Since many people have an oak tree in or near their yard, let’s look a little closer at one insect that uses the oak tree and the ground under the oak tree. Then, we can back up a little and see the larger picture, including more of this small ecosystem. Finally, we can consider ways that we can support ecosystems like those that can occur in our yards.
Have you ever been out in the woods or on the prairie and noticed a strange swelling in the branch of a plant or bumps on the leaves. If you have, you might have seen a gall. I used to think that these things indicated diseased plants. Instead, they indicate that an insect has laid an egg on the stem or leaf and that as the egg hatched and larva begin to grow, chemicals alter the growth of the plant to form a little incubator for the insect inside.
One of the many insects that rely on oak trees to survive is the Oak Gall Wasp (Amphibolips quercusinanis). In the spring, female oak gall wasps lay their eggs on newly unfurling oak leaves. When the egg hatches, the larva releases chemicals that change the growth of the leaf, in this case forming a green sphere. In the center of the sphere, the larva develops and eventually forms a pupa. In late summer to early fall, the adult wasp emerges from the pupa, chewing through the fibrous sphere and exiting the gall. Male and female wasps mate and then the female crawls underground to lay eggs on the tree roots.
Oak gall wasp eggs stay underground (under the oak) over the winter and hatch in the spring. When the eggs hatch, larva feed on the tree roots and will pupate and crawl out of the soil. All of the emerging wasps are females. Without mating, since there are no males, these females lay eggs on the new oak leaves. This is an asexual generation that will produce the males and females that reproduce in the autumn.
This seems simple, right? But ecology is complicated and fascinating! We have only accumulated small bits of knowledge of what is an intricate puzzle with almost endless questions to be answered. If you were to find an oak gall on the ground in early summer, you might find a hole in the gall. What is going on here? Why did this oak gall wasp hatch and emerge early? Surprise! What actually happened is that another insect laid and egg on the oak gall. The larva of this insect crawled inside and fed on the larva or pupa of the oak gall wasp.
Note: This type of an insect (usually a wasp also) is called a parasitoid. Where parasites usually do not kill their hosts, parasitoids do kill their host. Even more complicated, some parasitoids are second, third, fourth or fifth order parasitoids. This means that a certain parasitoid (already feeding on its host) is attacked by another parasitoid (this is an example of a second order parasitoid).
Now back up from the example of the oak gall wasp and consider that there are hundreds of insects that spend all or part of their life cycle in, on, or under oaks trees. There are moth and butterfly caterpillars that feed on leaves and are eaten by birds feeding their hungry spring and summer broods. There are beetles that lay their eggs in the bark of the tree. In some cases, bark beetle larva become the hosts for sawflies or wasps and are killed before they can emerge as adult beetles. Cicadas may lay their eggs on small branches and when the larvae hatch they fall to the ground, dig underground and attach themselves to the oak roots, sometimes for years until they are mature. If there is appropriate leaf cover under the tree, pupa or larva of some species may overwinter under the tree. Leaf cover and plants allow a dead animal or insect to decompose close to the soil and the organisms whose job it is to break down tissue. Birds, mammals and other organisms including all of those caterpillars will deposit fecal material that provides nutrients for the whole system.
As you consider the activity that is occurring in one oak tree, you may begin to understand that there is a complexity that is not apparent to the eye. Predators and parasitoids keep populations of other insects and animals in check, but predators and parasitoids can be eaten by species higher in the pecking order! These organism interactions, along with soil, rocks, humidity, temperature, water conditions, threats from non-native species, and more are ECOLOGY! And you can have a hand in helping ecology along as we will see in the next post.