What term describes the condition when one or both species cannot survive without the other?

The term that describes the condition when one or both species cannot survive without the other is mutualism. In mutualistic relationships, the interaction benefits both species involved, often to the extent that their survival and reproduction are interdependent. A classic example is the relationship between bees and flowering plants, where bees obtain nectar for food while simultaneously aiding in the pollination of the plants, which is crucial for their reproduction.

In contrast, commensalism involves one species benefiting while the other is neither helped nor harmed, which does not reflect the dependency described in the question. Parasitism indicates a relationship where one organism benefits at the expense of another, which also does not align with the notion of mutual survival. Protosymbiosis typically refers to a more general or unknown symbiotic relationship and is not specifically used to describe a situation of dependency. Thus, mutualism best captures the essence of a mutual survival requirement between species.