Each organism has energy costs that keep it alive, such as the costs of finding, capturing, eating,
digesting, and assimilating food and the costs of keeping its cells, tissues, and organs alive and
healthy. These costs are called respired costs. Thus, the amount of food and hence energy
available to each individual can be used for, essentially, three things: Metabolism, Growth, and
Reproduction. The amount of energy that a group of individuals can "give" to the next higher
feeding (or trophic) level is the amount of energy put into growth and reproduction, i.e., a
carnivore can only eat the flesh, offspring or reproductive products of the plant/animal that it preys
upon. It cannot eat the amount of energy used by that organism to keep its body alive and healthy.
Each stack of the pyramid below is a feeding level, also called a trophic level. The bottom-most
level is that of primary producers. Primary producers are autotrophs and consist of plants,
photosynthetic protists, photosynthetic bacteria, and chemosynthetic bacteria depending on the
particular ecosystem. The second level is that of primary (1°) consumers which consists of
herbivores. The third level is that of secondary (2°) consumers which consists of carnivores. All
levels above the third level consist of carnivores and omnivores. If you took the amount of energy
represented by the biomass of all the organisms in each feeding level and created stacked boxes, it
would generate a pyramid shape, as seen below. At each higher trophic level, there are fewer and
fewer organisms and less biomass or energy. The size of the level often corresponds to the number
of individuals in that level. Generally, there are far more primary producers than primary
consumers, than secondary consumers. A very general "rule of thumb" is that each level consumes
90% of its energy intake in respiration, that is, they leave only 10% of their biomass that is
consumable by the next trophic level. That means that 100 pounds of plants are required to keep 10
pounds of a herbivore alive. Using this "rule of thumb" how much biomass (and how many
animals and plants) is required to support 100,100 pound 4° degree carnivores?
Quaternary Consumers
Tertiary Consumers
Secondary Consumers
Primary Consumers
Primary Producers
A. 100,100 pound animals =
B. These animals have to eat
_ ____________ pounds.
__________pounds of 3° carnivores (A times 10).
C. If each 3° carnivore weighs 10 pounds, how many of these do the 4° carnivores have to
eat?________________________(B divided by 10).
D. Each 4° carnivores has to eat___________________of them (C divided by 100).
E. To keep __________________ pounds (answer B) of 3° carnivores alive they have to eat a large
amount of 2° carnivores biomass.
F. How many pounds of 2° carnivores are required?__________________________(E times 10).
G. If each 2° carnivores weighs 1 pound, how many of these do the 3° carnivores have to
eat? ___________________________ (F divided by 1).
H. Each 3° carnivore (answer C) has to eat ____________________of 2° carnivores (F divided by C).
I. To keep ___________________________ pounds (answer F) of 2° carnivores alive they have to eat
a large amount of herbivore biomass.
J. How many pounds of herbivores are required?______________________________(F times 10).
K. To keep _______________________ pounds (answer J) of herbivores alive they have to eat
100,000,000 pounds of 1° producers (J times 10).
L. If each 1° producer weighs 0.1 pound, how many of these do the 2° consumers have to eat? (J
divided by 0.1).
M. Each herbivore has to eat_________________________________of 1° producer (K divided by G).
Draw a stacked histogram of the above numbers (as shown above). Use one 4° consumer and
plot the numbers of 3°, 2°, and 1° consumers and 1° producers needed to keep that one 4°
consumer (1 mm = 1000 individuals).
The above exercise shows the necessity of having both large numbers of individuals and a diverse
number of species available within an environment to allow it to withstand perturbations.
To complete the objectives of this exercise we will sample a community known as the benthic
macroinvertebrates. Benthic macroinvertebrates are the animals that live on and in the bottom of
streams, lakes, ponds, etc. As do all other living organisms, these have a range of tolerance to
physical, chemical and biological conditions. Some can live in only pristine habitats and others can
live in either mildly or heavily impacted (polluted, disturbed, etc.) habitats. The presence or
absence of organisms and their relative abundance (species diversity) in a community of similar
organisms can be used as an indicator of stream health, that is, if it is a pristine habitat or has been
mildly or severely impacted. When environmental quality declines, the first species to disappear
are those that must have a pristine environment. This usually decreases the variety of species in
that habitat (lower species diversity) and leaves a "vacuum" which is filled by greater numbers of
more tolerant species. The community then becomes less stable since there are fewer species, to
support a variety of food webs.