A
tree hollow or
tree hole is a semi-enclosed cavity which has naturally formed in the trunk or branch of a tree. These are predominantly found in old trees, whether living or not. Hollows form in many species of trees, and are a prominent feature of natural forests and woodlands, and act as a resource or habitat for a number of vertebrate and invertebrate animals.
Hollows may form as the result of physiological stress from natural forces causing the excavating and exposure of the heartwood. Forces including wind, fire, heat, lightning, rain, attack from insects (such as termites or beetles), bacteria, or fungi. Also, trees may self-prune, dropping lower branches as they reach maturity, exposing the area where the branch was attached. Many animals further develop the hollows using instruments such as their beak, teeth or claws.
The size of hollows may depend on the age of the tree. For example,
eucalyptEucalypts are woody plants belonging to three closely related genera:Eucalyptus, Corymbia and Angophora.In 1995 new evidence, largely genetic, indicated that some prominent Eucalyptus species were actually more closely related to Angophora than to the other eucalypts; they were split off into the...
s develop hollows at all ages, but only from when the trees are 120 years old do they form hollows suitable for
vertebrateVertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony...
s, and it may take 220 years for hollows suitable for larger species to form.
Hollows in fallen timber are also very important for animals such as echidnas, numbats, chuditch and many reptiles. In streams, hollow logs may be important to aquatic animals for shelter and egg attachment.
Hollows are an important habitat for many wildlife species, especially where the use of hollows is obligate, as this means no other resource would be a feasible substitute. Animals may use hollows as
diurnalDiurnality is an animal behavior characterized by being active during the day and sleeping at night. Animals that are not diurnal might be nocturnal or crepuscular . Many animal species are diurnal, including many mammals, insects and birds...
or nocturnal shelter sites, as well as for rearing young, feeding,
thermoregulationThermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different...
, and to facilitate ranging behaviour and
dispersalBiological dispersal refers to a species movement away from an existing population or away from the parent organism. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for individual fitness, but also for population dynamics, population...
. Hollows are found in living and dead standing trees; and in the form of fallen timber; and hollow logs in streams, which can provide shelter to aquatic animals or are used for egg attachment. While use may also be opportunistic, rather than obligate, it may be difficult to determine the nature of a species' relationship to hollows — it may vary across a species' range, or depend on climatic conditions.
Animals will select a hollow based on factors including entrance size and shape, depth, and degree of insulation. Such factors greatly affect the frequency and seasonality of hollow use.
The use of hollows by
invertebrateAn invertebrate is an animal without a vertebral column. The group includes 95% of all animal species — all animals except those in the Chordate subphylum Vertebrata ....
s has not been broadly studied..
Description
A tree hollow is a cavity in a living
treeA tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...
. Tree holes can be caused when an injury to the tree, such as breakage of a limb, creates an opening through the
barkBark is the outermost layers of stems and roots of woody plants. Plants with bark include trees, woody vines and shrubs. Bark refers to all the tissues outside of the vascular cambium and is a nontechnical term. It overlays the wood and consists of the inner bark and the outer bark. The inner...
and exposes the
sapwoodSapwood may refer to:* sapwood: a part of the wood, as distinct from the heartwood* SS-6 Sapwood, the NATO reporting name for the R-7 Semyorka intercontinental ballistic missile....
. The sapwood is attacked by fungi and
bacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, which form a cavity in the
boleIn botany, trunk refers to the main structural member of a tree that supports the branches and is supported by and directly attached to the roots. The trunk is covered by the bark, which is an important diagnostic feature in tree identification, and which often differs markedly from the bottom of...
of the tree. The resulting cavity can fill with water, thus becoming a type of phytotelma. Therefore, there are wet and dry tree holes. Tree holes are important habitats for many animals, such as
CeratopogonidaeCeratopogonidae, or biting midges , are a family of small flies in the order Diptera...
,
ChironomidaeChironomidae are a family of nematoceran flies with a global distribution. They are closely related to the Ceratopogonidae, Simuliidae, and Thaumaleidae. Many species superficially resemble mosquitoes but they lack the wing scales and elongate mouthparts of the Culicidae...
, the
Common MerganserThe Common Merganser , also known as a the gulaund or goosander, is a large-sized duck, which is distributed over Europe, North Asia and North America. It is most common on lakes and rivers. Its nests can be found in treeholes.There are three subspecies* M. m. merganser, the Goosander of Europe*...
,
toucanToucans are a family, Ramphastidae, of near-passerine birds from the neotropics . The family is most closely related to the American barbets. They are brightly marked and have large, colorful bills. The family includes five genera and about forty different species...
s,
woodpeckerWoodpeckers are near passerine birds of the order Piciformes. They are one subfamily in the family Picidae, which also includes the piculets and wrynecks. They are found worldwide and include about 180 species....
s, and
bluebirdThe bluebirds are a group of medium-sized, mostly insectivorous or omnivorous birds in the genus Sialia of the thrush family . Bluebirds are one of the few thrush genera in the Americas. They have blue, or blue and red, plumage...
s. Tree holes can be important in the maintenance and spread of some diseases, for example
La Crosse encephalitisLa Crosse encephalitis is an encephalitis caused by an arbovirus which has a mosquito vector .-History:...
.
Artificial hollows
Animals have been found to use artificial structures as substitutes for hollows. For example
pygmy possumThe pygmy possums are a family of small possums that together form the marsupial family Burramyidae. There are five extant species of pygmy possum, grouped into two genera. Four of the species are endemic to Australia, with one species also co-occurring in Papua New Guinea and Indonesia.Pygmy...
s in the chute of a grain silo; or
pardalotePardalotes are a family, Pardalotidae, of very small, brightly coloured birds native to Australia, with short tails, strong legs, and stubby blunt beaks. This family is composed of four species in one genus, Pardalotus, and several subspecies. The name derives from a Greek word meaning "spotted"...
s in the top, horizontal pipe of a children's swing. Purpose built
nest boxA nest box, nestbox or birdhouse) is a man-made box provided for animals to nest in. Nest boxes are most frequently utilized for wild and domesticated birds, but some mammal species may also use them.-Construction:...
es, such as birdhouses and bat tubes, are also constructed for conservation and for wildlife observation. The size of the next box, entry hole and placement height may be chosen in consideration of certain species. Natural hollows are still generally preferred for
habitat conservationHabitat conservation is a land management practice that seeks to conserve, protect and restore, habitat areas for wild plants and animals, especially conservation reliant species, and prevent their extinction, fragmentation or reduction in range...
.
Around the world
Conservation of hollow-using fauna is an issue in many parts of the world. In North America, recovery of the
Eastern BluebirdThe Eastern Bluebird, Sialia sialis, is a medium-sized thrush found in open woodlands, farmlands and orchards, and most recently can be spotted in surburban areas. It is the state bird of Missouri and New York....
(
Sialia sialis) has required nest boxes due to the loss of natural hollows. The scarcity of dead, hollow-bearing trees in
ScandinaviaScandinavia is a geographical region in northern Europe that includes, and is named after, the Scanian Province. It consists of the kingdoms of Norway, Sweden, and Denmark...
n forests is a key threatening process to native bird life. In
SwedenSweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe...
, almost half of
red-listedRed-listed species includes any indigenous species or subspecies considered to be extirpated, endangered, or threatened in their locale. Extirpated taxa no longer exist in the wild in the locale in which they are listed , but do occur elsewhere. Endangered taxa are facing imminent extirpation or...
species are dependent on dead hollow-bearing trees or logs.
Australia
In
AustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the continental mainland , the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans...
, 304 vertebrate species are known to use tree hollows in Australia: 29 amphibians, 78 reptiles, 111 birds, 86 mammals. Approximately 100 of these are now
rareA rare species is an organism which is very uncommon or scarce. This designation may be applied to either a plant or animal taxon, and may be distinct from the term "endangered" or "threatened species" but not "extinct"....
,
threatenedThreatened species are any species which are vulnerable to extinction in the near future.World Conservation Union is the foremost authority on threatened species, and treats threatened species not as a single category, but as a group of three categories: vulnerable, endangered, and critically...
or
near-threatenedNear Threatened is a conservation status assigned to species or lower taxa that may be considered threatened with extinction in the near future, although it does not currently qualify for the threatened status...
on Australian State or Commonwealth legislation, in part because of the removal of hollow-bearing trees.
Threats to hollows include: native forest
silvicultureSilviculture is the art and science of controlling the establishment, growth, composition, health, and quality of forests to meet diverse needs and values of the many landowners, societies and cultures.-Regeneration:...
,
firewoodFirewood is any wood like material that is gathered and used for fuel. Generally, firewood is not highly processed and is in some sort of recognizable log or branch form....
collection, rural
diebackDieback may refer to a number of plant problems and diseases including:* Forest dieback caused by acid rain, heavy metal pollution, or imported pathogens* The death of regions of a plant or similar organism caused by physical damage, such as from pruning...
(such as from inundation and
salinitySalinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
), grazing by
cattleCattle are the most common type of large domesticated ungulates. They are a prominent modern member of the subfamily Bovinae, are the most widespread species of the genus Bos, and are most commonly classified collectively as Bos primigenius...
, and land clearing. Additionally, pest and introduced species such as the
Common MynaThe Common Myna or Indian Myna also sometimes spelled Mynah, is a member of the starling family. It is a species of bird native to Asia with its initial home range spanning from Iran, India and Kazakhstan to Malaysia and China...
and Western honey bee (
Apis mellifera) compete with native species for hollows; domestic and feral
catThe cat , also known as the domestic cat or housecat to distinguish it from other felines and felids, is a small carnivorous mammal that is valued by humans for its companionship and its ability to hunt vermin and household pests...
s and
black ratThe Black Rat is a common long-tailed rodent of the genus Rattus in the subfamily Murinae...
s prey on hollow-using animals and have been damaging especially to island populations; and some native hollow-using species have increased population densities or expanded their ranges since European settlement, such as the
GalahThe Galah , Eolophus roseicapilla, also known as the Rose-breasted Cockatoo, Galah Cockatoo, Roseate Cockatoo or Pink and Grey, is one of the most common and widespread cockatoos, and it can be found in open country in almost all parts of mainland Australia.It is endemic in Australia , where its...
,
Common Brushtail PossumThe Common Brushtail Possum is a nocturnal, semi-arboreal marsupial of the family Phalangeridae, and the largest of the possums....
and the
Little CorellaThe Little Corella, Cacatua sanguinea, also known as the Bare-eyed Cockatoo, is a white cockatoo native to Australia and southern New Guinea....
and compete with less common native species.
Russia, China, Korea
Asiatic Black bears, also known as Himalayan bears (Lat.:
Ursus thibetanus), in northern parts of their range, such as Russian province Primorye, China, and both Koreas, prefer spend winter periods in large tree hollows, where females also give birth to cubs. Threats include massive deforestation in these countries, combined with direct poaching of wintering bears - with selective destruction of the best hollow trees. (Details: www.geocities.com/new_jalsomino/dens_engl.html). In Russia, attempts (sometimes successful) are made to restore such broken trees:
Denning trees of Himalayan bears are being restored in Primorye.(in Russian; cyrillic Windows-1251). Unfortunatelly, only little portion of all damaged trees can be restored in Primorye, where forests are basically logged without taking to account needs of large fauna.
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