by Tony Pople and Gordon Grigg
Department of Zoology, The University of Queensland
for Environment Australia, August 1999
Chapters 10,11,12 and 13 and Appendix 1 provided by staff at Environment Australia
KANGAROOS AS PESTS, KANGAROOS AS A RESOURCE
In the management of kangaroos, their multiple status as pest, resource and national symbol are all tightly interwoven. The main reason an industry is approved is almost certainly because of the extent to which kangaroos are regarded as a pest; and their commercialisation has provided a self-funding pest control agent. Had kangaroos had no commercial value, pest control would have been a direct and continuing cost to graziers. On the other hand, their status as national symbol, with a very high conservation value, has guaranteed sufficient public interest for government conservation agencies to implement extensive monitoring and regulatory procedures which ensure that annual harvests are conservative.
The present situation is a compromise which has been responsive primarily to the opposing demands for much greater pest control on one hand and complete protection on the other.
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. More will be said later about the interactions between these conflicting values ( Chapter 8).
Kangaroos as a resource
Apart from their intrinsic value, kangaroos have instrumental value for their tourism appeal, for their meat and for their hides. Ramsay (1994) has provided a comprehensive quantitative review of the extent of the export of kangaroo meat and hides and its history. Another source, though somewhat outdated, is Kirkpatrick and Amos (1985).
The value of kangaroos to Australia's tourism industry is probably impossible to quantify and is, anyway, beyond the scope of this review. Suffice to say that the value would be very high, putting a premium on the maintenance of sufficient kangaroos for them to be highly visible in the Australian landscape.
Ramsay (1994) reviewed the value and quantities of the export trade in hides and meat, recording a period of comparative stability through the 1980s, with annual values of exported products in the vicinity of $10-15 million dollars, rising rapidly to about $30 million by 1992. The total value of the kangaroo industry to Australia is hard to estimate but is likely to be in the vicinity of $100 million.
Kangaroo hides and leather
Kirkpatrick and Amos (1985) described the trade in skins as the backbone of the industry, and it remains so. Until the late 1980s most of the exported product was in the form of pickled skins. Since then, however, there has been a very significant increase in the proportion exported as leather.
Kangaroo leather is recognised as a very valuable commodity. It has a particularly high strength to weight ratio and is very fine and pliable. It is particularly good for sports and other shoes, belts, gloves, leather clothing and footballs, and its strong place in the world leather industry is well deserved. Red kangaroo furred hides are sometimes used in fashion garments, while others may end up as floor rugs and souvenirs.
The value of leather exports has increased greatly in recent years as the superior quality of the product for particular high class applications has become more widely recognised. Additionally, local production capacity and efficiency has increased, and Australia has a significant and specialised kangaroo leather tanning industry. Twenty-one countries imported kangaroo leather in 1991-92 (Ramsay 1994). Although the trend in prices has been upwards, skin prices rise and fall according to demand and there is increased competition when prices rise from goat and calf leather. Because manufacturers can turn to lesser quality but cheaper products when prices are high, the capacity for price rise is always limited.
The situation with meat may be different, because perceptions about the value of the meat is undergoing change, with the realisation that the meat is not only a very good game meat but also has nutritionally desirable attributes (O'Dea 1988). Grigg (1988, 1997) has argued that the qualities of kangaroo meat, coupled with its comparatively short supply (limited ultimately by quotas, see Chapter 4) give it a potential to achieve high prices on the world market as a speciality game meat ( Chapter 8). Whether or not this is possible remains to be seen.
Use of meat for human consumption, extent of the resource
Despite the quality of the meat, more than half of the kangaroos shot commercially in Australia each year are used for their hides only, so meat production could be increased without more kangaroos being killed, or quotas being raised. Queensland and Western Australia are the only States which approve shooting only for skins. New South Wales and South Australia permit carcass shooting only. All of the States, in their Management Programs, recognise a need to maximise usage of the shot animals, but in Queensland and Western Australia large distances between the harvest areas and markets, coupled with higher temperatures and lower human population densities, present particular difficulties and militate against implementation of similar regulations in those States, unless prices should rise significantly.
Although the use of kangaroo meat for human consumption has been legal in South Australia since 1980, in other States the sale of kangaroo meat other than for pet food was not permitted until 1993, when it was legalised in New South Wales. Simultaneously, mutual recognition legislation came into force, under which products allowable for sale in one State cannot be disallowed in another. This made the sale of kangaroo meat in all States possible from that time, although there are still some legal anomalies.
Since 1993, kangaroo meat has shown considerable penetration into the restaurant market within Australia, particularly, but not only, in tourist destinations. It has become a routine component of the menu in many top restaurants and enjoys a particular focus in restaurants which concentrate on 'bush tucker'.
The reasons that kangaroo meat was prohibited for human consumption for such a long time are not straightforward but an important element was a fear by producers of traditional red meats that kangaroo would become a significant competitor for beef and lamb. This fear is unreasonable, as shown by figures from Switala (1995) who calculated that approximately 55,000 tonnes of kangaroo meat (excluding wallaby) would have been available if the 1993 quota had been taken fully. In the same year, about 2.8 million tonnes of beef, veal, mutton, lamb and pork were used in Australia. Similar results would be obtained for comparisons made in years since then. That is to say, even if all of the kangaroos shot under the commercial quotas each year were used for human consumption, this would be an almost insignificant fraction of Australia's annual red meat production, less than 2%.
This finding was part of the most comprehensive review of the potential supply and potential value of kangaroo meat so far undertaken. Switala (1995) took into account the typical sex ratio of harvests in each of the States, the meat yield from carcasses and the 1993 quotas and calculated the availability of 55,000 tonnes of meat, of which 80% could be supplied by Queensland (approx. 26,000) and New South Wales (approx. 19,000) together. At present, only an insignificant amount of that meat production is going for human use, and most of it is staying in the paddocks where the animals are shot.
Taking retail prices of $3-6 per kilogram, Switala calculated that, at 100% of the quota utilised, this represented a meat industry with a potential value of $171-341 million. He concluded that a quota utilisation of 40% and an average retail price of $5/kg were reasonable in the near future, implying a meat industry worth about $100 million annually.
Kangaroo meat is so far almost unrecognised as a significant game meat outside Australia, although there has been an export trade in game meat for human consumption since 1955 (Macfarlane 1971; Corrigan 1988). This was suspended for about 10 years during the 1970s because of a Federal ban on the export of kangaroo products and a ban on their import by the U.S. Government (Shepherd and Caughley 1987). These bans reflected concern, now resolved ( Chapter 4), about the extent to which the harvests may have been impacting adversely upon kangaroo populations. The amount of information now available about the population ecology of kangaroos and their abundance has shifted the focus of opposition by animal rights groups and some conservation groups instead upon animal welfare issues (see Chapter 9). Indeed a large supermarket chain in the U.K. has very recently decided to discontinue stocking kangaroo meat as a result of opposition from animal rights groups.
Nutritional and public health issues
Apart from the conservation plusses which attend the use of kangaroo products, taken in a regulated and humane way (See Chapter 9), the meat itself has attributes which make it attractive. Typically, kangaroo meat contains less than 2% fat, about 40% of which is polyunsaturated (Sinclair 1988). Nutritional benefits of kangaroo meat for people advised to follow cholesterol-lowering diets were discussed by O'Dea (1988) who conducted research which showed that both Aboriginal Australians and Australians of European origin had 19-24% lower plasma cholesterol levels following 2 weeks on a diet containing 500g/day of kangaroo meat. Actual cholesterol intake remained the same throughout the test- diet periods as in the control periods before and after, and the reduction was attributed to the high proportion of polyunsaturated lipids in kangaroo meat. Additionally, the kangaroo diet led to a rise in the plasma levels of the polyunsaturated fatty acid arachidonic acid, which is high in the meat, and which has a positive effect on blood flow. O'Dea and her colleagues concluded that their results could 'explain, at least in part, how the diet consumed traditionally by Aborigines in northern Australia protected against diseases of the cardiovascular system: its particularly low content of saturated fat (O'Dea 1984; O'Dea and Sinclair 1985) helped to keep cholesterol at a low level even when the diet was rich in cholesterol from organ meats and eggs; and its relatively high content of arachidonic acid derived from lean meats such as kangaroo and some tropical fish may have reduced the risk of thrombosis by favouring the production of the arachadonic acid-derived prostanoid, prostacyclin'. This has been recognised in the form of advice by some cardiac specialists to their patients.
The bottom line from a nutritional point of view, according to O'Dea (1988) seems to be that kangaroo meat provides 'a consistently very lean red meat which can be included with confidence in a cholesterol-lowering diet'. She advocated that it was 'important that kangaroo meat be made generally available throughout Australia', a view which is unfortunately inconsistent with its comparatively limited supply and with the notion of it becoming sold as a speciality meat at a much higher price, with the ecological benefits which that could bring (see Chapter 8).
O'Dea advocated further that because these desirable nutritional attributes may not be obtained in farmed individuals, it was important for meat to come from free-range kangaroos, in their wild, undomesticated state.
Apart from nutritional qualities, taste and texture are very relevant to acceptability. These attributes are hard to quantify but the extent of its penetration into the restaurant trade since 1993 sends a clear positive signal about that. Its taste and texture may be said to be similar to beef, but sufficiently different to be interesting. Cooked correctly (grilled, medium rare), it is tender and, because of the low fat content, is usually served with a light sauce, often from native fruits or berries.
Public health issues are often raised in the context of the acceptability of kangaroo meat. Claims are often made by people who are opposed to kangaroos being exploited commercially that the meat is unfit because of parasites or pathogens.
It is often claimed that kangaroo meat is a particular risk for toxoplasmosis and salmonellosis. Certainly, like other meat, kangaroo can become infected with Salmonella, but it is not more prone than other meats, and the usual hygiene standards and practices are required. As far as Toxoplasma is concerned, if kangaroo meat, or other red meat is contaminated with cat faeces, it can transmit toxoplasmosis. Hygienic handling prevents this occurring. Because the preferred way to eat kangaroo meat is rare rather than well cooked, contaminated meat would be less likely to be neutralised. However, although there has been a fuss raised from time-to-time, there is no known case of toxoplasmosis being transmitted by eating kangaroo.
Andrew (1988) reviewed the issue of kangaroo meat and public health, including the records of inspections between 1980 and 1987 made of carcasses by Australian Quarantine Inspection Service officers at export game meat establishments (this pre-dated the change of legislation in New South Wales in 1993). There were records for 204,052 red, eastern and western grey carcasses of which 196,104 were passed as fit for human consumption. Of the 7,948 rejected, 81% were rejected for reasons not associated with parasites or pathology, mainly poor handling, particularly inadequate refrigeration. Of the rest, only 1,452 were rejected because of a parasite, and that was for a nematode, Pelicitus roemeri, which is quite harmless, anyway, to humans, but is unsightly.... it is uncommon, but can infect the muscles of the lower leg.
Andrews' conclusion was '.... that provided harvesting, transportation, refrigeration, inspection and distribution are carried out in accordance with established criteria, kangaroo presents little or no danger to human health when compared to other forms of meat and that there are no public reasons why it could not be considered as a viable alternate to meat from domestic animals'.
Interestingly, with greater awareness of animal welfare issues, kangaroo meat is the meat of choice for some people who prefer not to eat meat from traditional livestock, including chicken, because of the way many of them are housed during life (feed-lots or other high-density farming) and transported, handled and slaughtered. Many see kangaroo harvesting as an analogue of free- range chickens and free-range pigs, with the swift slaughter of free- range kangaroos, unaware of danger and having lived under natural conditions, as preferable to the way traditional livestock is handled.
Kangaroos as pests
The reasons behind the pest status of the most abundant macropod species are
- damage to rangeland and crops
- damage to fences
- competition with stock for drinking water in droughts
- damage and injury resulting from collisions between kangaroos and vehicles
The last three are less controversial than the first.
We will concentrate on the damage to rangelands, because it is in the rangelands that most commercial harvesting occurs ( Figure 14) and where pastoralists look towards the kangaroo industry as a source of pest control.
There is no doubt that kangaroos compete with domestic livestock for food and water (Shepherd and Caughley 1987) and, although the extent of this damage and the strength and nature of competition are very difficult to quantify, there is no doubt that kangaroos are clearly perceived as pests by the bulk of primary producers whose land they share (Gibson and Young 1988). The main concern is that they consume pasture and, when this is scarce, the concern is heightened. It is not easy to put a dollar figure on the costs, but the Gibson and Young study found that individual landholder's perceptions of their losses to kangaroos, totalled, came to $A113 million, 3% of the gross agricultural production in the areas under the national kangaroo management program. This was broken down into 51% through the cost of fodder eaten by kangaroos, 27% lost crop production, 14% fence repairs and 8% through the cost of water consumed. This was based on a survey of 906 farmers and graziers in five pastoral zones across Australia. It was based on their perceptions. While the survey was based solely on the perceptions of farmers and graziers, it is nonetheless a measure of the extent to which kangaroos are seen as pests.
It is, however, a measure of the extent to which kangaroos are seen as pests.
As a result of this perception, commercial harvesting is encouraged by graziers throughout the sheep rangelands and this is supplemented by killing animals solely for damage mitigation. This may be done legally by shooting animals on pest destruction permits, but also includes illegal methods such as poisoning, clearance of sheltering vegetation and shooting outside the guidelines of the Code of Practice for the humane shooting of kangaroos. In the past, animals were shot en masse after being herded in organised drives or battues (Kirkpatrick and Amos 1985; Barker and Caughley 1992).
Much of Australia's rangelands are degraded as a result of the introduction of domestic grazing animals with the coming of Europeans. This has been recognised for nearly a century. Indeed, a Royal Commission into the condition of lands in western New South Wales, called in response to perceptions of a developing crisis, reported extensive land degradation in 1901 (for an interesting summary of its findings, see Lunney 1994). Estimates of the geographical extent of the rangelands which is degraded vary. Newman and Condon (1969) estimated 1.85 million km2. Woods (1984) estimated 2.18 million km2, and concluded that much of the substantially degraded land will become desertified if landuse and management do not change.
It is worth pointing out, in this context, that, according to the Australian Bureau of Statistics (ABS) 1990 yearbook , the area of utilisation of land for grazing in the 1980s ranged from 424 to 440 million hectares. The numbers of sheep grazed in Australia increased more than sevenfold in the 100 years after 1860, and averaged approximately 150 million over the next 20 years. Sheep numbers peaked at 180 million in 1970. In contrast, kangaroo numbers in the whole of Australia are likely to be in the vicinity of 25-35 million.
There is now a strong recognition that land management practices must change, and graziers and government agencies have identified the management of 'total grazing pressure' as a very high priority, particularly in the sheep rangelands.
Options available for the rehabilitation of sheep rangelands include:
- changing the stocking rate
- altering the season of use
- resting or 'spelling' regimes
- culturing and seeding treatments
Kangaroos are seen as being inimical to all of these, because a de- stocked or spelled paddock, or a seeded paddock, is likely to attract kangaroos to the resultant green pick, because of their mobility through fences. Norbury and Norbury (1993) described preferential grazing of kangaroos in spelled paddocks, and discussed the potential benefits of closing waters to discourage this, either through turning the water off or the use of Finlayson troughs or similar devices (see later).
Apart from the immediate effects of kangaroos as they constrain opportunities for graziers to modify grazing pressure by moving sheep about, there is the longer term consideration about the extent to which competition between sheep and kangaroos is of economic significance.
Competition between sheep and kangaroos
The largest concern about kangaroos as pests derives from a belief that kangaroos are using fodder which could otherwise be available to sheep (see Gibson and Young 1988), because sheep too generally have a strong preference for grasses and forbs over a large range of environmental conditions. Not surprisingly therefore, much of the focus by ecologists attempting to assess the extent of this competition has focussed on the diets of kangaroos and sheep, and findings of dietary overlap translated into evidence for competition (e.g. Griffiths and Barker 1966; Wilson 1991b; Dawson and Ellis 1994). McLeod (1996) has pointed out that dietary overlap may indicate resource sharing rather than competition, with an essential element of competition being harm to one or more individuals as a consequence.
McLeod (1996) has reviewed what he considered to be the most useful studies on possible competition between kangaroos and sheep. The first to experimentally manipulate populations of both species, were Wilson (1991a) and Edwards et al. (1995,1996). These three studies focussed on red kangaroos.
Edwards' study concluded that there was little evidence of competition above the very low pasture biomass of 50-60 g/m2, whereas the presence of kangaroos reduced the liveweight of sheep at pastures below that threshold. Despite some acknowledged design weaknesses in the study, McLeod judged that the results are likely to be representative of what occurs in usual populations of sheep and kangaroos throughout the shrub-steppe rangelands such as those at Fowlers Gap where that study was conducted.
Wilson's study, on the Darling River near Louth in the semi-arid woodlands of New South Wales, claimed to present unequivocal evidence of direct competition between sheep and kangaroos. McLeod asserts that this should be treated with scepticism, for a variety of experimental design-related reasons, not the least of which was that that the experimental paddocks carried sheep at unrealistically high densities.
McLeod, Edwards and Wilson all recognised that the main issue was not so much whether or not there is competition but whether or not there is economic harm as a result of that competition. Hence, all measured sheep condition and/or elements of sheep productivity such as body mass, fecundity and wool growth under different treatments. This is an important point, because it is implicit in the perception of kangaroos as pests that sheep productivity is lowered as a result of the presence of kangaroos. The implication is that, if kangaroo numbers are lowered, then economic performance of a property will improve.
McLeod, following on at the same study site used by Edwards, also found that exploitative competition was rare, but that interference competition was more common because he found that kangaroos consistently avoid areas used by sheep. In terms of decreased productivity by sheep, he concluded that this occurred only in unusual circumstances, at very low pasture biomasses combined with high kangaroo densities, and that interspecific competition was asymmetric, with sheep dominating.
McLeod drew attention to the need for more research into these questions. Indeed, similar manipulative experiments are needed, in circumstances which allow for better experimental design and at realistic stocking densities, and at different geographic locations. What occurs in the semi-arid woodlands of New South Wales, for example, may not be obtained in the Mitchell grasslands of Queensland.
It is worth noting, however, that all of the studies so far have been comparatively short term (less than ten years). The history of an experimental exclosure at Koonamore, in South Australia, with sheep removed in 1925, shows how important long-term studies are (Sinclair 1996). At Koonamore, there has been considerable rehabilitation within that exclosure, but it took a very long time. The rehabilitation occurred in the presence of kangaroos and its rate increased greatly after the removal of rabbits in the early 1970s and, particularly, following a very large rain event in 1978.
Much more experimental research is needed not only to resolve questions about competition between kangaroos and sheep, and its effects, but to determine appropriate land management practices which could encourage rangelands rehabilitation.
An early step in this would be a comprehensive review of what is known so far, and Environment Australia has identified a need for this to be done for the role of kangaroos as a component of Australia's grazing systems. If the continuing pressure on the rangelands towards deserts is not reversed, not only will the rangeland not maintain or recover economic productivity, but the habitat could deteriorate to the point where the long-term persistence of kangaroos is compromised.
Effectiveness of the kangaroo industry in pest
Pest control, sustained-yield harvesting and conservation need not be mutually exclusive (see Chapter 2). For these to be in concert, damage needs to be reduced to acceptable levels without reducing either population viability or the commercial viability of the industry harvesting the population. However, there is the potential danger that the commercial industry may drive the population down to such a level that it puts itself out of business and is no longer available to maintain control. These factors need to be balanced by Australian governments in making policy relevant to kangaroo management.
Naturally, harvesting does reduce populations. The question is whether or not the reductions achieved are enough to satisfy the perceived needs of graziers for pest control. Using a population modelling approach, Caughley (1987a) estimated that at a long- term offtake of 10-15% per annum, kangaroo populations would be reduced by 30-40% in the long term. This harvest rate is close to the actual harvest rates of recent years, suggesting that the industry has been lowering numbers by about that much.
Has the industry been effective? The answer to this has more to do with perceptions than anything else, for there are not yet any quantifiable measures of effectiveness, in terms of increased sheep production or pasture regrowth as a consequence.
There is little doubt that the kangaroo industry is not perceived to have been effective in pest control (e.g. Martin 1995), and there are frequent calls from grazier's organisations and, sometimes, from production-focussed government agencies, for higher quotas or, recognising that economics usually dictates that the quotas are not taken fully anyway, for the implementation of additional control measures.
Additional (legal) control measures include the issue of 'shoot-and- let-lie' permits, which are issued by all States, Finlayson troughs and, most recently, proposals to use immunocontraceptive techniques.
Electrification of watering points (Finlayson troughs etc.)
The electrification of troughs and other watering points in a way which prevents kangaroos drinking, but does not inhibit sheep, has been used in New South Wales and in Western Australia, with varying degrees of success. The installation comprises low-lying wires, carrying high-voltage and low current powered by a solar recharged battery (electric fence technology), placed so that the tail of a drinking kangaroo contacts the wire and it receives a shock (Norbury 1992).
A recent report which evaluates these techniques is 'Tactical management of grazing pressure in rangelands by manipulating access to water supplies' (Hacker et al.1996). They found that the technique could be applied around dams and other open watering points, as well as at troughs, and that it was 'largely, though not universally effective' in excluding kangaroos from water'. Both species of grey kangaroos were more persistent than reds in trying to cross the barrier, but 'most kangaroos are unable to drink normally in the presence of the barrier'.
The effectiveness of the installations depends, however, not so much as the extent to which they stop kangaroos from drinking but, rather whether or not that leads to any beneficial result. Clearly they are ineffective except in very dry times, because kangaroos otherwise have many sources of water from which to choose. Hacker et al. (1996) found that, under hot, dry conditions, kangaroos became concentrated around the electrified troughs and suggested that this offers opportunities for improved harvesting efficiency. Under mild conditions, similar concentrations did not occur. They discussed the applicability of the method for protecting de-stocked areas by encouraging kangaroos away, but pointed out that this can be managed, anyway, by closure of the watering points in destocked paddocks. Although there is said to be a general awareness of the technique, few graziers have attempted to utilise it. This is possibly because, although they could have a role as an aid to harvesting or to protect destocked areas, the success is very dependent on specific seasonal conditions.
In the Gascoyne region of Western Australia, King et al. (1996) found that >80% of wallaroos and red kangaroos were repelled from Finlayson troughs when alternative sources of water were available. However, when the only sources of water in a 1,000 km2 area were Finlayson troughs, kangaroos and wallaroos quickly found ways of drinking from troughs. Furthermore, both the distribution and abundance of kangaroos and wallaroos in this large area was unaffected by having Finlayson troughs as the only source of water.
The idea of controlling kangaroos by immunocontraception has received some attention recently, particularly from animal rights groups which see it as more humane than shooting. On this promise, The Cooperative Research Centre for Marsupial Conservation and Management, based at Sydney's Macquarie University has been funded to develop techniques by which, using immunogenic antigens, kangaroos could be made sterile for long periods, probably up to about a year.
While this has obvious practicality for controlling kangaroos in contained or sub-urban situations such as golf courses, wildlife parks and public reserves, its use in the rangelands is more problematical.
Some of the issues were raised by McCallum (1996), who pointed out that kangaroos are considered to be a problem for landholders primarily in a drought, when there is little net recruitment anyway, making it hard to see how immunocontraception has much to offer. Additionally, he offered the view that 'the notion that one can 'control' species with highly variable populations, both temporally and spatially, given that a response will not be apparent immediately, is one which would surprise most population ecologists'.
This comment alludes to the particular difficulties of using immunocontraception in a tactical, limited way. It may be very useful, of course, for controlling feral possums in New Zealand, for example, where extinction would be greeted with delight, and this is part of the CRC's brief. Application to species with a high conservation value, however, presents serious difficulties; managing the application of sufficient 'dose' without compromising the conservation goals would present quite a challenge.
The risk of regional extinction as a consequence of lowered fecundity has been explored quantitatively using a model for red kangaroo populations, developed by P. Timmers, University of Queensland (unpublished data).
The results are not encouraging. Figure 24 shows the dynamics for red kangaroos in two areas with different rainfall, Longreach and Emerald. At Longreach, immunocontraception at various levels of reduction in fecundity is certainly effective at reducing density. However, the relationship between risk of extinction and percentage reduction is not linear; it increases quite remarkably above a particular threshold. A different approach is shown in Figure 25, which models a series of immunocontraceptive control events over a 50 year timespan. The model considered a starting population with a density of 30 kangaroos per sq. km. The aim was to work out how many control events in 50 years would be needed to keep the population below 10 per sq. km, with different levels of fecundity control, in the Longreach and Emerald districts, respectively. In Longreach, 6 control events of 80% reduction over the 50 years were sufficient.... but the population went extinct. At a lower level of fecundity reduction with each application, eight control events of 60% reduction would be required, with a risk of extinction of 60%, or 14 events of 40% reduction, at a risk of extinction of 'only' 15%.
Figure 24. Assessment of the risk of extinction (solid lines) of a closed population of red kangaroos following different reductions in fecundity by immunocontraceptive treatments in two areas, Longreach (431 mm rain p.a.) and Emerald (634 mm rain p.a.). Population densities as a percentage of the untreated population are shown as dashed lines. Note that in the Longreach area, a reduction in fecundity of 30% leads to a very marked drop in population density (>80%) and a >60% chance of local extinction. (after Timmers, unpublished data).
Figure 25. Assessment of the required frequency of immunocontraceptive treatments of populations of red kangaroos (initially at 30 animals per square km) to keep the population at a target density of 10 animals per square km over a 50 year period. Treatments have three levels of severity: 40, 60 and 80% reduction in fecundity. The associated extinction risk is shown on the y-axis. In the Longreach area, for example, 8 treatments per 50 years, each resulting in a 60% reduction in fecundity, or 14 of 40% reduction, would be required to keep the population below 10 animals per square km, with extinction risks of 60% and 15%, respectively. (after Timmers, unpublished data).
That is to say, the risk of extinction would have to be minimised by employing frequent, less severe applications. This will be more expensive, but there are also serious difficulties in managing the timing and severity of the application, given the lag referred to by McCallum (1996). The effect would be well after the application and likely confounded with other influences as well.
Another significant concern is the lack of specificity of the technique. It is quite unlikely that any immunocontraceptive agent would be likely to be any more than family specific. That is, it would affect any exposed member of the Macropodidae.
This has major implications. Many vulnerable and endangered macropods, such as yellow-footed and brush-tailed rock-wallabies, bridled nailtail wallabies, rufous bettongs and spectacled hare- wallabies occur in the same places where graziers would be interested in reducing the numbers of kangaroos. These species commonly make use of the same watering points and would, presumably, be just as attracted to the transgenic immunocontraceptive grain/bait as any other macropod. How this apparent impediment will be addressed is not yet identified.
In conclusion, immunocontraception could be effective in small areas, if immigration is insignificant, but there is a considerable risk of local extinction if it is applied over a long time and on a large scale. Control would have to be on a large scale at a low level to be effective, which is very expensive. There would also need to be very close monitoring, which is particularly difficult in applications of immunocontraception because of the time lag between the application of the control event and the ensuing effect. In species where there is no concern about risking extinction, such as with attempts to control or eradicate an introduced and unwanted pest species, immunocontraception is not subject to this constraint. Where some measure of regulated control is required, however, as would be the case with kangaroos, significant difficulties arise.
Immunocontraception may have its place, but is unlikely to be of significant value in the sheep rangelands because it is likely to be too expensive, too difficult logistically, and too risky. Also, as yet there is no suitable delivery mode, except by direct injection. Garrot (1995), writing about options for the future management of ungulate populations in the U.S.A came to the same conclusions: 'Contraceptive techniques .... will not replace to any appreciable extent traditional management tools such as public hunting' and 'Practical applications .... will be restricted to specialised situations where lethal techniques are difficult or impossible to implement due to safety or social constraints and where financial and personnel resources are available to support such management programmes'.