I'd add to Iris's good comments that in a large enough forest that the animals
never leave it, they are not a source of fertilizers at all. Not even
slightly. This is because they don't "make" fertilizer nutrients; they simply
recycle them and move them around. For rabbit manure to have N, P, K, or other
elements in it, the rabbit had to eat a plant that contained those elements.
The plant extracted the elements from the soil. So at best, the rabbit manure
puts the nutrient back onto the soil. Agricultural use of manure works since
it is applied to land where the animal's feed source did NOT grow, so in a
sense, you are importing nutrients from somebody else's field. Nitrogen-fixing
bacteria can be important, especially those associated with legumes, bayberry,
and other plants.