Why are pollutants so detrimental to ecosystems?

Answer 1

How are they not?

1. Destroy habitats: chemicals may cause soil erosion, leading to plant die-off. Plants are both food and shelter for many organisms. You also make the soil unavailable for future use, since the chemicals don't go away. Similar consequences occur in marine environments.

2. Directly kill organisms: some toxins lead to immediate death of individuals.

3. Indirect killing: along with destroying the habitat and food sources, some chemical build up within body tissues (like DDT, a fat-soluble insecticide). As you go up the food web, the levels of such chemicals increase, since every consumer acquires all the chemicals from whatever he eats. This may lead to death of the top consumers, or otherwise an inability to reproduce. DDT makes the shells of bald eagles weak, so that they can not reproduce.

Ecosystems are a delicate balance of flora, fauna and abiotic elements (water, nutrients like sulfur, iron, oxygen, etc). When you tip the scales just a little, either by killing off a whole population or just a few individuals, everything else is affected. Pollution has the propensity to destroy ecosystems.

Answer 2

They upset the balance, chemically, of the ecosystem. This changes the bottom of the food chain, the plants, bacteria etc and everything above them in the food chain.

That being said, they are creating another ecosystem, one in which organisms which can survive in polluted systems thrive. Unfortunately organisms which find the new ecosystem toxic, will soon die out.