a drought occurs over a period of months / years & drought is when available water decreases in an environment ->  drought is when available water decreases in an environment over a period of months / years  drought is when available water decreases in an environment over a period of months / years & as available water decreases , the population of plants will decrease ->  a drought that occurs over a period of months / years may cause the population of plants in an environment to decrease  a drought that occurs over a period of months / years may cause the population of plants in an environment to decrease & a marsh willow herb is a kind of plant -> a drought that occurs over a period of months / years may cause the population of marsh willow herb in an environment to decrease