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60 Environmental effects of grazing The environmental effects of grazing are readily observed. Differences in vegetation across fence lines are sometimes marked in terms of biomass, structure, composition and colour (due to curing, flowering, growth stage or composition). While the causes of these changes can be confounded with other treatments, such as fertilising, sowing, harrowing and herbiciding, enough evidence can be seen close to the fence to indicate when changes are largely due to grazing. This is not an attempt to comprehensively review the literature on the effects of grazing; rather the purposes are to provide examples of grazing effects and to elicit principles. Recent overviews of the impacts of grazing by domestic animals on biodiversity in Australia include those by Dorrough et al. (2004), Lunt (2005) and Lunt et al. (2007). Caution is required because, as Dorrough et al. (2004) points out, ‘We still have only a very basic understanding of the effects of different grazing strategies and pasture management on biodiversity’. Information on the effects of grazing The main sources of information about grazing effects are: • Historical observations – ‘A striking effect of the wool industry on Australian vegetation has been the virtual elimination of T. [hemeda] australis’ (Moore 1962). Moore also noted, ‘The trend under grazing [by exotic animals] is towards communities of short-lived alien species’. Woinarski and Fisher (2003) quote Finlayson (1945) as saying, ‘It is not so much, however, that species are exterminated by the introduction of stock, though this has happened often enough, but the complex equilibrium which governs long established floras and faunas is drastically disturbed or even demolished altogether’. Lunney (2001) attributed the extinction of 24 medium-sized mammal species in western New South Wales during the first 60 years of settlement – ‘the greatest period of mammal extinction in Australia in modern times’ – to the presence of ‘sheep, and the way that the land was managed for the export wool industry’. • Historical legacies – quantitative studies of the effects of grazing practices (including burning) have been carried out in north-eastern New South Wales by Henderson and Keith (2002) and Tasker and Bradstock (2006); quantitative comparisons across different land tenures that have historically experienced different, unknown grazing regimes, such as paddocks, roadsides and reserves (e.g. Prober and Thiele 1995). • Observations made across fence lines (Noble 1997) or at camping spots, sheltering spots, dung heaps (for certain species), watering points and yards compared with areas outside them. Moore (1962) noted that in sheep ‘camps’ there was an increase in the exotic genera Marrubium (Horehound), Urtica (Stinging Nettle) and Onopordon (Scotch Thistle). The effects of different historical grazing regimes have manifested in spring in Canberra, ACT, where massive blue flowerings of the exotic Patterson’s Curse (Echium plantagineum) can occur in paddocks grazed by domestic animals. In contrast, across the fence and along the adjacent roadside, numerous yellow flowers of the native Bulbine bulbosa and the absence of Echium have been seen (Plate 4.2). In farm surveys Reseigh et al. (2003) found that ‘the never grazed and infrequently grazed sites had significantly higher native species richness than grazed sites’. Driessen et al. (1990, pp. 75–76) showed that the greater the intensity of grazing by stock, the fewer the number of indigenous Tasmanian Bettongs (Bettongia gaimardi) found (as indicated by their diggings). • Direct observation of diet – bite rates for sheep and goats in a Mediterranean shrubland were directly observed by Papachristou (1997). • Measurements made at different distances from water sources (Landsberg et al. 1997) or from intensively grazed areas, such as near rabbit burrows (Leigh et al. 1989). • Experiments – ‘Rabbits reduced the cover and biomass of 39 species of forb, in some cases to zero’ (Leigh et al. 1987; Plate 4.3). In the studies analysed by Kemp et al. (2003), ‘where the herbage mass was maintained between 2 and 4 t DM/ha [dry matter per hectare] then species were maintained and productivity [for sheep] was optimised’. Fire and adaptive management Underpinnings of fire management for biodiversity conservation in reserves
Underpinnings of fire management for biodiversity conservation in reserves Dung counts may be used to help quantify the grazing intensities of different animals (Allcock and Hik 2004; Archibald et al. 2005; Kirkpatrick et al. 2005), especially when uncontrolled numbers of native and feral animals are involved, as well as controlled numbers of domestic stock. Plate 4.2 The yellow native Bulbine bulbosa predominates over the exotic Patterson’s Curse (Echium plantagineum) on the roadside (foreground), while the reverse is true over the fence in the horse-grazed paddock (background) (Gill 2003). Just what grazing is needs to be outlined. Grazing encompasses the relevant actions of herbivorous animals – eating, trampling, lying down, pawing or digging the ground, rubbing against trees, eating bark (ringbarking), defecating, urinating and carrying seeds externally or internally, all with flow-on effects to biodiversity, resource distribution (e.g. by camping) and water quality (e.g. Alexiou 1983; Friedel and James 1995; Robertson 1997; Noble 1997, p. 47). Needless to say, different animals have different grazing characteristics. Added to the direct effects of grazing are the effects of infrastructure associated with grazing, such as fences, watering points, yards and tracks. These have their own effects on the environment or on aims of management, such as recreation and biodiversity conservation. In some places, the use of horses and ground vehicles for mustering may have an effect. Plate 4.3 Grazing exclusion plot (on right) established in 1989 at Wilsons Promontory to study the impact of native and introduced herbivore grazing pressure in Coastal Grassy Woodland communities. The abundant themdia sp and other palatable species present inside the exclusion plot have been heavily suppressed outside by grazing (Whelan 2008). Fire and adaptive management 61
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