One particularly severe impact humans have on natural environments is the introduction of invasive species. invasive species have particularly devastating effects on islands because

Invasive species have particularly devastating effects on islands because island species have typically evolved with few competitors and predators.
And thus if an invasive species is introduced into these islands, it will have severe consequences given that islands don't normally have a lot of predators and competitors. This is why humans need to take care of what they are doing if they want to preserve their natural environment. 

Ecological footprint is determined by measuring carbon dioxide emissions for a given region.