Each change taking place in an ecosystem, either provoked by man or being a natural consequence, causes a reaction. For instance, species may leave the habitat because they can't find food or certain types of plants perish because tall trees don't block the sun's rays. As different species of animals and plants depend on each other and the terrain for their survival, the first change provokes a second one, which in turn can provoke a third change and a whole chain of reactions unfolds.
Chain reactions are easier to predict or follow when they are a result of disruptions in the food chain. For example, if a river is polluted and fish perish, then organisms consuming fish -- piscivores -- won't be able to find food. Therefore, piscivores will either die or leave the habitat, searching for food. However, piscivores are also part of the food chain, meaning that someone consumers them as well. Fewer piscivores means less food for animals higher in the food chain, affecting their numbers as well.
The terrain determines the flora and fauna present in an ecosystem. For example you won't find a frog in a desert environment. When there is a sudden change in an ecosystem, such as a high number of trees being removed or water resources drying out, organisms using them for protection or for basic needs such as hydration are forced to move, provoking similar reactions when the food chain is disrupted.
Even when changes happen in an ecosystem, nature manages to restore balance in a similar fashion as the economy's supply and demand mechanism. When herbivores becomes scarcer, then species feeding from them -- carnivores -- become scarcer as well. However, fewer carnivores means less "demand" for food, leaving space to herbivores to raise their numbers again. With more herbivores in the ecosystem, carnivores can find food more easily and the balance is restored.