From Our Co-Founder: A story about two islands

Feral Herald |

Author and biologist Tim Low was one of the Invasive Species Council’s co-founders 20 years ago. He wrote and published Feral Herald for many of those 20 years, and brings us another story in this edition.

This is a story about two islands, both in Western Australia.

One of them, Bernier, on the edge of Shark Bay, stands out as the Australian island that has saved more mammals from extinction than any other, five species all up. The djoongari or Shark Bay mouse (also known as the Gould’s mouse) survived on this 42 square kilometre landmass and nowhere else. The mala, burrowing bettong, banded hare-wallaby and Shark Bay bandicoot each survived on one or two other islands as well. All disappeared from the mainland long ago.

The other island is a case study in degradation. Goats were taken there in 1899 and not removed until 1984. Biologists who visited in 1959 found networks of goat trails in the better parts and the worst parts reduced to mobile dunes, running from one side of the island to the other, blamed on goats removing the plant cover. Goats were climbing into the crowns of shrubs and low trees to browse the tops of branches, many of which were broken off. Culls of 500 goats in the 1970s did little to improve the habitat.

I said two islands, but both descriptions actually apply to Bernier.

This island provides the kind of evidence that implicates cats and foxes rather than habitat degradation for Australia’s appalling tally of extinct and endangered mammals. There are many places on the mainland that kept better habitat than Bernier, but what mattered was its lack of cats and foxes. The island’s mammals survived savage droughts among those destructive goats, and at one stage they had a flock of sheep to contend with as well.

Bernier Island is close to Dorre, an island that saved the same species as Bernier apart from the mouse. No fire has ever been recorded on Bernier, but Dorre had serious fires in 1860, 1909 and 1973. Proactive fire management is vital for many species on the mainland but obviously not on these islands. One reason for the difference is foxes and cats on the mainland hunting with ease after fires. The last fire on Dorre was so destructive that rangers feared for the hare-wallabies and moved some to nearby Dirk Hartog Island. It had cats and they did not survive. The hare-wallabies did survive on scorched Dorre Island.

The five rare species have been helped with extra habitat by moving some individuals to fenced reserves on the mainland. One site chosen was Heirisson Prong, a peninsula of land in Shark Bay, close to Bernier Island. A predator-proof fence was installed across the neck of the peninsula, and bettongs and bandicoots added after cats and foxes were removed. The bandicoots thrived, multiplying to about 470, until cats breached the fence and killed them all. Foxes also breached the fence and killed bettongs. The project failed because the fence proved too difficult to maintain, corroding quickly in the salt air.

But Dirk Hartog Island has become very important for mammals today. The cats were removed and in 2018 it was stocked with hare-wallabies and mala, then with bandicoots and djoongari. They all seem to be doing well.

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