Ecological PyramidsA key to understanding the structure and function of ecosystems
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:• Pyramid of numbers• Pyramid of biomass• 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 Pyramid of Numbers in a Aquatic Ecosystem carnivores.
‘inverted’ + ‘spindle- shaped pyramids
Evaluating the pyramid of numbers ADVANTAGES DISADVANTAGES Numbers of a speciﬁc species Simple easy method (number can be too great to measurecounting) of giving an overview accuratelyGood for comparing changes to Does not take into accountthe ecosystem at different times ‘juveniles’ or immature forms of year e.g. between seasons 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 biomassIn an aquatic habitat the pyramidof biomass is inverted or spindleshaped where the biomass oftrophic level depends upon thereproductive potential andlongevity of the member.
Evaluating a pyramid of biomass ADVANTAGES DISADVANTAGESOvercomes the problems of pyramids Only uses samples from populations, so of number 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 productivityPyramid of productivity is agraphic representation of theﬂow of energy through eachtrophic level of a food chainover a ﬁxed time period. Theinput of solar energy may beindicated as an extra layer atthe base.
pyramid of productivity The energy level of each trophic levelOnly 10% of the energy is available to has two arts i.e. Net Production (NP)next trophic level (as per Lindemanns and Respiration (R) and is measured in ten percent rule). KJ m-2 yr-1
pyramid of productivity Advantages Disadvantages It is difﬁcult and complex to No inverted pyramids are collect energy data (rate of obtained biomass production over time Shows actual energy Problem required) is always exists in transferred and allows for assigning a species to a rate of production speciﬁc trophic level, Can compared different especially omnivoresecosystems 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.
consequences of ecological pyramids and ecosystem function• 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 contaminantincreases in quantity. This is because a ﬁsh feeds on large quantity of smaller plants and man eatsﬁsh. These contaminants like DDT remain in the fats and are not degraded in the body. Over theyears the amount of DDT increases in the body. This is called biomagniﬁcation.