Elaborate how invasion by an alien species reduces species diversity of an area. 

When an alien species is deliberately or accidentally introduced into an area, it may become invasive and threaten other species. Invasive species can be a serious competitor; competing for the same resources. It can also turn out to be predator for some species. There are many examples of invasive species which played havoc with the local biodiversity. Some examples are as follows: 
(a) Water Hyacinth: Water Hyacinth is originally from amazon basin. It has proved to be invasive species in many countries. This plant grows at rapid rate and covers the whole pond and lake. By covering the surface of the pond. this plant reduces the supply of sunlight and oxygen to the bottom of the pond. Thus, it proves detrimental for other organisms in the pond. 
(b) Parthenium or Carrot Grass: This plant came as contaminant along with imported wheat from America. This plant proved to he highly Invasive in India. This is a notorious breed which becomes a big problem for the farmers in India.