Are they just aliens?
By Chris Chandler
Whilst crazed adrenaline searching adventurers hurtle down white water rapids and dive off 50 metre high bungee jumps, keen bird watchers take to hills for a mellower version of outdoor recreation, unaware of the potential dangers in tow. Any passionate bird watcher making a trip to Australia with the hope of viewing some interesting species may not have been aware of the potential life threatening risks. The invasive Common Myna is a concern for it potential health issues from droppings, stealing food off plates at restaurants, spreading bird mites, avian malaria, lice and thread worms. If that wasn’t enough, they are suspected to cause dermatitis, irritable rashes, asthma, spread Arboviruses and Salmonellosis leading to vomiting, fever, diarrhoea and, oh yeah, death in some cases, if not treated with antibiotics.
The movement and introduction of species from their native range to foreign environments has increased significantly over the past decades as a result of increased global human travel. This accidental or intentional introduction of non-native species is causing catastrophic impacts to global ecosystems, decreasing biodiversity and driving homogenisation of the world’s biota.
First of all, what is an invasive species? The general definition of an invasive species is a non-native species that has been introduced to an ecosystem and has then gone on to establish a self-sustaining population that is spreading and therefore causing environmental or economic harm. I disagree that an invasive species has to be a non-native species introduced by humans. Native species can, and have, become highly populated in ecosystems causing significant environmental problems. Considering the amount of human modified land and effects of climate change, environments are becoming altered so much so it is as if native species are in non-native environments. I think that the invasive species definition should include both the movement of a species to a non-native range and the alteration of an environment to influence the role of native species becoming invasive. Otherwise, what’s the point in terms such as, Invasive Non-Native Species (INNS) and Invasive Alien Species (IAS) if all non-natives are invasive?
Often there has been a focus towards studying invasive plants or small mammals such as rats in order to assess the environmental impacts as a result of the spread and establishment of self-sustaining populations. Invasive birds, on the other hand, have received relatively little attention, possibly because they are commonly found in human rich or degraded landscapes, where the impact of invasive species is considered not such a big problem. That then brings us to the question; what is the impact of invasive bird species? Are there any?
It has been suggested that invasive birds can have an impact on competition for nesting sites, food sources and are a common vector for the transmission of diseases, however quantifying the impact of invasive species on native populations is still a challenge. Studies have observed that invasive birds are more aggressive and generally win in competition for a nesting site, however the evidence for this having an impact on the environment is a bit feathery (excuse the pun).
This is somewhat amusing when you consider the IUCN to state in 2000, despite a lack of evidence, that the Common Myna (Acridotheres tristis) is the third most invasive species in the world, more so than Rattus rattus (Common ship rat) and Tamarix spp. (Salt Cedar), for example. Whilst this bird is highly abundant and noticeable in the urban landscape, it is important this does not mask systematic scientific judgment concerning the true impact of the species. Scientific evidence in fact conveys the opposite. The Common Myna was observed to display no more aggression towards native bird species than any other species. They showed no signs of hindrance during feeding, and furthermore it was found that the Common Myna showed little competition for nesting sites and were commonly found in artificial structures, imposing little impact on resource use . This is backed up in Sydney where the presence of the Common Myna is negligible and it is thought the bird has little, if any, impact on native birds.
As far as we know, there has been no empirical evidence of negative impacts as a result of bird invasions. Until now. A study published in July  provides the first evidence to address the impacts of an invasive bird. Hallelujah! This study also assesses the impact of the Common Myna on native bird populations in Australia, in a natural habitat. They found that this bird does in fact have a negative impact on three cavity-nesting birds and eight other small birds when conducting experiments. So I guess we can say that an invasive bird is not an oxymoron!
I think it important that conservation management fully understands species impacts. Often it is noted that introduced species provide benefits, such as new bird colonists acting as pollinator compensation for a decrease in pollinator populations. Most of all, we need to fully understand the impacts of invasive bird species on the environment and the interaction with native fauna. A lack of empirical evidence will only lead to species management being based on observations within a particular location. It is essential that studies focus on invasive bird impacts, with particular attention to species interactions in natural habitats. This will begin to increase our understanding of the impact of invasive birds, and help to answer questions such as, why some invasive species have a greater impact than others and just how much invasive birds are truly invasive?
1 Duncan, R.P., Blackburn, T.M and Sol, D. (2003) The Ecology of Bird Introductions. Annual Review of Ecology, Evolution, and Systematics, 34: 71-98
2 Lowe, K.A., Taylor, C.E and Major, R.E. (2011) Do Common Mynas significantly compete with native birds in urban environments? Journal of Ornithology, 152: 909-921
3 Grarock, K., Tidemann, C.R., Wood, J and Lindenmayer, D.B. (2012) Is it benign or is it a pariah? Empirical evidence for the impact of the Common myna (Acridotheres tristis) on Australian birds. PLoS ONE, 7 (7): doi:10.1371/journal.pone.0040622