy contained in different kinds of organims.
The trophic pyramid of an ecosystem, either biomass or energy based, can tell you a lot about that ecosystem. For example, a trophic pyramid of a stable, healthy ecosystem will look something like this:
Note something extremely important about the shape of this pyramid. By far the biggest of the levels is the Producer level. In fact, if I'd created this truly to scale, the difference would be a lot greater than it looks in this image. And the secondary consumer level is much smaller than the primary consumer level. The tertiary consumer level is much smaller still, etc. This relative size relationship exists in both bioimass and energy trophic pyramids, but it is much more exaggerated in energy pyramids than biomass pyramids.
This will always be true in any stable ecosystem because of the simple reality of any energy transfer process (like one animal eating another to get its energy, which is why we eat each other). Energy transfer processes are always a long way short of 100% efficient. In fact, it's been calculated that the average transfer of energy from prey to predator is only about 10%. And it's usually even less efficient when the organism consumed is a plant. This means that fully 90% (or more) of the energy stored in an organism is not transfered to whoever eats that organism--it is lost as waste. The term used for this phenomenon is ecological efficiency. The ecological efficiency of energy transfer from any level of an ecosystem to the next highest level is typically about 10%.
This explains some very interesting things about ecosystems. It explains why the populations of herbivores are always so much larger than the populations of carnivores. It explains why there are more plants than everything else combined--the plant populations support all other populations in the ecosystem, either directly or indirectly.
It's also instructive when we examine issues like theories about dinosaur extinction. The most popular current hypothesis about what happened at the end of the Cretaceous Period is the Alvarez impact hypothesis. At the core of this hypothesis is the notion of an ecological disaster--debris in the atmosphere reducing the incoming sunlight, thus decreasing the amount of photosynthesis in every ecosystem on the planet. The direct result of this would be the dramatic reduction of the size of the Producer level of each of those ecosystems' trophic pyramid. Just think about the implications for all of the animals in those ecosystem.
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