How do you express a natural habit of a species?

Solution 1:

In general, the plural form of the species name should be used (both as subject and object).

• Lions do not eat wolves (although I bet they would, if hungry enough).
• Hyenas do not eat wildflowers.
• Horses sleep standing up.
• Blue whales eat krill, and the occasional Jonah.

Note that "krill" is both singular and plural (actually collective, since an individual of a krill species is rarely referenced). A similar example is

• Deer eat moss

Since nobody looks at moss and says "Oh, look! Mosses!" From a technical point of view, a specialist might say "mosses" or "krills" when speaking of multiple individual species. "There are 98 mosses," but this is not used in common parlance. Most people, probably even specialists, would say "There are 98 species of moss (or krill)." And of course, "deer" is both singular and plural.

Now, you could use the definite article to generalize upon the species, and in that case you would use the singular:

• The lion does not eat wolves.
• The blue whale prefers krill over plankton.

But this sounds rather pretentious, even if correct.