Ecosystems and food chains
Everything in an ecosystem is connected, so removing or adding one population changes the populations above and below it.
A worked example
In grass → rabbit → fox, disease that kills most of the rabbits leaves the foxes short of food, so fox numbers fall as well.
The mistake people actually make
Abiotic means the non-living conditions, such as sunlight, water, temperature and soil minerals. Earthworms and decomposers live in the soil but are alive, so they are biotic.