ECOLOGICAL PYRAMIDS

ECOLOGICAL PYRAMIDS

ECOLOGICAL PYRAMIDS
• If we arrange the organisms in a food chain according to their trophic levels, we often form a pyramid with a broad base representing primary produces & only a few individuals in the higher trophic level.
• This pyramid arrangement is especially true if we look at the E content of an ecosystem.
• Following the 2nd law of thermodynamics, less food E is available to the top trophic level than is available to the preceding level.

• For example, it takes a large # of plants to support a modest colony of grazers such as squirrels. Several squirrels might have been required to feed an ocelot.
• There is less E in each succeeding level due to the following reasons:
1) Some of the food that organisms eat is undigested & doesn’t provide reusable E.
2) E that is absorbed is used in the daily processing of living or loss as heat when it is transferred from one form to another & thus cannot be stored as biomass that can be eaten.

3) Predators such as the ocelot do not operate at
100% efficiency. A general rule of thumb is that only 10% of the E in one consumer level is represented in the higher level.

• (The amount of E available is expressed in biomass). • For example, it generally takes ~ 100kg of grass to make 10kg of agouti & 10kg of agouti to make 1kg of ocelot.
• The total # of organisms & the total amount of biomass of each successive trophic level of an ecosystem may form pyramids similar to those describing E content.

PYRAMID OF NUMBERS
• Refers to the number of individuals at each trophic level in an ecosystem.
• The length/area of each bar/rectangle is proportional to the number of individuals.
• Plants in the 1st trophic level often outnumber animals at the 2nd trophic level, though this will depend on the relative sizes of organisms, such as a tree compared with phytoplankton.
• This is often an upright pyramid but not always.

Pyramid of numbers (upright)
9
carnivores
105 carnivores

50,000 herbivores 100,000 producers Ocean pyramid of numbers

For the oceans as shown above, the bottom level would be quite large, due to the enormous number of small algae.

Pyramid of numbers (inverted)

Top carnivore

Carnivore
Herbivore
Producer

The number of individuals at the producer level is small. Problems with pyramid of numbers
1. Deciding to which trophic level an organism belongs. 2. The producers vary is size, but a single grass plant or alga, for example, is given the same status as a single tree.
3. The range of numbers is so great that it is often difficult to draw pyramids to scale.

PYRAMID OF BIOMASS
• This is a representation of the amount of energy contained in biomass, at different trophic levels for a given point in time.
• The amount of energy available to one trophic level is limited by the amount stored by the level below.
• Because energy is lost in the transfer from one level to the next, there is successively less total energy as you move up trophic levels.
• In general, we would expect that higher trophic levels would have less total biomass than those below, because less energy is available to them.

• To produce this pyramid, the dry weight of each species is calculated.
• In other words, the total mass of the organism
(biomass) is estimated for each trophic level.
• The dry weight of an individual is the mass of organic material contained within that individual. • To determine this the organism is dried in an oven to remove water then weighed.

• This is done because the water content of an organism (which has no impact on the amount of E or nutrients available to the ecosystem) varies considerably.
• Again the length/area of each bar is equal to the dry weight/biomass of all individuals at a trophic level.
• Biomass decreases at each trophic level.
• The unit for this pyramid is dry weight of organic matter per square metre.

• Biomass at the time of sampling, is known as standing biomass/standing crop bimass.
• This gives no indication of the rate of production
(productivity) or consumption of biomass.
• This can be misleading in 2 ways:
1. If the rate of consumption (loss through use as food) more or less equals the rate of production, the standing crop does not give any indication of productivity, i.e. the amounts of material & E passing from one trophic level to the next in a given time period such as one year.

– For example, a fertile, intensively grazed pasture may have a smaller standing crop of grass, but a higher productivity, than a less fertile & ungrazed pasture. 2. If the producers are small, like algae, they have a high turnover rate (i.e. a high rate of growth & reproduction balanced by a high rate of consumption or death).
– Thus although the standing crop may be small compared with large producers such as trees, the productivity may be the same as trees accumulating their biomass over a long time period. • In other words, an amount of phytoplankton with the same productivity as a tree would have a much smaller biomass than a tree, even though it can support the same amount of animal life.
• In general, larger, longer-lived plants & animals have lower turnover rates than the smaller shorter-lived plants/algae and animals and accumulate materials & E over a longer time period.

• Just as with the inverted pyramid of numbers, in some rare exceptions, there could be an inverted pyramid of biomass, where the biomass of the lower trophic level is less than the biomass of the next higher trophic level.
• The oceans are such an exception because at any point in time the total amount of biomass in microscopic algae is small.
• Thus a pyramid of biomass for the oceans can appear inverted.

One possible consequence of this is shown to the right. The pyramid at the bottom shows an inverted pyramid for an English Channel community. The zooplankton has a higher biomass than the phytoplankton on which it feeds.
This is seen in ocean & lake planktonic communities at certain times of the year; phytoplankton biomass exceeds zooplankton biomass during spring bloom but at other times the reverse might be true.

PYRAMID OF ENERGY
• The most fundamental & ideal way of representing relationships between organisms in different trophic levels.
• It shows the amount of enery (in kilojoules {kJ}) present at each trophic level.
• This pyramid is always upright.
• The full unit for a pyramid of energy is kJ m-2 year-1. • This pyramid has many advantages:
1) It takes into account the rate of production, in contrast to pyramids of numbers & biomass which depict the standing states of organisms at a particular moment in time. Each bar of a pyramid of E represents the amount of E per unit area or volume that flows through that trophic level in a given time period. (see pyramid on Silver Springs).
2) Weight for weight, 2 species do not necessarily have the same E content. Hence comparisons based on biomass maybe misleading.

Pyramid of E for an aquatic ecosystem (Silver Springs, Florida)
Carnivores to top carnivores Herbivores to carnivores
Producers to herbivores Gross production

Units used are for E flow.

88
1603
14098

87110

Pyramid of E for ocean

3) Apart from allowing different ecosystems to be compared, the relative importance of populations within one ecosystem can be compared & inverted pyramids are not obtained. 4) Input of solar E can be added as an extra rectangle at the base of a pyramid of E.

•Pyramids of E are considered the most useful of all 3 types.
•They are the most difficult to obtain data for because they required more measurements than pyramids of biomass.