Most of the time we think of organisms existing on their own in the world, but oh are we wrong! Most of the organisms in an environment really depend on other species to survive and exist. Even us humans depend on the smallest organisms (such as intestinal flora) to survive. At Las Brisas Reserve one very interesting relation between two organisms has to do with ants and plants. The photo depicts one small aspect of this relation. The leaf in the photo is of a wild passion fruit of the genus Passiflora, and the ants are drinking from the yellow extra-floral nectaries on the leaf's underside. But what is going on exactly? Plants don't just give away costly resources like nectar; or as we say "no such thing as a free lunch."
Plants in the genus Passiflora are the host plant of the Heliconian butterflies, which lay their yellow eggs on the leaves for their offspring to feed on. Of course the plant does not want this to happen, so it seeks ways to deter this event from happening. First of all, as the leaves mature they develop yellow spots on them, which from a distance appear to be yellow eggs, and the female Heliconian does not want competition for her young so she may not choose those leaves. Second, the underside of these yellow spots are the extrafloral nectaries depicted in the photo, which attract the ants. These ants are constantly patrolling the plant looking for nectar, and if by chance they happen to find a tiny larva they will attack it for food. So the butterfly tends to lay eggs on the tender, young growth which still has not developed yellow spots with nectaries and manages to provide food for the larvae.
This is a constant, on going battle where one species always tries to overcome the other, but evolutionary pressure always leaves them at about the same place they started. In biology this is known as the Red Queen hypothesis, relating to a line said by the Red Queen to Alice in Lewis Carrol's Through the Looking-Glass:
"Now, here, you see, it takes all the running you can do, to keep in the same place."
- Ernesto Carman