Tree hollow
Encyclopedia
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, eucalypt
s develop hollows at all ages, but only from when the trees are 120 years old do they form hollows suitable for vertebrate
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 echidna
s, numbat
s, 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 diurnal
or nocturnal shelter sites, as well as for rearing young, feeding, thermoregulation
, and to facilitate ranging behaviour and dispersal
. 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.
Especially in Europe, entomologists are interested in the use of hollows by invertebrate
s. One beetle associated with hollow trees, Osmoderma eremita
, has been given the highest priority according to the European Union's Habitat Directive.
. Tree holes can be caused when an injury to the tree, such as breakage of a limb, creates an opening through the bark
and exposes the sapwood
. The sapwood is attacked by fungi and bacteria
, which form a cavity in the bole
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 Ceratopogonidae
, Chironomidae
, the Common Merganser
, toucan
s, woodpecker
s, and bluebird
s. Tree holes can be important in the maintenance and spread of some diseases, for example La Crosse encephalitis
.
s in the chute of a grain silo; or pardalote
s in the top, horizontal pipe of a children's swing. Purpose built nest box
es, such as birdhouses and bat tubes, are also constructed for conservation and for wildlife observation. The size of the nest box, entry hole and placement height may be chosen in consideration of certain species. Natural hollows are still generally preferred for habitat conservation
.
(Sialia sialis) has required nest boxes due to the loss of natural hollows. The scarcity of dead, hollow-bearing trees in Scandinavia
n forests is a key threatening process to native bird life. In Sweden
, almost half of red-listed
species are dependent on dead hollow-bearing trees or logs.
, 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 rare
, threatened
or near-threatened
on Australian State or Commonwealth legislation, in part because of the removal of hollow-bearing trees.
Threats to hollows include: native forest silviculture
, firewood
collection, rural dieback
(such as from inundation and salinity
), grazing by cattle
, and land clearing. Additionally, pest and introduced species such as the Common Myna
and Western honey bee (Apis mellifera) compete with native species for hollows; domestic and feral cat
s and black rat
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 Galah
, Common Brushtail Possum
and the Little Corella
and compete with less common native species.
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, eucalypt
Eucalypt
Eucalypts 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 vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
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 echidna
Echidna
Echidnas , also known as spiny anteaters, belong to the family Tachyglossidae in the monotreme order of egg-laying mammals. There are four extant species, which, together with the platypus, are the only surviving members of that order and are the only extant mammals that lay eggs...
s, numbat
Numbat
The numbat , also known as the banded anteater, or walpurti, is a marsupial found in Western Australia. Its diet consists almost exclusively of termites. Once widespread across southern Australia, the range is now restricted to several small colonies and it is listed as an endangered species...
s, 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 diurnal
Diurnal animal
Diurnality is a plant or animal behavior characterized by activity during the day and sleeping at night.-In animals:Animals that are not diurnal might be nocturnal or crepuscular . Many animal species are diurnal, including many mammals, insects, reptiles and birds...
or nocturnal shelter sites, as well as for rearing young, feeding, thermoregulation
Thermoregulation
Thermoregulation 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 dispersal
Biological dispersal
Biological dispersal refers to 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...
. 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.
Especially in Europe, entomologists are interested in the use of hollows by invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s. One beetle associated with hollow trees, Osmoderma eremita
Osmoderma eremita
Osmoderma eremita, the hermit beetle or Russian leather beetle, is a species of European beetle in the family Scarabaeidae. The larvae develop in hollow trees. Oak is the most important tree species, but the larvae may develop in any tree species with suitable hollows. Due to habitat loss and...
, has been given the highest priority according to the European Union's Habitat Directive.
Description
A tree hollow is a cavity in a living treeTree
A 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 bark
Bark
Bark 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 sapwood
Sapwood
Sapwood may refer to:* Sapwood, the part of living wood where sap flows, as distinct from the heartwood, where it doesn't* SS-6 Sapwood, the NATO reporting name for the R-7 Semyorka intercontinental ballistic missile...
. The sapwood is attacked by fungi and bacteria
Bacteria
Bacteria are a large domain of prokaryotic 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 bole
Trunk (botany)
In botany, trunk refers to the main wooden axis 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 the...
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 Ceratopogonidae
Ceratopogonidae
Ceratopogonidae, or biting midges , are a family of small flies in the order Diptera...
, Chironomidae
Chironomidae
Chironomidae are a family of nematoceran flies with a global distribution. They are closely related to the Ceratopogonidae, Simuliidae, and Thaumaleidae...
, the Common Merganser
Common Merganser
The Common Merganser or Goosander Mergus merganser is a large duck, of rivers and lakes of forested areas of Europe, northern and central Asia, and North America. It eats fish and nests in holes in trees...
, toucan
Toucan
Toucans are members of the 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, often colorful bills. The family includes five genera and about forty different species...
s, woodpecker
Woodpecker
Woodpeckers 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 bluebird
Bluebird
The 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 encephalitis
La Crosse Encephalitis
La 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 possumPygmy possum
The 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 pardalote
Pardalote
Pardalotes or peep-wrens 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...
s in the top, horizontal pipe of a children's swing. Purpose built nest box
Nest box
A nest box, also spelled nestbox is a man-made box provided for animals to nest in. Nest boxes are most frequently utilized for wild and domesticated birds, in which case they are also called birdhouses, but some mammalian species may also use them. Birdhouses are the most common types of nest...
es, such as birdhouses and bat tubes, are also constructed for conservation and for wildlife observation. The size of the nest box, entry hole and placement height may be chosen in consideration of certain species. Natural hollows are still generally preferred for habitat conservation
Habitat conservation
Habitat 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 BluebirdEastern Bluebird
The Eastern Bluebird, Sialia sialis, is a small thrush found in open woodlands, farmlands and orchards, and most recently can be spotted in suburban 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 Scandinavia
Scandinavia
Scandinavia is a cultural, historical and ethno-linguistic region in northern Europe that includes the three kingdoms of Denmark, Norway and Sweden, characterized by their common ethno-cultural heritage and language. Modern Norway and Sweden proper are situated on the Scandinavian Peninsula,...
n forests is a key threatening process to native bird life. In Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
, almost half of red-listed
Red-listed
Red-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
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, 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 rare
Rare species
A rare species is a group of organisms that are 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"....
, threatened
Threatened species
Threatened species are any speciesg animals, plants, fungi, etc.) which are vulnerable to endangerment in the near future.The 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,...
or near-threatened
Near Threatened
Near 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 silviculture
Silviculture
Silviculture is the practice of controlling the establishment, growth, composition, health, and quality of forests to meet diverse needs and values. The name comes from the Latin silvi- + culture...
, firewood
Firewood
Firewood 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 dieback
Dieback
Dieback 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 salinity
Salinity
Salinity 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 cattle
Cattle
Cattle 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 Myna
Common Myna
The Common Myna or Indian Myna also sometimes spelled Mynah, is a member of family Sturnidae native to Asia. An omnivorous open woodland bird with a strong territorial instinct, the Myna has adapted extremely well to urban environments...
and Western honey bee (Apis mellifera) compete with native species for hollows; domestic and feral cat
Cat
The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...
s and black rat
Black Rat
The black rat is a common long-tailed rodent of the genus Rattus in the subfamily Murinae . The species originated in tropical Asia and spread through the Near East in Roman times before reaching Europe by the 1st century and spreading with Europeans across the world.-Taxonomy:The black rat was...
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 Galah
Galah
The 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 on the mainland and was...
, Common Brushtail Possum
Common Brushtail Possum
The Common Brushtail Possum is a nocturnal, semi-arboreal marsupial of the family Phalangeridae, it is native to Australia, and the largest of the possums.Like most possums, the Common Brushtail is nocturnal...
and the Little Corella
Little Corella
The 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.