When is the serious threat developed to an ecosystem ?
A. When any one or two species become extinct.
B. When key species that drive ecosystem become extinct.
C. When native species are replaced by exotic species.
D. When human beings take conservation measures.
Answer:B
When is the serious threat developed to an ecosystem ?
A. When any one or two species become extinct. B. When key species that drive ecosystem become extinct. C. When native species are replaced by exotic species. D. When human beings take conservation measures.