Ecological Pyramids

Ecological Pyramids

A key to understanding the structure and function of ecosystems

Intended Learning Outcomes

At the end of this session, students will be able to

• Explain ecological pyramids

• Classify ecological pyramids

• Explain consequences of ecological pyramids and ecosystem function

• Explain Bio-magnification

Content

• Ecological Pyramids

• Types of ecological pyramids

• Advantage and disadvantages of ecological pyramids

• Bio-magnification

What are Ecological Pyramids?

• Ecological pyramids are graphical representations of the trophic structure of ecosystems.

• Trophic levels are the feeding position in a food chain such as primary producers, herbivore, primary carnivore, etc.

Types of ecological pyramid

Three types of ecological pyramids can usually be distinguished namely:

1. Pyramid of numbers

2. Pyramid of biomass

3. Pyramid of productivity

Pyramid of numbers

• Is the graphic representation of number of individuals per unit area of various trophic levels

• Large numbers of producers tend to form the base

• Lower numbers of top carnivores occupy the tip

 

Pyramid of Numbers in a Grassland Ecosystem

Pyramid of numbers - example

• The shape of the pyramid of numbers vary from ecosystem to ecosystem.

• In aquatic ecosystems and grassland communities, autotrophs are present in large numbers per unit area.

• They support a lesser number of herbivores, which in turn support fewer carnivores.

Pyramid of Numbers in a Aquatic Ecosystem

‘Inverted’ + ‘spindle- shaped pyramids

Evaluating the pyramid of numbers

ADVANTAGES	DISADVANTAGES
Simple easy method (number counting) of giving an overview	Numbers of a specific species can be too great to measure accurately
Good for comparing changes to the ecosystem at different times of year e.g. between seasons	Does not take into account ‘juveniles’ or immature forms
	All organisms are included regardless of size, hence ‘inverted’ pyramids

Pyramid of biomass

• Is  the  graphic  representation  of  biomass  present  per  unit  area  of  different trophic levels, with producers at the base and top carnivores at the tip.

• Biomass is calculated by the mass of each individual x number of individuals at each trophic level (g m-2 or g m-3)

Inverted pyramid of biomass

• In an aquatic habitat the pyramid of biomass is inverted or spindle shaped where the biomass of trophic level depends upon the reproductive potential and longevity of the member.

Evaluating a pyramid of biomass

ADVANTAGES	DISADVANTAGES
Overcomes the problems of pyramids of number	Only uses samples from populations, so difficult to measure biomass exactly
	Time of year that biomass is measured influences result e.g. trees in summer
	Organisms of the same size do not necessarily have the same energy content
	Inverted’ pyramids may result from producers with high turnover rate

 

Pyramid of productivity

• Pyramid of productivity is a graphic representation of the flow of energy through each trophic level of a food chain over a fixed time period. The input of solar energy may be indicated as an extra layer at the base.      

• Only 10% of the energy is available to next trophic level (as per Lindemann's ten percent rule).

• The energy level of each trophic level has two arts i.e. Net Production (NP) and Respiration (R) and is measured in KJ m^-2 yr^-1

Pyramid of productivity

Advantages	Disadvantages
No inverted pyramids are obtained	It is difficult and complex to collect energy data (rate of biomass production over time)
Shows actual energy transferred and allows for rate of production	Problem always exists in assigning a species to a specific trophic level,
Can compared different ecosystems based on relative energy flows

 

Consequences of ecological pyramids and ecosystem function

Bioaccumulation - when plants / animals take up a chemical from the environment and do not excrete it. The chemical builds up in the organism over time to a potentially lethal level.

Biomagnification - refers to the sequence of processes that results in higher concentrations of the chemical in organisms at higher levels in the food chain (at higher trophic levels). In this way the chemical’s concentration is magnified from trophic level to trophic level. The concentration of the chemical may not affect lower levels of the food chain but the top levels take in so much it can cause disease or death.

• There are many pollutants like the DDT that are not bio-degradable. These accumulate in the organisms and cause serious health problems. The contamination of water with these pollutants results in their entry into the microscopic plants and animals. These organisms are fed upon by higher aquatic life like the fish. The fish in turn are fed upon by the land animals including man.

• Thus, the pollutant reaches the body of man. At each step in the food chain, the contaminant increases in quantity. This is because a fish feeds on large quantity of smaller plants and man eats fish. These contaminants like DDT remain in the fats and are not degraded in the body. Over the years the amount of DDT increases in the body. This is called bio-magnification.

Summary

• Ecological pyramids are graphical representations of the trophic structure of ecosystems

• Ecological Pyramids can be classified as

        Pyramid of numbers

        Pyramid of biomass

        Pyramid of productivity

• Bio-accumulation – when plants / animals take up a chemical from the environment and do not excrete it

• Bio-magnification - refers to the sequence of processes that results in higher concentrations of the chemical in organisms at higher levels in the food chain