Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Occupancy–abundance relationship wikipedia , lookup
Biodiversity action plan wikipedia , lookup
Reconciliation ecology wikipedia , lookup
Invasive species wikipedia , lookup
Assisted colonization wikipedia , lookup
Theoretical ecology wikipedia , lookup
Habitat conservation wikipedia , lookup
Molecular ecology wikipedia , lookup
Invasive species in the United States wikipedia , lookup
Introduced species wikipedia , lookup
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 James Grundy, St Peter’s College 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 James Grundy, St Peter’s College 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