The greater the distance an island is from a continent (assuming islands are of similar size and elevation), the fewer plant and animal species it will support but the greater the distinctiveness of those species. As an example, compare Trinidad and Tobago with Puerto Rico. Trinidad and Tobago are two sister islands lying off South America, to which they were once connected by a land bridge. Their combined land mass is little more than half that of Puerto Rico, but their native avifauna totals approximately four hundred species, nearly twice that of Puerto Rico's. However, all the bird species on Trinidad and Tobago, with the sole exception of an endemic guan, are found either on the South American mainland or elsewhere. Contrarily, Puerto Rico’s native avifauna totals only two hundred and forty species but includes sixteen endemic species, which, except for two occurring in the nearby Virgin Islands, are found nowhere else in the world.