Spokespeople for the Kangaroo Industry Association of Australia and some other pastoralist organisations, are warning that in the current land of “flooding rain”, landholders will be overwhelmed by burgeoning kangaroo numbers unless commercial offtake (or harvesting) is ramped up (with government support).

Should landholders be worried? Should we be commercially killing more kangaroos?

On the mainland, the commercial industry kills four species of large kangaroo – the red kangaroo, the eastern and western grey kangaroo and the common wallaroo. The offtake is from a mixed stock of four species of quite different biology, even though they are usually presented generically as “kangaroo” to consumers.

In 2011, the combined commercial quota for killing kangaroos is 3,730,710. The 30-year average total population of the four species of kangaroos in the commercial zones is about 27 million. The quota varies between species (highest for red kangaroos), state management programs and harvest zones and this distils down to about 14% of the combined population of the four species.

How quickly do kangaroo populations jump back with the rain?

The red kangaroo is noted for its quick reproductive response to drought-breaking rain. Even so, development is slow: it is about a year from conception to weaning. The common wallaroo is similar but the two grey kangaroos have slower development, taking about 18 months to weaning.

Fecundity, or the ability to reproduce, is increased in kangaroos because these animals have overlapping generations. In kangaroos, the uterus and pouch are rarely left vacant. Thus the red kangaroo will give birth in about a month and then mate and conceive 1-2 days later.

The joey spends its first seven months largely in the pouch. When it leaves the pouch permanently, the next young is born and shortly thereafter another baby is conceived. The potential annual fecundity for the red kangaroo is 1.36 offspring, 1.25 to 1.32 in the common wallaroo, 1.03 in the eastern grey kangaroo and 1.00 in the western grey kangaroo.

How do kangaroos’ reproductive rates compare to those of sheep?

Let’s compare the kangaroo to a sheep that’s bred for meat and lives in a similar habitat (while there are more common meat species, they tend to live in higher-rainfall areas).

The Dorper – which is growing in popularity as a source of organic sheep meat – has an annual fecundity of 1.5 with a capacity to increase this through producing twins. Gestation is around five months. Farmers decide when to wean, but it’s usually around four to six months later.

Producers aim for about 80% of lambs to survive to weaning. A field estimate of reproductive success of a red kangaroo is 3.7 offspring over a lifetime – about 41% of their potential.

Clearly Dorper sheep can greatly out-reproduce the most fecund of the large kangaroos in the rangelands.

Despite common perceptions, kangaroos do not rapidly increase to so-called “plague proportions”, compared to sheep.

Balancing growth and the land’s carrying capacity

Even so, the populations of any of the four kangaroo species will grow while immigration and reproduction exceed emigration and mortality.

Kangaroos are counted in “management areas” and may move between them – this is known as immigration and emigration. Red kangaroo males can move 250 kilometres or more in six months (though other species tend to stay in one small area for most of their life). A good run of seasons will raise reproduction and lower mortality.

Likewise the populations of sheep and other livestock will grow rapidly. Here re-stocking stands in for immigration and sales represent emigration. Mortality is typically kept low through good management. Eventually a carrying capacity or production target will be approached.

The commercial kangaroo industry operates with a quota proportional to the population. As the population goes up, so does its stock. It gets harder to take this quota when the landscape is flooded.

There is also a minimum size limit for offtake. The kangaroo must weigh at least 14kg (for pet consumption) or 15kg (for human consumption) once it’s field dressed (the basic cleaning up of a carcasse that’s done after it is shot). Hunters cannot kill kangaroos until the animals grow to at least that size (around 22-23kg live-weight).

Kangaroos and their kind live in the reproduction and growth slow lane compared to their counterparts among placental ruminants.

Should graziers be alarmed about incipient kangaroo plagues?

Scientists predict that the wet period in northern Australia will continue. With vegetation growth and drying off will come fire, as is already evident in Central Australia and far western NSW.

El Niño-Southern Oscillation cycles are accelerating with climate change and we are likely to return to drought sometime soon. It can take some time for rainfall and pasture production to affect kangaroo numbers, so populations are as likely to be cut-back before they climb to the high levels seen in 2001.

In the meantime, there are ways to identify damage to individual properties. Equally, there are methods of auditing and monitoring the killing to ensure that specific objectives for damage mitigation are achieved.

The ups and downs of kangaroo populations are a normal and enduring pattern.

Further Reading

• THINKK, the think tank for kangaroos, University of Technology Sydney
• Advocating kangaroo meat: towards ecological benefit or plunder? – THINKK
• Shooting our wildlife: an analysis of the law and policy governing the killing of kangaroos – THINKK
• The welfare implications of commercial kangaroo killing: do the ends justify the means? – THINKK