You should label it density unless you're doing something really peculiar.

At any given point of time, an animal (or group of animals, if this is an aggregation or population density) possesses a single density. They probably possess multiple units of density measurement - e.g. Wikipedia lists the average density of a human as 1062 kg/m³ - but only a single density^*.

Your Y axis counts units of density measurement, not numbers of densities; it quantifies the quality rather than asking how many different instances of that quality exist. Unless you're using density as the metric for determine number of populations of animals (e.g. a density of 10 animals/km² and another location having a density of 15 animals/km² demarcates them as different populations). But that's a really crazy way to count population groups.

^* - Yes, I realize the density will be different at different places in the body, but at that level we're talking about tissue densities, not human density. Even a heterogeneous body has only a single density when considered as a whole. I also realize this is most likely about population density, but it's a lot cooler to me to think of it in terms of physical body density, and that doesn't change the answer, or even the above asterisk (with appropriate terminological substitutions of course).