Changes In The Species Diversity Biology
As urbanisation spreads and the diversity and abundance of vegetation and habitats change so does the diversity and species richness of the Australian avifauna. The noisy miner, Manorina melanocephala, is potentially the major reason for the decline of small sized passerines in such urbanised environments, particularly along the eastern side of Australia. The noisy miner's physical, habitual and personality traits all point to this hypothesis.NOISY MINER INFORMATION
The noisy miner is a species of bird that has adapted and become well habituated with the continuously increasing presence of disrupted or urbanised environments. This is not just through feeding and habitat adaptations but also through behavioural traits and reactions (Lowry 2011). They are a native honeyeater species that live in large colonies of up to more than a hundred individuals (Dow 1977). It has been found that noisy miners in general are quite a bold species which means they are quite tolerant of outside stimuli and allows them to more easily adapt to the closeness to humans and our lifestyles. From the individual birds studied it was shown that noisy miners in the urban locales were even more explorative and aggressive than counterparts from more rural or isolated areas. This difference was most evident along roadways or noisy quadrants where urban noisy miners would exhibit the Lombard effect and produce a much louder call than regular to compensate for the external interference.
A large portion of bird communities in the suburban matrix of Australia is comprised of nectarivores and there is a strong correlation between nectar production and the abundance of nectarivores within a given area (Brown & Hopkins 1996). This modified matrix consists of fragmented pieces of natural habitat, gardens, parklands and man-made construction. Urban environments continue to expand and grow which in turn leads to a decrease in the available area left for natural habitats to thrive and exist. Gardens and parks in particular are places where humans have introduced a higher amount of flowering and nectar producing vegetation than would normally be found in the natural environment. French, Major & Hely (2004) conducted research to identify which avian species were utilising nectar producing trees and plants in this urban landscape. It was found that the noisy miner fed on nectar at a much lower rate than other honeyeaters despite the fact that the noisy miner constitutes a larger percentage of the local avian biomass. It was also discovered that nectarivores, specifically noisy miners, preferred to feed and interact on native varieties of flowering trees and hence were found in higher density within areas containing Banksia and grevillea plants. These findings from French, Major & Hely (2004) reveal one of the reasons why noisy miners can be found in such large numbers among native feeding sites.
In addition to the preference of native trees, the noisy miner also has other habitat preferences that explain their local distributions. These include soil type, availability of yellow gum trees, tree density and shrub density in declining importance (Oldland, Taylor & Clark 2009). Fertile, deep soil types that support eucalypt groves and year-round vegetation were the most influential factor because nutrition rich land mass allowed for higher amounts of nectar as well as more concentrated nectar to be produced by the plants. Nectar being more available and accessible is a large benefit to the noisy miner and due to its higher body size the noisy miner attempts to control such valuable areas. Edge geometry of habitat areas was also found to influence where noisy miners congregate. Taylor et al. (2008) found corner patches, corridors between remnants and protrusions along remnant edges significantly increased the likelihood of larger abundance of noisy miners (Kutt et al. 2012). Typically, the noisy miner will inhabit areas that are less than 20 Ha in area (Ford et al. 1995) and will frequently use zones that range from 100-200 m from the edge (Piper & Catterall 2003). Combining the knowledge of preferred food and preferred feeding and nesting sites it can be seen that noisy miners are expected to be predominantly encountered in areas close to humans and ranging slightly into the fringe areas between natural and man-made locations.
Noisy miners tend to express very aggressive and competitive behaviour within a local territory (Montague-Drake et al. 2011) and this interspecific competition can be attributed to the relatively large body size for the particular niche that the noisy miner fills (Piper & Catterall 2003).DECREASE IN SMALL PASSERINES
According to research investigating species diversity by Major & Parsons (2010) there is a difference between historical evidence of Sydney bird communities and more recent survey statistics. Museum specimens from museums in Sydney and Melbourne were compared to the modern composition of bird communities identified by field observations. The top ten most frequently recorded bird species circa 1900 included 6 insectivores, 3 honeyeaters and a granivore all under 26 grams in weight. This is an avian diversity heavily favoured towards small native birds feeding on insects and nectar and does not include the noisy miner as a major proponent. This contrasts greatly with the more recent composition from 1998-99 where only 3 insectivores and 1 honeyeater remained in the top ten and only one species was below 26 grams. This is a definitive change in species composition over a period of less than a century with several exotic species becoming more widespread and larger bodies becoming the mean. The noisy miner was the only honeyeater present in the top ten and again exhibited a larger body size than birds of a similar niche. While there is no authoritative reason for this drastic downturn it is obviously a general trend that there is a historic decrease in small passerines over the last 100 years. Figures 1 and 2 (Major & Parsons 2010) illustrate this fact, in particular showing the decrease in avifauna in the 10-25 gram bodyweight range and the less abundant populations of insectivores and honeyeaters.
Habitat loss through deforestation for agricultural use is one of the major factors leading to the decrease in biodiversity on worldwide and successively smaller environments (Oldland, Taylor & Clark 2009). Local extinction of varied native fauna is still generally ascribed to this habitat loss however ideal locations for small native passerines are still readily accessible in the urban landscape (Parsons, Major & French 2006). In the already heavily urbanised realm of eastern Australia the habitat degradation has already had a significant impact on species richness and diversity so there must be other major effects that continue to contribute to the shift in avifauna species. The influx of the noisy miner is credited as being at the apex of the list of possible conditions that caused this transformation (Kutt et al. 2012).NOISY MINERS VERSUS PASSERINES
Noisy miners can be described with the ecological term 'reverse keystone species' (Montague-Drake et al. 2011; Grey, Clarke & Loyn 1998) that defines a species taking over a habitat after changes in the environment and leads to a population or abundance reduction of other contemporary species. The hyper-aggressive nature of urban noisy miners leads to great success in excluding small birds from normally occupied habitats (Hastings & Beattie 2006). Parsons et al. (2006) were able to extensively investigate the interactions of noisy miners and other birds and found that there were severe negative associations between the noisy miner and all other studied birds with a body mass under 59 grams. Of particular note were the superb fairy-wren and silvereye, which were both significantly affected. The only species that coexisted with the noisy miner without detriment were all over 100 grams and had a different and varied diet. These exceptions were the Australian magpie, galah, pied currawong and the rainbow lorikeet. The aggressive behaviour of the noisy miner aided in the decline of other species by displacing them from an area entirely or restricting unwanted competitors to a distribution that included less productive opportunities, either food wise or habitat availability ( Montague-Drake et al. 2011). This fact alludes to earlier information where noisy miners prefer higher grade soil types and thus better provisioned vegetation and will act extremely territorial to protect this very valuable resource. In figure 3 it can be seen that the abundance and productivity of noisy miners within a sample area directly correlates with a decrease in both abundance and productivity of small passerines and the reverse is also true. Whereas this competition is absent with the larger adjacent species and therefore the impact of noisy miners on non-competitor abundance is negligible or non-existent altogether.
Directed group mobbing is the main method noisy miners use to deter competitors from an area and it was observed that noisy miners performed between two and three aggressive mobbings per hectare per hour (Piper & Catterall 2003). It has also been suggested that this behaviour is unique among avifauna and has been witnessed being targeted against other animals including reptiles and humans. This suggestion implies that noisy miners are not necessarily discriminatory in their target choice and will attempt to ward off any intruders in their chosen territory. It is just the fact that they are as much as two to six times larger than the smaller insectivores and other honeyeaters that there is such a deleterious effect. The larger birds are still attacked by groups of noisy miners but instead withstand or ignore the offensive behaviour (Piper & Catterall 2003).
The results from Maron & Kennedy (2007) provided that small passerine abundance and small passerine richness were 2.9 and three times greater respectively when noisy miners were not present. In addition survey sites that had less than 3 noisy miner observations contained 8.1 times more abundance and 7.2 times more species richness than sites that had >3 noisy miners. The evidence suggests that it is the density of Manorina melanocephala that is the primary problem along remnant edges (Maron et al. 2011). Piper & Catterall (2003) performed similar abundance and richness testing and the results were even more conclusive. This second investigation demonstrated that within a noisy miner colony the abundance of small passerines with similar diets to the noisy miner were between 20 and 25 times less than assemblages outside a noisy miner colony. Species richness was also found to be approximately 10 times less. The plentiful presence of the noisy miner can be attributed to the diminishing occurrence of 57 of the identified 71 small-bodied passerines (MacNally et al. 2012) and is possibly the first species to influence the avifauna on an assemblage level through the use of aggression and competitive tendencies. These are all statistically significant findings that support the hypothesis that noisy miners are a major contributing factor to the decline of many species in urban and peri-urban areas of Australia. In addition, the reduction in small bird species is further compounding the problems faced due to urbanisation. With less abundance and richness of avifauna the health of the surrounding trees and their canopies are detrimentally affected. This is known as dieback (Grey, Clarke & Loyn 1998) and can cause further chain reactions to species abundance due to the declining quality of food sources and protective foliage.HOW TO HELP THE PASSERINES
The first of several possible solutions to the decrease in species richness and abundance in small passerines is to alter and engineer the vegetation composition to be more favourable to the smaller birds while also hindering the aggressive tactics of the noisy miner. The first way to differentiate the vegetation diversity is to add plants that have a positive relationship with smaller insectivores and nectarivores. After examining multiple arrangements of plant life Hastings & Beattie (2006) concluded that Acacia forests that included a shrubby understorey and Acacia forests without understorey were the combinations that had the most positive effect on species composition in regards to noisy miners. Acacias with an understorey attracted no noisy miners at all and 82 percent of the small birds were noticed to be feeding. Acacias without an understorey had similar statistics except that noisy miners were present, although there was no evidence of chasing or mobbing activity in this environment. Maron & Kennedy (2007) also found that a cypress understorey and a lack of eucalyptus or spotted gum grove showed a marked decrease in the presence of noisy miners while also helping drive an increase in the smaller avifauna. A second resolution could be to stop the removal of the invasive Lantana weed species in certain areas. It has been seen that smaller passerines thrive in the Lantana dominated understorey and its removal is a decrease in available resources for feeding and nesting while at the same time its removal is also a danger because it opens up the understorey allowing noisy miners a more suitable habitat (Kath, Maron & Dunn 2009) to aggressively dominate.
Another potential solution that has some surprising yet very informative results so far is the removal or translocation of entire noisy miner colonies (Grey, Clarke & Loyn 1998). In the experimental removal of noisy miners by Grey, Clarke and Loyn (1998) at four sites in Australia it was found to have an encouraging effect. At three of the four sites the abundance of small insectivores and nectarivores increased in the twelve months after the experimental removal. In many of the sample areas noisy miners did not return at all, even after twelve months of observation. These results imply that the removal of the highly abundant noisy miner from selected ideal habitats could lead to the permanent increase in the previously dominated smaller species and hence increase the species diversity, abundance and richness. This is an advantageous option to consider, as side-effects of increasing the abundance of insectivores is the control of the prey insects and the possible reversal of the dieback process.CONCLUSION
The findings presented are almost unanimously definitive in that noisy miners certainly have a detrimental effect on species abundance, species richness and overall diversity within the avian ecosystem. The hyper-aggressive nature of the noisy miner in addition to their large colony sizes and comparatively large body size allows them to take over the most suitable insectivore and nectarivore habitats in the urban and peri-urban environment at the expense of alternate species. It is possible for drastic actions to be taken to curb this unique phenomenon, such as translocation of colonies and complete vegetation altering of an area if the problem continues to worsen.
Figure 2: Differences in bird community composition. Black bars are 1900 specimens and white bars are 1998-99.
Figure 1: Changes in body size between 1900 and 1998-99. Black bars are 1900 specimens and white bars are 1998-99.
Figure 3: Joint effects of Noisy Miner abundance and productivity on abundance for competitors (a) and non-competitors (b). Dashed line represents greater productivity. Solid line represents lesser productivity (Montague-Drake et al. 2011).
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