Lessons 5 · 1 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
5
Habitats of organisms. The aquatic environment
Textbook: pp. 23–29
GoalKnow the concepts of habitat, ecological factor and adaptation; name the four habitats; explain the properties of the aquatic environment and adaptations to it (plankton, nekton, benthos, hydrophytes).
A habitat is the set of biotic and abiotic conditions that act directly or indirectly on the life of an organism, population or species; any component of it is called an ecological factor if it triggers adaptive responses in the organism. In evolution organisms developed structural, physiological and behavioural adaptations to the environment; the unity of organism and environment was formulated by V. I. Vernadsky. Earth has four main habitats: water, ground-air, soil and the body of another organism; life first appeared in water. In the aquatic environment density is high (about 800 times that of air), temperature varies little, oxygen is scarce and light fades with depth; in deep lakes in winter the bottom water is about +4 °C because water is densest at that temperature. Hydrobionts form three ecological groups: plankton (small organisms drifting with the current), nekton (active swimmers: fish, whales, squid) and benthos (bottom dwellers: shelled molluscs, crabs, rays). Hydrophytes have weakly developed mechanical tissue and roots, well-developed air-filled aerenchyma and thin leaves.
Worked examples
A flatfish lies on the bottom on its flat body and matches its colour: a benthic animal with the adaptations of a flat shape and camouflage. A dolphin, with its streamlined body and flippers, is nekton; a jellyfish drifts with the current – plankton.
The floating leaf of a water lily is thin with stomata on its upper side, and its stalk has air-filled spaces: the aerenchyma carries oxygen to the roots and keeps the leaf on the surface. In water the root need not absorb water – so root hairs are few.
Class activity
Observe the school aquarium (or a picture): which creatures are plankton, nekton, benthos? Make a three-column table. Do not approach natural water bodies without a teacher or adults, do not take water samples, and observe water animals without touching them.
Practice
1
What is an adaptation? Give one example in an aquatic animal.
A trait that lets an organism live and reproduce in its environment; e.g. a fish’s streamlined body and fins.
2
Distinguish plankton, nekton and benthos in one sentence each.
Plankton drifts with the current; nekton swims actively against it; benthos lives on or in the bottom.
3
Why does temperature change less in water than on land?
Water has a high heat capacity, so it warms and cools slowly.
4
Why are green algae replaced by brown and red algae as depth increases?
At depth light is weak and its composition changes; algae with additional pigments can use this dim light.