Lessons 7–8 · 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
7
Environmental factors and their classification
Textbook: pp. 39–46
GoalExplain the groups of ecological factors, optimum and tolerance, the limiting factor (Liebig’s law), eury- and stenobionts and the ecological niche.
New words
limiting factor · cheklovchi omilzone of tolerance · tolerantlik zonasieurybiont / stenobiont · evribiont / stenobiontecological niche · ekologik nisha
Explanation
Ecological factors fall into three groups: abiotic (climatic, edaphic, topographic, physical), biotic (phytogenic, zoogenic, mycogenic, microbiogenic) and anthropogenic (human activity). The effect of any factor is most favourable in the optimum zone, becomes more unfavourable (pessimum) away from it and at the minimum and maximum limits the organism dies; the range between them is the zone of tolerance. Liebig’s law (of the minimum): survival is decided by the factor furthest from its optimum – the limiting factor – even if other factors are abundant. Species with a wide tolerance range are eurybionts (rat, fly), those with a narrow range are stenobionts (the koala feeds on eucalyptus leaves); a species tolerant to one factor may be intolerant to another. Factors act together: heat is easier to bear when humidity is low. The ecological niche is the sum of all abiotic and biotic conditions and the way of life of a species (its food, active time); a habitat is merely its «address». By the competition rule, two species in a community cannot hold the same niche for long – one displaces the other, or they divide the niches.
Worked examples
Suppose cockroaches are inactive below 12 °C, die above 38 °C and are most active at 22–30 °C. The tolerance zone is 12–38 °C (width 26 °C), the optimum 22–30 °C. At 40 °C temperature becomes the limiting factor even if food is abundant.
Two birds live on one lake: the flamingo wades in shallow water and filters small crustaceans, while the cormorant dives into deep water to catch fish. The place is the same, but the niches differ, so they can coexist without strong competition.
Class activity
«Weak link» game: make a chain of paper rings, make one ring thin, and see where the chain breaks when pulled. Write in one sentence how this resembles Liebig’s law. Use scissors only under your teacher’s supervision.
Practice
1
Distinguish optimum, pessimum and zone of tolerance.
Optimum – most favourable level; pessimum – unfavourable but survivable; tolerance zone – the whole range between minimum and maximum.
2
A wheat field got plenty of fertiliser but no irrigation all summer, and the yield is low. What is the limiting factor?
Water (moisture) – the factor furthest from its optimum.
3
A grey rat (cosmopolitan) and a koala: which is a eurybiont and which a stenobiont?
Rat – eurybiont; koala – stenobiont (eats only eucalyptus leaves).
4
Why can an ecological niche not be replaced by the habitat?
A niche includes way of life, food, active time and all factors; a habitat is only the area occupied.