2.1 Ecosystem Structure

IBESS

Food chain in a Taiga forest

Sun

Producer

Primary consumer

Secondary consumer

What would you call the trophic level of something that ate the lynx?

Sprat

Baltic cod

Ringed seal

• 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.

Pyramid of Numbers

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).

Biomass Pyramid

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 ProductionJoules 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???

LE 54-23

Zooplankton0.123 ppm

Phytoplankton0.025 ppm

Lake trout4.83 ppm

Smelt1.04 ppm

Herringgull eggs124 ppm

Conc

entr

ation

of P

CBs

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?