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3. BIOLOGY, ECOLOGY AND CONSERVATION OF KANGAROOS

3.1. Introduction

Kangaroos are among the most widely studied species in Australia, largely as a consequence of the commercial harvest. The biology, ecology, conservation status, threats and issues relating to the conservation and harvesting of the kangaroo species that are the subject of this plan have been comprehensively documented in a large number of widely available publications. It is beyond the scope of this plan to reiterate the contents of these publications as they relate to kangaroos and their management. Accordingly, the following sections provide only a summary of the variety of publications that address specific aspects of kangaroo biology, ecology, conservation, management and harvesting and provide references to more detailed information where appropriate.

3.2. Biology and Ecology

The information in this section has largely been adapted from the background information for kangaroo management in commercial harvesting of kangaroos in Australia (Pople & Grigg 1999).

3.2.1. Introduction

The four kangaroo species that are the subject of this plan are abundant over a broad area of Australia and NSW (Figures 3 to 6). The three most abundant species (red kangaroo, eastern grey kangaroo and western grey kangaroo), which comprise about 97% of the commercial harvest, are particularly common over the sheep and cattle grazing pastures of western NSW. Within sheep rangelands, the provision of permanent watering points means kangaroos are now more likely to be limited by food than water (Oliver 1986). This has had a profound effect on their distribution as well as their abundance (Newsome 1965a). It has been suggested that sheep and cattle also improved the habitat of kangaroos through facilitative grazing; creating a sub-climax pasture (Newsome 1975). These changes to the environment would have been most pronounced in the late 1800s when average sheep numbers in the rangelands of NSW were nearly twice what they are today (Caughley 1976). Other changes were also wrought upon Australia's rangelands following European settlement – numerous species of eutherian herbivores and predators were introduced and became established in the wild; at the same time numerous small native mammal species disappeared and many are now extinct. As Caughley (1987b) explained, not only was the habitat modified, but the ecological system was 'changed beyond recognition'. The current distribution and abundance of kangaroos may therefore bear only a vague resemblance to what it was prior to European settlement.

3.2.2. Red kangaroo (Macropus rufus)

Red kangaroo is the most abundant species of kangaroo. It is distributed over much of dry, inland Australia and is the only species exclusively restricted to the arid zone (Tyndale-Biscoe 2005) (Figure 3). This distribution reflects the interaction between mean annual precipitation and mean annual temperature (Caughley et al. 1987). Red kangaroo occupies a wide range of habitats including mulga and mallee scrub, scrublands, woodlands, grasslands and even desert (Caughley 1964; Russell 1974; Johnson & Bayliss 1981; Low et al. 1981; Short et al. 1983; Strahan 1995). However, Strahan (1995) and Russell (1974) describe a preference of this species for open plains habitat.

Many scientists consider that vegetation clearing, provision of artificial watering points and control of dingo (Canis lupus dingo) populations to facilitate the grazing of domestic stock in the pastoral zone have ‘improved’ the habitat for red kangaroo and thus resulted in a general population increase from pre-colonial times (Russell 1974; Newsome 1975; Caughley et al. 1980; Squires 1982; Grigg 1982). Conversely, intensive agriculture is not regarded as beneficial to the species (Grigg 1982; Short & Grigg 1982). However, little red kangaroo habitat has been altered by intensive agriculture.

As the red kangaroo is a herbivore, its role in the ecosystem can be defined as a primary consumer. Several detailed dietary studies have been undertaken on this species (Griffiths & Barker 1966; Chippendale 1968; Storr 1968; Bailey et al. 1971; Ellis 1976), all indicating a preference for green herbage including grasses and dicotyledonous plants. Although they prefer to eat grasses and forbs, when these become scarce red kangaroo will switch to chenopods and black bluebush, and in some areas will even browse shrubs (Tyndale-Biscoe 2005).

Red kangaroo have significantly lower energy and water requirements than sheep; Munn et al. (2008) found the grazing pressure of red kangaroos to be equivalent to about 0.35 of a Dry Sheep Equivalent (DSE), rather than the previously assumed 0.7 DSE. Furthermore, water turnover was only 13 per cent of that of sheep (Munn et al. 2008). This implies that the relative contribution of kangaroo populations to overall grazing pressure is smaller than previously thought. Consequently, reducing kangaroo populations will have less impact on grazing systems than control of feral herbivores and management of domestic livestock.

The reproductive biology of red kangaroo has been thoroughly studied (Frith & Sharman 1964; Newsome 1964a, b, 1965b; Sharman 1964; Sharman & Pilton 1964). Females come into oestrus at approximately 35-day intervals and are therefore potentially fertile throughout the year. Periods of extreme drought, however, may lead to suppression of the oestrus cycle, which is cued to body condition (Moss & Croft 1999). Females can come into breeding condition almost immediately after drought-breaking rains. Pregnancy does not interrupt recurrence of oestrus. The female may give birth 33 days after mating and the result from this post-partum mating remains a quiescent blastocyst until the previous young is about to leave the pouch or is lost prematurely (embryonic diapause). Bilton and Croft (2004) studied the lifetime reproductive success of female red kangaroos from an un-harvested population north of Broken Hill, and found that on average, females achieve only 41 per cent of their maximum reproductive potential. The number of droughts experienced in a female’s lifetime affected both her lifespan and reproductive capacity. Given the relatively high and stable population of red kangaroos in the study area, Bilton and Croft (2004) suggested that in addition to drought, the population was limited by mechanisms affecting juvenile survival.

Studies of behaviour and social organisation have been conducted by Caughley (1964) and Croft (1980). Red kangaroo is a gregarious species (Kirkpatrick 1967) and although relatively large groups may sometimes form, these groups are unstable in their composition (Croft 1980). The only enduring red kangaroo relationship is between the mother and her young. The mating system of the red kangaroo appears to be based on polygamy (Croft 1980).

Several studies have examined the movement patterns of red kangaroo (Frith 1964; Bailey 1971; Denny 1980; Croft 1980; Priddel 1987). These studies indicate that the majority of the population is relatively sedentary, moving distances of no more than 10 kilometres, although a small proportion of animals may move tens or hundreds of kilometres. Individual home ranges have been found to overlap. In western NSW, Croft (1991) found that red kangaroos had weekly home ranges of 259 to 560 hectares. Natal dispersal is male-biased (Edwards et al. 1994), and dispersal distances tend to increase during drought (Johnson 1989).

The population dynamics of red kangaroo have been studied in detail, largely derived from regular aerial surveys. These surveys provide a means of assessing the response of macropod populations to environmental conditions, particularly rainfall. J Caughley et al. (1984), working in NSW, found that the rate of increase in numbers was related to rainfall. Populations decreased when rainfall was approximately 90 millimetres below average and, except when rainfall was extremely high, increased when rainfall exceeded the 90 millimetres below average level. The maximum annual rate of increase was approximately 45 percent per annum, but under average rainfall, populations increased at 30-35 percent per annum. In poor conditions, populations declined at a maximum rate of 55 percent per annum. Robertson (1986) observed a 30 percent per annum decline in the red kangaroo population at Kinchega National Park in western NSW during the 1982-83 droughts. Similar population changes have been observed in South Australia by Grigg (1982). Moss and Croft (1999) found young males were the most significantly impacted by nutritional stress under drought conditions; female sub-adults maintained body condition but the onset of sexual maturity was delayed.

The red kangaroo is subject to predation by the dingo. Shepherd (1981) has made direct observations of dingo predation of red kangaroo, concluding they prefer juveniles as prey and the dingo might be able to limit the rate of increase of red kangaroo populations. Caughley et al. (1980) were more definite in their conclusions concerning dingo predation, and attribute the high densities of red kangaroo in the sheep country of South Australia, Queensland and NSW to the elimination of the dingo from these areas.

3.2.3. Eastern grey kangaroo (Macropus giganteus)

Eastern grey kangaroo is distributed across eastern Australia from northern Queensland to Tasmania between the inland plains and the coast (Russell 1974; Strahan 1995) (Figure 4). The distribution corresponds with areas where rainfall either has little seasonal trend or where rainfall in summer exceeds rainfall in winter (Caughley et al. 1987). Eastern grey kangaroo is abundant and occupies a range of habitats including woodland, scrublands, open forest, and semi-arid mallee and mulga scrubs (Caughley 1964; Calaby 1966; Bell 1973; Russell 1974; McCann 1975; Taylor 1980; Hill 1981; Strahan 1995; Southwell 1987).

Poole (in Strahan 1995) considers it likely the development of the pastoral industry has led to a marked increase in the abundance of this species. Furthermore, the eastern grey kangaroo has been moving westward for the past 70 years due partly to the increase in watering points for sheep and cattle (Tyndale-Biscoe 2005). Conversely, intensive agriculture with its associated widespread tree clearance has not been beneficial to the species (Short & Grigg 1982). The western boundary of the eastern grey kangaroo range is probably maintained by competition with red kangaroos and wallaroos because the latter species have a better tolerance of high temperatures and uncertain rainfall (Tyndale-Biscoe 2005).

The eastern grey kangaroo is a herbivore and therefore a primary consumer. Detailed dietary studies indicate the species is a grazer with a preference for grasses, such as spinifex (Triodia metchelli), growing in woodlands (Kirkpatrick 1965; Griffiths & Barker 1966; Southwell 1981; Taylor 1983b).

Reproductive biology of eastern grey kangaroo has been well studied (Kirkpatrick 1965, 1967; Poole 1975; Kirsch & Poole 1972). Breeding occurs throughout the year but there is a peak of births in summer. The oestrus cycle is 46 days and the gestation period 36 days. Post-partum ovulation does not occur in eastern grey kangaroo and quiescent blastocysts are rarely found in this species.

The social behaviour of eastern grey kangaroo reflects their seasonal breeding and preference for woodland habitat. Eastern grey kangaroo is gregarious (Southwell 1984a), forming groups that are unstable in their composition (Southwell 1984b). There are three common associations related to essential life functions: male-male agonistic behaviour to establish hierarchical rank; males courting oestrus females – this species has a polygamous mating system (Jarman & Southwell 1986); and the mother-young association (Tyndale-Biscoe 2005).

Eastern grey kangaroos are less mobile than red kangaroos. Studies of eastern grey kangaroo movement by Jarman and Taylor (1983) and Jarman and Southwell (1986) indicate the species occupies well-defined, overlapping home ranges. Both sexes are relatively sedentary and females migrate to a lesser extent than males (Zenger et al 2003). However, genetic analysis undertaken by Zenger et al. (2003) indicated only weak genetic structuring of populations, suggesting there are high levels of dispersal at both a local (<50 km) and regional (50-230 km) scale.

The population dynamics of eastern grey kangaroo were examined during the aerial surveys of J. Caughley et al. (1984), which were undertaken at two sites to the east and west of the inland plains of NSW. The eastern site contained both eastern grey kangaroos and western grey kangaroos, which cannot be reliably distinguished from the air. Eastern grey kangaroos were far more abundant than western grey kangaroos (J Caughley et al. 1984), so the changes observed can be attributed almost entirely to eastern grey kangaroos. J Caughley et al. (1984) found populations had a maximum rate of increase of 35 percent per annum where rainfall was above average, and a rate of increase of 25 percent per annum at average rainfall. Populations declined only when rainfall was well below average.

Aerial survey has been the main means by which broad-scale estimates of eastern grey kangaroo populations have been obtained. Prior to 1987, the only broad-scale estimate for the eastern highlands, where fixed-wing aerial surveys are not possible, was a ‘plausible guess’ of five per square kilometre (Caughley et al. 1983). Preliminary results from the recent helicopter survey in NSW indicate an average density in suitable habitat of 11 per square kilometre. Recalculating this estimate for all of the survey area (including both suitable and unsuitable habitat) gives a density of nine per square kilometre, considerably higher than that of the five per square kilometre guess of Caughley et al. (1983). Taylor (1983b) recorded localised densities of 14 per square kilometre and 31 per square kilometre for eastern grey kangaroo on his two study areas on the New England Tablelands of NSW.

Eastern grey kangaroos are subject to predation by the dingo (Robertshaw & Harden 1985). Removal of dingoes from areas of eastern grey kangaroo habitat has reduced the effects on populations of this natural predation.

3.2.4. Western grey kangaroo (Macropus fuliginosus)

Eastern and western grey kangaroos have probably diverged from a common ancestor quite recently, and the biological and ecological differences between the two species are subtle. Indeed, western grey kangaroo was only confirmed as a separate species from eastern grey kangaroo in 1972 after detailed investigation of electrophoretic, serological, morphological and reproductive evidence (Kirsch & Poole 1967, 1972). Poole (in Strahan 1995) in reviewing information on western grey kangaroos commented that many aspects of the species's biology and ecology are so similar to eastern grey kangaroo that they hardly needed to be described separately. Accordingly, only the principal points of difference are addressed in this summary.

The western grey kangaroo is, perhaps, named inappropriately because the species actually occurs across the south of the continent, with a distribution extending northwards through western NSW and into a small area of southern central Queensland (Figure 5). This distribution corresponds to areas of aseasonal or winter rainfall (Caughley et al. 1987). Where the range of western grey kangaroos overlaps with eastern grey kangaroos, the latter are more abundant. Both species have similar habitat preferences and western grey kangaroo, too, has benefited from pastoralism but been disadvantaged by intensive agriculture (Short & Grigg 1982).

Neave (2009) examined the genetic structure of the western grey kangaroo across its range, and determined that eastern, central and western populations could be distinguished on this basis. There was further genetic structuring within the western population, however all western grey kangaroo in NSW fell into the eastern population.

Coulson and Norbury (1988) found that, like the eastern grey kangaroo, the western grey kangaroo feeds mainly on grasses. Working in north-western Victoria, Norbury (1987) found they ate more than 75 per cent grass in a mixed pasture but, as pasture biomass declined, shifted to forbs and shrubs. Barker (1987) described a similar shift from forbs and grasses to shrubs for western greys feeding on pastures in western NSW and southern Queensland. This contrasted with red kangaroos and eastern grey kangaroos, which continued to feed on grasses and forbs as pasture biomass declined.

The reproductive biology of the western grey kangaroo shows some minor differences from the eastern grey kangaroo: the mean lengths of oestrus cycle (35 days) and gestation (30.5 days) are shorter, and western grey kangaroo does not exhibit embryonic diapause (Poole, in Strahan 1995). Breeding may occur year round, except in very poor seasons.

Both eastern and western grey kangaroos are less mobile than red kangaroos. Studies of the eastern grey kangaroo by Jarman and Taylor (1983) and Jarman and Southwell (1986) indicate the species occupies well-defined, highly overlapping home ranges. Few individuals have been shown to disperse, those that do being young males. Western grey kangaroos were studied by Priddel (1987) and Priddel et al. (1988a, b) and show the same general patterns, with individuals occupying relatively small home ranges that overlap extensively.

3.2.5. Wallaroo (Macropus robustus)

The wallaroo has the widest distribution of the larger macropod species. It occurs across the entire mainland continent and is only absent from the extreme northern and southern portions of Australia (Russell 1974; Strahan 1995) (Figure 6). Despite their relative abundance, members of this group are infrequently seen because of their association with mountains and rocky hill country (Dawson 1995). A consequence of their close association with such habitats is that wallaroo distribution is discontinuous. This discontinuity has resulted in the wallaroo being a species that shows considerable variation in external characteristics such as coat colour, coat texture and ear length. In the most recent review of the species, Richardson and Sharman (1976) suggested four sub-species should be recognised, reflecting the extremes of variability present. Two wallaroo sub-species are found in NSW – the eastern wallaroo (M. robustus robustus), which is common on the eastern and western slopes of the Great Dividing Range; and the euro or inland wallaroo (M. robustus erubescens), which is found in the drier areas of the state (Dawson 1995).

The wallaroo occupies a wide range of habitats but prefers areas with steep escarpments, rocky hills or stony rises (Calaby 1966; Kirkpatrick 1968; Russell 1974; McCann 1975; Strahan 1995; Taylor 1985). Newsome (1975) considers the alteration of vegetation communities to sub-climax spinifex by the grazing of sheep in north-west Western Australia has enabled wallaroo to invade previously unoccupied valley areas.

Wallaroo appears to occur at lower overall densities than the other large macropods, but high densities can occur in localised areas. Surveys over small-scale areas of favourable habitat have revealed densities of 16 to 44 per square kilometre at Fowlers Gap in western NSW (Croft 1981) and 7 to 55 per square kilometre on grazing properties of the New England Tablelands (Taylor 1983a). Recent broad-scale ground surveys across the eastern highlands in Queensland and NSW give a more representative picture of overall density. In south-east Queensland, wallaroos attained an average density of 11per square kilometre across 65,000 square kilometres of suitable habitat (Southwell & Fletcher 1989). In the NSW New England Tablelands, aerial surveys conducted since 2004 indicate average densities of between 1 and 5 per square kilometre across management zones (Cairns et al. 2011; Cairns 2007; Cairns 2004).

The wallaroo is a herbivore, and hence a primary consumer. Detailed dietary studies have been undertaken by Ealey and Main (1967), Storr (1968), Ellis (1976), Squires (1982), and Taylor (1983b). Taylor (1983b) found that in the NSW tablelands, wallaroos had a broadly similar diet to eastern grey kangaroos, consisting primarily of grasses. In the arid Pilbara region of Western Australia, the wallaroo was found to concentrate on spinifex (Ealey & Main 1967). The species is thus a grazer.

The reproductive biology of wallaroo has been studied by Sadlier (1965), Ealey (1963), Kirkpatrick (1968) and Poole and Merchant (1987). Like red kangaroos, wallaroos are opportunistic breeders. Under normal conditions females breed continuously, giving birth to a single young every eight to nine months. However, if drought persists for more than six months, female wallaroos enter a state of anoestrus until they either die or the drought breaks (Tyndale-Biscoe 2005).

The wallaroo is less gregarious than the other large macropod species (Kirkpatrick 1968; Croft 1981; Taylor 1982). Croft (1981) studied their social behaviour, which is broadly similar to that of other large macropod species. Social groups within groups are highly unstable, the only enduring relationship being between a female and its progeny. The wallaroo also appears to have a similar mating system to the other large macropods.

Studies of movement by Ealey (1967), Croft (1981), and Jarman and Taylor (1983) indicate the species is relatively sedentary, occupying small home ranges that overlap broadly with those of other individuals. Clancy and Croft (1989) found that males of M. r. erubescens in the Fowlers Gap area progressively shifted their centres of activity within their home ranges on a short term basis, a trait shown by some of the females as well. Movements were quite small-scale, however, within a couple of kilometres and home ranges remained stable from year to year.

Figure 3: Distribution of red kangaroo (Macropus rufus)

F
igure 4: Distribution of eastern grey kangaroo (Macropus giganteus)
Figure 5: Distribution of western grey kangaroo (Macropus fuliginosus)

Figure 6: Distribution of wallaroo (Macropus robustus)