Lessons 11–12 · 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
12
Biotic factors. Relationships between living organisms
Textbook: pp. 67–75
GoalDistinguish the types of biotic relationships (neutralism, symbiosis, competition, parasitism, predation, amensalism) by symbols and give examples.
Biotic factors are the effects of living organisms on one another, divided into intraspecific and interspecific relationships. The result is written with symbols: (+) benefit, (–) harm, (0) no effect. Neutralism (0 0): species do not interact directly; it is rare in nature. Mutualism (+ +, obligatory): fungus and alga in lichens, legumes and nodule bacteria, mycorrhiza; protocooperation (+ +, not obligatory): a bee and a flowering plant. Commensalism (+ 0): synoikia (an epiphyte on a tree), commensalism of food (a jackal eating the remains of a lion’s kill, a remora attached to a shark) and sharing of a food source. Antibiosis (– –, + –, – 0): competition (– –), parasitism and predation (+ –), amensalism (– 0: a shading tree to a grass; a mould releasing an antibiotic against bacteria). In 1932 G. F. Gause grew two infusorian species with similar needs in one vessel and one disappeared – the competitive exclusion principle; if species divide their niches, they coexist. Predator–prey and parasite–host relations regulate numbers and clean populations (serving natural selection); a parasite usually does not kill its host at once.
Worked examples
On a tomato field a weed competes with the crop for water and light (– –); tall maize shading a low grass is amensalism (– 0); bacteria in alfalfa roots fix nitrogen to the plant’s benefit (+ +).
Eagle–marmot: the eagle benefits, the marmot is harmed (+ –): predation. Tick–dog: the tick benefits, the dog is harmed (+ –): ectoparasitism. A predator kills and eats its prey, while a parasite usually does not kill its host at once.
Class activity
Class game «Relationship cards»: write species pairs (e.g. bee–flower, wolf–saiga, dodder–alfalfa) on 12 cards; students draw a card and find its symbol (+ +), (+ –), (– –), (+ 0), (– 0) or (0 0). Do not touch dodder or other plants outdoors; do not catch unfamiliar insects.
Practice
1
State the difference between mutualism and protocooperation.
In both partners benefit; in mutualism living together is obligatory, in protocooperation it is not.
2
Give the symbols: (a) cellulose-digesting bacteria in a cow’s gut–the cow; (b) substances released by walnut-tree roots–the grasses beneath it; (c) fox–field mouse; (d) sheep and rodents feeding on the same grass on one pasture.
(a) (+ +); (b) (– 0) (the grasses are harmed, the tree is unaffected); (c) (+ –); (d) (– –).
3
What conclusion follows from Gause’s experiment?
Two species with similar ecological needs cannot coexist for long: the stronger competitor displaces the other.
4
Why can removing all predators also harm the prey population?
The control on numbers disappears: prey overpopulate, food runs out, disease spreads and the population crashes.