July 2024 – Discover-Nature

In the past couple of blogs, we learned that oak trees are a keystone species in their habitat, especially for the vast network of relationships between oaks and insects.

^{Ladybug on oak tree leaf by EMFA16, Getty Images}

^{Leaf litter and under-bark room for rent, by Wayne S. Grazio, Nov 2013}

^{Insect holes & homes on cork oak, Quercus suber, by Bloor4ik, Getty Images}

Oak trees and their surroundings provide shelter and food from leaves, wood, bark, sap, flowers, pollen, and acorns to thousands of species of insects and other invertebrates. Shelter is provided on the tree, inside cavities or other fissures, among the leaf litter, and around the root systems. As oaks age, there are more opportunities for shelter and food, including after branches or the whole tree dies and falls to decay on the forest floor.

^{Chickadee nest by Jack VandenHeuval, Getty Images}

^{Chickadee eating a caterpillar by Bettina Arrigoni, May 2018}

Over 900 caterpillar species have been identified in oak habitats, more than three times the number found in forests dominated by maple trees. Canopy leaves and branches provide shelter and nesting opportunities for dozens of bird species who find a ready source of food within the insect populations. Ninety percent of bird species feed insects, especially caterpillars, to their young. The Carolina Chickadee feeds four to six chicks for 16 days before they fledge. Mealtimes require 400 to 500 caterpillars each day.

^{Aphids and ants by oday222, Getty Images Pro}

^{Philodromus praedatus spiders live mainly in mature oak trees, by Nikk, May 2016}

^{Oak gall by Michael Boubin, Getty Images}

Several species of flies and wasps lay their eggs on oak trees, stimulating a growth hormone in the tree to grow a gall, an outgrowth of bark, around the eggs. When the larvae hatch, secretions from the gall feed the larvae, and the gall provides protection to the developing insects. The abundance of insects attracts other invertebrates from higher up on the food chain. Spiders frequently inhabit mature oak trees hunting and feeding on many of the insect species. Aphid colonies are often found on oak trees, but cause little damage to the tree. Aphids feed on the sap from the tree and secrete a sticky, sugary substance called honeydew, a favorite food for ants. The ants protect the aphid colonies, keeping them underground during inclement weather and herding them up the tree in summer to ensure the aphids are well fed to keep producing more honeydew.

^{Fall webworm, Hyphantria cunea, by Anita Gould, Sep 2015; Polyphemus moth caterpillar, Antheraea polyphemus, by Ed Uthman, Sec 2018; Speckled wood caterpillar, Pararge aegeria, by Dean Morley, May 2013; Linden looper caterpillar, Erannis tillaria, by sankax, Jun 2009}

Insects and oak trees have created an environment rich in food for many of its inhabitants. According to Doug Tallamy, a leading entomologist at the University of Delaware, “caterpillars transfer more energy from plants to other animals than any other type of creature.” The loss of habitat for insects puts considerable strain on all other environments, including ones supporting ourselves. Insects do most of the work of pollination, allowing plants to reproduce. They also provide for quick decomposition and the return of nutrients to the soil for growing new plants. Insects are a driving force of the world’s food webs and are necessary for a healthy environment.

There are over 400 species of oaks worldwide, with about one quarter found in North America. As we have briefly touched upon in this series of blogs, oak trees provide opportunities for healthy habitats for many other species. For information and further discussion on the relationships between caterpillars and oak trees, please see The Nature Of Oaks, by Doug Tallamy.

In the last blog, we discussed oak trees as a keystone species, substantially affecting the types and abundance of other species in their habitat. There are 400-500 species of oaks worldwide, supporting an extraordinary 2,300 wildlife species not including fungi, bacteria, and other microorganisms. Oak trees in all stages of life and death have numerous biotic relationships with wildlife.

^{Badger in tree cavity by Byrdyak, Getty Images||Golden millipede in Lagos by efeghali, Getty Images||Neanura springtail by Henrik_L, Getty Images||Pileated woodpecker in oak tree by mtruchon, Getty Images||Slug & rollie pollies by Sonya Kate Wilson, Getty Images||Wood Mouse in tree cavity by byjohn, Getty Images}

Fungi are often identified as attacking oak trees, but they can have very beneficial relationships. Fungi growing in the ground assist an oak tree in getting nitrogen, phosphorous, and other nutrients out of the soil, and in return get carbon and energy from the sugars in the tree’s sap. Oaks provide a place to live for over 700 species of lichens, plus uncounted liverworts and mosses. Lichens covering the oak’s surface help to keep them free of fungal infections and invasive insects. Liverworts and mosses help oaks to retain water and release it into the soil over a long period of time. As oak limbs fall and decay, water retention softens the wood providing food and habitat for decomposers.

Green moss on oak tree by tntemerson, Getty Images

Lichen & peat moss on oak by Sean Gardner, Getty Images

Fallen oak leaves are slow to decompose due to their tannin and lignin content, both preservative compounds. Leaf litter, composed of several layers of fallen leaves, becomes habitat for many millions of species including millipedes, springtails, woodlice, slugs, beetles, ants, and other organisms that feed on decaying plants. Through their activities, the creatures of the leaf litter help to return nutrients to the trees.

Oak forest leaf litter by Tina_C_Olsen, Getty Images

Acorns, with high amounts of fats, proteins, and carbohydrates, are one of the healthiest and most sought after foods in the woodland. White oaks, living between 200 and 300 years, will produce about 3 million acorns during their lifetime. Many bird species eat acorns, but oak trees enjoy a special relationship with blue jays, whose beaks are strong enough to pry an acorn open. In autumn, blue jays enjoy feeding on acorns, but also cache them for winter, burying several at a time under a few inches of soil in various locations up to a mile from the originating tree. Although blue jays cache large quantities of acorns, they can only remember and retrieve about 25% of them, allowing many new oak trees to grow and spread into new areas.

Blue jay with red oak acorn by mirceaux, Getty Images

Oak trees support 31 mammal species who consume acorns for food, use the tree for shelter, or use the surrounding habitat for hunting. Gray squirrels and red squirrels favor acorns in the autumn and throughout the winter from caches hidden in nearby locations. They remember far more cache locations than blue jays, but there are still uneaten acorns that will germinate in spring to start new trees. Acorns are also a favorite food for badger, deer, wild board, and wood mouse.

^{Gray squirrel eating acorn by viktor2013, Getty Images}

^{Red squirrel with acorn by Neil_Burton, Getty Images}

Oak trees offer shelter for many species. Squirrels often build their nests high up among the leaves that provide protection from weather and predators. Dead branches, especially those with cavities, may remain on the tree for many years, providing excellent roost sites for bats. Open limbs, leafed areas, and cavities all provide nest sites for a variety of birds. In addition, the many insects drawn to oak trees provide a ready source of food for all of these nesters. Humans have been acorn consumers for millennia. Raw acorns, high in tannin content, can be toxic in larger quantities; but, properly prepared, acorns are high in calories, vitamin C, starch, magnesium, calcium, phosphorous, and antioxidants. They have been a food staple in many cultures and can be found in the records of ancient Greeks, Iberians, Japanese, and English.

Oak trees have many direct and indirect relationships with insects, which we will explore in the next blog.

Month: July 2024

Oak Tree Habitat, pt.3

Oak Tree Habitat, pt.2