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Welcome to this presentation on Ecological Pyramids, which is designed to support students studying the IB Environmental Systems and Societies course.\n
So to begin with, what are ecological pyramids?\n\nEcological pyramids can be defined as being graphical representations of the trophic structure of ecosystems. To briefly recap, trophic structures are the feeding position of organisms in the food chain.\n
There are three main types of ecological pyramid that can be distinguished from one another.; these are:\nA pyramid of numbers\nA pyramid of biomass\nand a pyramid of productivity\n\nEach of these three pyramid types will now be described.\n
A pyramid of numbers is the graphic representation of number of individuals per unit area of various trophic levels stepwise with producers forming the base and top carnivores the tip.\n\nThe pyramid here of a grassland ecosystem shows large numbers of plants, then less numbers of mice, then less snakes and, finally, relatively small numbers of hawks.\n\nGiven what has been said previously about the flow of energy in ecosystems, this graphical representation appears to make sense, as there is less and less energy available to the higher trophic levels owing to the loss of heat energy experienced through respiration at each stage of transfer.\n
Here is a pyramid of numbers for an aquatic ecosystem. Again we can see a large number of producers and then the number of organisms at each trophic level declines, as you move up through the food chain.\n\nThe amounts of organisms at each trophic level varies from ecosystem to ecosystem. Grassland and aquatic ecosystems are similar to that shown here.\n\nHowever, it should be noted that some ecosystems do not show the traditional pyramid shape.\n
In the diagram on the right showing a parasitic food chain, in this case for an oak tree ecosystem, notice that the large tree provides food to several herbivorous birds. The birds support still larger population of ectoparasites leading to the formation of an inverted pyramid with less numbers at the bottom and more numbers at the top.\n\nThe diagram on the right shows that when a large tree supports a large number of herbivorous birds which in turn are eaten by carnivorous birds like falcon and eagle, which are smaller in number, it forms a spindle shaped pyramid. Here there are less numbers at the bottom, more in the middle and less again at the top.\n\n\n
There are a number of advantages and disadvantages of using a pyramid of numbers to examine the trophic structure of an ecosystem. \n\nOn the positive side, using a pyramid of numbers can be viewed as having a simple easy method of giving an overview of the trophic structure, as it involves number counting. Counter to this is the difficulty of determining the numbers of many species whereby their size is so small and numbers are so great for accurate measurement to take place.\n\nAnother drawback is that the size of the organisms are not taken into account, so while we are able to determine the number of organisms, we have not taken into account the overall biomass present. For example, we do not get an idea of the number of young or juvenile species that will grow over time, nor do we get the numbers of older more mature species.\n\nOn the up side, we are able to produce a pyramid of numbers at different times during the year, so we can look at seasonal changes to the trophic structure. For example, animal migrations may change the number of species available at different trophic levels at particular times during the year.\n\nFinally, as we have stated earlier, sometime counting the number of species results in an inverted or spindle shaped pyramid, which is not reflective of typical energy loss in an ecosystem. The decrease in available energy through each trophic level should always result in the typical pyramid shape.\n
A 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.\n\nAs has been stated earlier in the course, 'Biomass' is the total amount of organic matter in an ecosystem at any time. To determine the biomass, the mass of each individual is calculated and the multiplied by the number of individuals. This gives a weight of biomass per unit area. Environmentalists will often determine the dry weight of biomass.\n\nIn a terrestrial or land based ecosystem, as shown in the diagram, the maximum biomass occurs in producers, and there is progressive decrease in biomass from lower to higher trophic levels.\n\n
However, in aquatic habitats, as indicated by the diagram, the pyramid of biomass is inverted or spindle shaped where the biomass of trophic level depends upon the reproductive potential of each species and how long each individual may live for.\n
The benefit of a pyramid of biomass is that it overcomes the limitations of a pyramid of numbers because it can show how much biomass exists at each trophic level.\n\nHowever, it takes a lot longer to construct a pyramid of biomass because of the difficulties associated with measurement. Additionally, the time of year that biomass is measured influences the result. For example, deciduous trees in summer will have a greater biomass than in winter when they lose all of their leaves.\n\nFurther to this, biomass is not an accurate measurement of energy content. Organisms of the same size do not necessarily have the same energy content. This is further emphasised in aquatic ecosystems where pyramids are inverted because the producers and primary consumers have short lifespans, so the energy availability may be \n
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.
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.