Invertebrates in the Treetops

Skyrail Nature Diary: December 2010

It has long been known that various insects, mites, spiders and worms are vital to the ecosystems that reside in the treetops. Many epiphytes that produce compost from falling leaves, mostly ferns, contain huge numbers of tiny insects and earthworms that help aerate the compost and break it down.

Springtails, millipedes, beetles and mites are important members of these tiny, often overlooked animals, many of them less than half a millimetre long. Several groups of insects including grasshoppers, bugs, thrips and beetles, are important herbivores in rainforest ecosystems.

There are numerous examples of cooperation in the treetops, generally known as symbiosis. Cooperation can occur in many forms. Several ant species protect certain trees from harm in exchange for nectar and pollen, while others protect aphids (plant lice) in return for a sugar solution. However, there are also insects which are parasites to their host tree including scale insects, spittle bugs, aphids and gall wasps. They will suck sap from the tree without giving anything in return.

Mites are an often overlooked group in the treetops. They are arachnids, distantly related to spiders. They have only one obvious body part with prominent jaws protruding and eight legs. There are three types: ticks, water mites and true mites. Mites are usually tiny and can be either herbivores or carnivores. The carnivores often have a close relationship with the leaves of several tree species. Trees make small pockets for the mites to live in, known as domatia.

Some types of herbivorous mites are also important for the leaves. Many leaves are covered with communities of moss, lichens and fungi that bind up nutrients, known as epiphylls. Mites feed on these communities and release the locked up nutrients as faeces for re-absorption by the leaf, while also freeing covered leaf area for photosynthesis (process by which plants use energy from the sunlight to make sugar).

Beetles make up by far the largest proportion of the invertebrates in the treetops. There are an estimated 500,000 species in the world. Most are smaller than half a millimeter in length. A lot of these species appear to be restricted to certain trees. Experiments have shown that each tree has a unique complement of beetles, up to 80% not found on neighbouring trees. They fulfill countless roles in the canopy (treetop), among them the breakdown of compost in large epiphytes, and predation of other invertebrates. Some have unusual defense mechanisms. The most amazing is the bombardier beetle, spraying chemicals at boiling point.

Ants are the most important group in the treetops after beetles. Ant plants usually grow on paperbark trees in swampy areas and occasionally on some rainforest trees. The ant plant has spiny leaves and otherwise consists of a large disc covered in bumps and hollowed out in the form of an ant nest. Ants will move into these ready made nests and defend it from intruders. It saves them the bother of having to make one themselves. In addition to this, the ants defecate inside the chambers, fertilizing the host.