Cat and fox: agents of Australian extinctions

Australia’s drylands are famous for their assemblage of ultra-cool mammals. As one example, it is difficult for us non-Australians to imagine a more endearing creature than the rock-wallaby pictured below.

Black-foted Rock-wallaby

Black-footed rock wallaby. Credit: Peter McDonald.

Unfortunately, numerous species of Australia’s dryland mammals are going extinct. Many of these extinct species weigh between 35 and 5500 grams – a weight range that researchers have described as the critical weight range (CWR). Peter McDonald and his colleagues wanted to know what was causing these extinctions, and why were they most prevalent in the CWR. They considered two hypotheses. First, perhaps the land was becoming less productive, either from habitat destruction by humans, or as a result of changing climate. Reduced plant abundance could cause herbivorous mammals to go extinct. An alternative hypothesis is that perhaps newly introduced predators, notably feral cats and red foxes, were killing the native mammals so effectively, that they were disappearing from the Ausralian drylands.

Previous research indicated that extinction rates were lower in areas that had more species living in trees and around rocks, leading McDonald to think that maybe habitat was influencing extinctions in important ways. In particular, he realized that rugged mountainous areas might have fewer predacious cats and foxes, and secondly that these two predators tend to go for prey within the CWR. Putting these ideas together, perhaps mountainous areas are refuges for Australia’s dryland CWR species, protecting them from predator-driven extinction. If so, mammal species richness would be highest in rugged, protected areas, and lowest in more open areas. If, on the other hand, mammals are going extinct because overall productivity is declining, we would expect overall species richness to be greatest in the most productive areas.
McDonald and his colleagues tested these two competing hypotheses by censusing mammals in four different types of habitats in Tjoritja National Park within the MacDonnell Range of central Australia. These were (1) mountain areas dominated by a sparse assemblage of shrubs and clumps of spinifex grass, (2) spinifex grasslands (with a more abundant cover of spinifex than found in the mountains), (3) Acacia shrublands, and (4) alluvial woodlands, which were most productive with richest soils.

 

 

Mountain refuge habitat_PeterMcDonald

Mountains. Credit: Peter McDonald

 

spinifex

Spinifex grasslands

acacia

Acacia shrubland

alluvial

Alluvial woodland

The researchers set up a variety of different mammal traps in 90 different sites representing these four habitats to capture and identify small mammals, and they detected larger mammals by searching for fresh scat at each site. The researchers estimated productivity with the normalized difference vegetation index (NDVI), which uses satellite imagery to measure the green-ness, and hence productivity, of a site or region.

In support of the predation hypothesis, more mammal species were found in the most rugged terrain.

McDonaldFig1A

Number of mammal species per site in relation to ruggedness of terrain. The curve is the fitted value of the regression equation.  The shaded area represents the 95% confidence interval.

In contrast to the productivity hypotheses, fewer mammal species were found in the most productive sites

McDonaldFig1c

Number of mammal species per site in relation to productivity of terrain as measured by the NDVI. The curve is the fitted value of the regression equation.  The shaded area represents the 95% confidence interval.

While it’s useful to evaluate both hypotheses by measuring current species richness, the researchers also needed to know how many species were actually driven to extinction in the time since cats and foxes invaded. They reconstructed historic species richness for each habitat based on subfossil remains (remains of organisms that are only partially fossilized), from indigenous knowledge supplied by aboriginal Australians, and from historical accounts in the early literature.

They discovered that CWR extinctions were most prevalent in alluvial (12/12 species) and acacia (7/7 species) habitats. Spinifex habitas lost 5/6 CWR species, while mountainous habitats only lost 2/6 CWR species. Importantly, species outside of the CWR have survived relatively well in all habitats, further implicating cats and foxes as the agents of extinctions.

McDonaldFig2

Current (extant) and historic (pre-invasion by cats and foxes) mammalian species richness in the four habitats. The dots are the mean weight, and the lines are the weight ranges for each species.  The shaded area represents the critical weight range (CWR)

More support for the the predation-habitat link comes from recent research that indicates that red foxes are absent from the mountain habitat, while feral cats are substantially less abundant. Even when present, cats are much less efficient hunters in the mountain habitat because the complex rock structure affords more refuges to prey items.

Feral cat with fat-tailed antechinus_NTG

Feral cat captured on camera with a fat-tailed Antechinus. Credit Tony Griffiths.

Across Australia, many CWR species have gone extinct in regions colonized by cats and foxes. McDonald and his colleagues provide solid evidence that these introduced predators are responsible for these extinctions. They urge researchers to explore other mountainous regions in Australia to see if they too are acting as refuges for CWR mammals.

note: the paper that describes this research is from the journal Conservation Biology. The reference is McDonald, P. J., Nano, C. E. M., Ward, S. J., Stewart, A., Pavey, C. R., Luck, G. W. and Dickman, C. R. (2017), Habitat as a mediator of mesopredator-driven mammal extinction. Conservation Biology, 31: 1183–1191. doi:10.1111/cobi.12905. doi:10.1111/cobi.12908. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2017 by the Society for Conservation Biology. All rights reserved.

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