This is a work in progress – comments are welcome (and moderated so will not appear immediately, and spam not at all)
A healthy ecosystem consists of a large number of species (types of organism) interacting in balance. Depending on the environment different species will be more prevalent, but there is normally a balance – and a range. In a area we might think of a natural grassland (for example) there will be a range of grasses, and also other vegetation, as well as insects, small mammals, birds etc.