Food web of the Little Desert

The Little Desert has a fascinating food web (trophic structure) which is central to the entire ecosystem. Involving plants, herbivores, omnivores and predators in ascending trophic levels with decomposers to return nutrients for plants, this food web is the basis of biodiversity. 

On the IFFA camp of spring 2016 we viewed distribution maps and discussed the ecology of vertebrate herbivores, omnivores, mesopredators and top predators in the Aboriginal and European novel Little Desert ecosystems. Predators can be ground, semi-arboreal or aerial. Primary species occur throughout the sandy desert (Lowan Mallee bioregion) while secondary species are mainly on the surrounding plains (Wimmera and Murray Mallee bioregions). 

Curiously, dingoes and quolls are not recorded. Dingoes are relatively rare in waterless desert while foxes are relatively common.1 Much of the country lacks surface fresh water but were they near the Wimmera River? Did they go extinct from early European persecution? The Aboriginal ecosystem large predators were humans, goannas and possibly dingoes and quolls. Now they are goannas and red foxes. Cats probably occur in the Lowan Mallee but they appear to be secondary species and are not recorded from the national park.2 

In the Lowan Mallee novel ecosystem, the major predators are raptors and owls (the largest is brown falcon) and three goannas (lace monitor, sand goanna, Rosenberg’s goanna) along with red fox and probable cat. 

Collectively these predators control all populations of native and novel vertebrate herbivores and omnivores (especially western grey kangaroo, common brushtail possum, silky mouse, goat, sheep, rabbit). These prey species have the potential for overpopulation and require top-down control for ecosystem stability and diversity. 

As the top ground predator, the fox preys on juvenile goannas which may prevent these mesopredators from increasing in numbers and wiping out rabbits. However dingoes would also have controlled goannas. Foxes and goannas control rabbits everywhere. They also control brushtail possums in yellow gum woodland in the desert and in river red gum woodland along the Wimmera River. The fox alone appears to substitute for humans and possibly dingoes by preying on emergent pouch young of western grey kangaroos, preventing this largest of the herbivores from becoming overabundant and degrading the ecosystem. Eagles take young kangaroos but rarely visit the desert, preferring open country. 

In this novel ecosystem it is a management choice between dingo and fox to control kangaroo grazing and browsing unless the kangaroos are hunted by humans as the Aboriginal people did. Dingo (re)introduction may conflict with sheep farming in the surrounding area. Otherwise it appears only the fox can provide this ecosystem service. 

A study of the fox diet in Victoria found that in the Wimmera, which includes the Little Desert, the diet consists mainly of insects (28.6%), black wallaby (15.9%), common brushtail possum (12.7%), rabbit (11.6%), sheep (11.6%), undetermined bird (7.9%), western grey kangaroo (5.8%) and plant material (5.8%).3 Listed threatened native fauna are recorded in <2.5% of the diet and none of these species (squirrel glider, smoky mouse) occur in the Little Desert.4 Presumably malleefowl are part of the undetermined bird component. 

Despite the widespread belief that foxes impact negatively on malleefowl, a recent study found that foxes in the Mallee are ‘positively correlated with malleefowl conservation’.5 The fox preys on the iconic malleefowl Leipoa ocellata but they appear to be in widespread predator-prey balance. Malleefowl are fecund. A female may produce some 400 eggs in her lifetime of which only two hatch and survive to the adult stage in a stable population. The other >99% either fail to hatch (some 20%) or the birds die young as eggs or immature birds mainly from predators or heat stress, factors that have been operating for millions of years. The malleefowl’s main problems lie elsewhere: habitat loss, fragmentation and alteration. 

Does the link between foxes and malleefowl reflect fewer dingoes as they kill foxes and could be more efficient predators of malleefowl? Or do foxes suppress cats? Do foxes suppress rabbits and goats which compete for food with malleefowl? Fortunately the malleefowl has effective preadaptations against the fox – it has been dealing with ground predators for millions of years. The giant malleefowl Leipoa gallinacea was not as fortunate – it didn’t survive humans possibly due to its larger size and larger nest mound. It was one of many large megapodes sent extinct by humans in the Australia–Pacific region.6 

We then viewed dramatic evidence of brushtail possum associated decline (BPAD) without leaving the campground. The river red gum woodland canopy was overbrowsed by 

brushtails and was in moderate to poor condition. Trees at the north end of the campground were severely defoliated. Trees showed the diagnostic symptom of BPAD: uneaten leaves on hanging branches. Abundant possum scats were seen on the path under one tree. Doug Frood found scratch marks on a stag tree indicating it was home to a possum. At least one tree in long grass beside the Wimmera River was dead. To avoid foxes, brushtails prefer to change food trees in long grass where there is little or no slashing or grazing, and on such sites browsing is more concentrated and tree decline is more evident. No fox scats were seen. The previous day we saw a disused fox den over the road. A stick placed in front of the entrance indicated fox control had taken place. This may have caused the local trophic imbalance. 

The trophic structures of Victoria’s 28 bioregions7 all warrant detailed study. Each bioregion has a unique trophic structure. What controls the population size of each major herbivore, omnivore and predator? What do they eat and not eat? The structure and function of the Aboriginal and European ecosystems can be modelled and compared. Research on bioregional ecology has considerable scientific interest and many practical applications.

For more on the trophic ecology of southeast Australia: 

Yugovic J 2017. When predators go missing: native mammal herbivore imbalance and the predator-prey ecology of south east Australia. http://www.spiffa.org/do-ecosystems-need-top-predators ;

  1. Southgate R, Paltridge R, Masters P, Ostendorf B. 2007. Modelling introduced predator and herbivore distributions in the Tanami Desert, Australia. Journal of Arid Environments 68: 438–464. 
  2. Victorian Biodiversity Atlas www.depi.vic.gov.au/environment-and-wildlife/biodiversity/victorian-biodiversity-atlas
  3. Atlas of Living Australia www.ala.org.au/
  4. Davis NE, Forsyth DM, Triggs B, Pascoe C, Benshemesh J, Robley A, Lawrence J, Ritchie EG, Nimmo DG, Lumsden LF 2015. Interspecific and geographic variation in the diets of sympatric carnivores: dingoes/wild dogs and red foxes in south-eastern Australia. PLoS ONE 10(3): e0120975. 
  5. Walsh JC, Wilson KA, Benshemesh J, Possingham HP 2012. Unexpected outcomes of invasive predator control: the importance of evaluating conservation management actions. Animal Conservation 15: 319–328. 
  6. Worthy T 2016. A case of mistaken identity for Australia’s extinct big bird. The Conversation 14 January 2016. theconversation.com/a-case-of-mistaken-identity-for-australias-extinct-big-bird-52856 
  7. DELWP 2016. EVC benchmarks: bioregions. Department of Environment, Land, Water and Planning, Victoria. www.depi.vic.gov.au/environment-and-wildlife/biodiversity/evc-benchmarks 

– Jeff Yugovic