What has the greatest effect on the current decline of biodiversity?
a. plants
b. animals
c. humans
d. natural environment

Humans

Human activity has greatly distorted the ecosystems in the environment which resulted to invasive species.
Invasive species can threaten an ecosystem and the biodiversity of the organism that exist in the present environment when the invasive species increase rapidly in number. 
Exotic species are a threat to biodiversity because they alter the ecosystem of that area. They share food and habitat resulting in unbalanced ecosystem. Also, habitat destruction destroys the habitat of all the life forms in that area.