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closeComment from Chris Johnson
Posted by chrisjohnson on 17 Jul 2012 at 01:00 GMT
This paper makes a useful contribution to knowledge of the diet of the dingo. On that score, it is a great paper. It also includes a discussion of the possible impacts of dingo predation on small mammals. Here, there are serious problems.
Allen and Leung show that dingoes occasionally prey on hopping mice, which appear in up to 8% of dingo scats. They then argue that this low rate of predation could potentially eliminate hopping mouse populations. This argument uses the following chain of reasoning:
1. Allen and Leung assume that dingoes prey on hopping mice at a constant rate, so that the maximum incidence of predation recorded in their study (8% of dingo scats containing hopping mice) would continue to apply as hopping mice declined in abundance. This is biologically implausible.
2. They further assume that all dingoes that eat hopping mice eat nothing else, so each mouse-eating dingo must eat lots of mice (i.e. 20 mice/dingo/day). This is also biologically implausible, and not supported by Allen and Leung’s own data.
3. Then Allen and Leung apply a predation rate estimated from assumptions 1 and 2 to a very low-density hopping mouse population. They assume further that there is zero reproduction in this population, so it is unable to replace animals removed by predation. Inevitably, the hopping mouse population goes extinct.
As an argument for a significant effect of dingo predation on the demography of hopping mice, this is an extremely shaky flight of speculation.
I don’t object to speculation in scientific papers. Allen and Leung develop this argument in the Discussion section of their paper, which is where authors ought to feel free to indulge themselves a bit. I do, however, object to such speculation being elevated to the status of a firm conclusion in the abstract of the paper, which is what Allen and Leung do when they state “… dingo predation alone has the potential to depopulate local hopping-mice populations within a few months”.
Readers of this abstract, be warned: do not assume that what the authors say here is supported by evidence that they present in the paper.
RE: Comment from Chris Johnson
BenAllen replied to chrisjohnson on 19 Jul 2012 at 07:34 GMT
The research and management of Australian dingoes is highly controversial and hotly debated. And although we know a fair amount about dingoes, there is still much to learn.
We're grateful that Chris Johnson (CJ) recognises the valuable contribution our study makes to knowledge of dingo diets. Given that we report the results of the second largest dingo diet study ever conducted, we hoped this would be the case. But we are a little concerned about CJs misrepresentation of our work and the distortion of statements we actually made. Such practices are not helpful to anyone.
In our paper, we supplement data on dingo diets with other information to assess and discuss the potential risks of a generalist canid predator on a couple of threatened rodents which feature in their diet. We use correlative analyses and explore some hypothetical scenarios to do this. We make several assumptions during this exercise, the reliability of which are discussed at length in the paper. CJ has mentioned only a few, and has misunderstood some of these.
Regarding the first point CJ raised, of course predation rate will change as the prey population declines. To avoid repeating ourselves however, we refer readers to Page 6 for a discussion of this assumption.
Regarding the second point CJ raised, there are many studies (including, but not limited to ours) which show small rodents to be a major feature of dingo diets at times - especially in the arid areas where we worked. We also showed from our diet data that a substantial proportion of scats contained only rodent hair. Considering the energy requirements of dingoes, we thought it reasonable to assume (in line with other studies) that dingoes can persist solely on rodents at times, and are by no means forced to consume larger prey in order to survive.
The third point CJ raised is incorrect. Nowhere do we state, assume or imply that there is zero reproduction in low-density rodent populations. We did say (on Page 6) that deteriorating environmental conditions (such as drought) limit the breeding success of rodents. This phenomenon is well-known to arid zone ecologists who understand that many small mammal populations decline during drought times - hopping-mice included. If breeding success didn't decline during these times, rodent populations would remain stable or increase under such conditions. Now this would be biologically implausible.
Perhaps the most concerning of CJs comments is the truncation of a sentence to misleadingly change its meaning. For those who read the whole Abstract and not just CJs comments, they will note that the full sentence that CJ objected to reads: "Assessing the hypothetical predation rates of dingoes on dusky hopping-mice revealed that dingo predation alone has the potential to depopulate local hopping-mice populations within a few months." For those still wondering whether or not statements in our Abstract are supported by the data in our paper, we encourage such to familiarise themselves with the meanings of the words 'hypothetical' and 'potential'.
We hope that people read all of our paper, and not just the Abstract, and especially not just part-sentences. Readers who do the latter are unlikley to glean any useful information from our work.
For such a controversial topic, we would warn readers not trust misleading comments, but to instead spend their time evaluating the merits of the results presented and the methods which generated them. For that you will have to read the paper.