Tree Bark Habitats

     At first appearance, tree bark seems sterile, but it is a habitat for certain adaptable and commonplace kinds of life in the Middle Atlantic States, as elsewhere.  But one has to look closely to see most of the life on tree bark.  Squirrels can be obvious scurrying up trees and out their limbs, but most life on bark is small, often still, and much of it blends into the gray or brown of the bark.
     Mosses, lichens and tiny mushrooms are the bulk of the non-animal life living on bark, forming little gardens that are most apparent during and just after a rainfall.  Mosses, lichens and mushrooms absorb the water, which swells them and makes them more visible. 
     Lichens are a flat combination of fungi that holds a place on the bark and absorbs rain water, and alga that has green chlorophyll that is powered by sunlight (photosynthesis) to combine carbon dioxide from the air and hydrogen from water to make a sugar that is the food for both the fungi and the alga. 
     Mosses also have chlorophyll and make their own food.  But the mushrooms get their nutrition from breaking down a little of the bark.
     Two kinds of daddy long-legs, eastern and brown, are spider relatives most noted for their eight, very long legs, compared to the sizes of their bodies.  They are the critters most likely seen on tree trunks and branches in woods and older suburban areas where they catch and eat tiny invertebrates during warmer months.  Both kinds of daddy long-legs are camouflaged on tree bark, but some are seen, caught and eaten by a variety of birds and other critters anyway.
     Carpenter ants and other kinds of ants commonly run up and down tree bark on missions known only to them.  Carpenter ants are the big, black ones that live in colonies in cavities they dig themselves in dead wood under the bark.  When excavating tunnels, they poke their heads out of holes in the wood and bark to dump bits of wood from their mandibles.  That wood piles up on the ground, giving away the presence of carpenter ants at work.
     Willow aphids and other kinds of aphids live on soft, young bark and suck sugary sap from it.  Certain types of ants protect aphids and are rewarded with ingesting the sugary waste aphids exude from their rears, a waste called honeydew.  But the attractive, little lady bug beetles, and their nymphs, consume the aphids, creating another food chain of who eats whom.
     Brown creepers and two species of nuthatches are small birds in the Mid-Atlantic States that walk up and down tree bark and peer into protective crevices for invertebrates and their tiny eggs.  Creepers have slightly down-curved beaks they pry into cracks to get food.  Nuthatch beaks are stout and a tiny bit up-turned to pick food from tree bark crevices. 
     Creepers flutter to the bottom of a tree trunk and slowly spiral up the tree in search of invertebrate food.  But nuthatches are the only family of birds that can walk down tree trunks head first.  Not even woodpeckers can do that.  It is amazing how adapted some species of life are to their niche, which allows them survival in that niche.
     Mostly at night, sometime in August, many annual cicada grubs emerge from the soil of suburban areas and climb trees and other objects to varying heights.  Coming to rest, the brown, outer shell of each grub splits along the back and the winged, adult insect crawls out of its old exoskeleton.  After gaining strength, each adult cicada flies away in search of a mate or mates, leaving the empty shell on the tree bark as a reminder to us that it was there.  And now, during August days, we hear the almost incessant, pulsing whining of several male annual cicadas high in the tree tops.  That buzzing whine brings the genders together to mate and lay fertilized eggs.
     Though seemingly uninhabited and dull, tree bark is alive with life in the Mid-Atlantic States, if one looks closely.  It is amazing where life is.  It seems every niche on Earth has life in it.