Ecosystem diversity means all of the creatures that live in an ecosystem. If the system is tropical, the creatures there are different than the ones that are found in a dessert ecosystem. In the system itself, the creatures all are adapted to live in a very dry area. Some desserts are colder and some are very hot. There are usually more diferent ones in tropical ecosytems. These creatures can be as small as bacteria and as large as very large cats, etc. For example a cat in a tropical ecosystem would be able to live there but perhaps not do as well in the desert. The opposite would be true.