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Most of the southwest Pacific birds are derived from New Guinea . A lot of these birds have to expand their niches and colonise other islands to reduce the competition in species-poor islands . By inhabiting altitudinal bands, different types of habitats, and/or areas in the forest even if they are suboptimal, they can expand their niche spatially to be excluded from other species on species-rich islands, areas such as the vertical strata.By moving to areas excluded of other species, they reduce selection pressure, avoid poor gene pools and low endemism . Frequent expansions by these birds, are from lower to higher altitudes, or from secondary growth, to mature forest . It is common for the birds to colonise areas of the newly inhabited island that are similar to their original niche. As a result, there is no need for a change in diet if the niche they newly occupy has the same trees, thus a change in diet is rare . Niche shift is not usually endured in approximately half of the colonising populations . Unless birds have been isolated long enough to undergo morphological divergence, their foraging technique will stay the same .
Animal and plant species may be supported by islands with similar climate and similarly structured habitats, depending on the size of the island and the distance from the colonisation source area . Problems such as competition effects on niche breadth, fitness, and abundance can be explained by these ecological comparisons of faunas of such islands . Pacific Islands Land and fresh-water birds on the southwest provide a favourable way to study niche shifts . Thousands of islands are in the Polynesian, Melanesian, and Papuan zoogeographical subregions, varying largely in size . The species of birds range from 1-512, the flat atoll elevates up to 17,000 ft. and can be isolated from the source area from up to 5000 miles . Only a small number of species are derived from New Zealand and Australia, with most of them derived from the species-rich Island New Guinea. New Guinea acts like an “island archipelago” in terms of montane bird distribution, since it consists of mountain ranges of different sizes isolated from each other by a “sea” of unoccupiable lowlands. To understand why and how the niche varies in terms of competition between different birds species, studying the same bird species on different islands or mountains is a good way to start .
Altitudinal shifts are one of the most common mechanisms for ecological sorting of congeneric birds species in New Guinea . There are many ecologically similar species which share altitudinal bands under interspecific competitions . Some species find it hard to establish themselves but the ones that can expand their altitudinal range, upwards or downwards of mountainous Pacific islands, smaller than New Guinea.
Turdus poliocephalus,for example, is confined to altitudes > 9000 ft on New Guinea, yet to sea-level on many small islands such as Viti Levu, and descending to 4000 ft on Karkar. Furthermore, in the absence of Z. fuscicapilla, white-eye Zosterops atrifrons, their altitudinal range on New Britain expands upwards, and also downwards to sea-level when in low numbers .
When competition is low, the broadening of habitat is a lot easier and thus, is observed frequently in southwest Pacific birds. Two species C. indica and C. stephani are mutually exclusive in habitat selection on New Guinea and Bagabag . Where C. indica is absent on KarKar and New Britain, the C. stephani occupies in the centre of the forest, and coastal forest. Vice versa, on Espiritu Santo, where C.indica occurs in the interior forest, and coast whilst C. stephani is absent . On New Guinea, one or more species of true grass-warblers are confined to dry grassland, whereas in Gudalacanal where the warblers have not reached, is the dense population of sunbird Nectarinia jugularis found inside the tall dry grass, or otherwise confined to the open country in bushes, forests, and trees . Furthermore, , two unrelated small birds with elongated curved bills on New Britain and New Guinea come together when drinking nectar from flowers; the sunbirds are strictly confined to the open country lowlands and to the forest edges. The honey-eaters of the genus Myzomela confined to the mountains and lowlands . Due to the gathering of these species at feeding sights, the honey-eaters and sunbirds fight . The Honey-eater is largely confined to elevations above 4000 ft and the sunbird species In the lowlands, forest interior and up mountain slopes to the elevation at which the honey-eater inhabits. Contrastingly the sunbirds are absent when the honey-eaters inhabit open country of lowlands and the forest on Espiritu Santo and Upolu, and Viti Levu .
Within New Guinea, many of forest bird species are vertically restricted . Some forage 10-30 ft above the ground, others in the treetops, and others within 5 ft. of the ground. For example, in New Guinea, the Flycatcher Myiagra alecto is strongly confined to the understorey but in Bagabag and Karkar, is found ranging from the understorey to the treetops . In New Guineas’ mountain ranges, whistler Pachycephala hyperythra shares much of its altitudinal range with Pachycephala soror. P. soror is very abundant in the understorey so is very easy to catch but the P. hyperythra is 10 ft above the ground, therefore, is never caught. On Mt.Turu, S.virgatus occurs alone on the eastern end of the range and therefore is caught half the amount it usually would because it is foraging high enough to avoid the net. The white-eye Zosterops lateralis ranges from the understorey to the treetops within New Zealand and Australia . Contrastingly, on invading Espiritu Santo, it encountered an endemic white-eye Z.flavifrons which is not found in New Zealand and Australia. The two species now coexist in forest-edge habitats and therefore Z.flavifrons forages above 15 ft, and Z. lateralis confines within 15 ft of the ground . This represents the opposite to a usual trend, in that the invader Z. lateralis onto a small island with more family, rather than less, and hence had to contract its niche rather than expand it .
Vertical, altitudinal and habitat shifts endorse the behaviour of a species to occupy more space, potentially to attain a bigger population . Moreover, an abundance shift may also be observed e.g. regardless of whether a species occupies more space on a small island, a species become more abundant on a small island than it does on a larger island in its prefered optimal habitat . The Paradise Kingfisher Tanysiptea galateax,for example, has double the amount of density on KarKar, than New Guinea, despite they occupy very similar habitats, and have the same vertical foraging range in the lowlands of both islands .
Another distinctive feature of niche variation is apparent on islands surrounding the mainland. Species-rich islands seem to have a larger abundance of species of insect, bird, reptile or mammal that a species-rich island or mainland . This is somewhat dependent on how much is competition, and how much is an intrinsic adaptation. Furthermore, the effects of factors such as longevity on fitness, population size and isolation . Of all the bird’s species, reduced competition can have an effect on total population density . When similar habitats were observed, Karkars’ population density was much lower than New Guinea, extremely in higher altitudes. During breeding seasons, in temperate zones, counting the singing territorial males in a given area is a good method for determining population densities . This is not easy to do in southwest Pacific islands as the breeding of species is spread over the whole course of the year. Some species have breeding peaks at certain seasons, but some are not big breeders, nor very territorial . To allow for this, birds were caught in nets within a surface area of 22.5m2 to compare the total population densities on the different islands, under similarly structured habitats . Where birds species decline from 132 to 31 in Bagabag, this local number shows a yield (birds caught per day) reduction in proportion to the species number. At higher altitudes in the montane forest of New Guinea, New Britain, and Karkar, the yield decreases more vigorously than the number of species . The montane forest of Karkar with 46% more species than New Guinea montane forest, has only an 11% yield . Likewise, when comparing New Britain and New Guineas’ subalpine forest, the yield declines even more rapidly than the number of species . The understorey is up to nine times lower in netting yields on a species-poor island than on a species-rich island of similar habitat . It seems a bit surprising that the total population density declined faster than or just as fast as the number of species on the Pacific Islands, given the fact that some of the island species become more abundant on New Guinea in the preferred habitats whilst others expand their habitat. This means that the individuals per species either declines or remains linear. Two factors may underlie this effect, first being competition. The New Guinea species (512), is confined to a niche space that can favour them efficiently and in which can keep a constantly high population density. Species-poor islands have the expansion of some species into new niche spaces, from which they were prohibited from on New Guinea, to exploit space more efficiently due to the failure to colonise a small island. New Guinea occupants will likely outwhey the expansion of new species,meaning high population densities of newly colonised species will be hard to maintain. The second-growth species may be able to colonise but not be able to exploit that habitat area very efficiently. On small islands, the density of individuals per species is lower at high altitudes than in the lowlands. Species that colonise small satelite islands, as a result of impoverishment on the source island New Guinea, are found to be tropical species that have expanded to higher elevations due to reduced competition. Karkar and New Britain montane forest maintains most of the species, in contrast to half the tropical species in the subapline mossy forest of New Britain and only 3-8% of tropical species in New Guineas high-altitude habitats. The tropical species are most likely only able to maintain smaller population densities on Karkar and New Britain due to their lack of ability to adapt to cooler conditions than the New Guinea montane species that are being replaced.
 Diamond, J. M. (1970). Ecological Consequences of Island Colonization by Southwest Pacific Birds, I. Types of Niche Shifts. Proceedings of the National Academy of Sciences, 67(2), 529-536. doi:10.1073/pnas.67.2.529
 Diamond, J. M. (1970). Ecological Consequences of Island Colonization by Southwest Pacific Birds, II. The Effect of Species Diversity on Total Population Density. Proceedings of the National Academy of Sciences, 67(4), 1715-1721. doi:10.1073/pnas.67.4.1715
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