Question

Something that causes a population to decrease

Answer 1

Some possibilities of why a population decreasing are:

-invasive species
-deforestation
-pollution
-habitat loss
-change in food chain/web
-oil spills

Hope this helps!

Answer 2

if environment is favorable than growth will be good but

If not than growth will be less and population will decrease.

like in case of competition, food and water availability is less, unfavorable conditions, rainfall, disease, disasters et,.