Answer
The population of black swallowtail butterfly will decrease when the Canby’s dropwort plants coverage decreases
Step-by-step explanation
The larvae of the black swallowtail butterfly thrive well when there is enough numbers of Canby’s dropwort because the larva feeds on the plant. A decrease in Canby’s dropwort plant due to clearance for agriculture and construction will reduce the number/coverage of the Canby’s dropwort which will directly reduce the number of black swallowtail butterfly because their larvae feed on the plant for survival.