Home to many endangered species, islands around the world are the last strongholds for exotic animals like the Banded Hare Wallaby. So why is it that these amazing epicenters of biodiversity face crazy high rates of extinction compared to other locations?
According to Emily Hanna and Marcel Cardillo from the National Australian University, animal species are being wiped out by a number of factors like geographic isolation and animal life history. However, the scientists found that the most important determinant of whether a species survives is surprisingly based on the presence or absence of the invasive, black rat.
Inspired by their curiosity and need to protect Australian animals, Hanna and Cardillo found these results after conducting a large-scale analysis of 323 Australian islands. They looked at the relationships between geography, species history, and human-brought predators. They discovered that animals weighing above 2.7 kg were more likely to survive on islands both farther away from the mainland and lacking a large population of rats. On the other hand, the survival of animals weighing below 2.7kg was strongly correlated with rat population only, where more rats meant more death.
You may be asking yourself, “how in the world is this possible?” Well, according to the researchers, rats are able and more than willing to prey on anything smaller or equal in size to itself. Thus, they can prey on the young and juvenile forms of almost any animal as long as the rat can beat it in a fight. As seen in the study’s results, the size constraint of the rat’s diet translates into a higher survival of larger animals. But the rat’s voracious appetite combined with its ability to reproduce, spread, and adapt quickly makes it a most dangerous foe and a conservationist’s nightmare. So, how does one defeat the invasion of rats?
The answer to controlling rat populations is a simple one. According to scientists Hanna and Cardillo, “Adverse effects of black rats on island mammal populations is strongly mitigated by the presence of larger predators, with cats, foxes, and dingoes all providing such a mitigating effect.” This effect is known as mesopredator suppression, where apex predators, or predators at the top of the food chain, can outcompete or directly prey upon mesopredators, predators on lower rungs of the food chain. In other words, the island animals are protected from non-native predators by other non-native predators; just like how in Lord of the Rings, the elves can protect the commoners from an army of goblins.
These findings have strong implications on the future direction of conservation practices. Currently, it is common practice for people to immediately extract or kill the large invasive predators like the dingo because they are perceived as being more of a threat. Dingoes are especially treated as the big bad wolf by farmers because of the dingoes’ penchant for feeding on livestock; they are usually shot on site by farmers. However, it is the dingo that exerts the most control over rat populations. Thus for the good of the continuation of native island species, it is important to protect these larger predators. And, as our authors put it “[future conservation efforts] should aim to balance the eradication of smaller and larger predators.” Without the correct balance, many amazing species like the cute, cuddly Wallaby could go extinct and be replaced by hordes of black rats.
Hanna, E. and Cardillo, M. (2013), Island mammal extinctions are determined by interactive effects of life history, island biogeography and mesopredator suppression. Global Ecology and Biogeography. doi: 10.1111/geb.12103