The term niche refers to the role and space that an organism fills within an ecosystem. An organism’s niche encompasses how and where it finds shelter and the resources that it consumes. Competition for resources as well as biological constraints dictate the niche an organism will fill in its environment. Additionally, competition for limited resources among two organisms can drive their respective niches in opposing directions, leading to less competition (see graph). When an organism that formerly filled an important niche such as bees with crop pollination, their functional niche (pollination) is no longer filled, and there are negative consequences.
Citations
http://zoology.muohio.edu/oris/cunn06/graphics/cunningham06es_s/ch04/others/fig4_07.jpg