Ecology, biodiversity and immunity

ZIMSEC A Level · Biology · Form 5, Form 6

Energy flow: producers fix about 1–3% of light energy (gross primary productivity); net primary productivity = GPP − respiration. Transfer between trophic levels is about 10%.
Nitrogen cycle: nitrogen fixation (Rhizobium in legume nodules), ammonification, nitrification (Nitrosomonas, Nitrobacter), denitrification.
Succession: from pioneer species to a climax community.

Biodiversity includes species diversity, genetic diversity and ecosystem diversity. Simpson's index measures diversity.
Sampling: quadrats, transects, mark–release–recapture (population = M × C ÷ R).
Threats: habitat loss, overexploitation, pollution, invasive species and climate change.
Conservation: national parks (Hwange, Gonarezhou), community schemes (CAMPFIRE), seed banks, zoos and captive breeding, international agreements (CITES).

Non-specific defences: skin, mucus, phagocytes.
Specific: B lymphocytes make antibodies (humoral response; plasma cells and memory cells); T lymphocytes (helper T cells release cytokines; killer T cells destroy infected cells).
Active immunity (infection or vaccination) is long-lasting; passive immunity (antibodies across the placenta or in breast milk) is short-lived.
Diseases: malaria (Plasmodium, spread by Anopheles), tuberculosis, HIV/AIDS (infects helper T cells), cholera. Antibiotics act on bacteria, not viruses; resistance spreads through misuse.

Exam tip: Use the mark–release–recapture formula carefully and state its assumptions (no births, deaths or migration; marks not lost).

40 beetles are marked and released. Later 50 are caught, 10 of them marked. Estimate the population.
Distinguish between active and passive immunity.
Why does HIV weaken the immune system?

Show answers

40 × 50 ÷ 10 = 200
Active: the body makes its own antibodies and memory cells (long-lasting). Passive: antibodies are received ready-made (short-lived).
It infects and destroys helper T cells, which coordinate the immune response.

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