The answer to that is one word; man. Man is responsible for the incredible amounts of polluting agents that are being spewed into the ecosystem each day. These pollutants harm the ecosystem in an unimaginable way. Overhunting and overutilization of resources by man has also caused considerable harm to the ecosystem.
Humans destroy ecosystems more than natural disasters--which can also be beneficial. Humans will fill the air with CO2 gasses, which trap in heat and cause global warming, melting icecaps, raising water levels, and making beach and water environments flooded. And how are we doing this? Well, the most well-known reason is cars. Car engines, as a byproduct, give off CO2 emissions. Meat production contributes even more so, between transporting and refrigerating and packing meat, as well as the food production FOR the animals and the waste. (Go vegan, kids.) We also destroy ecosystems by polluting when waste is not disposed of properly, stripping forests, killing populations of animals, fracking, toxic waste, human organic waste (you know what I mean by that), forcing plants into endangerment with beauty products--Chanel No. 5 & the rosewood tree, flushing chemicals, and disturbing natural habitats. So what harms an ecosystem? Us. That's the ugly truth, and anyone that thinks differently has to open their eyes to the world.