B. OAK TREES: THE SERVE YOURSELF BUFFET

Sand live oak Turkey oak Chapman's oak Myrtle oak Scrub oak Some species of oaks produce acorns with higher tannins than other oaks. When the Florida scrub-jay caches acorns, it buries ones heavy in tannins so they are less likely to rot or be eaten by animals. More than 50 kinds of gall wasps live on scrub oaks. These wasps are so tiny that they couldn't sting a person, even if they wanted to. Different kinds of gall wasps usually cause the tree to produce different-looking galls. These oak galls are formed by the oak in response to chemicals produced by both the gall wasps as they insert their eggs into leaves, stems, flowers, and fruits, and by the larvae that develop inside the gall. The wasp larvae feed on the soft and nutritious flesh of the plant available to them from inside the gall. Larvae remain in the protective and edible gall until they mature into adults, and then gnaw their way out, leaving tiny neat holes. Just how the different gall wasps stimulate oaks to form a specific kind of gall is not completely understood. Because galls are often complicated structures with several layers, it appears the wasp gives the plant a set of instructions (in the form of growth-stimulating chemicals) that result in the special residence and restaurant the wasp larva needs.

Introduction

A few generations ago a large percentage of children grew up on farms. From an early age these children were responsible for tending gardens. They observed and, unconsciously, began to understand the complexity of the biological systems. These children would have fought specific pests on a variety of different plants. Tomatoes had hornworms, potatoes had several kinds of leaf beetles, squash vines had borers, apples had fruit maggots and fruitworms, and so on. These children not only understood that different plants had their own enemies, but they would have known that different parts of a plant may be eaten by particular insects. They would have known to look for cabbage worms on the underside of the leaf, and that cutworms hide in the soil during the day and emerge at night to fell and consume bean plants under cover of darkness. They would have known that some insects attack young plants, other insects attack older plants. They would have understood that a plant does not need to be pest-free in order to thrive and produce a crop.

Old farm circulars and rural school leaflets from 50 or 70 years ago included sections on beneficial insects called, "the farmer's friends." Children would also have been familiar with these "friends." When children got bored with pulling weeds and squashing caterpillars all day, their sharp eyes and curious minds were quick to appreciate the dramas of predation. A wasp pounces on a furiously writhing caterpillar, a lady beetle cuts a swath through a patch of aphids, a team of ants find a grasshopper changing its skin, and attack it in its weakened condition. A spider wraps a big June beetle in silk, an assassin bug on a flower subdues a bee, a praying mantis carefully eats the head of a fly that is still buzzing. As crops matured, insects visited the flowers, one kind of bee for the squash, another for the alfalfa. Bumblebees pollinated blueberries; honey bees and solitary bees visited apples and plums.

When work was finished in the garden, children possibly postponed the next round of chores by exploring adjacent pastures where dung beetles rolled their trophies home from the cow pat. Or they made little discovery trips to the woods, where caterpillars dropped from the beech trees on invisible silken threads when chased from their rolled leaves by quick-probing warblers.

Life of the average child today is much different. They are expected to know and understand very little about the complexity of ecological systems. Does this lack of understanding matter? Yes! Most of the photosynthesis that provides food energy for humans and other animals still takes place in very complex systems. The forests that give us wood and paper need thousands of species in order to function properly. The decontamination of water in lakes and rivers requires a whole web of interacting organisms. Humans have the ability to disastrously simplify these systems so that they no longer work well. Talking about the nitrogen cycle or the hydrologic cycle as if they were mere chemical and physical processes is not enough! If we want our students to become a generation of responsible citizens, we need to introduce them to the complexity of ecological systems. The oak tree project will, for many students, be their first direct observation of the complexity of familiar ecological systems.

This one project cannot completely change the way students view the world. However, it can be an important step toward a more mature and realistic view of the planet they will inherit. Or, to look at it another way, this section may help students enjoy natural diversity and drama in a way that all children might have just a few hundred years ago.

Background Information

During this activity, students will approach ecological complexity by investigating how plant-eating insects divide up the resources provided by oak trees. A big plant, such as a tree, offers as many different places to live as a good-sized city does to a human, and the choice of different kinds of restaurants is equally large. A single tree has leaves, flowers, buds, twigs, bark, and roots. A single leaf may be old or newly developed, it may be exposed to the sun, or it may be shaded. These distinctions are important to insects. The leaf itself has veins that can be tapped for sap, it has several layers of tissue that can be eaten, and it can even be induced to grow special structures called galls.

The insects that feed on trees are fussy about where and how they feed. They are picky because these insects are specialized to feed in particular ways. They have no choice in what they do: their mouthparts and digestive systems are adapted for a particular way of eating and processing some special part of the tree. Part of the specialization seen in tree-eating insects is the result of plant defenses.

Plants do not tamely submit to being gobbled up by insects. Different parts of a tree have different kinds of tough, inedible protective coverings. Tender tissues, such as young leaves, may have a protective coat of barbed or hook-shaped hairs. Most tissues have chemical defenses, usually several kinds. In movies, a special agent who is trying to retrieve a stolen document or invention is equipped with all kinds of fancy gadgets to bypass the elaborate security measures of the enemy stronghold and storage vault. Insects that feed on plants are like those secret agents with their fancy gadgets. Like real secret agents, insects are not always successful in penetrating the defenses and gaining the prize.

A question often comes up as students begin to explore the diversity of insects that may feed on a single species of tree. "With all these bugs, why does the tree still have leaves on it?" This topic is considered at the end of the project, when the students list the habits of the animals they see on the tree, including predators. The plant-eating insects all have many predators. The predators, the "farmer's friends" of the old days, keep the tree-eating species under control.

A question that almost never comes up, but which is still interesting is, "What is the value of the plant-eating insects?" The value of these insects is that they are, to a large extent, responsible for the diversity that we see in plants as well as the number of interesting and useful features found in each species of plant. Beyond this, the more general value of all biological diversity is that it makes ecological systems more versatile and efficient. Most people, including students, understand and value diversity at some level. For example, we all understand that each person is different, and while life would be simpler if we were all the same, in the long run, humans are very dependent on this diversity. We need people with a variety of different skills and abilities to keep mankind versatile and efficient. (Imagine a town full of dentists!) Extending this principle to the scale of ecological systems is not easy, but it is a step that all biologists and students must eventually take.

About Oaks

This activity focuses on oaks because the Florida is full of them! The six species that occur in scrub include: scrub oak, sand live oak, myrtle oak, Chapman's oak, runner oak, and the less frequent turkey oak. Except for the turkey oak, all of the oaks are evergreen, which means leaves stay on the trees all year and drop off only as they die. Turkey oaks lose their leaves in winter and tender new leaves emerge in early spring. (Scrub oaks drop almost all their old leaves before new ones emerge, but stay leafless for a very short time.) Most of the leaf eating occurs in March and April when new growth is easy to eat, or after a fire, when the new oak sprouts emerge.

While many species of plants have flowers with male and female parts to ensure pollination by insects, oak trees have separate male and female flowers that occur on the same tree. Rather than relying on insects for pollination, oak trees need wind to blow pollen from male flowers, or catkins, to petal-less female flowers. Several species of native bees, gall-making wasps, and caterpillars eat pollen produced by male flowers and can be found on the catkins when oak trees bloom in early spring.

Oak trees provide essential food for many animals in the scrub. Leaves are eaten by a wide variety of insects, such as grasshoppers, katydids, caterpillars, leafminers, gall-making wasps, and beetles. Tree- and leafhoppers suck sap from twigs and leaves. Acorns, the fruit of the oak tree, are favored by weevils and other insect larvae, squirrels, deer, bear, and birds, including the Florida scrub-jay

Oaks and acorns contain chemicals called tannins that help protect them from being eaten. However, some insects and animals have adaptations that help them deal with these tannins so they can eat oak leaves anyway!

Many kinds of moth caterpillars, and a few butterfly caterpillars, feed on scrub oak leaves. Some caterpillars eat entire oak leaves. Others make irregular holes or notches in the leaves. Others eat the green parts of the leaves and do not eat the leaf veins, leaving a brown network of veins. Some very flat moth caterpillars feed inside the leaf, between the upper and lower surfaces, and leave behind a brownish or whitish blotch or twisting line. Some kinds of caterpillars use silk to sew together overlapping scrub oak leaves and then feed within this shelter.

With so many caterpillars eating scrub oak leaves, it is amazing that oaks have any leaves left at the end of summer! However, so many birds and insects feed on caterpillars, their numbers are usually kept down to a reasonable level. Florida scrub-jays eat lots of caterpillars, and so do digger wasps and twig-nesting wasps. Dozens of kinds of little parasitic wasps and flies prey on caterpillars. When biologists try to raise oak caterpillars to see what kind of moths will develop, frequently wasps or flies emerge instead.