They're only 1st level? Well then, they won't be set back by much compared to the other PCs when they die. Kill 'em! Well, not deliberately, but do let the jungle be deadly enough to pose the level of threat you've already decided on, and let the "tough lone wolf" types die if they take on challenges they can't handle and the dice kill them. If they survive, great, if not, not much loss.
Build up the danger as they progress, though, by telegraphing the the threats before they're unavoidabletelegraphing the the threats before they're unavoidable. A player is much more likely to accept a death that they deliberately, knowingly courted than one that just pounces out of a tree and eats their head without warning.
When they make fresh characters (other shipwreck survivors that washed up on a different, nearby shore), perhaps they won't make lone-wolves this time.