eating something decreases the amount of that something & scavengers eat dead organisms ->  scavengers decrease the amount of dead organisms  dead organisms are a source of disease & as the amount of a source of something decreases , the amount of that something will decrease ->  as the amount of dead organisms decreases, the amount of disease will decrease  scavengers decrease the amount of dead organisms & as the amount of dead organisms decreases, the amount of disease will decrease ->  scavengers decrease the amount of disease  decreasing something negative has a positive impact on a thing & disease has a negative impact on an organism ->  decreasing disease has a positive impact on organisms  decreasing disease has a positive impact on organisms & an ecosystem contains a large community of / large numbers of living organisms ->  decreasing disease has a positive impact on an ecosystem  scavengers decrease the amount of disease & decreasing disease has a positive impact on an ecosystem ->  scavengers have a positive impact on an ecosystem by decreasing the amount of disease  scavengers have a positive impact on an ecosystem by decreasing the amount of disease & helping something has a positive impact on that something -> scavengers help an ecosystem by decreasing the amount of disease