Ecology and energy flow: study guide

Key ideas

Ecology measures how many organisms there are, how diverse they are and how energy moves between feeding levels. Most questions are short calculations from field data.

• Population density is N ÷ area. The mark–recapture estimate is N = (M × C) ÷ R, with M marked in the first sample, C caught in the second and R recaptured.
• Net primary productivity is gross primary productivity minus the producers’ respiration: NPP = GPP − R.
• Only about 10 % of the energy at one trophic level reaches the next, which is why food chains are short and pyramids narrow.
• Simpson’s diversity index D = 1 − Σ(n ÷ N)² rises with more species and with more even numbers.

Key equations

• N = (M × C) ÷ R (Lincoln–Petersen)
• NPP = GPP − R
• density = N ÷ area
• D = 1 − Σ(n ÷ N)²
• transfer efficiency ≈ 10 %

Worked example

In a mark–recapture study, 50 animals are marked and released. A later sample of 40 contains 8 marked animals, so N = 50 × 40 ÷ 8 = 250. If producers fix 10 000 kJ, primary consumers get about 1000 kJ and secondary consumers about 100 kJ. A producer with GPP = 5000 kJ and respiration 2000 kJ has NPP = 3000 kJ.

Common mistakes

• Using the recaptured number as C. C is the total number caught in the second sample.
• Assuming mark–recapture works when marked animals die or leave. Its assumptions need to hold.
• Expecting energy to be recycled in an ecosystem. Matter cycles, but energy flows through and is lost as heat.

Practice

Test yourself with practice questions on this topic: Ecology and energy flow