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Chapter 1 LITERATURE REVIEW 1.1 Nectarivore Communities

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Chapter 1 LITERATURE REVIEW 1.1 Nectarivore Communities Chapter 1 LITERATURE REVIEW 1.1 Nectarivore communities There is considerable debate regarding the role of competition in producing patterns of niche partitioning in ecological communities (e.g. Connell 1983; Roughgarden 1983; Simberloff 1983; Stong Jr. 1983; Mac Nally 1995; Walter and Paterson 1995; Wisheu 1998; Gordon 2000; Mac Nally 2000). Competition is thought to refine the niche of a species when two or more species compete for some resource that is limiting. Natural selection favours conspecifics that compete less with individuals of other species, tending to widen any niche separation between the competing species. There are a number of lines of argument against niches being competitively structured: individualistic, logical structure and lack of field evidence (Gordon 2000). The individualistic argument suggests that species have individual tolerances, preferences and limitations in relation to environmental gradients that govern their distribution and abundance, not pressure from sympatric species (e.g. Stong Jr. 1983). The logical structure argument attacked the idea of competitively structured communities, suggesting that studies had failed to adequately discount a ‘null’ hypothesis that communities are actually assembled at random from species in a regional pool (e.g. Simberloff 1983). Finally, reviews of field evidence suggested that other factors are more important in limiting the growth of natural populations, such as predation, parasites and environmental heterogeneity (e.g. Connell 1983). Despite the arguments of the previous paragraph, studies of nectarivore communities usually invoke competition as the driving force behind community organisation (e.g. Keast 1968a; Ford and Paton 1976b; Lyon 1976; Ford 1977; Ford and Paton 1977; Ford 1979; Feinsinger et al. 1985; McFarland 1986a). In the case of nectarivore communities, competition does appear justified as the primary factor in structuring communities. This is partly due to obvious interference competition, particularly by the larger species within the community, and partly due to the fact that nectar is often a resource in demand by all the component species, and has been shown to be inadequate to meet the requirements of the local nectarivore community at least temporarily (Ford 1977; 1979; Feinsinger et al. 1985; McFarland 1986a; Armstrong 1991). Particularly in Australian systems there are likely to be a number of different nectarivore species 1 attempting to access nectar from the same plant at any one time. Two factors contribute to this shared preference for nectar: the flowering seasons of Australian plants provide a series of different nectar sources at different times of the year (Ford 1977; 1979; Paton 1979; 1986a); and there is no specificity of nectarivore species to plant species (Paton and Ford 1977; Collins and Briffa 1982). Any fluctuations in nectar availability are therefore likely to cause nectarivores to compete for resources during times of shortage. Alternative sources of renewable carbohydrate (Section 1.5.3.2) are likely to be used by nectarivore communities in similar ways to nectar resources (Paton 1980). A further piece of evidence suggesting the importance of competition in honeyeaters are the consistently male skewed sex ratios recorded by studies on honeyeaters (Paton 1979; Pyke et al. 1989; Foster 2001). Male honeyeaters are usually larger than female honeyeaters (Collins and Paton 1989; Paton and Collins 1989), and have been implicated in aggressively excluding females from the best resources, thus forcing them to both move further and forage from inferior resources leading to increased mortality (Paton 1979). Studies of nectarivore communities suggest a number of ways through which competition drives community organisation: size (Ford and Paton 1977; Ford 1979; Paton 1979; Wykes 1985; McFarland 1986a; Collins and McNee 1991), beak length (Ford 1977; Ford and Paton 1977; Paton 1986a; Paton and Collins 1989), habitat (Ford and Paton 1976b; Ford 1977; Recher 1977; Ford 1979; Loyn 1985; Wykes 1985) and behaviour, which includes a mix of social and feeding strategies. The best documented of behavioural strategies is the dominance of an area by Manorina (miners) through group territorial defence (Dow 1977; Loyn et al. 1983; Poiani et al. 1990; Pearce et al. 1995; Grey et al. 1997; Clarke and Schedvin 1999) but also includes concepts that have received only passing comments in the literature (e.g. 'prostitution', Wolf 1975; Paton 1979). Two behaviours, termed here swamping and stealth, have been reported by a number of authors. Stealth behaviour is the use of secretive behavioural techniques to access resources that are being protected (Lyon 1976; Paton 1979; McFarland 1996). Flocking behaviour, or swamping, is the use of a combined direct approach by a number of individuals to access resources that are being protected (Paton 1979; 1980; McFarland 1986a; Slater 1994; Timewell 1997). 2 Ford (1979) suggested size as the most important niche axis dividing honeyeater communities; the largest species aggressively exclude the smaller species from the best resources. Size has been criticised as a niche axis on the basis that it really includes a variety of actual niche axes such as resource harvesting efficiencies, metabolic rates and home range size (Gordon 2000). However, in the case of nectarivores it appears justified, as it plays a direct role in giving the largest species access to the best resources. Large honeyeaters require access to the best available nectar resources to meet their energy requirements and due to their size are able to aggressively dominate those resources (interference competition). The smaller honeyeaters, will gladly also use the best nectar supplies if they can access them (shared preference for resources) but are often compelled to use inferior nectar resources due to the aggression of larger honeyeaters. Smaller honeyeaters are able to use inferior nectar supplies as their overall energy requirements are less (Ford 1979). Collins and McNee (1981) supported this hypothesis, finding the largest honeyeaters relied on the most productive plants, while the smaller honeyeaters were forced to use less rewarding plants, as did Paton (1979). McFarland (1986a) provided further support, finding that a spatial gradient in nectar richness enabled several species to coexist. The largest species dominated the richest areas and the smaller species exploited the poorer areas. However, temporal variation was also found to be important, with most species, ‘unhindered in terms of where, and on what resources, they can forage’ during times of either very low or very high nectar resources (McFarland 1986a). Nectar resources are also divided amongst honeyeaters based on beak length. The long- beaked species take nectar from the flowers of all plant species, while the short-beaked honeyeaters may have trouble reaching the nectar from tubular or gullet-shaped flowers (e.g. Astroloma or Epacris) and are therefore primarily limited to open flowers (e.g. Eucalyptus) (Ford and Paton 1977; Paton 1986a). Dow (1977) documented indiscriminate interspecific aggression of dense colonies of a species of Manorina (miners) leading to its domination of an area. Studies have since shown that the removal of Manorina colonies results in an influx of other honeyeaters and insectivorous birds (Loyn et al. 1983; Pearce et al. 1995; Grey et al. 1997; Clarke and Schedvin 1999). A long term study (7 years) at one site in south-eastern Australia showed a decrease in honeyeater populations corresponding to an increase in a 3 Manorina melanophrys (Bell Miner) population (Poiani et al. 1990). Similar studies have not been published with other suspected aggressive species, such as Anthochaera spp. (Wattlebirds), Philemon spp. (Friarbirds) (Higgins et al. 2001) or New Holland Honeyeaters (Phylidonyris novaehollandiae). However, one study did demonstrate an increase in small honeyeater species after a decrease in abundance of the dominant Anthochaera chrysoptera (Little Wattlebird) from a site following removal of nectar sources (Pyke 1989). There is also some evidence that removal of P. novaehollandiae from an area that they previously dominated (in which flowers were grown commercially) resulted in an influx of other smaller honeyeaters (D. Paton, unpublished data.). Besides the arguments against community organisation being competitively structured, outlined in the first paragraph, there is also an argument that on the Australian mainland, the scale at which many birds move and the heterogeneity of the landscape preclude the conditions necessary for competitive interactions to develop patterns of community organisation (Mac Nally 1995). Mac Nally (1995, pg. 378) suggests that, ‘Local diversity at any time appears to be determined by a complex relation between the available regional pool of species potentially able to occupy a location, idiosyncratic habitat requirements and large-scale dynamics of individual species, resource irruptions and habitat architectures.’ This complex relation supposedly leads to a situation in which, ‘it is not surprising that similar species should frequently co-occur. To the contrary, it would be surprising were ecological differentiation to emerge from small- scale competitive interactions (1995, pg. 378).’ Mac Nally (2000) provides data outlining a situation in which three similar species (insectivorous birds) co-occur in relatively high densities without substantial differences in foraging, providing one example of how
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