Underpinnings of fire management for biodiversity conservation in ...

Underpinnings of fire management for biodiversity conservation in reserves
More generally, it has been noted that, ‘Historically, stock grazing has caused enormous damage to
many Australian ecosystems’ and ‘there is no scientific dispute that grazing stock continue to degrade
ecological values in other areas, such as alpine grasslands in Victoria’ (Lunt 2005, p. 1). However,
Leigh (1994) suggested that, ‘Too often, conclusions about the alleged deleterious effects of grazing
are drawn without there being adequate controls in the form of ungrazed plants’.
Effects of grazing through diet selection
‘Grazing of native plants by introduced livestock is … highly selective as regards the sites and the
species of plants …’ (Costin 1983).
Importation of exotic grazing animals into a reserve, especially a grassy one, can lead to a quick
reduction in herbaceous fuel load (non-woody biomass), according to the density of animals per
hectare and the palatability of the dominants. A first impression would be that such a practice would
compromise biodiversity values. However, just what the effects of grazing would be depends on the
diet of the animals, in the first instance. While there are many processes in the plant-life cycle that
can be affected by grazing animals – and many animal habitat features that can be affected directly
and indirectly – attention is drawn here to the immediate effects of grazing through diet selection.
Animals are selective feeders to the extent that some herbivores may even prefer certain parts of a
plant above others, or plants of a certain sex within a population of a single species, as in the case
of sheep feeding on the native Bladder Saltbush, Atriplex vesicaria (Walsh et al. 2005). If the animals
feed preferentially on certain flowers or seeds, they may disadvantage one species of plant over
another. By reducing biomass of one species, they may allow another to out-compete it. By removing
seedlings of trees, they may influence the gross structure of the vegetation.
Some examples of plant species’ selection follow:
• Between broad categories of plants – Australian Swamp Wallabies (Wallabia bicolour) have a
reputation as browsers (i.e. feeding on shrubs) rather than grazers (i.e. feeding on herbaceous
species). So too do feral goats (Capra hircus) (Henzell 2004), whereas feral camels (Camelus
dromedarius) in the arid zone prefer both shrubs and forbs to grasses (Dorges and Heucke 1995).
An extreme example is in parts of western New South Wales, where the grasses have been
completely eliminated by grazing and unpalatable woody plants now dominate; fires which may
have kept shrubs at relatively low cover in the past became much rarer, thereby allowing the shrubs
to persist and dominate at the expense of the grasses (Friedel and James 1997; Noble 1997, p. 39).
Shrub-grass relativities can be altered.
• Between plant species by livestock – ‘Sheep showed a high degree of species selection when grazed
in different seasons on 3 contrasting pastures’ (Leigh and Holgate 1978). Free-ranging cattle in the
United States had preferences in the following order: Smooth Bromegrass (Bromus inermis) > Crested
Wheatgrass (Agropyron desertorum) > Western Wheatgrass (Pascopyrum smithii) > ‘native range’
plants (Fehmi et al. 2002). In Israel, the ‘changeover from goat to cattle grazing encouraged the
expansion and invasion of thorny shrubs formerly eaten by goats’ (Bonneh et al. 2004).
• Between plant species within native vegetation – Leigh et al. (1987) provided a list of palatable
and unpalatable species, from the point of view of the European Rabbit, in the high country of
New South Wales. Included in the palatable-forb category were 32 species, including Brachycome
spp. (daisies), Bulbine bulbosa (a lily), a number of orchids, Wahlenbergia spp. (Bluebells) and some
exotic species. Among the 54 non-palatable forbs were native Geranium spp., Ranunculus spp.
(Buttercups) and Stylidium graminifolium (Trigger Plant). Leigh et al. (1989) provide lists of species
in similar categories for central-western New South Wales.
Herbivores do not recognise plants as native or exotic; they choose food in terms of palatability (see
Lunt 2005 p. 20, from Vallentine 2001, for characteristics of palatability) and accessibility. Some native
and exotic plants are toxic (Everist 1981) to some animals, and may therefore affect their selection.
There is no reason to expect that certain native or exotic animals will select exotic weeds over natives
or dominant fuel species over other less common species. The abundances of plant species in relation
to their cover or biomass, in addition to their palatabilities, will help determine the effects of grazing
on potential fuels.
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