Determinants of Island Species Extinction

We are currently living in an era of mass species extinction, where island species face faster rates of extinction compared to other areas. A recent study conducted by scientists Emily Hanna and Marcel Cardillo at the Australian National University suggests that the cause can be attributed to introduced predators like feral cats, dogs, and more importantly black rats.

Islands are some of the last strongholds for exotic, endemic species, but they are home to mostly endangered animals. Thus, it is critical to ensure the continued existence of island animal life; otherwise, the consequences would strongly and adversely impact biodiversity worldwide.

With this mindset, researchers Hanna and Cardillo set out to find the cause behind the increased rate of island animal extinction.

Using Abbot and Burbage’s database of island species, the researchers conducted a large scale analysis of 323 different Australian islands. They created extinction models applied to decision trees to study the relationships between extinction rates, presence or absence of apex predators and mesopredators, and island geography.

Unexpectedly, Hanna and Cardillo found that the black rat population is driving the increased rates of extinction on islands for both large and small animals.

They found that the longevity of large animals above 2.7 kg was largely correlated with island distance from the mainland in the absence of black rats. One mentioned theory proposes the idea that isolation could increase extinction rate because farther island populations have become unused to adapting to new threats such as introduced predators, and are therefore less able to defend against predation.

For animals smaller than 2.7 kg, it is the presence of the black rat only that is strongly correlated with extinction, and this even stands true for islands beyond Australian waters.

Having the capability to prey on animals equal to its size, rats are known to eat the young and juvenile forms of almost all animal species, continually killing new generations. This combined with the rat’s ability to reproduce, spread, and adapt quickly makes it one of the most widespread and dangerous invasive species around the world.

So, how does one stop the spread of black rats? According to the research conducted, allowing the continued existence of apex predators, or predators at the top of the food chain, would result in a top-down control of the black rat population, otherwise known as mesopredator suppression where larger predators can control the population growth of smaller predators.

Hanna and Cardillo realized that when islands with black rats also had the presence of larger predators, specifically dingoes, the rate of native species extinction decreased significantly; however, many of the larger predators that control rat populations are also invasive species. Thus, the study’s findings have many implications for the future direction of conservation biology.

The current status quo towards invasive species is to eradicate and/or extract the animal instantly, including the apex predators, but according to both authors, “[future conservation efforts] should aim to balance the eradication of smaller and larger predators.” Otherwise, the quick loss of one species like the dingo could result in “mesopredator release and trophic cascades”.

Without the presence of dingoes, foxes, and cats, islands could face the complete extinction of several species due to the uncontrolled force of the black rat. Thus, new methods of invasive species extraction should be looked into before it is too late, and native island species disappear forever.

Source:

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

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