madppiper, the agrarian American South was not the only place during the antebellum period that fostered the plantation system. Plantations existed in most parts of the western hemisphere during that time where it was agriculturally feasible.
The plantation system also existed in Jamaica, Haiti, the Bahamas, and in "Middle America" from Guatemala all the way to Panama, and even in South American countries like Brazil.
Just because we know it was a plantation house doesn't mean the folks who inhabited it were "Americans" from the United States.
The plantation house was probably owned by wealthy planters from one of the surrounding counties, likely Costa Rica.
According to
this page, Costa Rica's chief agricultural exports are coffee, bananas, sugar and pineapples. I imagine one or all of these products would have been cultivated on Isla Sorna during the period we think the plantation house was "operational."