Not necessarily. Elves and Humans would be classified as being part of the same genus of species, in this case homo, and members of the same genus can sometimes breed together under abnormal circumstances.
This is how you get ligers, a cross between a lion and a tiger. Both different species, but part of the panthera genus. Or mules, also a cross between two species, but part of the equus genus.
But they can still produce offspring with humans (e.g. Elrond is half-elven), which is usually what defines a species.
Not necessarily. Elves and Humans would be classified as being part of the same genus of species, in this case homo, and members of the same genus can sometimes breed together under abnormal circumstances.
This is how you get ligers, a cross between a lion and a tiger. Both different species, but part of the panthera genus. Or mules, also a cross between two species, but part of the equus genus.