On my road trip north of Traverse City, I noted that the trees were full of ants. What are the ants doing in the trees? Aphid ranching! Ants are unable to feed on the tree leaves themselves. The leaves contain toxins and physical barriers to feeding by ants. However, the ants can indirectly feed on the trees by ranching aphids.
Aphids feed on the trees by avoiding the leaf cells (and their toxins) altogether and inserting their mouthparts directly into the plant phloem, which does not contain toxins and is not defended against aphids. The phloem contains more sugar than aphids need, so aphids excrete a liquid (honeydew) that is high in sugar. The aphid excretion is in the form of droplets, a convenient size for the ants to ingest. The ants wait patiently next to the aphids to ingest each honeydew droplet as it appears. When the ants are full, they return to the colony and regurgitate some of the sugar to share with their nest mates and feed the brood.
In most cases, the ants will tend aphid colonies that are already established. In more complex interactions, ants carry aphids back to the nest and place them on conveniently located leaves. The ants in the trees are predators that obtain protein by eating other insects. This benefits the aphids because the ants will aggressively attack other insects including predators of the aphids. The aphids can feed and reproduce in peace, without interference from predators and the ants are well fed by the aphids. The interaction is mutually beneficial. Interactions between aphids and ants go back tens of millions of years, long before humans began our own ranching a little over 10,000 years ago.