The Catastrophic Effect of Invasive Species on Native Biodiversity

Biological diversity, or more commonly known as biodiversity, describes the world’s variety and richness of species with their unique traits, genetic variability, and distribution. One of its simpler definitions refers to the arrangement of all life on Earth. This also includes ecosystem processes that occur through species interaction, such as pollination. Islands accommodate a large amount of Earth’s biodiversity with many unique and endemic plant and animal species. Species end up on islands by drifting, like castaways, or by dispersing across oceans. These species are confined to isolated areas located with some distance from other landmasses. This isolation wields unique evolutionary forces that develop a distinct genetic pool and species become highly specialized with unique adaptions and characteristics. Many island species are endemic and found nowhere else in the world. Islands have a higher concentration of endemic species than continents and it increases with isolation.

However, biodiversity is declining rapidly due to land-use change, climate change, overexploitation, and invasive species. These are described by Diamond as the ‘evil quartet’ as they are human causes of extinction. Invasive species, in particular, are organisms that are introduced, directly or indirectly, through human action to habitats outside their range. They often reproduce rapidly and spread at alarming rates. This is a big problem on islands as endemic species are unable to adapt and coexist with introduced species. Invasive species have a catastrophic effect on native biodiversity and are one of the primary drivers on islands. This threatens the existence of native organisms and can also create serious economic and ecological damage.

Feral cats are among the invasive species which have been implicated in many extinctions, especially in Australia. Feral cats (Felis catus) may have arrived already in the 17th century onboard ships, and in the 1800s they were released intentionally in a failed attempt to help control pests like rats and mice. By 1850 the feral cats had Australia their home and can now be found everywhere in the wild. Feral cats are different from domestic cats as they can survive without human assistance and according to science are the main driver of extinction for Australia’s mammals, birds, lizards, and frogs. Because of this, they have become the main target of the Australian Governmental control program Threatened Species Strategy. The Australian government announced in 2015 that they intend to cull more than 2 million feral cats by 2020 through a range of methods which include shooting, trapping, and poisoning.

This paper aims to discuss the implementation of mass culling the feral cat (Felis catus) in Australia. The economic, environmental, and ethical issues raised will be presented to demonstrate the complexity of making conservation strategies concerning the native Australian flora and fauna.

Issues

Prevention of feral cats creates a lot of conflicts with commercial and community interests such as agriculture, hunting, and the pet industry. The costs of removing cats rise with time and opportunities are lost due to a lack of funding. Constant observations are required as the potential for re-invasion is high. The lack of funding creates insufficient and poorly aimed control programs which can only be applied short-termed. Invasive species are seen as a money trench and governments are unwilling to put in enough funding as they rather spend that money on other issues. Broad-scale control for feral cats in Australia still remains indefinable and it comes at a great economic loss to be able to achieve the best outcome.

Economic Impacts

There are no optimistic economic costs in regard to invasive species, and it is unsure exactly how much and where the government spends money on their pest control. New South Wales has been estimated to spend around 30% of the national funds on pest control. However, there is no specific document that describes exactly how much they spend. A national study was made to investigate how much is being spent and landholders fund $22 million with an extra $5 million through taxes and including government spending of $39 million, this makes up a total of $61 million.

It has been estimated that around $2 million are put into management and research each year to control feral cats. Feral cats prey on birds and it has been estimated that the predation on birds alone has been a loss of $144 million per year. Control methods are expensive and require a long-term application. Feral cats are shy and are very varied when it comes to baiting as they normally avoid human contact which makes them even harder to shoot. Fencing has been proven to work the best when it comes to controlling methods, but it is only good for smaller habitats, and it is expensive and could be as high as $18,000 to 55,000/km. Feral cats are usually spread out over a large area and hang out in low densities which makes baiting extremely hard.

Environmental Impacts

Feral cats pose a significant threat to endangered and threatened species in Australia and according to the threat abatement plan (2008), it has been estimated that around 80 species are under threat.

Competition

Feral cats compete with both omnivores and carnivores for food sources. Studies have shown that many different species' own diets overlap with feral cats, such as dingoes, red foxes, and raptors. Researchers have found that red foxes actively exclude feral cats from resources, and they also predate on feral cats. Raptors have a direct competition with feral cats as their biggest diet comprises of rabbits, however, feral cats are not seen as a threat for the raptors. Dingoes are a natural predator for the feral cat, however, it does not exist in areas where the feral cat resides.

Predation

Feral cats are opportunistic and generalist carnivores that feed on several different prey, however, when available rabbits are preferred. In a study by Dickman and Newsome, it showed that rabbits stood for up to 50% of the feral cat’s diet. Doherty et al found that the rest of the cat’s diet was made up of small mammals that nested or spent time on the near-ground. In regions where the rabbit had been controlled, the feral cats fed on smaller mammals.

In a field experiment, it was found that feral cat predation is higher in habitats where there are fewer foxes. However, even though they pose a threat to native species, it was found that feral cats together with foxes control introduced rabbits which also cause huge ecological damage. In Australia, feral cats have no natural predators except dingoes, and this has made them top predators in environments where dingoes don’t exist. Read and Bowen estimated that around 700 reptiles per square kilometer is being consumed by feral cats annually. Birds are also an important part of the cat’s diet and in coastal areas, seabirds form a big part of it.

In 1995, Heirisson Prong Project reintroduced bandicoots from Dorre Island to a cape nearby on the mainland. They used fencing to try to keep out feral cats and foxes and for a short time, their population numbers increased. However, it later quickly declined and was later followed by extinction, and the primary cause was found to be the feral cat.

Disease

Feral cats are a major threat to public and animal health as they are known to be carriers of several different diseases such as sarcosporidiosis and Toxoplasma gondii. The risk of disease being spread increases as breeding between feral and domestic cats are high. These diseases can easily transmit to native flora and fauna, as well as humans. T.gondii can cause miscarriage and defects at birth, and it has also been found in marine species such as dugongs and dolphins. Moreover, it can cause blurred vision and difficulty walking which will make them more prone for predation. In Western Australia, Adams found that the infection was present in 6.5% of native mammals and 4.9% of feral cats and persisted in 15.5% of the marsupials in Perth city. Furthermore, feral cats are good carriers of disease and if rabies were to break out, feral cats would then become a high risk to all species susceptible to the disease.

Ethical Issues

Including ethical interests is a fundamental part to creating control measures against feral cats. The value of feral cats differs depending on the observer’s opinion. Many see feral cats as a pest and a threat to native fauna, however, others are concerned about the health of domestic cats as they might be threatened by the chosen actions in the abatement plan. Others agree that they should be controlled to protect native species but without the culling, while some think that culling all feral cats is the best option.

Different cultural values connected to feral cats is also extremely important to take into consideration. Some indigenous people acknowledge feral cats as a part of the ecosystem and do not see them as feral, but as a newcomer, and some argue that they came before European settlement and thus are protected by Aboriginal Law. However, in some circumstances, it is debated whether feral cats threaten native species to the point that it can be acceptable to use harmful actions. It should always be heavily backed with research that allows actions to be put where they will be the most applicable as to not waste any lives.

Discussion

In Australia, feral cats are implicated in many extinctions and they also cause economic and ecological impacts as well as ethical issues concerning mass culling. The government stated that they would kill a target of 2 million feral cats by 2020, however this target was based on information without any reliable source of their population size. This estimate was based on the fact that 5-18 million feral cats currently reside in Australia. In a recent study by Legge et al, they estimated that there are around 2-6 million feral cats which throw their initial number off. If they were to go through with the plan with this new number, 32-95% of the population would be killed instead of 11-40% which was the initial aim. The target was made without any reliable sources and should therefore be reconsidered.

Feral cats are known as generalists and therefore have a fast reproduction rate and can easily recolonize to other areas which makes them able to re-invade habitats. For control measures to have a long-term effect, it has to be constantly monitored which is very expensive and it has to be done in areas where there are threatened native species present. It is still unclear whether the controls have an effect on feral cats. Research done by the government stated that around 211,500 feral cats were killed within a year. However, another research conducted in Tasmania found that eradicating feral cats had an adverse effect with cat numbers increasing instead of decreasing within the target sites. It was made in a small scale and showed that new cats came into the area after the dominant individuals were gone. This verifies that for control measures to have an effect, monitoring of the cats is essential to successfully eradicate them and needs to be an approach with several different methods.

Feral cats are one of the primary drivers of extinction, yet other factors are also a part of the problem, such as habitat loss. In the threatened species strategy, habitat loss is barely mentioned while feral cats are continually cited. Australia is considered one of the world’s worst when it comes to a land clearing with the clearing of vegetation, in New South Wales, rising to 800% between 2013 and 2016. In 2017, NSW passed the new Biodiversity Conservation Act giving farmers greater power to clear even more land without having approval from the government. The strategy does not mention enough approaches to tackle this problem and needs to be implemented.

Conclusion

There are several literatures recommending management methods as well as monitoring methods, and eradication on the small island has shown to be successful. However, these are not as successful on the mainland and additional research has to be done to acquire better methods. Eradication of feral cats is a complicated issue with many sponsors and other participants having different opinions with some opposing the plan while others agree. It is a sensitive issue that should be handled with care and every proposition should be heavily justified and reviewed. It is not an issue that the government can decide for themselves, but the whole community needs to be involved. Methods such as shooting, trapping, and poisoning are expensive and poisoning can cause other species to be affected as well. More research is essential to understand how the different species interact with each other and how feral cats affect native species in long term. This is required to be able to come up with the best conservation strategy. Whether culling should go forward or not is a question which cannot be answered now considering the limited research that has been done on the different interactions. Another solution would be to focus on how to recover native species and how they might be able to co-exist with feral cats.