I wrote a paper on this as an undergrad. They reintroduced wolves across areas in the US, which had been wiped out by early human populations who considered them a threat and a nuisance. So they brought back the wolves, and felt warm and fuzzy about it. The wolves reconquered their territory and hunted their quarry. Unfortunately this included beavers, whose population had grown over the years in the absence of their natural predator. The beaver population grew and eventually stabilised based on available resources. Once the wolves were reintroduced their populations declined. The lack of beavers changed the direction of the rivers, which affected the water table. Not a problem for the wolves, but problematic for the long established human settlements which relied on the water table staying put. Nature is a complicated and unpredictable system.