Introduced mammals on seabird breeding islands
Encyclopedia
Seabirds include some of the most threatened taxa anywhere in the world. For example, of extant albatross
species, 82% are listed as threatened, endangered, or critically endangered by the International Union for Conservation of Nature. The two leading threats to seabirds are accidental bycatch
by commercial fishing operations and introduced mammals
on their breeding islands. Mammals are typically brought to remote islands by humans either accidentally as stowaways on ships, or deliberately for hunting, ranching, or biological control of previously introduced species. Introduced mammals have a multitude of negative effects on seabirds including direct and indirect effects. Direct effects include predation and disruption of breeding activities, and indirect effects include habitat transformation due to overgrazing and major shifts in nutrient cycling due to a halting of nutrient subsidies from seabird excrement. There are other invasive species on islands that wreak havoc on native bird populations (e.g. Brown Snakes on Guam), but mammals are by far the most commonly introduced species to islands and the most detrimental to breeding seabirds. Despite efforts to remove introduced mammals from these remote islands, invasive mammals are still present on roughly 80% of islands worldwide.
s (Felis catus) are one of the most commonly introduced mammals and one of the most effective predators on ground-nesting seabirds. On the Kerguelen Islands
in the Southern Ocean
, cats kill 1.2 million nesting seabirds annually. One study stated that introduced feral cats have been responsible for the extinction of 10 petrel
species in the 20th century alone. Feral cats are such effective predators on ground-nesting seabirds because seabirds did not evolve with terrestrial predators and therefore did not develop defenses against them. In many cases, seabirds do not even recognize feral cats as a threat. It has also been demonstrated that feral cats on islands preferentially eat seabirds, but during the nonbreeding season cats switch their prey base to insects, rats, and mice for subsistence.
are by far the most common introduced mammals on seabird islands. Currently, approximately 80% of islands worldwide have populations of introduced rats. The most common introduced species is the Norway Rat (Rattus norvegicus), but Black Rats
(R. rattus, also known as “Ship Rat”) and Polynesian Rat
s (R. exulans) have found their way to offshore islands and have subsequently decimated populations of breeding seabirds. Norway Rats are thought to be the most destructive due to their high rates of population growth and extreme ecological plasticity
that allows them to live in a wide variety of habitats and climates. Introduced rats have been shown to negatively impact 75 species of seabirds in 10 different families; smaller burrow-nesting seabirds such as storm petrels and shearwaters are the most negatively affected, while larger, surface-nesting birds such as gulls seem to be the least impacted due to their ability to defend their eggs and chicks.
(Mus musculus). Like rats, House Mice have a high intrinsic growth rate and are exceptionally adaptable to a wide range of ecological conditions and therefore can persist on a majority of the world’s islands. On the mainland, mice eat mainly grains and insects; however, on tiny, remote islands these food items are frequently unavailable, forcing the mice to adapt and prey on seabird eggs and chicks.
The most spectacular example of this is on Gough Island
, a small island in the middle of the south Atlantic
. Mice made it to Gough Island in the 19th century and began shifting their prey base to mainly seabird chicks. One of the largest and most endangered species of seabird in the world, the Tristan Albatross
(Diomedea dabbenena) nests mainly on this island and do so because of its historical lack of terrestrial predators. The mice introduced to Gough Island have evolved to eat exclusively albatross chicks and grow to twice the size of a normal House Mouse on average as a result. Even though the albatross chicks are massive compared to the mice, the chicks do not recognize mice as a threat and allow the mice to eat them alive while they sit at their nests.
(Alopex lagopus) and Red Fox
es (Vulpes vulpes) are extremely adept predators on ground-nesting seabirds. Foxes are able to depredate both adult and young seabirds because these seabirds have developed no defenses against these highly-evolved predators. Foxes have been present on the Aleutian Island chain in Alaska
since the 1700s and have drastically reduced seabird populations and even extirpated species from certain islands. Patagonian Fox
(Dusicyon griseus) has also been introduced to the Falkland Islands
, home to some of the largest seabird colonies
in the world.
s (Procyon lotor) are omnivorous mammals that can subsist on a diet of seabird eggs and chicks where they are introduced. Raccoons decrease seabird breeding success; for example, in 2004 Raccoons were introduced to gull breeding colonies in the Isles of Shoals
, Maine
. Islands without Raccoons were 17 times more likely to hatch and fledge chicks than islands with introduced Raccoons present. In another example, Raccoons introduced to the Scott Islands, British Columbia in the 1930s devastated what was the largest aggregation of breeding seabirds in the eastern Pacific south of Alaska.
s have been introduced on over 800 islands worldwide. However, this report is nearly 20 years old and more introductions have certainly happened since that time. Through grazing, rabbits alter the vegetative structure of islands that seabirds depend on for breeding activities.
to offshore islands in an attempt to incite ranching and establish permanent human residence on these islands. The negative effects of grazing animals on island ecosystems is mainly through change in the vegetation cover, but these animals also have direct effects on breeding populations of seabirds through the trampling of nests and chicks. There is even an instance of pigs
digging up burrows and consuming endangered Townsend's Shearwaters (Puffinus auricularis) in the Revillagigedo Islands
off the western coast of Mexico
.
means. People have brought mammals to islands both intentionally and accidentally. Regardless of their method of introduction, once invasive mammals establish populations on these islands, their populations grow rapidly due to the quick and effective filling of empty niches as well as the seemingly endless supply of easy-to-capture food in the form of unsuspecting seabirds, mainly eggs and chicks.
worldwide, over 30% of them are listed as threatened or endangered by the IUCN, and rodents introduced accidentally to islands are thought to be seabird’s greatest terrestrial threat. Usually, these rats and mice have to adapt their diets upon reaching islands, but once they do so, there is very little keeping their populations in check. Furthermore, in cases where islands already have native populations of insular
, small mammals they are often driven towards extinction by the more aggressive and plastic
invaders. Because of these reasons, rat and mouse eradication on islands is prioritized over any other foreign species removal.
is the most common form of direct effect and greatly influences hatching and fledging success of seabirds. Predation by introduced rats alone has been implicated as the major cause for at least 11 seabird extinctions worldwide. In some cases, skilled predators such as foxes or cats even prey on adult birds. The introduction of cats and their subsequent predation on adult Guadalupe Storm Petrels (Oceanodroma macrodactyla) is believed to be the main cause of this species’ extinction in the early 20th century.
In addition to predation, there are other direct ways in which introduced mammals can negatively affect breeding seabird populations. Larger grazing animals such as cattle can trample seabird nests, burrows, eggs, and chicks. One study examining the influence of Norway Rats on the reproductive success of Least Auklet
s (Aethia pusilla) found that even though there wasn’t much evidence for predation, the overall reproductive success of the colony decreased significantly due to rats disrupting adult auklets attempting to brood
or provision young chicks. Rabbits also affect seabird nesting success. They contend for nest sites, alter existing burrows, and cause physical damage to eggs and chicks.
Introduced mammals can also have compound direct effects on colonies of breeding seabirds. In addition to consuming eggs and chicks, introduced pigs also dig up and destroy burrows of nesting seabirds. Feral cats often disturb breeding seabirds to the point where adults abandon their nests. Cats also frequently kill more seabirds than they actually consume, a behavior called surplus killing.
(Pisonia umbellifera).
There have also been more complex indirect interactions describing how introduced mammals can indirectly affect seabird breeding populations. On the Aleutian Island chain in Alaska, over 10 million seabirds form dense colonies and many are greatly affected by the direct predation of introduced Arctic Foxes. However, recent research has shown that seabirds drastically alter the flora of their breeding islands by subsidizing nutrients for plants with their excrement. Once these colonies are decimated by introduced predators, the nutrient cycling is interrupted and the plant communities on the islands change drastically. Some of the remaining seabirds have trouble adapting to this rapid change in vegetation and are forced to breed elsewhere or suffer lower reproductive success.
is well known and widely reported; therefore, large-scale efforts are currently in place to either mitigate these impacts or eradicate introduced mammals off islands completely. However, the effectiveness of methods currently employed is still being debated.
There have been attempts on 387 islands worldwide to eradicate rodents and the most recent synthesis of results show that on 284 of those islands, rodents have been completely exterminated. Rodenticides have been used in over 99% of these eradication campaigns utilizing three methods of deployment: 1) bait stations 2) hand broadcasting and 3) aerial broadcasting. Bait stations are optimal for minimizing nontarget impacts and other forms of unintentional environmental harm from the poison, but this method is labor intensive and can be impossible given the topography
of some islands. Aerial distribution of rodenticide by helicopter is now the most common method, but a combination of methods is often the most effective.
Feral cats have proven more difficult to completely remove from islands. Of the 48 islands feral cats have been completely removed from in the past half century, only 10 have been on islands larger than 10km2. Similar to goat eradications, the most effective cat eradication technique is to combine several approaches including hunting, live trapping, poisoned bait stations, and even releasing feline viruses amongst cat populations. On smaller islands, cat eradication is more straightforward, but it can prove extremely difficult on larger islands with human populations. Due to the high level of interaction between humans and feral cats on these islands, poisoned bait stations and feline viruses have to be used sparingly or not at all.
Albatross
Albatrosses, of the biological family Diomedeidae, are large seabirds allied to the procellariids, storm-petrels and diving-petrels in the order Procellariiformes . They range widely in the Southern Ocean and the North Pacific...
species, 82% are listed as threatened, endangered, or critically endangered by the International Union for Conservation of Nature. The two leading threats to seabirds are accidental bycatch
Bycatch
The term “bycatch” is usually used for fish caught unintentionally in a fishery while intending to catch other fish. It may however also indicate untargeted catch in other forms of animal harvesting or collecting...
by commercial fishing operations and introduced mammals
Introduced species
An introduced species — or neozoon, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its indigenous or native distributional range, and has arrived in an ecosystem or plant community by human activity, either deliberate or accidental...
on their breeding islands. Mammals are typically brought to remote islands by humans either accidentally as stowaways on ships, or deliberately for hunting, ranching, or biological control of previously introduced species. Introduced mammals have a multitude of negative effects on seabirds including direct and indirect effects. Direct effects include predation and disruption of breeding activities, and indirect effects include habitat transformation due to overgrazing and major shifts in nutrient cycling due to a halting of nutrient subsidies from seabird excrement. There are other invasive species on islands that wreak havoc on native bird populations (e.g. Brown Snakes on Guam), but mammals are by far the most commonly introduced species to islands and the most detrimental to breeding seabirds. Despite efforts to remove introduced mammals from these remote islands, invasive mammals are still present on roughly 80% of islands worldwide.
Species commonly introduced
Cats
Feral CatFeral cat
A feral cat is a descendant of a domesticated cat that has returned to the wild. It is distinguished from a stray cat, which is a pet cat that has been lost or abandoned, while feral cats are born in the wild; the offspring of a stray cat can be considered feral if born in the wild.In many parts of...
s (Felis catus) are one of the most commonly introduced mammals and one of the most effective predators on ground-nesting seabirds. On the Kerguelen Islands
Kerguelen Islands
The Kerguelen Islands , also known as the Desolation Islands, are a group of islands in the southern Indian Ocean constituting the emerged part of the otherwise submerged Kerguelen Plateau. The islands, along with Adélie Land, the Crozet Islands and the Amsterdam and Saint Paul Islands are part of...
in the Southern Ocean
Southern Ocean
The Southern Ocean comprises the southernmost waters of the World Ocean, generally taken to be south of 60°S latitude and encircling Antarctica. It is usually regarded as the fourth-largest of the five principal oceanic divisions...
, cats kill 1.2 million nesting seabirds annually. One study stated that introduced feral cats have been responsible for the extinction of 10 petrel
Petrel
Petrels are tube-nosed seabirds in the bird order Procellariiformes. The common name does not indicate relationship beyond that point, as "petrels" occur in three of the four families within that group...
species in the 20th century alone. Feral cats are such effective predators on ground-nesting seabirds because seabirds did not evolve with terrestrial predators and therefore did not develop defenses against them. In many cases, seabirds do not even recognize feral cats as a threat. It has also been demonstrated that feral cats on islands preferentially eat seabirds, but during the nonbreeding season cats switch their prey base to insects, rats, and mice for subsistence.
Rats
RatsRATS
RATS may refer to:* RATS , Regression Analysis of Time Series, a statistical package* Rough Auditing Tool for Security, a computer program...
are by far the most common introduced mammals on seabird islands. Currently, approximately 80% of islands worldwide have populations of introduced rats. The most common introduced species is the Norway Rat (Rattus norvegicus), but Black Rats
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...
(R. rattus, also known as “Ship Rat”) and Polynesian Rat
Polynesian Rat
The Polynesian Rat, or Pacific Rat , known to the Māori as kiore, is the third most widespread species of rat in the world behind the Brown Rat and Black Rat. The Polynesian Rat originates in Southeast Asia but, like its cousins, has become well travelled – infiltrating Fiji and most Polynesian...
s (R. exulans) have found their way to offshore islands and have subsequently decimated populations of breeding seabirds. Norway Rats are thought to be the most destructive due to their high rates of population growth and extreme ecological plasticity
Phenotypic plasticity
Phenotypic plasticity is the ability of an organism to change its phenotype in response to changes in the environment. Such plasticity in some cases expresses as several highly morphologically distinct results; in other cases, a continuous norm of reaction describes the functional interrelationship...
that allows them to live in a wide variety of habitats and climates. Introduced rats have been shown to negatively impact 75 species of seabirds in 10 different families; smaller burrow-nesting seabirds such as storm petrels and shearwaters are the most negatively affected, while larger, surface-nesting birds such as gulls seem to be the least impacted due to their ability to defend their eggs and chicks.
Mice
Mice are another commonly introduced mammal to islands. The species usually introduced is the cosmopolitan House MouseHouse mouse
The house mouse is a small rodent, a mouse, one of the most numerous species of the genus Mus.As a wild animal the house mouse mainly lives associated with humans, causing damage to crops and stored food....
(Mus musculus). Like rats, House Mice have a high intrinsic growth rate and are exceptionally adaptable to a wide range of ecological conditions and therefore can persist on a majority of the world’s islands. On the mainland, mice eat mainly grains and insects; however, on tiny, remote islands these food items are frequently unavailable, forcing the mice to adapt and prey on seabird eggs and chicks.
The most spectacular example of this is on Gough Island
Gough Island
Gough Island , also known historically as Gonçalo Álvares or Diego Alvarez, is a volcanic island in the South Atlantic Ocean. It is a dependency of Tristan da Cunha and part of the British overseas territory of Saint Helena, Ascension and Tristan da Cunha...
, a small island in the middle of the south Atlantic
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
. Mice made it to Gough Island in the 19th century and began shifting their prey base to mainly seabird chicks. One of the largest and most endangered species of seabird in the world, the Tristan Albatross
Tristan Albatross
The Tristan Albatross, Diomedea dabbenena, is a large seabird from the albatross family. One of the great albatrosses of the genus Diomedea, it was only widely recognised as a full species in 1998.-Taxonomy:...
(Diomedea dabbenena) nests mainly on this island and do so because of its historical lack of terrestrial predators. The mice introduced to Gough Island have evolved to eat exclusively albatross chicks and grow to twice the size of a normal House Mouse on average as a result. Even though the albatross chicks are massive compared to the mice, the chicks do not recognize mice as a threat and allow the mice to eat them alive while they sit at their nests.
Foxes
Where introduced, ArcticArctic fox
The arctic fox , also known as the white fox, polar fox or snow fox, is a small fox native to Arctic regions of the Northern Hemisphere and is common throughout the Arctic tundra biome. The Greek word alopex, means a fox and Vulpes is the Latin version...
(Alopex lagopus) and Red Fox
Red Fox
The red fox is the largest of the true foxes, as well as being the most geographically spread member of the Carnivora, being distributed across the entire northern hemisphere from the Arctic Circle to North Africa, Central America, and the steppes of Asia...
es (Vulpes vulpes) are extremely adept predators on ground-nesting seabirds. Foxes are able to depredate both adult and young seabirds because these seabirds have developed no defenses against these highly-evolved predators. Foxes have been present on the Aleutian Island chain in Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...
since the 1700s and have drastically reduced seabird populations and even extirpated species from certain islands. Patagonian Fox
Argentine Grey Fox
The South American gray fox , also known as the Patagonian fox, the chilla, or the grey zorro, is a species of zorro, the "false" foxes.-Range and habitat:...
(Dusicyon griseus) has also been introduced to the Falkland Islands
Falkland Islands
The Falkland Islands are an archipelago in the South Atlantic Ocean, located about from the coast of mainland South America. The archipelago consists of East Falkland, West Falkland and 776 lesser islands. The capital, Stanley, is on East Falkland...
, home to some of the largest seabird colonies
Bird colony
A bird colony is a large congregation of individuals of one or more species of bird that nest or roost in close proximity at a particular location. Many kinds of birds are known to congregate in groups of varying size; a congregation of nesting birds is called a breeding colony...
in the world.
Raccoons
RaccoonRaccoon
Procyon is a genus of nocturnal mammals, comprising three species commonly known as raccoons, in the family Procyonidae. The most familiar species, the common raccoon , is often known simply as "the" raccoon, as the two other raccoon species in the genus are native only to the tropics and are...
s (Procyon lotor) are omnivorous mammals that can subsist on a diet of seabird eggs and chicks where they are introduced. Raccoons decrease seabird breeding success; for example, in 2004 Raccoons were introduced to gull breeding colonies in the Isles of Shoals
Isles of Shoals
The Isles of Shoals are a group of small islands and tidal ledges situated approximately off the east coast of the United States, straddling the border of the states of New Hampshire and Maine.- History :...
, Maine
Maine
Maine is a state in the New England region of the northeastern United States, bordered by the Atlantic Ocean to the east and south, New Hampshire to the west, and the Canadian provinces of Quebec to the northwest and New Brunswick to the northeast. Maine is both the northernmost and easternmost...
. Islands without Raccoons were 17 times more likely to hatch and fledge chicks than islands with introduced Raccoons present. In another example, Raccoons introduced to the Scott Islands, British Columbia in the 1930s devastated what was the largest aggregation of breeding seabirds in the eastern Pacific south of Alaska.
Rabbits
The most recent report indicates that rabbitRabbit
Rabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world...
s have been introduced on over 800 islands worldwide. However, this report is nearly 20 years old and more introductions have certainly happened since that time. Through grazing, rabbits alter the vegetative structure of islands that seabirds depend on for breeding activities.
Livestock
There is a long history of introducing livestockLivestock
Livestock refers to one or more domesticated animals raised in an agricultural setting to produce commodities such as food, fiber and labor. The term "livestock" as used in this article does not include poultry or farmed fish; however the inclusion of these, especially poultry, within the meaning...
to offshore islands in an attempt to incite ranching and establish permanent human residence on these islands. The negative effects of grazing animals on island ecosystems is mainly through change in the vegetation cover, but these animals also have direct effects on breeding populations of seabirds through the trampling of nests and chicks. There is even an instance of pigs
PIGS
PIGS is a four letter acronym that can stand for:* PIGS , Phosphatidylinositol glycan anchor biosynthesis, class S, a human gene* PIGS , the economies of Portugal, Italy , Greece and Spain...
digging up burrows and consuming endangered Townsend's Shearwaters (Puffinus auricularis) in the Revillagigedo Islands
Revillagigedo Islands
The Revillagigedo Islands or Revillagigedo Archipelago are a group of four volcanic islands in the Pacific Ocean, known for their unique ecosystem...
off the western coast of Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
.
Methods of introduction and establishment
Most seabirds breed on islands that are hundreds or even thousands of kilometers from the mainland; therefore, the most common way mammals get to these islands is by anthropogenicAnthropogenic
Human impact on the environment or anthropogenic impact on the environment includes impacts on biophysical environments, biodiversity and other resources. The term anthropogenic designates an effect or object resulting from human activity. The term was first used in the technical sense by Russian...
means. People have brought mammals to islands both intentionally and accidentally. Regardless of their method of introduction, once invasive mammals establish populations on these islands, their populations grow rapidly due to the quick and effective filling of empty niches as well as the seemingly endless supply of easy-to-capture food in the form of unsuspecting seabirds, mainly eggs and chicks.
Accidental
Rodents usually make it to islands as stowaways aboard ships. Once these ships reach islands, mice and/or rats escape and establish populations on islands. Due to their high intrinsic rates of population growth and extreme ecological plasticity in both their diet and habitat preferences, their abundance often increases astronomically just a few years after their introduction. Of the 328 species of true seabirdsProcellariiformes
Procellariiformes is an order of seabirds that comprises four families: the albatrosses, petrels and shearwaters, storm petrels, and diving petrels...
worldwide, over 30% of them are listed as threatened or endangered by the IUCN, and rodents introduced accidentally to islands are thought to be seabird’s greatest terrestrial threat. Usually, these rats and mice have to adapt their diets upon reaching islands, but once they do so, there is very little keeping their populations in check. Furthermore, in cases where islands already have native populations of insular
Island
An island or isle is any piece of sub-continental land that is surrounded by water. Very small islands such as emergent land features on atolls can be called islets, cays or keys. An island in a river or lake may be called an eyot , or holm...
, small mammals they are often driven towards extinction by the more aggressive and plastic
Phenotypic plasticity
Phenotypic plasticity is the ability of an organism to change its phenotype in response to changes in the environment. Such plasticity in some cases expresses as several highly morphologically distinct results; in other cases, a continuous norm of reaction describes the functional interrelationship...
invaders. Because of these reasons, rat and mouse eradication on islands is prioritized over any other foreign species removal.
Intentional
There are also many examples of humans purposefully introducing mammals to remote islands. There are three main reasons why people deliberately introduce mammals to islands. The first is to initiate ranching and agriculture on the islands as a precursor to establishing permanent human residence. The second reason is for hunting purposes with the financial backing of sport-hunters and fur-trading companies. The third reason is as a form of biological control on other invasive species that have been previously released on islands. Releasing cats to control rodent populations is the most common example of this; ironically, the cats that are released are far more efficient predators on native seabirds than the rodents they were meant to control.Effects of introduced mammals
The negative effects of introduced mammals on seabird breeding islands are extreme, highly varied, and can be felt almost immediately after the introduction event.Direct
Direct effects of introduced mammals on breeding seabirds are the most noticeable effects. PredationPredation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...
is the most common form of direct effect and greatly influences hatching and fledging success of seabirds. Predation by introduced rats alone has been implicated as the major cause for at least 11 seabird extinctions worldwide. In some cases, skilled predators such as foxes or cats even prey on adult birds. The introduction of cats and their subsequent predation on adult Guadalupe Storm Petrels (Oceanodroma macrodactyla) is believed to be the main cause of this species’ extinction in the early 20th century.
In addition to predation, there are other direct ways in which introduced mammals can negatively affect breeding seabird populations. Larger grazing animals such as cattle can trample seabird nests, burrows, eggs, and chicks. One study examining the influence of Norway Rats on the reproductive success of Least Auklet
Least Auklet
The Least Auklet, Aethia pusilla, is a seabird and the smallest species of auk. It is the most abundant seabird in North America, and one of the most abundant in the world, with a population of around nine million birds. They breed on the islands of Alaska and Siberia, and spend the winter close to...
s (Aethia pusilla) found that even though there wasn’t much evidence for predation, the overall reproductive success of the colony decreased significantly due to rats disrupting adult auklets attempting to brood
Avian incubation
Incubation refers to the process by which certain oviparous animals hatch their eggs, and to the development of the embryo within the egg. The most vital factor of incubation is the constant temperature required for its development over a specific period. Especially in domestic fowl, the act of...
or provision young chicks. Rabbits also affect seabird nesting success. They contend for nest sites, alter existing burrows, and cause physical damage to eggs and chicks.
Introduced mammals can also have compound direct effects on colonies of breeding seabirds. In addition to consuming eggs and chicks, introduced pigs also dig up and destroy burrows of nesting seabirds. Feral cats often disturb breeding seabirds to the point where adults abandon their nests. Cats also frequently kill more seabirds than they actually consume, a behavior called surplus killing.
Indirect
Introduced mammals also have a multitude of indirect effects on seabirds that might not be as obvious as the direct effects. Cattle, rabbits, goats, and other introduced grazing animals drastically change the vegetative cover of islands, increasing soil erosion by an order of magnitude. This drastically changes island form and habitat types, to which breeding seabirds are very sensitive. An example of this can be seen on Cabbage Tree Island, Australia where the threatened Gould’s Petrel (Pterodroma leucoptera) nests. As introduced rabbits overgrazed the forest understory, these burrowing petrels became exposed to predators and vulnerable to entanglement in the sticky fruits of the Bird-lime TreePisonia umbellifera
Pisonia umbellifera, commonly known as the Bird-lime Tree, is a species of plant in the Nyctaginaceae family. It grows throughout the tropical Indo-Pacific. It is native to the Andaman Islands, Indonesia, Malaysia, the Philippines, Thailand, Vietnam, China, Taiwan, Hawaii and Madagascar and the...
(Pisonia umbellifera).
There have also been more complex indirect interactions describing how introduced mammals can indirectly affect seabird breeding populations. On the Aleutian Island chain in Alaska, over 10 million seabirds form dense colonies and many are greatly affected by the direct predation of introduced Arctic Foxes. However, recent research has shown that seabirds drastically alter the flora of their breeding islands by subsidizing nutrients for plants with their excrement. Once these colonies are decimated by introduced predators, the nutrient cycling is interrupted and the plant communities on the islands change drastically. Some of the remaining seabirds have trouble adapting to this rapid change in vegetation and are forced to breed elsewhere or suffer lower reproductive success.
Mitigation/eradication of introduced mammals
The massive negative impacts of introduced mammals on breeding seabirds and island biodiversityBiodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
is well known and widely reported; therefore, large-scale efforts are currently in place to either mitigate these impacts or eradicate introduced mammals off islands completely. However, the effectiveness of methods currently employed is still being debated.
Exclosure
In certain situations, predator exclosure structures (allowing adult birds to freely move in and out of the nest site to brood/provision chicks, but preventing mammalian predators from attacking the eggs, chicks, or adults) have proven successful. Most studies investigating the effectiveness of predator exclosures have used ducks or shorebirds as a study system. Applying these ideas for true seabirds might prove impractical due to the density and locations where seabirds nest.Eradication
The most common method used today to nullify the effects of introduced mammals on seabird colonies is by partial or complete eradication. Introduced goats have been successfully eradicated from 120 islands worldwide. To do so, scientists use a combination of tools to locate and remove goats including global positioning systems (GPS), geographic information systems (GIS), aerial hunting via helicopter, hunting dogs, and herding (Judas) goats. Using this multifaceted approach, large populations of goats (>20,000 individuals) have been eradicated from islands in only a few years.There have been attempts on 387 islands worldwide to eradicate rodents and the most recent synthesis of results show that on 284 of those islands, rodents have been completely exterminated. Rodenticides have been used in over 99% of these eradication campaigns utilizing three methods of deployment: 1) bait stations 2) hand broadcasting and 3) aerial broadcasting. Bait stations are optimal for minimizing nontarget impacts and other forms of unintentional environmental harm from the poison, but this method is labor intensive and can be impossible given the topography
Topography
Topography is the study of Earth's surface shape and features or those ofplanets, moons, and asteroids...
of some islands. Aerial distribution of rodenticide by helicopter is now the most common method, but a combination of methods is often the most effective.
Feral cats have proven more difficult to completely remove from islands. Of the 48 islands feral cats have been completely removed from in the past half century, only 10 have been on islands larger than 10km2. Similar to goat eradications, the most effective cat eradication technique is to combine several approaches including hunting, live trapping, poisoned bait stations, and even releasing feline viruses amongst cat populations. On smaller islands, cat eradication is more straightforward, but it can prove extremely difficult on larger islands with human populations. Due to the high level of interaction between humans and feral cats on these islands, poisoned bait stations and feline viruses have to be used sparingly or not at all.
Effectiveness
There have been many who have debated whether invasive mammal eradication campaigns are effective enough considering the non-target and environmental impacts of the methods used, the cost, and even those who believe the problem of introduced mammals on islands is overstated . However, since introduced mammals pose such a threat to biodiversity worldwide, the pervading thought is that action must be taken to remove these invasive mammals from islands. Many studies have shown overwhelming effectiveness of individual eradication campaigns, including partial or complete recovery of seabird populations. Reviews have shown that over 70% of rat and 90% of goat eradication campaigns that have been attempted have successfully ridded the islands of these invasive mammals. Additionally, while concerns persist that island ecosystems take centuries to recover after the removal of invasive mammals, recent research suggests that, if managed properly, islands might take only a couple years to a couple decades to recover.See also
- SeabirdSeabirdSeabirds are birds that have adapted to life within the marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same environmental problems and feeding niches have resulted in similar adaptations...
- Introduced speciesIntroduced speciesAn introduced species — or neozoon, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its indigenous or native distributional range, and has arrived in an ecosystem or plant community by human activity, either deliberate or accidental...
- ProcellariiformesProcellariiformesProcellariiformes is an order of seabirds that comprises four families: the albatrosses, petrels and shearwaters, storm petrels, and diving petrels...
- AlbatrossAlbatrossAlbatrosses, of the biological family Diomedeidae, are large seabirds allied to the procellariids, storm-petrels and diving-petrels in the order Procellariiformes . They range widely in the Southern Ocean and the North Pacific...
- PenguinPenguinPenguins are a group of aquatic, flightless birds living almost exclusively in the southern hemisphere, especially in Antarctica. Highly adapted for life in the water, penguins have countershaded dark and white plumage, and their wings have become flippers...
- International Union for Conservation of Nature
- BiodiversityBiodiversityBiodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
- Threats to biodiversity
- Invasion biology terminologyInvasion biology terminologyThe need for a clearly defined and consistent invasion biology terminology has been acknowledged by many sources. Invasive species, or invasive exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats...
- Human impact on the environment
- Island restorationIsland restorationThe ecological restoration of islands, or island restoration, is the application of the principles of ecological restoration to islands and island groups. Islands, due to their isolation, are home to many of the world's endemic species, as well as important breeding grounds for seabirds and some...