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Talking Caterpillars: Obligate versus Facultative Interactions.

Obligate interactions are those in which one or both partners must participate in the interaction to survive.

Facultative interactions are those in which the partners may participate in the interaction, but they don't have to.

Example: certain aphids cannot survive unless they are tended by ants. Thus they have an obligate interaction with ants. However, the ants don't need the aphids to survive. Their interaction with the aphids is facultative.

As with types of interactions, these two categories represent end points on a continuum.

Interactions can also be specialized or generalized. Specialists are organisms that only interact with one or a few species. Generalists interact with many species.

  • Obligate and specialized: organism interacts with only one or a few species, and they must interact to survive.

  • Facultative and specialized: organism doesn't have to have the interaction, but if they do, it is with only one or a few other species.

  • Obligate and generalized: organism must have the interaction, but it can be with many other species.

  • Facultative and generalized: organism is capable of interacting with many other species, but they don't have to have the interaction.

Next: ant interactions with lycaenid butterfly larvae.

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