The fact that the global ecosystems form broad latitudinal belts is mostly because of climatic reasons. It works very simply, the shape of the Earth doesn't allow the sun to heat up the surface equally in every place, so on the equator and around it the sun heats up the Earth the most, as the latitude changes the sun rays become weaker and weaker because they fall at a smaller angle and are dispersing much more, this forms the different climatic regions on Earth thus creating different ecosystems with it, ecosystems that have their borders mostly on the lines of change in climate from one region to another, so we mostly have latitudinal belts of ecosystems.