Up to now, we have focused on interactions between two species. What happens when there are more than two species? Consider, for example, what occurs when two different organisms both attack another organism. In this case, the attack of one organism can sometimes aid the attack of the other. Such an indirect interaction between the two attacking species that benefits them both is called facilitation.

For example, both fungi and beetles attack pine tree phloem, and the attack of each is facilitated by the other. Bark beetles facilitate fungi that attack pine trees by providing entryways through their tunnels into the tree, while fungi facilitate bark beetles by lowering the amount of the pine’s resinous defenses. It is clear that a single species can have both positive and negative effects on others that broadly overlap in a niche.

Like insects, viruses and other pathogens also usually attack only one particular type of host tissue such as the gastrointestinal, respiratory, nervous, or circulatory system. Severe infections can sometimes cause greater susceptibility of their hosts to infection by other pathogens.

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