One change leads to another

In an ecosystem, one small change can set off many more, like a line of dominoes: knock over the first and the rest fall. Follow one line through.

Suppose many plants in a pond start dying because of pollution. Fewer plants means less oxygen is produced in the water.

Less oxygen means the fish population drops, and that knocks on: there are now fewer consumers in the pond.

With fewer fish eating them, insects increase, and they spread to nearby farmland.

So farmers have to use pesticides to save their crops, which can harm the environment again and lead to further problems. One act of pollution ends, several steps later, in more pesticide on the fields.

A real case (Activity 12.9). In the 1980s, India was a big exporter of frog legs, especially from the Indian bullfrog, Hoplobatrachus tigerinus. So many were caught that frog populations fell.

Frogs eat insects, so with fewer frogs, agricultural pests rose. Farmers used more synthetic pesticides, which harmed the environment, the soil and water quality, and the health of people and the environment. So the Government of India banned the export of frog legs to prevent further damage.

An ecosystem is in balance when the interactions among organisms and their surroundings keep populations and resources stable. But that balance is dynamic, not fixed: it shifts all the time and can be disrupted by natural changes (floods, droughts, forest fires) or human-made ones (pollution, overfishing, cutting forests, too much pesticide).