>Parasites can offer a beneficial service to the host but cannot live without it. Incidentally, this is not what biologists consider parasitism. To a biologist

>Parasites can offer a beneficial service to the host but cannot live without it. Incidentally, this is not what biologists consider parasitism. To a biologist, a parasitic relationship is one that benefits one while harming the other. Do you think that reddit was harmed by imgur? If imgur benefited but reddit was not harmed, their relationship was commensal. If they both benefited, their relationship was mutual. To capture the idea that imgur was totally dependent on its relationship with reddit (but not vice versa), you would say that their mutualism was obligate for imgur but facultative for reddit.