Lessons 17–18 · 2 lessons · A. G‘afurov, A. Abdukarimov, J. Tolipova, O. Ishankulov, M. Umaraliyeva, I. Abdurahmonova. Biology Grade 11, 1st edition. Sharq Publishing and Printing Joint-Stock Company, Tashkent, 2018
17
The trophic structure of ecosystems
Textbook: pp. 93–99
GoalExplain the trophic structure of an ecosystem: orders of consumers, detritivores, food chains and webs, and trophic levels.
New words
food chain · oziq zanjirifood web · oziq to‘ritrophic level · trofik darajadetritivore · detritofag
Explanation
Organisms in an ecosystem are linked by food: producers occupy the first trophic level, primary consumers (herbivores) the second, secondary consumers (carnivores) the third, and so on. A food chain is a series through which matter and energy pass from one organism to the next; the term was proposed by C. Elton in 1934. There are two types: the grazing chain begins with producers (grass → grasshopper → lizard → snake), the detritus chain with dead organic matter (fallen leaf → earthworm, fungi → predatory insect → bird). Detritivores (earthworm, woodlouse, carrion beetle) are consumers that fragment detritus, while reducers break it down to mineral substances. An omnivorous species (bear, rat) occupies several levels at once, so chains intersect into a complex food web. Chains usually have 3–5 links because energy is lost at each step. In a web, if one species disappears, other food sources remain, so an ecosystem of high species diversity is stable.
Worked examples
Lake chain: phytoplankton → daphnia → small fish → pike → cormorant. Phytoplankton – level 1 (producer), daphnia – level 2 (primary consumer), small fish – level 3, pike – level 4, cormorant – level 5. Fish may also take part in a detritus chain.
A fox is at level 2 when eating rose hips, level 3 when eating grasshoppers and level 4 when eating a lizard that fed on grasshoppers (grass → grasshopper → lizard → fox), so it links several chains in one web. In a year when mice are scarce, the fox switches to more insects and fruit – the web provides stability.
Class activity
Draw a food web of at least 10 organisms from your area (garden, lakeside, field): arrows show «who eats whom»; write each organism’s trophic level. Use pictures or reference books instead of disturbing animals in nature.
Practice
1
What is meant by a trophic level of a food chain?
The set of organisms belonging to one link of the chain.
2
Order this detritus chain correctly: bird, fallen leaf, earthworm, predatory beetle.