2.1 Ecosystem Structure IBESS
Food chain in a Taiga forest Secondary consumer Primary consumer Producer What would you call the trophic level of something that ate the lynx? Sun
Ringed seal Baltic cod Sprat
Normally ecosystems are made up of more than one food chain…they are food webs.
We can’t forget decomposers! • Decomposers have the important role of feeding on the remains or wastes of other organisms. • Many bacteria and fungi are decomposers.
Why a pyramid? • Between the trophic levels, about 90% of the energy is lost to the environment, usually as heat. Energy lost as heat
More on pyramids • Three types of pyramids you should know • Pyramid of numbers • Pyramid of biomass • Pyramid of production
Pyramids of Numbers • The size of each block is proportional to the number of individual organisms present in that trophic level. • Because of the drop in energy at each trophic level, the number of predators is generally much lower than the trophic level below.
Pyramids of Biomass • Each tier represents the standing crop (total dry weight of all organisms) in one trophic level. • Most biomass pyramids narrow sharply from primary producers at the base to top-level carnivores at the apex because energy transfers between trophic levels are so inefficient. Shown in grams per meter squared (gm-2) or sometimes in Joules (J).
Pyramids of production • The loss of energy with each transfer in a food chain is represented by this pyramid. • Trophic levels are stacked in blocks with primary producers forming the foundation of the pyramid. • The size of each block is proportional to the net production, expressed in energy units per square meter per year (Jm-2yr-1)
Pyramid of Production Joules per square meter per year: Jm-2yr-1 m-2yr-1 m-2yr-1 m-2yr-1 m-2yr-1
Pros and cons • Pros • Productivity are rates of flow, where biomass are stores at one time. • Shows actual energy transferred. • Energy from solar radiation can be added. • Cons • Difficult and complex to collect energy data as the rate of biomass production is needed over time. • Like other pyramids…where to assign omnivores???
Herring gull eggs 124 ppm LE 54-23 Lake trout 4.83 ppm Concentration of PCBs Smelt 1.04 ppm Zooplankton 0.123 ppm Phytoplankton 0.025 ppm
Practice 1 • An ecosystem consists of one oak tree on which 10 000 herbivores are feeding. These herbivores are prey to 500 spiders and carnivorous insects. Three birds of the same species are eating these spiders and carnivorous insects. The oak tree has a mass of 4000 kg, the herbivores insects have an average mass of 0.05 g, the spiders and carnivorous insects have an average mass of 0.2 g and the three birds have an average mass of 10 g. • Construct, to scale, pyramids of numbers and biomass
Practice 2 • Assuming an ecological efficiency of 10%, 5%, and 20% respectively, what will be the energy available at the tertiary consumer level (4th trophic level, given a net primary productivity of 90 000 kJ m-2 yr-1? • What percentage is this figure of the original energy value at the primary producer level?
Practice 3 • In an aquatic ecosystem, plankton are eaten by minnows, which are eaten by pickerel, which are eaten by cormorants. • DDT (a toxin) is introduced into the ecosystem and after ten years is found in the following concentrations. Water (0.00005), Plankton (0.04), Minnow (0.23), Pickerel (1.33), Cormorant (26.4). • How many trophic levels are there in this food chain? • How many times more concentrated is the DDT in the body of the cormorant than in the water?