Competition between sheep and kangaroos in Australia

by Yadav Sharma Bajagai

Summery  

Millions of sheep and kangaroos share the same habitat in the sheep rangelands of southern Australia and dietary competition between these two species has been a matter of concern to pastoralists, conservation ecologists and animal scientists for long time. This issue has been tried to be addressed in this article. It is found that sheep and kangaroos both positively select grass and forb resulting considerable overlapping in their diet during flush season. But when grasses are in short supply during drought, sheep are forced to eat more of less preferred vegetation (chenopods). Sheep are more flexible than kangaroos to shift into chenopods and other shrubs decreasing the overlap in diet during dry season. Dietary competition is not significant when pasture biomass production is high (>30 g DM/m²) but it requires due attention when there is low biomass production during drought. Issue has been discussed and recommendation has been drawn.

Introduction

Kangaroos are probably the most iconic animal of Australia and draw attention of individuals from many backgrounds. Some consider this as an agricultural pest [2, 3] while others see them as important creature to be conserved [2] and even other regard them as source of meat and fur [3]. From the farmers' perspective, they are serious competitor for feed resources in pastures [4, 5]. In the past, government even used to pay bounty to kill the kangaroos [2].

Millions of sheep and kangaroos share the same habitat in the sheep rangelands of Australia [1]. Productivity from sheep farming has been found to be lowered due to competition between sheep and kangaroos in this area (Wilson 1991; cited in Edwards, Dawson et al. 1995) affecting livelihood of pastoralists [5]. The issue is also important for conservation of kangaroos in this ecosystem [5]. The interaction between these two herbivores in these rangelands will be reviewed and discussed in this text.

Area under consideration  

The sheep rangelands lie in the arid and semiarid landmass of Australia forming a crescent shape extending from northern Queensland, through New South Wales, South Australia to Western Australia covering 22% area of Australia (1.7 million km²) [6]. Average annual rainfall is 236 mm per year and this is the major determining factor for pasture biomass production of the area [1]. Temperature is very high during summer and cool to cold during winter [6, 7].

Figure 1: Distribution of sheep and sheep rangeland [16]

Low altitude flat land with depositional floodplains, sandplains and dunefields, and erosional surface are major land type of the sheep rangelands [6]. Vegetation in the sheep rangelands can be classified into forest, woodlands, shrublands and grasslands [6]. Shrublands, which is study site for this task, covers 10% of the sheep rangelands and predominantly consists of saltbushes and bluebushes (chenopods) [6].

Chenopods shrublands are fertile land with saline soil and suitable for livestock if water resource is available [8]. Atriplex spp. (saltbushes) and Maireana spp. (bluebushes) are two major vegetations in this area. Botanical and common name of some important plants species are given in table 1.

 

Low production of biomass is major nutritional constraint in this type of land due to erratic and low rainfall [6, 8]. Therefore energy is one of the major limiting nutrients in the area. Similarly, phosphorus is regarded as deficient in majority of the Australian land ­[9, 10]. 

Animals under consideration

Sheep

 

Sheep, a true ruminant herbivore, is the major livestock in sheep rangelands of Australia [1]. There are more than 70 million sheep in Australia and approximately 20 million are concentrated in sheep rangelands [5, 11].

These are seasonal polyoestrus animal and mostly breed when day length are short [12, 13]. They have 5 to 7 month long breeding season during short days with multiple 17- days estrus cycle [13]. This seasonality in reproduction is mainly controlled by photoperiod [12] and breeding season in southern hemisphere (e.g. Australia) generally begins in December/January  [14]. Some Mediterranean breeds like Merino, Karkul etc. tend to breed round the year but most of the breeds are seasonal [14]. Gestational length varies from 140 to 159 days (average 150 days) [14]. Some key reproductive parameters of sheep are given in box 1 [14].

 

Kangaroo

Kangaroos are most abundant leaping marsupials native to Australia [15, 16]. The study site (the sheep rangelands) is the major kangaroo zone of Australia sharing habitat with grazing domestic sheep [1]. Four major species of the kangaroos are found in the sheep rangelands which are red (Macropus rufus), eastern grey (M. giganteus), western grey (M. fuliginosus) and the euro (M. robustus) [17].

Size of the animal depends upon the species [17]. Red kangaroos are the largest of all species [17] and probably the most competitive species with sheep for diet [4, 5]. Some of the adult male may reach up to 80 kg but females are smaller (25-30 kg in average) [17]. In harvested population male female ratio may be less than one [17].

The peculiarity of the reproduction in kangaroos is that they have shorter gestation period than oestrus cycle (35 days in average)  [17, 18]. Due to its short gestation period and prolificacy their numbers can increase fourfold in five years [15, 17]. Kangaroos are continuous breeder and special pouch on abdomen is peculiar structure in which the young one spend their early life [17].

Trend in national kangaroo population in Australia is shown in figure 2.

Figure 2: Trend of kangaroo population in Australia [16]. 

Grazing behavior and diet selection

Sheep

Sheep are primarily grazer [19] with some degree of browsing [20]. They are good diet selector due to presence of narrow bite [21] and this selection of feed is the result of palatability and characteristics of feed; nutritional history and past experience with the feed, learning from mothers and nutrient requirement of the animals [21, 22]. Recognition and prehension of particular feed is done after visual, olfactory and gustatory assessment of the feed (figure 1) [21]. Gustatory perception is well developed and important than other sense in sheep for diet selection [21]. 

  Figure 1: Process of diet selection  
Sheep in rangelands are generally more selective feeder than cattle and goat but tend to eat range of  plants as far as possible [23] to fulfill their nutrient requirement and have ability to choose feed according to their need [21]. The range of acceptance widens during drought [23]. Generally large and small sheep eat 23.5 g and 37 g DM per kg body weight daily [20].

 

Figure 2: Plant factors affecting selection 

Quality of the leaves and ease of harvesting are two major factors determining the particular plant to be eaten (figure 2) [23]. They tend to select high quality grass with more soluble carbohydrate and try to avoid unproductive plants and the plants with toxic chemicals [21]. Forbes (2007) has used the concept of minimum total discomfort (MTD) to describe the selection of feed by herbivores. According to this concept, sheep tend to achieve MTD by manipulating the duration of grazing and type of feed consumed [21].

Sheep select the landscape in which there is maximum return per unit foraging time but this selection also depends upon location of water points, predation, ease of harvesting etc. [23]. O'Reagain and Garu [24] have described three phase selection pattern when they graze in a paddock of mixed vegetation. In first phase, the animal choose majority of the preferred plant species with some quantity of the plants of intermediate preference. When quantity of first priority plants decline, plants of second priority will be grazed with further defoliation of first rank plants (if present). When more than two third of first and second rank plants are grazed, then the animal start to graze any species of plant (not preferred) available [24]. But this selection pattern is altered if there is other accessible landscapes to supply quality forage and sheep shift to new landscape to graze [25]. If new landscapes are not available to graze, animal change their grazing behavior such as increasing bite rates, extending grazing time [26] and increasing chewing and ruminating time to improve digestion [23] and urea recycling in rumen [27].

They tend to graze more in the area of water hole especially in arid climate zone avoiding hottest part of the day to eat and go for the shelter [21]. Sheep grazing on good quality lush pasture can fulfill major quantity of water requirement from plant and may require drinking in a few days interval but sheep foraging dry pasture should drink water more frequently [23]. Vegetation near to water points often tend to be overgrazed [28].

In grasslands of Australian sheep rangelands, daily water consumption varies from 0.6 l per day to 6.2 l per day per adult and average daily distance walked for drinking ranges from 4.7 km to 6.2 km depending upon drinking frequency [29]. But in saltbush area of sheep rangeland, volume of water consumed per day ranges from 2.3 l to 9.3 l according to season and sheep walk 8 km to 14 km per day according to drinking frequency [29]. Drinking frequency is once daily in early summer and twice daily in late summer [29]. They start to graze within one hour of sunrise [29] and seven percent of daylight hour is generally consumed to travel to and from water point [28]. This duration may extend up to forty percent of daylight our (Burnside et al. 1990; as cited in O'Reagain and McMeniman 2002). Squires (1981; cited in O'Reagain and McMeniman 2002) has recommended to keep water points in 5 to 7 km apart.

Kangaroo

 

Kangaroos are principally grazers [20, 30] and prefer to live in the area with short green grass avoiding long herbage and dry pasture [31]. The diet preponderantly consists of grasses and succulent herbs (Chippendale 1962; cited in Priddel 1987). They select the best quality grasses among available [32] and prefer to spend more time in the area with greater herbage quantity [33]. 

Major part of kangaroo diet in sheep rangelands of Australia contains grasses and chenopods with small but regular amount of composites [30]. Large and small kangaroos eat 21 g and 35 g DM per kg body weight daily [20]. Grasses are first choice in lush season [30] and forbs are in second rank [20, 32] but chenopods are preferred only when grasses are scarce during dry period [30, 32]. They don't select particular parts of plants except avoiding woody stem and root [30].

Major species of grasses grazed by kangaroos in sheep rangelands of Australia are Enneapogon avenaceus, Eragrostis spp., Lophochola pumila, Stipa spp., Agrostis avenacea, Sida spp., Solanum esuriale, Solanum karsensis, Senecio spp., Myriocephalus stuarti, Centipeda cunninghami, Centaurea meliensis, Sisgmbrium spp., Brassica tornefortii, Plantago drummondii etc. [32].

Kangaroos are nocturnal in habit, therefore most of the grazing takes place at night [31]. Grazing generally starts shortly before sunset [31] and continue till 10 PM then starts again in the morning till 10 AM [20].

They are very adaptable to dry condition and able to maintain only in dry forage for a few months [30]. Mating generally takes place when there is enough green pasture and majority of females show anoestrus during drought [17]. Forbs of genus Bassia are important feed source during drought which remains green and fleshy even in dry times [30]. 

Although some of the scientist [30, 34] have reported about exceptionally long distance travelled by some kangaroos, they are sedentary animals and like to remain in certain territory [31, 32, 34, 35] and tend to utilize feed within this range [32]. When feed resources are scarce in home range the kangaroos become weak and some even may die in extreme cases [32]. Male are more mobile than female and size of home range for adult female is 18 km and that for male is 36 km [35]. They generally prefer to live in small group [17]. 

 

Dietary overlap and competition between sheep and kangaroos

Although sheep are ruminant and kangaroos are non-ruminant foregut fermenter, significant overlap between diet of sheep and kangaroos has been reported [2, 4, 5, 36, 37]. Edwards et al. [5] have reported 52 to 73% overlap between diet of sheep and kangaroos in sheep rangelands of Australia and this dietary overlap is more serious during drought [37]. Sheep has similar diet selection with kangaroo but with greater variability [37]. However, Behavioral confrontation is generally not occurred between sheep and kangaroos because former is diurnal in habit while the later is nocturnal [4].

Sheep and kangaroos eat similar amount of biomass if expressed as per unit body weight when food is available ad libitum [38]. Generally sheep consume 1200-1500 g/d (2-3% of body weight) and kangaroos eat 700-1000 g /d (2-3% of body weight) assuming adult body weight 50 and 30 kg respectively for sheep and kangaroo [38].

Both being generalist herbivores, grass is all time first preference for sheep and kangaroos  whereas forbs are selected usually by sheep and occasionally by kangaroos [5]. While grazing in the same paddock, kangaroos will get better quality grass (green blades) than sheep and compete for legumes with sheep thus being detrimental to wool production [20]. Similarity and differences between sheep and kangaroos in diet selection and foraging behavior in sheep rangeland of Australia are summarized in table 2. 

 

Competition for pasture between two species is mainly dependent upon quantity of pasture supply [5]. Competition is generally non-significant when pasture biomass is greater than 30 g DM/m² [38]. Response to shortage of preferred vegetation is different between sheep and kangaroos. Sheep adapt to drought by increasing the amount of chenopod in their diet but kangaroos have lesser ability to consume increased amount of chenopods during drought [38]. For example, when quantity of biomass in rangeland decreased up to 250 kg/ha, sheep consume about 20% Maireana pyramidata whereas kangaroos consume only 10% of this plant [39]. Kangaroos are more affected than sheep by interspecific competition in rangelands during drought due to nutritional management of sheep [1]. Observation and conclusion of different authors about dietary interaction between sheep and kangaroos have been summarized in table 3.

 

Discussion 

There is significant overlap in diet between sheep and kangaroos in sheep rangelands of Australia but competition between these two herbivores is sporadic due to different strategies in sheep and kangaroos to adopt in food deficit condition. Although there is overlap in selection of plant species, there is no enough information found about parts of plant selected. The effect of competition is found to be less severe in comparison to overlapping in diet.

In contrast to above conclusion, some scientists (Newsome 1975, cited in Squires 1982; Squires 1982) have indicated that grazing by livestock creates more favorable diet condition for kangaroos in rangelands. This ratiocination may be the result of conducting research in flush season or lack of geographical uniformity with others or different species and breeds of animals or different numbers of animals sharing the same habitat.

Even though results of different researchers are comparable, some discrepancies are also found in results of different scientists. Norbury et al. [35] reported that male kangaroos move farther than female from their home range but Priddel [34] had found a female red kangaroo to be moved the farthest distance from home range. Similarly, size of home range given by Norbury et al. (1994) and Priddel (1987) are also very contrasting as former has given it as 18 to 36 km while the later suggested it as 8 km. This variability may be due to different environmental condition of the study site.

In addition, most of the authors cited in this text have considered differences among different variety of kangaroos. Red kangaroos share more diet with sheep than grey kangaroos. Many of them have not considered the breed differences among sheep. Similarly, none of the researchers has accounted the seasonality in the reproduction in sheep which may affect diet selection and movement in sheep. In addition, transmission of disease between these two animals has not been studied in these researches.

 

Conclusion and recommendation  

From above review, it can be inferred with fair confidence that sheep and kangaroos share significant amount of rangeland resources in the sheep rangeland of Australia. Although both of the species are predominantly grass eater in ordinary condition, sheep is more flexible than kangaroos to select other plants like chenopods during scarcity. Forbs are fairly selected by both species with higher amount in the diet of sheep. They manage scarcity in feed supply by shifting into chenopods and into shrubs in more severe condition. Sheep start to eat chenopods and shrubs more easily than kangaroos. Overlap in diet is more in wet season and less in dry condition but effect of overlapping is more severe during dry condition due to overall shortage of biomass. Competition between sheep and kangaroos is intermittent drawing more attention during drought and kangaroos are probably more affected than sheep.

Nutritional management of sheep in rangeland should consider the dietary interaction between these species according to level of biomass production in rangeland. As competition between sheep and kangaroos are dependent upon climate related decrease in pasture production [4], provision for overlapping should me made at least during drought when biomass in rangeland is reduced to 30 g DM/m² (box 2). Nutritional management shown in box 2 is based on the assumption that there is no different in selection for plant parts between sheep and kangaroos. Further study and research should be done to find the detail of diet selection with respect to parts of plant by sheep and kangaroo.

 

Kangaroo meat is being exported in more than twenty countries and their skin is very popular for strength and light weight [15]. As there is little behavioral conflict between two species due to their nocturnal and diurnal habit, integration of these two animals is another option of management which can increase the overall return from the enterprise with issue of conservation of these iconic creatures being addressed. States national parks authority with approval from Federal conservation department, Environment Australia (EA) set the harvest quota for kangaroos [16].