Neighbours from Hell
Occasionally, a foreign species will be introduced to an environment to which it doesn't belong. In most cases, the new species dies off without being able to breed; sometimes though, these species do continue in their new home and can cause huge problems for the local wildlife. These are called invasive species. This is a particularly urgent problem in Madagascar where, because it has spent 120 million years away from mainland Africa, almost every species on the island is found nowhere else on Earth!
If a new species is able to establish itself, sometimes it can increase its population extremely quickly, rapidly out-competing the native species. This can lead to the newcomers driving the original inhabitants from their natural ranges. Sometimes, particularly with plant species, invaders can smother whole areas and clog up entire rivers.
On Madagascar these issues are amplified. As an island of highly specialised species, invaders represent a particularly dangerous threat for the natives - the homegrown species can't leave to another habitat! Once an invader has dominated an area of land, anything that was dependent on that habitat will have a particularly hard time surviving in the future.