Look, honey, he thinks there's honey in there!
We can't all be Zheleznogorsk.
The rule book comes closest to saying "only one per population" on page 7:
Rule Book wrote:
Each Population must eat 1 Food or it will starve at the end of your turn.
Place the Food on the Food spaces, starting with the top Population card for each species.
So, if you've got a 2-pop Long Neck, it'll draw two Food Tokens from the Excess Food and the third won't be drawn since there is no Food space available. Fat Tissue is the only Trait that enables a species to eat a second Food for each Population.