2.2.2.3 Ecological Pyramids

Ecological Pyramids Ecological pyramids represent numerical relationships between successive trophic levels in a community. Which pyramid is chosen for examination depends on the type and quantity of data collected.

Pyramids Graphic models of quantitative differences between trophic levels By second law of thermodynamics energy decreases along food webs Pyramids are thus narrower as one ascends

Losses in the pyramid Energy is lost between each trophic level, so less remains for the next level Respiration, Homeostasis, Movement, Heat Mass is also lost at each level Waste, shedding, …

Pyramids of Numbers One way to simplify food webs is to assign the organisms to trophic levels and then count the number of organisms in each trophic level.

Pyramids of Numbers Needs sampling similar to Biomass and therefore has the same limitations Also measures the storages

Pyramids of Numbers Represents the numbers of individual plants and animals present in the food web. It does not take into account the relative size of any organisms. Consequently, it is possible to have an inverted pyramid of numbers.

© 2004 Brooks/Cole – Thomson Learning Pyramids of Numbers Grassland (summer) Temperate Forest (summer) Tertiary consumers Secondary consumers Primary consumers Producers

Pyramids of Biomass A pyramid of biomass takes body size into account You have to find the dry mass of the organisms in your sample and their energy content (this must be measured in a fixed area). However, it does seem to be logical i.e. big organisms support lots of little ones.

Pyramids of Biomass Represents the standing stock of each trophic level (in grams of biomass per unit area g / m2) Represent storages along with pyramids of numbers

How do we get the biomass of a trophic level to make these pyramids? Why can’t we measure the biomass of an entire trophic level? Take quantitative samples – known area or volume Measure the whole habitat size Dry samples to remove water weight Take Dry mass for sample then extrapolate to entire trophic level

How do we get the biomass of a trophic level to make these pyramids? Evaluation  It is an estimate based on assumption that all individuals at that trophic level are the same The sample accurately represents the whole habitat

Pyramids of Biomass Biomass decreases at each trophic level because the transfer of energy from the primary producer to the primary consumer and between the consumers is inefficient. Energy is lost at each stage, that is, at each of the transfers.

Pyramids of Biomass See example on your sheet green plant  zebra  lion

© 2004 Brooks/Cole – Thomson Learning Pyramids of Biomass Abandoned Field Ocean Tertiary consumers Secondary consumers Primary consumers Producers

Pyramids of productivity Pyramids of productivity require much more data to construct since many samples, separated by intervals of time, are needed to estimate productivity, They provide a coherent view of how an ecosystem works.

Pyramids of productivity Production by primary producers must be greater than production by secondary consumers and so on. Do not need to consider numbers or differences in size because all organisms can be converted to their energy equivalent.

Pyramids of productivity Flow of energy through trophic levels Energy decreases along the food chain Lost as heat Productivity pyramids ALWAYS decrease as they go higher – 1st and 2nd laws of thermodynamics

Pyramids of productivity Shows rate at which stock is generated at each level Productivity measured in units of flow (J / m2 yr or g / m2 yr )

© 2004 Brooks/Cole – Thomson Learning Decomposers/detritivores Energy Input: 20,810 + 1,679,190 1,700,000 (100%) Energy Output Total Annual Energy Flow Metabolic heat, export Waste, remains 1,700,000 kilocalories Producers Herbivores Carnivores Top carnivores Decomposers, detritivores Transfers 20,810 (1.2%) Incoming solar energy not harnessed 1,679,190 (98.8%) 4,245 3,368 13,197 720 383 2,265 90 21 272 5 16 © 2004 Brooks/Cole – Thomson Learning Top carnivores Decomposers/detritivores 21 Carnivores 5,060 383 Herbivores 3,368 Producers 20,810

Figure 54.11 An idealized pyramid of net production

Efficiency of trophic levels in relation to the total energy Figure 54.14 Food energy available to the human population at different trophic levels Efficiency of trophic levels in relation to the total energy available decreases with higher numbers But efficiency of transfer always remains around that 10% rule

How does pyramid structure effect ecosystem function? Limited length of food chains Rarely more than 4 or 5 trophic levels Not enough energy left after 4-5 transfers to support organisms feeding high up Possible exception marine/aquatic systems b/c first few levels small and little structure

How does pyramid structure effect ecosystem function? Vulnerability of top carnivores Effected by changes at all lower levels Small numbers to begin with Effected by pollutants & toxins passed through system