Question

For what type of organisms is species a difficult term to define?

Answer 1

I would say for organisms which reproduce asexually. Also fossil organisms.

Explanation:

Species is defined as a group of organisms which can interbreed among themselves but are reproductively isolated from other groups.

In that case, asexually reproducing organisms are not able to 'interbreed' -- that is why species is often described as a group of organisms sharing a common gene pool .

Species is also defined as a distinct line of evolution (with particular ancestral affinity and evolutionary tendencies) which maintained or can maintain its hereditary integrity through time and space. This is even applicable for extinct and fossilised organisms.