There are two main definitions of symbiosis in biology, (1) symbiosis as any association and (2) symbiosis as persistent mutualism [7]. The first definition allows any kind of persistent contact between different species of organisms to count as symbiosis, even if the contact is pathogenic or parasitic. The second definition requires that all of the participating organisms must benefit for the persistent contact to count as symbiosis [7]. One reason for this disagreement over definitions is that there is a large gray zone of cases where it is difficult to know whether all of the participants benefit from the association. For example, in many cases the presence or absence of mutual benefit can vary over time, depending on environmental factors. There is also a problem with deciding how long mutual benefit must persist before we can call it persistent mutualism. The desire for simplicity explains the appeal of symbiosis as any association. On the other hand, the presence of mutual benefit is theoretically interesting, which motivates the narrower definition of symbiosis as persistent mutualism.