Besides plant biology and responses among pollinator interactions, other patch characteristics influence invertebrate diversity and insect composition. The main factor which influenced richness of more mobile invertebrates in this study was altitude. Differences in invertebrate diversity and insect family composition in response to altitude is a complex interaction. Environmental conditions may vary substantially at increasing elevations, including variation in temperature, precipitation, partial pressure of atmospheric gases, turbulence and wind speed, as well as ultra violet radiation wavelengths (Barry, 1992). These environmental variables can affect biological and morphological traits within insect groups and plant species, consequently affecting plant community composition and concomitantly invertebrate assemblages (Hodkinson, 2005). Previous research has found that species richness of Lepidoptera and Hymenoptera - with the exception of bumblebees (Galen 1989); probably due to greater thermoregulatory capabilities (Corbet et al. 1993) - tend to be lower with an increase in altitude (Warren et al. 1988), whereas flies tend to become more prevalent and important for pollination of montane plants at higher altitudes (Warren et al. 1988; Kearns, 1992; Totland, 1993). Large flies were well represented at sites within Tongariro at the highest altitudes and in the absence of honeybees. With this in mind, large flies may be less affected by displacement in being able to readily utilise higher altitude patches in conditions less favourable for other pollinators. For soil invertebrates caught in the pitfall traps, the manuka cover in patches was most influential. It has been well recognised that specific plant species can significantly affect the type of insects that occur within patches, and that the abundance of favourable plant species will influence the abundance of insect visitors (Price, 1997). As manuka attracts a diverse range of pollinators, albeit many generalists, it is not surprising that an increase in the proportion of manuka cover would attract a higher abundance of insect visitors, which could have an effect on invertebrate diversity.
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The proportion of manuka cover and plant composition within patches also had a large influence on insect family composition for insects caught in both the intercept and pitfall traps. Plant composition was a significant influence on insect family composition for both intercept and pitfall trap insects, but plant diversity (richness and evenness) in some cases was not, suggesting that plant species identity and plant community composition are more important drivers of insect assemblage than a rich diversity of available floral and/or plant resources. A previous study by Perner et al. (2005) on the affect of various habitat characteristics on arthropod abundance in montane grasslands in central Germany showed similar findings. The authors found that plant community composition significantly affected the abundance of specific invertebrate functional groups, whereas plant diversity alone did not have a strong influence. Altitude had strong effects while patch size, although significant, displayed a weaker influence on insect family composition. The range of patch size within my study was not very large and there were few large patches (Range = 0.1ha – 7.1 ha; median = 0.3), so the fact that patch size appears to be having some influence, even among patch sizes with little variation, is worthy of further investigation.
The majority of insects collected in the intercept traps were not from the families of insects that were visiting the manuka flowers in the behavioural observations. Some of them were parasitoids and insect predators that may have been drawn to patches by an increase in pollinator activity during the peak flowering period. As the main pollinators were not well represented in the intercept traps, we cannot assume that the patch characteristics which had an effect on insect family composition were a good indication of the influence driving pollinator composition specifically. Species experience landscapes differently, and flying insects may not be as affected as strongly as litter invertebrates by changes in habitat. Pollinators that are more readily adaptable to generalised foraging are also at less of a disadvantage. The presence of honeybees and altitude were the most important factors influencing pollinator guilds; also sites with a higher visitation of honeybees tended to have higher overall patch invertebrate diversity. This highlights the complexity of interactions that occur within ecosystems, especially pollination systems, and that different taxa are affected by patch characteristics in different ways. Very different results may also occur
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depending on the functional groups of focus. There were obvious factors influencing the composition of insect family communities, but many more may become evident if insects were identified to species level. Unfortunately, the data collected at the morphospecies level were too sparse to be able to apply a reliable ordination output. Even so, information about insect composition at the family level provides useful information as insects within family units generally display similar ecological characteristics.
5.4 Further applications
Experimental replications of this research repeated over a number of years would help validate the current findings and more direct measures of competition, such as consequent survival, fecundity, and population density, would provide valuable information on the longer term effects of resource competition. Nevertheless, indirect measurements of competition, such as resource overlap, visitation rates, and resource harvesting patterns are helpful in assessing trends warranting further investigation.
Experimental manipulation of mobile insect pollinators is no easy feat. In order to accomplish this, beekeepers are needed who are willing to supply and transport a large number of hives. Honeybees in search of resources have been reported to travel an average of 1.1 km in suburban landscapes (Waddington et al., 1994), and 2.26 km in temperate forest mosaics (Visscher & Seeley, 1982), and in some instances, may travel up to 10 km to forage (Steffan-Dewenter & Kuhn, 2003). Therefore, sourcing sufficient areas of manuka with few honeybees within at least a 10 km radius is also difficult considering the popularity of honey produced from manuka. One possibility is to utilise isolated and restricted patches of manuka on Department of Conservation land, in which honeybee hives are prohibited, although vigilance must be adhered to when choosing patches as hives may still be placed just outside of the boundaries, allowing honeybees to access plant patches within restricted areas. However, restricted areas such as these could be used as control patches in which to further validate the current findings.
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5.5 Conclusion
It appears that honeybees have an effect on the abundance of large flies among manuka patches, but the long-term consequences are not known. Pollen limitation was not occurring among manuka patches regardless of the presence of honeybees, therefore sufficient capsule and seed set was produced at all sites. Environmental effects of altitude appear to play a contributing factor in the taxa richness of flying and more mobile invertebrates, and the proportion of manuka within patches on taxa richness of litter invertebrates. These patch characteristics, including patch size and plant composition also have an influence on insect family composition. The fact that patches with a higher proportion of manuka have a higher diversity of invertebrates is a surprising result given that this would appear to produce a more uniform habitat than more mixed patches. This may be worthy of further investigation, especially in terms of the consideration for biodiversity conservation. Many ecosystem variables and interactions must be taken into account when examining the effect of introduced species into novel habitats. However, without direct experimental manipulation it is difficult to apply generalised causation on the hierarchical contribution of different habitat dynamics on native insect and plant species. It is fair to say that many complex ecological interactions have an influence on taxa abundance, diversity and composition; and such impacts may vary dramatically, depending on the taxonomic level or habitat type of focus.
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