Download James Grundy, St Peter`s College Invasive non

James Grundy, St Peter’s College
Invasive non-native species: The Ring-necked Parakeet
The Ring-necked Parakeet Psittacula krameri is a medium-sized parakeet,
native to south Asia and central Africa, though feral populations have become
established in at least 40 countries on 5 continents (Strubbe and Matthysen 2009). It is
sedentary (DAISIE 2009), only expanding its range through extremely limited postbreeding dispersal – the Greater London population is estimated to be expanding by
approximately 400m per year (DEFRA 2007). However, it is a popular exotic cagebird,
and as a result it has been able to colonise new areas through escapes or releases;
several hundred birds are thought to escape or be released annually in England alone.
Widespread introduction is not the only reason for their success however. The
UK population is thought to have grown from a mere 4 individuals in 1968 to 6000 in
2002, and the best estimate for the current population is about 20,000 birds (DEFRA
2007). The growth of several different introduced populations of Ring-necked Parakeets
appears to be exponential (Strubbe and Matthysen 2009; DEFRA 2007): the species is
clearly able both to sustain itself and to proliferate in novel environments.
Ring-necked Parakeets are found across a large range of several different
environmental variables. They are able to tolerate variation in temperature and
precipitation, for example, illustrated by their success in both sub-saharan Africa and
north-western Europe. They are not found in mountainous or arid areas however
(DAISIE 2009).
With the exception of extremes, the abiotic variables which determine the
distribution of a species are often intimately linked to the biotic factors of food and
habitat availability. Ring-necked Parakeets feed mainly on seeds, fruit and flowers, but
are also highly adaptable in their diet, eating kitchen scraps such as bread, fat and even
meat. This enables them to survive in a variety of habitats. They are frequent visitors to
gardens in areas of western Europe, taking advantage of the practice of supplying food
for birds in winter. This food supply may explain why they show a marked preference for
built-up areas in Europe (Strubbe and Matthysen 2007). However, whilst it has been
suggested that Ring-necked Parakeets depend upon these resources for survival during
the winter months, evidence has shown that this is not the case – a population in
Brighton did not use bird feeders for 8 years! They are simply opportunistic feeders, not
constrained by any particular food type and taking advantage of whatever food
resources they find (DEFRA 2007), enabling them to be successful in both urban and
rural areas.
The most important factor restricting the distribution of Ring-necked Parakeets
is probably the availability of nesting sites, since they are secondary cavity-nesting birds
– they nest in pre-existing holes rather than excavating the nest hole themselves
(Strubbe and Matthysen 2009). They are most commonly found in wooded areas,
particularly those with Ash Fraxinus excelsior in Europe, as Ash provides some of the
most favourable breeding sites. Intensively managed woodland is not suitable, as the
trees tend not to have many suitable cavities: in Europe, Ring-necked Parakeets are
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usually found in cavities made by woodpeckers (Strubbe and Matthysen 2007), which
feed on insects in the bark of older trees not found in intensively managed woodland. It
seems that the availability of suitable nesting cavities, combined with low dispersal, is
the main factor restricting the range expansion of non-native populations: although there
is plenty of suitable habitat (an estimated 22.5-28.9% of forested habitats in northern
Belgium are suitable) much of it remains uncolonised due to a lack of ‘stepping stone’
areas of suitable breeding habitat. However, Ring-necked Parakeets are mobile and
able to cover large distances – it is only a matter of time before most suitable sites are
colonised (Strubbe and Matthysen 2009).
Their specific requirements for nesting sites have brought Ring-necked
Parakeets into conflict with birds which share the same preferences. In Europe, the
Nuthatch Sitta europaea and the Starling Sturnus vulgaris are the species most likely to
be affected by Ring-necked Parakeets as a result of this because all three species
depend largely on cavities made by woodpeckers, although other cavity-nesting birds
may also be affected. Woodpecker cavities are thought to be preferable as they are
usually quite spacious (particularly those found in the trunks of trees, which are
preferred by Ring-necked Parakeets), allowing for a greater clutch size, but have a
relatively small entrance which could reduce predation or provide better protection from
the elements (Strubbe and Matthysen 2009). Although there has been little evidence of
any adverse effects on native wildlife so far, the rise of an introduced population of
Ring-necked Parakeets has been shown to correlate with a decrease in Nuthatches in
Belgium (Strubbe and Matthysen 2007). When nest-holes were blocked experimentally,
the population of Nuthatches declined significantly as a direct result of competition with
Ring-necked Parakeets. Their success in outcompeting the Nuthatch seems to be
largely due to their earlier breeding time: Ring-necked Parakeets start laying eggs
towards the end of February, whilst Nuthatches begin breeding during the second half
of April. This means that the most suitable cavities are already occupied by the time
Nuthatches begin to breed, forcing them to use less-suitable nesting sites such as
natural cavities, which may reduce their reproductive success (Strubbe and Matthysen
2009).
Although most introduced populations of Ring-necked Parakeets are fairly
small at present and hence have little obvious effect on the native wildlife, the study by
Strubbe and Matthysen shows that they can and will outcompete native species. This is
likely to become a problem once the parakeet populations have expanded to the point
where nesting sites become a limiting factor, bringing them into direct competition with
species such as the Nuthatch and Starling (which is already declining in Europe). It
seems likely that the Ring-necked Parakeet may be the most successful of these three
species as a result of its considerable plasticity combined with early breeding, and if so
the extent to which Nuthatches and Starlings are affected will ultimately be determined
by their phenotypic plasticity.
Nuthatches are quite specialised in their requirements and thus may be
seriously affected by invasive Ring-necked Parakeets (Strubbe and Matthysen 2009).
Conversely Starlings often utilise the rooves of houses as nesting sites in Europe, and
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therefore may be able to adapt sucessfully, allowing co-existence with Ring-necked
Parakeets. Biological interactions are complex, however, and in reality species and
communities often react to changes in unforeseen ways. A study of Starlings in Canada
showed that they were unable to adapt when nesting cavities were blocked – rather
than utilising other cavities they simply stopped breeding, resulting in a population crash
(Aitken and Martin 2008). Contrasting these birds with those in Europe, it is evident that
the differences between birds of the same species can be quite marked, and thus we
may not always be able to predict how Ring-necked Parakeets will affect native wildlife.
Since we cannot reliably predict how or if species will adapt, we cannot make highly
accurate predictions about the effects of interactions between native and non-native
species. Whilst it is possible that both Starlings and Nuthatches could adapt
successfully, it is equally possible that Ring-necked Parakeets may be able to adapt to
utilising other cavities when availability of tree-holes begins to limit population growth
(DEFRA 2007), resulting in significant negative effects on the populations of other
cavity-nesting species.
In their native habitats, Ring-necked Parakeets are regarded as one of the
most important crop pests, damaging both cereals and fruit such as maize Zea mays,
rice Oryza sativa and Guava Psidium guajava in India. They have been variably
estimated to reduce yields of maize from between 10 – 80% in India. Due to their high
dietary plasticity, Ring-necked Parakeets have had variable impacts in other parts of the
world where they have been introduced. In England they are increasingly becoming
pests of fruit crops such as apples and have caused considerable damage in some
instances, although they are not yet considered a serious pest. In Australia they strip
bark from young trees as a food source in autumn, causing serious damage to
plantations (DEFRA 2007).
The major problems with deciding what measures, if any, are to be taken in
order to regulate populations of alien species concern the extent of the damage caused
by the species, and when to implement management strategies. Damage may be
economical (for example the destruction of crops), environmental (disrupting
communities or causing the extinction of native species) or social (causing annoyance
to people). Environmental and economic reasons usually carry the most weight as the
damage has more significantly detrimental effects. However, the extent of the damage
caused by alien species is not easy to detect when the population size is still small,
mainly due to our lack of ability to detect fine changes in the environment. Nevertheless,
it is important to take action at the earliest sign of potentially significant damage, as by
their very nature invasive species are very difficult to control once they have established
a population and spread.
In order to take action at the right time, potential invasive species must be
monitored carefully. It is not economically viable to monitor all alien species in this way
(only 10% of successful alien colonisers become invasive), but invasive species tend to
share several traits which make them easier to identify. They are usually highly plastic,
adapting well to changing situations through their behaviour or through generalist
strategies of feeding and habitat selection, but they may have additional traits, such as
James Grundy, St Peter’s College
high aggression or fecundity, which give them an advantage over other generalists.
They often demonstrate unusually rapid population growth, which may be a result of
successful generalisation in selection of habitat or exploitation of a vacant niche, and
this is may also be coupled with a moderately high rate of dispersal, allowing them to
colonise new areas quickly. Species which adapt well to change genetically may be
predisposed to invasiveness as it allows them to compete successfully with other
species evolutionarily. This may be difficult to detect, but is often high in species with a
high rate of reproduction as this results in a higher mutation rate. Invasive species may
be pre-adapted to the habitat in which they become invasive, so knowledge of the
native habitat may be useful – most invasive plant species in Mediterranean-type
habitats come from other Mediterranean-type habitats with different communities.
Although it is not always possible to predict whether a species will become invasive or
not, a detailed knowledge of its ecology can give insight which may help in the
identification of invasive species. Ring-necked Parakeets display some of the traits
associated with invasiveness, with a high rate of population increase and great plasticity
in their choice of food.
At present, introduced Ring-necked Parakeets have caused some damage to
both native bird species and crops, though in general the effects have apparently been
quite insignificant. However, there is clearly potential for considerable damage if
populations continue to expand in both size and range. Since the current rates of
population growth appear to be exponential, this possibility seems likely to become a
reality – management will become necessary in the near future. Since the species is not
causing obvious economical or environmental problems at present population sizes,
perhaps the best management plan is to take a preventative approach. A complete
eradication would be costly, especially in light of the proven ability of Ring-necked
Parakeets to establish successful populations from small numbers of birds, requiring
considerable and meticulous effort which may not be proportional to the damage they
would cause if left alone. However, it has been estimated that at present population
levels an annual removal of at least 30% of the population would be enough to prevent
them from spreading further. Since Ring-necked Parakeets have a very low dispersal
rate and populations are currently quite localised, this could be achievable at a fairly low
cost (DEFRA 2007).
In conclusion, whilst it is not always possible to predict the negative effects of
the introduction of alien species, it may be possible to identify those with the potential to
become invasive. It is important to monitor any likely species carefully, and take action
at the first sign of damage – this is the point at which they have filled the niche gap
which enabled them to colonise successfully, and begin to compete with other species
(plants, animals or humans). Due to their unpredictable nature, it may be better to take
action which is unduly economically costly rather than wait and observe the effects of
invasive species – by this time it may be too late to prevent a disaster.
James Grundy, St Peter’s College
References
Aitken K. and Martin K. (2008) Resource selection plasticity and community responses
to experimental reduction of a critical resource, Ecology, 89(4), 971-980
DAISIE European Invasive Alien Species Gateway (accessed 27/11/2009),
http://www.europe-aliens.org/
DEFRA (compiled by Fletcher M. and Askew N.) (2007) Review of the status, ecology
and likely future spread of parakeets in England
Strubbe D. and Matthysen E. (2007) Invasive ring-necked parakeets Psittacula krameri
in Belgium: habitat selection and impact on native birds, Ecography, 30, 578-588
Strubbe D. and Matthysen E. (2009) Experimental evidence for nest-site competition
between invasive ring-necked parakeets (Psittacula krameri) and native nuthatches
(Sitta europaea), Biological Conservation, 142(8), 1588-1594
Strubbe D. and Matthysen E. (2009) Predicting the potential distribution of invasive ringnecked parakeets Psittacula krameri in northern Belgium using an ecological niche
modelling approach, Biological Invasions, 11(3), 497-513