Sheila Newman: Sociologist and demographer
The literature indicates an understanding among scientists researching in the industry, that these animals are artificially kept by hunting at greater numbers than they would be naturally, in order to both to offset the impact of commercial hunting and to keep it going. The numbers are being manipulated upwards, then the alleged spontaneous overcrowding is being used to justify arguments for the need for a commercial industry and damage mitigation culling. If the calculations of the program of artificial ‘management’ of kangaroo populations are correct in estimating kangaroo populations too numerous, then the management amounts to a form of farming, which is preventing kangaroos from exercising normal patterns of social organisation that would limit fertility opportunities.
“Caughley (1976, 1977) outlined the principles of wildlife harvesting. To harvest a sustained yield from a population at steady density, it must first be manipulated in some way to promote its rate of increase (e.g. reduce it below its ecological carrying capacity or supplement its resources). The second of the four theoretical principles he identified was that: “Harvesting theory rests upon populations being regulated by some combination of density-dependent reproduction and mortality. This has been described well for a number of large mammals (Fowler 1987). Harvest mortality is seen as being compensated to some extent by lowered natural mortality rates and increased fecundity rates.” (Caughley (1976, 1977) cited by Tony Pople and Gordon Grigg, “Commercial harvesting of Kangaroos in Australia,” Department of Zoology, The University of Queensland, for Environment Australia, August 1999, p.5).
“Even as a pest, however, kangaroos are still a resource, and they can only be that while they remain numerous, a necessity which puts an economic value on their conservation.” (Tony Pople and Gordon Grigg , Commercial harvesting of Kangaroos in Australia, Department of Zoology, The University of Queensland, for Environment Australia, August 1999, pp.38-39)
If my impression is correct, that kangaroo numbers are being manipulated upwards by commercial management practices, then my observations on how these numbers painlessly reduce themselves is more likely to serve the kangaroo conservation and tourism concerns. According to predator-prey theory, predators adapt to prey numbers. Would the kangaroo harvesting industry consider this adaptation, rather than continuing to farm? Like so many industries, it seems to want more growth, so more kangaroos and more territory: The South Australian Commercial Kangaroo Management Plan 2018-2022 indicates a desire to increase the commercial kangaroo meat catchment area by formally extending it and by getting more farmers to work for it by shooting kangaroos on their property, through a DEWNR regional staff program.
The population model and assumptions in the South Australian Commercial Kangaroo Management Plan 2018-2022 seem to have a number of theoretical flaws and biases, some of which at least have been pointed out in recent research.
– There seems to be an underlying assumption in the population model used (exclusively predator/prey biomass dependent) that all the kangaroo species under consideration breed to the maximum (W.D. Hamilton (1964)) unless prevented from doing so by natural catastrophe, big predators and human culls for commercial or other reasons. Whilst Hamilton’s rules for inclusive fitness and Dawkins’ theory on selfish genes are very important, they should not cause a failure to investigate countervailing forces to genes seeking maximum reproduction.
Countervailing forces to reproductive urges exist in the subtleties of endogamy (breeding within a local population) and exogamy (incest avoidance) which also limit fertility opportunities, including, in some species, sexual maturation. For instance, young male kangaroos need to leave their natal group because there is only room for one dominant male breeder and he monopolises all the females unless another mature male successfully challenges him. The dispersing young male may not find a group that has available females or he may fail to successfully compete for them. Aspects of cooperative breeding may be present and delay sexual maturation in both males and females. This would explain some variations in breeding ages in different populations and environmental circumstances. These should limit fertility opportunities in a natural ecology, i.e. one not impacted by the commercial management considerations noted above.
My own theory is that undisturbed local populations respond to environmental cues like rainfall and soil richness by a variation in genetic algorithms for fertility. Sexual maturation and pairing is limited by the availability of territory. In humans these algorithms translate into kinship and marriage rules that vary in degree according to rainfall and other environmental cues.
But we have abundant examples demonstrating that, because of hunting and culling, more fertility opportunities are being created for younger, smaller animals.
“Commercial harvesting may affect the demography (e.g. size, growth, distribution and birth and death rates) of harvested kangaroo populations by selecting the larger kangaroos, which tend to be the older males (Allendorf et al. 2008). Commercially harvested populations may have a lower average age compared to that of unharvested populations. The average size of kangaroos in harvested populations may be lower, and populations contain a higher proportion of young animals than unharvested populations, but these differences are lessened during drought when older animals are lost from unharvested populations (Pople 1996).The sex bias (i.e. the percentage of harvested kangaroos that are male) has increased from 60-70% male to 92-97% male for red and western grey kangaroos (DEWNR 2017). The increase in sex-bias is due, in part, to some meat processing plants only accepting male carcasses. The sex bias of the euro harvest has historically been higher due to the small size of female euros, but the sex-bias has also increased from 75-95% male to 99% male (DEWNR 2017).” (South Australian Commercial Kangaroo Management Plan 2018-2022, p.25.)
Massive bias for shooting large males leaves remaining, smaller males, without the suppression of sexual maturation (possibly) or sexual behaviour (definitely) that those large competitive mature males would have caused. The remaining smaller males do not need to disperse from their natal group to breed. The natal groups may be so fragmented that most adult relatives have disappeared and the remaining young does and bucks may lack the normal incest avoidance due to disturbance of Westermarck relationships. It is also possible that the presence of mature related males and females may delay sexual maturity or behaviour in female kangaroos, so that loss of big males and females would then also favour early breeding in young does.
And the industry knows this:
“To harvest a sustained yield from a population at steady density, it must first be manipulated in some way to promote its rate of increase (e.g. reduce it below its ecological carrying capacity or supplement its resources).” (Caughley (1976, 1977) cited by Tony Pople and Gordon Grigg (1999))
If there were really a desire to achieve smaller kangaroo populations naturally, then a number of things could be tried. Assuming that the disorganisation caused by hunting and culling is responsible for managing the population numbers upwards, we might create reliably safe areas and corridors local to various kangaroo clans in the harvesting area so that they could adapt to protect dependent young and females, encouraging the Westermarck effect, normalisation of patterns of incest avoidance and dispersal, weight, rate of maturity, sexual competition and fertility response to environment. If this succeeded it would reduce both the perceived need and the opportunity for the commercial industry as well as damaging the kangaroo’s reputation for overpopulation. These refugia could also provide a safe and permanent environment away from the massive human population expansion and landscape transformation which also uproots, scatters and kills kangaroos.
More demographic Brakes: Sexual Segregation/Gender pathways
Some other variations in population organisation can affect fertility opportunities. Examples include separate gender pathways, with ‘sexual segregation’ where male and female populations live apart.
“Sexual segregation is a phenomenon seen in many species, with segregation occurring along behavioural or ecological dimensions. Sexual segregation in western grey and red kangaroos in semi – arid Victoria has been the subject of intensive investigations since the last review.
[…] MacFarlane and Coulson (2005) investigated the effects of mating activity, group […] composition, spatial distribution and habitat selection on sexual segregation in western grey and red kangaroos. The synchrony and timing of mating activity was seen to influence the magnitude and timing of social segregation in these species, with mixed sex groups predominating during the breeding season. …
… Spatial segregation and habitat segregation were also seen. Although the magnitude of these types of segregation were weaker, they were both still significantly influenced by synchrony and timing of breeding.
Coulson et al. (2006) discussed sexual segregation at three levels (habitat, social and dietary) and confirmed that both size and sex influence segregation.
MacFarlane and Coulson (2009) showed that the need for males to maintain contact with other males (perhaps to develop important fighting skills, evaluate rivals and establish a dominance hierarchy) might also promote sexual segregation.
Similarly Nave (2002) reported evidence of sexual segregation in eastern grey kangaroos in Victoria.” (Review of Scientific Literature Relevant to the Commercial Harvest Management of Kangaroos http://www.environment.nsw.gov.au/resources/nature/110641Kangaroolitreview.pdf)
What are the consequences of loss of sex-specific territory?
Years ago, Glen Marshall, who was a teacher and missionary in PNG between 1960 and 1974 told me that fertility shot up when churches convinced men and women to cohabit, where previously they had separate land and houses. I was later able to study this concept in detail and wrote a book about how Pacific Islander land-tenure and inheritance traditions kept populations within the limits of small islands.
What effect could reduction of habitat, forced cohabitation, forcibly changed migration routes and wiped out populations have on male/female kangaroo territory and consequently on fertility opportunities?
How do we know that the female bias (recorded by Fletcher, 2006) at Tinbinburra, for instance, is not due to that area being female territory?
Female elder kangaroos
Daughters seem to learn from their mothers to look after joeys. Where female kangaroos are early orphaned their parenting skill may increase risks in joey upbringing. The extraordinary rates of joey mortality may have something to do with this. (See Faces in the Mob for a study of success and failure in raising joeys in one mob.)
How far do kangaroos migrate?
Another aspect of population theory is migration. The range of kangaroo movement is probably greatly underestimated in calculations that do not take more recent MT DNA studies into consideration, such as Zenger et al DNA study 2003. Zenga et al look at Eastern kangaroo populations, but I note that South Australia uses the NSW model anyway. Seeing as the South Australian Commercial Kangaroo Management plan encourages research, I would suggest it undertake MT DNA studies similar to Zenger et al’s for South Australian populations. Underestimation of real geographical range risks skewing the estimation of population numbers by confusing seasonal or reactive population movement with permanent populations: Pople et al (2007) acknowledged that temporal and spatial kangaroo population movement had been ignored in the models and it does not seem to be taken into consideration in the South Australian Management Plan yet. Pople et al did not, however, consider the discrete dynamics within local populations and their interactions with other populations within the overall area. They were looking at a commercial hunting model seeking to predict when and where populations would be grouped together and easier to harvest economically.
Effectively the counting method and population model in the South Australian Management plan seems to assume an undifferentiated ‘metapopulation’ and to ignore the local populations that actually make up that metapopulation and which have their own local characteristics of endogamy, exogamy, dispersal and philopatrie) within that metapopulation.
“A metapopulation is a population of populations (Hanski & Gilpin 1991). Wright (1940) laid the groundwork for a genetic theory of metapopulations, while Andrewartha & Birch (1954, Ch.14) did the same for metapopulation dynamics: ‘A natural population occupying any considerable area will be made up of a number of local populations or colonies. In different localities the trend may be going in different directions at the same time.’ They emphasized the influence of dispersal on the number of patches occupied at any given time.” (Caughley, Directions in Conservation Biology, Journal of Animal Ecology, Vol. 63, No. 2 (Apr., 1994), pp. 215-244, p. 221)
ENVIRONMENTAL DAMAGE MITIGATION & ROO DENSITY: THEORY & EXPERIMENT
Goal: […] to provide an alternative management option for reducing the damage to land condition caused byoverabundant kangaroos. (The South Australian Commercial Kangaroo Management Plan 2018-2022, p.4.)
The South Australian Commercial Kangaroo Management Plan 2018-2022 seems to rely, as mentioned above, on a predator/prey, biomass dependent population theory, and to cite a small group of scientists who have apparently confirmed this theory time and again, describing how kangaroos wear down grasslands and then starve to death. One scientist not cited is ACT Ecologist Donald Fletcher, whose extensive field research failed to confirm the theory. The ACT Kangaroo Management model relies on the same literature as the South Australian Commercial Kangaroo Management Plan, and is one of high fertility sedentary populations that rarely migrate, grazing grasslands down to the subsoil. But, in his 2006 thesis, p. 237, Senior ACT Kangaroo management ecologist, Donald Fletcher, tested this model and found, to his surprise, that,
“The study did not provide evidence that high densities of kangaroos reduce groundcover to the levels where erosion can accelerate.
Unmanaged kangaroo populations did not necessarily result in low levels of ground cover. Groundcover had a positive but not significant relationship to kangaroo density, with the highest cover at the wettest site where kangaroo density was highest. Weather has an important influence on groundcover.”
“The results from the study as a whole indicate that unmanaged kangaroo populations did not necessarily result in unacceptably low levels of ground cover.” (Fletcher Phd: p.231.)