First reason for Biodiversity change is biological invasion and introduction of new aggressive weeds. These are very important reasons for the elimination of existing local species with poor competitive ability. There are several examples of spread of noxious weeds overtaking and even eliminating local species in different parts of the world.
Second reason is deforestation. Man has failed to check deforestation.If we fail to arrest deforestation, we may lose thousands of plant and animal species for ever. Both the tropics and temperate belts are facing deforestation in the developed and developing countries alike.