Week 8 · 1-9 Effect of Water Pressure on Organisms: pressure inside a liquid, P = Pa + Pwater, swim bladders, skeletons, cell membranes; 1-10 Ecological Balance and the Role of Humans in Sustaining Aquatic Life · 3 lessons
3
Ecological balance and how people can protect it
Integrated Science Sec 1 Term 1, Unit 1: pp. 56–66
GoalExplain ecological balance in water, trace effects through a food chain, and describe how people can protect aquatic life.
New words
ecological balance · a dynamic stability in which living things and non-living parts interact so that life continuesalgal bloom · an abnormal, rapid growth of algae, often caused by too many nutrientsoverfishing · catching fish faster than their numbers can recoversustainable development · meeting today’s needs without harming the ability of future generations to meet theirs
Explanation
Ecological balance is a dynamic stability, and in water it has three parts. Nutrient balance: nitrogen and phosphorus feed algae and plants, but too much of them can cause an abnormal algal bloom. Balance among species: predatory fish keep the numbers of prey in check, so fewer predators, for instance through overfishing, can let small fish grow too numerous and eat too much. Energy flow: energy passes from producers (algae and plants) to consumers (herbivorous and predatory fish). In a coral reef, predators control sea urchins, which can destroy the reef if they grow too numerous. Effects often form chains: more zooplankton can mean less algae. People harm aquatic life by pollution (pesticides, heavy metals), overfishing and destruction of habitats such as reefs and wetlands. People can help by conserving resources, spreading knowledge, supporting sustainable development, and through small habits: saving water and energy, sorting waste, and using buses or bicycles.
Worked examples
fewer predatory fish→more small fish→less zooplankton→more algae
Trace a chain. Predatory fish become fewer because of overfishing. Then small fish increase. They eat more zooplankton, so zooplankton decrease. With fewer zooplankton eating them, algae can increase. One change at the top moved through the whole food chain.
Why can too much fertiliser washed into a pond disturb the balance? It adds nitrogen and phosphorus, which feed algae, so algae can grow abnormally fast (an algal bloom). This upsets the balance of nutrients and of the species that depend on the water.
Is a family’s habit, such as saving water, “sustainable development”? It is a small daily action that helps. Sustainable development is the larger idea: meeting today’s needs without harming future generations’ ability to meet theirs.
Class activity
Food-web cards, and a river plan. Make cards for algae, zooplankton, small fish, a predatory fish and a sea urchin. Arrange them as a food chain, then remove one card and trace what happens to the others. Then write a five-line plan to protect a river or pond near you: two ways to reduce pollution, two ways to save water, and one way to teach others. Use only science and daily habits; no names of officials.
Practice
1
Name the three parts of ecological balance in an aquatic ecosystem.
Nutrient balance, balance among species, and energy flow.
2
What may happen to small fish when predatory fish are overfished?
Their numbers may rise too much, and they eat too much food.
3
Name two harmful human activities and one helpful habit.
Harmful: pollution, overfishing, habitat destruction. Helpful: saving water, sorting waste, using a bicycle.
4
What causes an algal bloom?
Too many nutrients, such as nitrogen and phosphorus.
