If you look closely in your garden, you may find drama on a small scale.
Warriors & Slaves
In this photo, Argentine Ants are herding their aphid slaves for food. The food source the ants are exploiting is the sweet honeydew substance aphids excrete after ingesting plant juices. In return for supplying this nectar to the ants, the aphids get protection from some predators.
Trying to penetrate defenders, this Four-spot Aphid Fly (center) is looking for an opportunity to lay an egg. This fly species lays a single egg on vegetation near active aphid colonies. This strategy gives the hatching larva an opportunity to find food immediately and begin feeding on aphids. Even though this fly did lay an egg, the ants will kill the larvae before it ever has a chance to consume any aphids.
Unfortunately this is a very good example of a non-native species (the Argentine Ant) disrupting the natural balance in a native garden by hindering a native pollinator (the aphid fly) and helping a garden pest (the aphid).