Grassduck: where the grazers fly instead of hopping
(writing in progress)
The main grazer in Mitchell grassland was an unknown, extinct species of marsupial. Ducks are among the unlikely grazers here because they are aquatic omnivores.”
“Mammals have always been more vulnerable than birds to dehydration in Mitchell grassland because droughts and floods are so severe. Indeed, kangaroos, wallabies, hare wallabies and rodents can all be excluded as important grazers in Mitchell grassland for the following reasons:
- the red kangaroo1, which would be expected to be the main grazer in these grasslands, actually avoids treeless vegetation in northern Australia despite living in nearby woodlands;
- the northern nailtail wallaby2 eats herbs rather than grass and is associated mainly with the edges of Mitchell grassland;
- hare-wallabies3 are largely solitary, have slow metabolism and occur in sparse populations4; and
- the long-haired rat5 is scarce except during occasional irruptions, and is potentially omnivorous rather than being specialized to eat grass.
Compared with all these mammals, the plumed whistling duck6 seems well-suited to grazing Mitchell grassland for the reasons listed below:
- unexpectedly for a duck its staple diet is the soft parts of dryland grasses, with even its hatchlings eating herbaceous greens;
- during the hot, wet season it grazes at night and avoids the heat by commuting to scattered pools for the daylight hours, allowing a deceptively light, mobile herbivore to cover the length and breadth of Mitchell grassland;
- it would be superior to marsupials in tolerating heat because of insulating feathers and a body temperature up to six degrees higher than in kangaroos; and
its webbed feet and flight give it an advantage during the wet season in escaping predators on the slippery clay.
All these factors suggest that the extreme environment of Mitchell grassland has shaped the little-known plumed whistling duck into the closest counterpart to the conspicuous hoofed grazers of prairies on other continents.”
1 Megaleia rufa
2 Onychogalea unguifera
3 Lagorchestes
4 It is also unknown whether hare-wallabies were common in Mitchell grassland in the first place.
5 Rattus vilosissimus
6 Dendrocygna eytoni
A grazing bird under the marsupial radar
Mitchell grassland, the most extensive1 treeless grassland in Australia, is surprisingly poor in indigenous grazers. It is safe to assume that grazing marsupials were formerly common in this area (see the accompanying map) but have become extinct in the last tens of thousands of years.
Ducks have flown under the radar as important grazers in Mitchell grassland because they are usually assumed to be aquatic omnivores rather than terrestrial grazers. What sets the plumed whistling duck2 (pictured here) apart from related ducks is a staple diet consisting of the soft parts of dryland grasses, with even the hatchlings eating herbaceous greens. The specialised diet of this particular duck3 makes it competitively superior to marsupials4 as a nomadic grazer of a semi-arid grassland subject to occasional flooding after cyclonic rains.
1 covering an area the size of Britain
2 Dendrocygna eytoni
3 a species restricted to Australia
Lawnmowing by waterfowl instead of kangaroos.
Subtitle: Mitchell grassland is Australia’s most extensive treeless grassland on relatively fertile clay soils, yet lacked mammalian grazers. This role was instead taken by the plumed whistling duck, a waterfowl species peculiar to Australia.
The main grazer in mitchell grassland, the most extensive treeless grassland in Australia, can safely be assumed to have been an extinct species of marsupial although it remains to be seen which one. It goes without saying that waterfowl are unlikely grazers in such dry environments because they’re aquatic omnivores.
Marsupials have always been more vulnerable than birds to dehydration in mitchell grassland4 because droughts and floods are so severe. The red kangaroo (Megaleia rufa), for example, failed to act as the main grazer in these grasslands because it prefers woodlands in northern Australia and avoids treeless vegetation. The northern nailtail wallaby (Onychogalea unguifera) eats herbs rather than the grass preferred by kangaroos, and is typical of only the edges of mitchell grassland. And hare-wallabies (Lagorchestes) have become rare since Europeans introduced predators from other continents, which suggests that it was probably not a significant grazer of the mitchell grassland5. However, the plumed whistling duck (Dendrocygna eytoni) seems well-suited to grazing in mitchell grassland, compared with all these mammals. Part of its comparative advantage is that its staple diet is the soft parts of dryland grasses, with even its hatchlings eating herbaceous greens. During the hot, wet season, the plumed whistling duck grazes at night and avoids heat stress by commuting to scattered pools for the daylight hours, allowing this deceptively light herbivore the mobility required to cover the length and breadth of mitchell grassland. This duck would also tolerate heat better than competing marsupials because of insulating feathers and a body temperature up to six degrees higher than in kangaroos. And its webbed feet and flight gives it an extra advantage during the wet season in escaping predators on the slippery clay soils. All these factors suggest that the extreme environment of mitchell grassland have made the little-known plumed whistling duck the closest counterpart to the conspicuous hoofed grazers of prairies on other continents.
Is it possible that hare-wallabies had some sort of mutualism going (like wildebeest/zebra)? Perhaps the duck and the hare-wallaby facilitated each other in some way – either improving grazing or looking out for predators? Or perhaps the hare-wallaby fed around the pools, and the duck fed in the grasslands far away from the pools? i.e. a niche separation. Just brainstorming… We’d have to check whether any hare-wallaby has ever actually been recorded in mitchell grassland. As I recall, distribution records for the whole genus are sparse and I only mentioned them out of thoroughness because their general distribution is wide enough that it brackets mitchell grassland and this genus is elsewhere present in treeless grasslands of various kinds. Any mutualism between hare-wallaby and duck sounds far-fetched. Probably the only way now that we could get any idea of whether mitchell grassland was part of hare-wallaby habitat would be to attempt reintroduction in a mitchell grassland reserve free of livestock, if such a thing exists. So we have the uncomfortable situation that we can’t disregard hare-wallabies as a possible grazer but they are nebulous both in fact and in interpretation]
Main logical flaw is still our dismissal of hare-wallabies (which I stand by) without clear reasons (we need insight into the niches of these marsupials compared with the niche of the duck, even if this is only a clear hypothesis. Do the surviving hare-wallabies perhaps prefer areas near surface water? The big questions in my mind: how often do they need to drink and how far apart are pools in the mitchell grasslands? Unfortunately there are no interns to follow up on this for us right now. There is a literature on hare-wallabies owing to reintroduction attempts, but the quickest way would be to write to the people involved; I doubt that hare-wallabies depend on drinking water but then again I also doubt their capacity to reach population densities great enough to make them significant grazers. A significant grazer wouldn’t be extirpated from the landscape so easily. I suggest that we make this last point in green, but note that this warrants further research.
Title: Ultimately benign biological invasions.
1 initially brightly-coloured capsules which dry and split instead of ripening
2 i.e. deceptive mimicry
3 Extracts of Zanthoxylum also control parasites such as protozoans (which cause leishmaniasis and trypanosomiasis) and helminth worms.
4 covering an area the size of Britain
5 A significant grazer – i.e. which occurred in large numbers – would be expected to be resilient from predation, as is evident in certain kangaroo species [can we provide some details on examples?]
Lawnmowing waterfowl instead of kangaroos.
The main grazer in mitchell grassland was an unknown, extinct species of marsupial. Whistling ducks are unlikely grazers here because they’re aquatic omnivores.
Marsupials have always been more vulnerable than birds to dehydration in mitchell grassland because droughts and floods are so severe. The red kangaroo1, for example, was not the main grazer in these grasslands because it prefers woodlands in northern Australia and avoids treeless vegetation. The northern nailtail wallaby2 eats herbs rather than the grass preferred by kangaroos, and is typical of the edges of mitchell grassland. And hare-wallabies3 have become rare since Europeans introduced predators from other continents, highlighting their poor resilience from climatic extremes in the face of additional pressures4. The plumed whistling duck5 seems well-suited to using mitchell grassland, compared with all these mammals6. Part of its comparative advantage is that its staple diet is the soft parts of dryland grasses, with even its hatchlings eating herbaceous greens. During the hot, wet season, the plumed whistling duck grazes at night and avoids heat stress by commuting to scattered pools for the daylight hours, allowing a deceptively light, mobile herbivore to cover the length and breadth of mitchell grassland. This duck would also tolerate heat better than competing marsupials because of insulating feathers and a body temperature up to six degrees higher than in kangaroos. And its webbed feet and flight gives it an advantage during the wet season in escaping predators on the slippery clay soils. All these factors suggest that the extreme environment of mitchell grassland have made the little-known plumed whistling duck the closest counterpart to the conspicuous hoofed grazers of prairies on other continents.”7
1 Megaleia rufa
2 Onychogalea unguifera
3 Lagorchestes
4 It is also unknown whether they were common in mitchell grassland in the first place.
5 Dendrocygna eytoni
6 [Anthony, we should revise the last few sentences after thinking through more objectively the case against hare-wallabies. OK, Antoni. Is it possible that hare-wallabies had some sort of mutualism going (like wildebeest/zebra)? Perhaps the duck and the hare-wallaby facilitated each other in some way – either improving grazing or looking out for predators? Or perhaps the hare-wallaby fed around the pools, and the duck fed in the grasslands far away from the pools? i.e. a niche separation. Just brainstorming… ]
7 Main logical flaw is still our dismissal of hare-wallabies (which I stand by) without clear reasons (we need insight into the niches of these marsupials compared with the niche of the duck, even if this is only a clear hypothesis. Do the surviving hare-wallabies perhaps prefer areas near surface water? The big questions in my mind: how often do they need to drink and how far apart are pools in the mitchell grasslands?
(writing in progress)
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