3.9 Eradication of Invasive Species
The introduction in the 1950s to Guam of the exotic brown treesnake, Boiga
irregularis, which is native in Australia and New Guinea, exemplifies how a fast
growing population of an introduced species on an island can threaten the native
species diversity. The snake devoured 10 of 13 bird species on this island (Rodda
et al. 1992; Burnett et al. 2006).
Since trade continuously increased during the last five hundred years an
uncountable number of alien species have been transported and dispersed into
landscapes and waters all over the world. As it is impossible and in some cases
not necessary to remove every exotic or alien species and as also native species
under changing environmental conditions may behave as invaders, some basic facts
and rules should be considered with respect to the amount of investment and targets.
First of all, almost all species that went extinct on islands were extirpated by the
establishment of one or more new guilds; this means that the introduced species
represents a guild which did not exist in the landscape before (Whittaker and
Fernández-Palacios 2007). Guam was a snake-free island before the brown treesnake
was introduced. The snake simply caught all the birds that did not try to escape
because they had never seen a snake or any other aggressive predator before. On
many oceanic islands far from the continent, terrestrial snakes, other groups of
predators, amphibians, pathogens, humans and the related guilds are not native.
The amphibian decline mainly is the result of the anthropogenic introduction of an
alien fungus (Batrachochytrium dendrobatidis) which damages the skin of amphibians. In many tropical regions, such a pathogen did not exist before it was introduced
by accident together with the African clawed frog (Xenopus laevis).
Most bird species on islands that went extinct were extirpated not by competition
but by new predators (rat, cat), ecosystem engineers (goat, boar), killing men as in
the case of the dodo on Rodrigues and Mauritius, or a combination of these
(Whittaker and Fernández-Palacios 2007).
Thus, competition between alien and native species is not the largest threat to
native species. However, a few examples may be seen as an exception (cf. IUCN
Red List).
Furthermore, the situation on small oceanic islands in most cases is more problematic than on large continental islands or mainland regions. The populations of
Prunus serotina, native to North America, increased dramatically across Central
Europe since the tree species was introduced in the nineteenth century (Starfinger
1997). Certain countries in Central and Western Europe currently pay a lot of money
to remove Prunus serotina from forests and heathlands, for example, as this plant
species is well known as aggressive invasive alien. However, the usefulness of these
measures of removal depends on the target and the definition of success. The
measures are sometimes successful when looking at the removal itself. When
looking at structures and species compositions at landscape scales it will be difficult
to find any effect except that the whole number of species increased by one: the
exotic species. We do not know any example across continental Europe that alien
172
C. Hobohm
The introduction in the 1950s to Guam of the exotic brown treesnake, Boiga
irregularis, which is native in Australia and New Guinea, exemplifies how a fast
growing population of an introduced species on an island can threaten the native
species diversity. The snake devoured 10 of 13 bird species on this island (Rodda
et al. 1992; Burnett et al. 2006).
Since trade continuously increased during the last five hundred years an
uncountable number of alien species have been transported and dispersed into
landscapes and waters all over the world. As it is impossible and in some cases
not necessary to remove every exotic or alien species and as also native species
under changing environmental conditions may behave as invaders, some basic facts
and rules should be considered with respect to the amount of investment and targets.
First of all, almost all species that went extinct on islands were extirpated by the
establishment of one or more new guilds; this means that the introduced species
represents a guild which did not exist in the landscape before (Whittaker and
Fernández-Palacios 2007). Guam was a snake-free island before the brown treesnake
was introduced. The snake simply caught all the birds that did not try to escape
because they had never seen a snake or any other aggressive predator before. On
many oceanic islands far from the continent, terrestrial snakes, other groups of
predators, amphibians, pathogens, humans and the related guilds are not native.
The amphibian decline mainly is the result of the anthropogenic introduction of an
alien fungus (Batrachochytrium dendrobatidis) which damages the skin of amphibians. In many tropical regions, such a pathogen did not exist before it was introduced
by accident together with the African clawed frog (Xenopus laevis).
Most bird species on islands that went extinct were extirpated not by competition
but by new predators (rat, cat), ecosystem engineers (goat, boar), killing men as in
the case of the dodo on Rodrigues and Mauritius, or a combination of these
(Whittaker and Fernández-Palacios 2007).
Thus, competition between alien and native species is not the largest threat to
native species. However, a few examples may be seen as an exception (cf. IUCN
Red List).
Furthermore, the situation on small oceanic islands in most cases is more problematic than on large continental islands or mainland regions. The populations of
Prunus serotina, native to North America, increased dramatically across Central
Europe since the tree species was introduced in the nineteenth century (Starfinger
1997). Certain countries in Central and Western Europe currently pay a lot of money
to remove Prunus serotina from forests and heathlands, for example, as this plant
species is well known as aggressive invasive alien. However, the usefulness of these
measures of removal depends on the target and the definition of success. The
measures are sometimes successful when looking at the removal itself. When
looking at structures and species compositions at landscape scales it will be difficult
to find any effect except that the whole number of species increased by one: the
exotic species. We do not know any example across continental Europe that alien
172
C. Hobohm