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There is a prevalent apprehension all over the world about the impact of introduced species on the world's biodiversity due to the capability of these introduced species affecting native species (Blanvillain et al., 2003). Native species are affected by the interactions they are having with these species, interactions such as competition, predation, herbivory, habitat alteration, diseases and hybridization (Witmer and Fuller, 2011). The magnitude of the impacts of the introduced species can be variable; some are devastating while others are comparatively benevolent.
The impacts of these introduced species can pilot changes in population dynamics and community structure of the native species (Griffin and Boyce, 2009), also the ecosystem services can be altered as a result. For effective management of biodiversity, it is essential to understand the impact of these introduced species. Since resources can be a limiting factor for management, prioritization should be given to the introduced species that pose greatest undesirable impact (Grarock et al., 2012). Traditionally it was believed that all introduced species have a negative impact on native species (Grarock et al., 2012) hence this can lead to wasteful allocation of resources. Therefore understanding a species impact facilitates targeted management to ameliorate the impact.
From previous studies, by Grarock et al.,(2012) it is evident that it can be very difficult to obtain the impact of the introduced species for three key reasons being; (1) insufficient data prior to and then after the invasion. (2) Changes in the environment occurring alongside species introductions, making it difficult to differentiate species impacts from the impacts of environmental change, examples being climate change and habitat clearing. (3) Poor indulgence of the mechanisms of the impact such as competition versus predation.
Peacock et al., (2007) found out that alteration of biological communities in terms of species richness, species evenness, guild structuring, biomass and community composition are a result of continuous expansion of human population which causes massive land use changes. Coupling this with the increasing international trade and travel massive proliferation of introduced organisms has resulted. A major obstacle for the continued survival of a large proportion of the world's threatened species particularly at local scale is the adverse environmental effects of introduced species (Grarock et al., 2012). Introductions of species outside their natural range can either be accidental or deliberate. Examples of such introductions are for agriculture, hunting, biological control; habitat restoration and pet trade being deliberate mechanisms while ship aided haulage is said to be accidental (Carr, 1990). As a result of human induced habitat modification, the geographical ranges of many species are naturally increasing. It is not the case that deterioration or fragmentation of habitats is appropriate for introduced species establishment but this works interactively with the innate effects that the introduced species themselves pose to the enduring native communities (Philip, 2007).
In South Africa the association between highly successful introduced birds, human distribution and modified habitats has been postulated from the Indian myna or Common myna (Acridotheres tristis) (Peacock et al., 2007b) therefore human distribution and the rate at which habitats are modified influences the success and spread of the introduced species. Indian myna is one of the four species out of 48 that have established a viable population in South Africa (Carr, 1990). The Indian myna belongs to the Sturnidae (Starlings) which is a very adaptable and successful passerine family (Lovette et al., 2008). Its distribution is so large to an extent of 1 000 000 to 10 000 000 Km2 globally (Witmer and Fuller, 2011) furthermore it is a species of least concern in IUCN (Peacock et al., 2007b).
The Indian myna is native to the Indian subcontinent and the adjacent areas, throughout the 20th century it has expanded its range up to Malay Peninsula, Thailand, China and Vietnam possibly by introductions (Philip, 2007).According to Parsons et al.,(2006) in Melbourne, Australia, mynas were deliberately introduced around 1862 for biological control of insect pests in agricultural fields and market gardens. The birds quickly established and they became the source of other introductions in other parts of Australia (Parsons et al., 2006). Around 1900s mynas were released in Durban, South Africa (Peacock et al., 2007a) also for biological control of pests. Now they have expanded and occur from Kwazulu-Natal up to Northern Free State and Southern Transvaal (Carr, 1990). These birds were traced to have originated from populations of the subspecies Acridotheres tristis tristoides, native to Assam and Burma (Peacock et al., 2007a). Another introduction took place in Johannesburg around 1930s and these were from a population of nominate Indian subspecies Acridotheres tristis tristis (Peacock et al., 2007a). In addition to deliberate introductions, mynas in South Africa are currently undergoing rapid and extensive expansion, as a result of extensive natural vegetation clearing for agriculture and increasing human population densities (Carr, 1990). In Botswana single birds were observed in Mahalapye in March 1975 and in 1991 March in Gaborone (Brewster and Major, 2000) and also the birds were observed in Bobirwa, north eastern part of Botswana in 1996. Concerning these birds found in Botswana, it is not evident whether they were from populations in South Africa or they were from a separate introduction.
In the native range, the Indian myna prefers open farmlands and cities as well. The Indian myna is closely associated with humans since it is often found on the outskirts of towns and outlying homesteads (Griffin, 2009). Seasonal variation in the occurrence of the birds has not yet been documented. In South Africa, a high rate of occurrence of the birds in an area that normally does not have high populations of these birds probably coincides with the breeding season and also the use of artificial structures for nesting (Peacock et al., 2007a). Also in urban areas of South Africa, breeding can be all year around but nesting primarily occurs in October-March (Carr, 1990). Indian myna is often accused of displacing native bird species and small mammals from gardens and nesting holes. However this competition may be intense for example in Durban, South Africa it was reported that Mynas demolished weaver nests(Peacock et al., 2007b).
Concern has always been raised around the world that the mynas affects native birds in three ways thus completion for food, cavity-nesting sites and competition for territory (Grarock et al., 2012). Despite these allegations, it is often difficult to obtain empirical evidence of the impact of the Indian myna on the native bird species. But monitoring the continuing range increase of this bird species and determining the relative importance of the influential factors determining its spread and density provides valuable opportunity to ecologists to manage current and predict future effects of the bird species on native species. In Botswana, a landlocked country bordered by South Africa, Namibia, Zimbabwe and Zambia, ever since the birds were firstly observed around 1975, there are no studies that can be singled out to have tried to determine the possibility of the introduction of the Indian myna in the country or investigate the competition significance of the Indian myna to the native bird species or investigate the subsequent urbanization role in the interactions between the Indian myna and native birds. Botswana has about 12 Important Bird Areas, in which some of them coincide with the areas where the Indian myna have been observed. Due to this scenario, how does this bird species impact native bird species in Botswana owing to the fact that in all parts of the world where the myna have successfully established, there is concern that the myna is impeding the survival of native bird species through competition?
Therefore this research in Botswana would like to focus on;
Describing the relationship between the native bird grouping composition and the Indian myna,
Evaluate aggression initiated by the Indian myna to that of the native bird species and
Investigate the frequency at which the myna encounters native birds and the native birds are victims of its aggression.