Problems in paradise: the destruction of Hawaiian species

A few months ago I wrote a post on invasive species in Finland, and in particular on the North American beaver (Castor canadensis). I received a comment on how it is bold (or maybe the commenter meant reckless) to say that almost all invasive species are threatening the native species of the region. I began thinking of this comment, and tried to find some studies that proved that invasive species are beneficial for the subject ecosystem. Unfortunately, I only came up with sad tales. One very devastating example of invasive species is the Hawaiian Islands.

The Hawaiian Islands in the center of the Pacific Ocean are one of the most isolated islands in the world. Their endemic terrestrial species originate from some hundred species that migrated thousands of kilometers over the Pacific Ocean during several millions of years. Because of the immigration bottleneck and isolated evolution, the Hawaiian Islands have become a place for numerous distinctive and fascinating species. But it has also made the fauna and flora of the islands very vulnerable to various disturbances, such as human invasion and human-mediated invasions.

Nowadays almost a quarter of Hawaiian terrestrial species are non-native. Birds have probably suffered the most. Previously there were 11 native goose species in the Hawaiian Islands, but nowadays only one species is left: the nene (Branta sandvicensis), also known as the Hawaiian goose. The same has also happened to the native duck species; just two duck species are left (the Hawaiian duck, Anas wyvilliana and the Laysan duck, Anas laysanensis).

The nene, also known as the Hawaiian goose (Branta sandvicensis), is the only endemic goose species left in the Hawaiian Islands. © Sari Holopainen

The nene, also known as the Hawaiian goose (Branta sandvicensis), is the only endemic goose species left in the Hawaiian Islands. © Sari Holopainen

The main reasons for these extinctions are introduced predators (e.g. the feral cat and mongoose), and feral and game species (e.g. the mouflon, Axis deer and feral pig). There are almost 60 studies on domestic ungulates, but none have demonstrated any positive effects of them on native species. Ungulates stimulate the growth of grass among other things, leading to more grasses and less forest. And all this changes the light regime and fire resistance of an ecosystem. Grazing is therefore destructive to Hawaiian forests and to every native organism living in them. It has also been proven that the invasive vertebrate species of Hawaii have facilitated at least 33 invasive plant species. In addition to damages caused by grazing, feral pigs alter nutrient cycling and accelerate soil erosion.

The main problems caused by feral pigs are alteration to nutrient cycling and acceleration of soil erosion. © Sari Holopainen

The main problems caused by feral pigs are alteration to nutrient cycling and acceleration of soil erosion. © Sari Holopainen

There is still some light at the end of the tunnel, although it might be rather dim. The public has come to aid in the eradication of many species. Scientists and wildlife managers have concurrently begun multi-scale population monitoring, which includes aerial and ground-based visual surveys as well as trail cameras. To intensify and simplify the eradications even further, several hundred kilometers of management fences have been constructed. As an outcome of this some success stories have emerged; the eradication of rabbits and feral goats. Furthermore, the midway islands of Hawaii are now rat and non-native mammal free!

Unfortunately, it has been too late for some Hawaiian ecosystems. A key threshold has been crossed in some regions, and recovery of certain ecosystems may not be possible any longer. The populations of illegally introduced axis deer (Axis axis) have been reduced to some dozens, but their eventual eradication has been problematic, because assessing the number of remaining deers on private properties has proved difficult. The axis deer was introduced to provide game, so private properties owned by hunters act as reservoirs for the deer, from where they can be disperse to clean areas.

The main feral goat eradication was performed in 1980s, and nowadays the Hawaiian Islands are goat free. © Sari Holopainen

The main feral goat eradication was performed in 1980s, and nowadays the Hawaiian Islands are goat free. © Sari Holopainen

To conclude, I still dare say that almost all invasive species threaten native species. Even though some invasive species don’t harm all native species, we are always looking at nature as a complex ecosystem consisting of several species and functions. When introducing an alien species, we will always alter the pristine ecosystem.

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Not so cute and cuddly – it’s time for the cat wars

Feral cats (Felis catus) have spread globally thanks to their reputation as cuddly house pets. Their habitat use is very diverse, ranging from urban areas to e.g. agricultural, woodland, and grassland landscapes. Feral cats also rapidly become semi-wild if given the chance to move freely in the great outdoors. Combining their ability to spread fast and wide with their ability to eat very varying diets (small mammals, birds, reptiles, amphibians, everything goes) is apt to spell major trouble for native wildlife. And though they are fairly small-sized mammals, they do not only go for small animals, but have been recorded to kill animals weighing up to four kg (nearly 9 lbs).

Feral cats have also been dubbed “compulsive killers”. That means that not only do semi-wild feral cats hunt food for their survival, but house cats momentarily let loose in back yards etc. will go after anything moving, despite having a meal waiting back home. They effectively conquer the niches of several native small or medium-sixed predators around the world, thus disrupting food webs and ecosystems. And to top it all of, feral cats can also spread toxoplasmosis, a parasitic disease that threatens humans and livestock (e.g. sheep in Australia).

Native and endemic island populations have been particularly hard struck by the spreading of feral cats, but the numbers behind this feline invasion are massive everywhere: an estimated 18 million native animals are eaten by feral cats every year in Australia alone, and a single cat is believed to kill five to 30 animals a day. This is no small-scale problem; the US alone has an estimated 100 million pet cats, each of which kills around 5 to 15 birds per year. That’s 1,500 000 000 birds. Feral cat-caused mortality rates of birds in Great Britain are estimated at 50 million a year.

The Australian government has reacted to feral cats with the strongest measures. They are not hard-pressed to find reason, as 100 native Australian species are in direct extinction threat caused by feral cats. Australia has issued a national threat abatement plan for predation by feral cats, aiming to eradicate them from three islands, with plans to evict the species from two more in the near future. New Zealand also has a zealous plan for lessening the damage caused by small mammals (including feral cats), with the aim of eradicating the species completely (except as an indoor pet) from the country.

Urban and rural feral cats can devastate native wildlife populations. ©Stella Thompson

Urban and rural feral cats can devastate native wildlife populations. ©Stella Thompson

Cat wars are being waged across the world with one common denominator: high-tech technology. Eradication measures include using thermal imaging, motion and remote sensing cameras, and GPS tracking collars along with more traditional baiting, trapping, and using tracking dogs. Several countries have also issued desexing programs and cat-free zones around particularly important areas (e.g. nesting sites). Fines can also be issued in some countries (including Finland) to cat owners whose cats have killed native animals. These fines can reach thousands of euros/dollars, depending on the species in question.

It seems unlikely that pet cats are going anywhere in the near future (and why should they); however, maybe what remains a question is whether their freedom in the great outdoors should be restricted or whether laws should be passed mandating cat owners to fasten little bells or colorful cat collars around their pets’ necks. Both have been shown to significantly lessen the numbers of birds, reptiles, and amphibians falling prey to this miniscule feline predator. It is important to remember that what people think is cute play when witnessed at home is actually stalking that will result in native species mortality when the cat is let loose. Attitudes must also change; all too often people believe it is okay to abandon their pet cats after caring for them for a while. Such cats can form populations of tens or hundreds of semi-wild individuals, living in abandoned buildings around the countryside and cities. Many may believe this to be a random occurrence, but e.g. 20 000 pet cats are abandoned yearly in Finland.