Oak Tree Habitat, pt.3

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.

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.

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. 

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.