To successfully manage introduced species it is essential to understand how a species will interact in the new environment. Species invasions can often follow a multi‐step process (introduction, establishment, population growth and spread). Numerous hypotheses have been formulated on factors that influence each stage of the invasion process. I wanted to investigate how well theory matches with what occurs in the natural world. To do this, I developed a general conceptual model for the invasion process, based on invasion theory. Then using a composite 41‐year data set, I recreated the invasion sequence of the common myna to investigate the similarities between invasion theory and this observed invasion. The length and duration of most stages in the invasion sequence for the common myna correspond closely with what invasion process theory would anticipate. Therefore, this finding indicated that a conceptual model, coupled with basic species, environment and event information, could be a useful tool to enhance the understanding and management of invasions.
Is it benign or is it a pariah?
Community concern over the impact of the common myna is growing in Australia, despite a lack of strong scientific evidence for its impacts. To effectively manage a species its impact must first be understood. However, change in species abundance is often slow and gradual, coinciding with environmental change. I used a long‐term data set from the Canberra Ornithologists Group Garden Bird Survey to investigate the impact of the common myna on native bird abundance. These data allowed me to look at the abundance of various bird species pre and post common myna invasion. I also included measures of environmental change in the model, as the establishment of the common myna did not occur in isolation from other impacts. From this analysis I found a negative relationship between the establishment of the common myna and the long‐term abundance of three cavity‐nesting species and seven small bird species. These results highlight the extent to which the common myna can influence both cavity‐nesting and small bird species. The bird species in this study were neither rare nor threatened (three are introduced). However, these mechanisms of impact (competition for nest‐cavities and territory) also may influence threatened species. Therefore, the effect of the common myna on native bird species in the Canberra area is not benign. However, there are still questions regarding the seriousness of this impact and the type of management (if any) that is warranted.
Drivers and passengers of change
Habitat modification and invasive species are significant drivers of biodiversity decline. However, distinguishing between the impacts of these two drivers on native species can be difficult. For example, habitat modification may reduce native species abundance, while an invasive species may take advantage of the new environment. This scenario has been described as the driver‐passenger model, with ‘passengers’ taking advantage of habitat modification and ‘drivers’ causing native species decline. I wanted to investigate this driver‐passenger model using the common myna. Volunteers helped me survey bird abundance over 2.5 years across high medium and low tree density sites, in nature reserves and urban areas. I was then able to investigate change in species abundance in relation to differing tree density and common myna abundance. The results indicated that the abundance of many native bird species was greater in high tree density nature reserves, while the common myna was uncommon in these areas. Common myna abundance was almost three times higher in urban areas than nature reserves and declined rapidly as tree density in nature reserves increased. Therefore, the common myna is primarily a passenger of habitat change. However, I also observed negative associations between common myna abundance and some bird species. This indicates that in combination with habitat change, the common myna may also be a driver of some bird species decline. The results of this study suggest that the effects of habitat modification and invasive species are interrelated. Many species are strongly influenced by habitat. However, high‐quality habitat for one species may not constitute high‐quality habitat for another species. Given the influence of habitat on species abundance, I suggest that habitat restoration and tree planting may be useful tools to both control common myna abundance and aid native bird species recovery.
Influence of habitat on the impact of introduced species
Due to the strong influence of habitat on species abundance I wanted to investigate if habitat influenced cavity‐nesting competition, between the common myna and native species. To do this I built over 200 nest boxes and put them up in nature reserves that had high, medium and low tree density. I then checked these boxes over three nesting season using a bullet camera mounted on a pole. From these data I examined the relationship between tree density and the abundance and nesting success of the common myna, crimson rosella and eastern rosella in Canberra, Australia. I then investigated the impact of common myna nest box occupation on crimson rosella and eastern rosella abundance. I observed that tree density significantly influenced the abundance and cavity‐nesting of all three species. Common myna abundance was greatest at low tree density sites and declined with increasing tree density sites. The opposite pattern was observed for the crimson rosella, with greater abundance at high tree density sites, declining over medium and low tree density sites. The eastern rosella was more abundant at medium tree density sites. Despite the strong influence of tree density, I found a significant negative relationship between common myna nest box occupancy and the abundance of the crimson rosella and eastern rosella at low tree density sites. At these sites the common myna occupied a high number of nest boxes (up to 90%) and built ‘fake’ nests that further reduced cavity availability. I also observed a negative relationship between common myna nest box occupancy and crimson rosella abundance at high tree density sites. The relationship appeared to be more severe at high tree density sites. Therefore, management of the common myna may be more effective in areas that represent ‘high quality’ habitat for native species.
Understanding basic species population dynamics for effective control
Population manipulation of introduced species can be difficult and many widespread eradication or reduction attempts have failed. Understanding the population dynamics of a species is essential for undertaking a successful control program. Despite this, control attempts are frequently undertaken with limited knowledge of the species population dynamics. For example, in Australia, concern over the impact of the introduced common myna has led to community members culling the species. I wanted to assess the impact of a community‐led common myna culling program in Canberra, Australia. I worked with the Canberra Indian Myna Action Group and they provided me with details of the number and location of common mynas caught and euthanised in Canberra. I then used bird survey data, from volunteers and the Canberra Ornithologists group, to look at the change in myna abundance in relation to trapping. I observed a significant negative relationship between common myna abundance and culling at fine‐scales (per square kilometre). However, when I expanded this area across four regions of Canberra the relationship between common myna abundance and culling was not significant. To investigate this further I created a basic population model to enhance my understanding of common myna population dynamics and the potential influence of various culling regimes. This model indicated that culling at a rate of 25 birds per square kilometre per year would reduce common myna abundance, regardless of initial density. However, across the four regions in Canberra culling was less than 15 birds per square kilometre. This indicated that currently too few individuals are being removed from the Canberra population to effectively reduce population size. Natural reproduction, survival and/or immigration of the common myna is able to replace the culled individuals. This highlights the value of undertaking basic population modelling to assess if potential control measures are capable of achieving desired outcomes.
Due to the strong influence of habitat on species abundance, habitat restoration and tree planting may be useful tools to both control common myna abundance and to aid native bird species recovery. This will not only increase habitat quality for native species (including cavity availability in the longer term), but it is also likely to make the habitat less suitable for the common myna, reducing its abundance. Without restoring habitat and making these areas ‘less suitable’ for the common myna, attempts to control species numbers are only likely to succeed over the short term, with the species reinvading once control actions are eased.