From the limited context of the quote, it is referring to this definition of distribution (via Merriam-Webster):
> a : the position, arrangement, or frequency of occurrence (as of the members of a group) over an area or throughout a space or unit of time
>
> b biology : the natural geographic range of an organism
Or, if that isn't simple enough, it just means _where_ the members of the species are located. In different contexts it may be simply the range, i.e., the space where _any_ individuals of that species are located, or it might actually refer to the density as well.