robustness of classifications
The assessment of ‘trend’ groups on undeveloped transects indicates that, for species assigned to trend groups along single paddock transects, 3–6% of species transect−1 are likely to misclassified as affected by proximity to water. Reasons for misclassification could include chance occurrences, spatial proximity of sites, or the influence of environmental gradients other than the proximity to water. Taking misclassification into account, the method of classifying species according to trends along single transects appeared to be robust in detecting trends only for the 12% of ground layer species (±3–6%) that increased in abundance with proximity to water.
One of our motivations in undertaking the regional study was to investigate how valid our earlier conclusions were of substantial widespread grazing impacts, based on apparent trends detected along single paddock transects in widely scattered localities (Landsberg et al. 1999a). For the eight different transects surveyed in the earlier study, the proportions of species classified as increasers and decreasers were much higher: increaser species ranged from 16% to 51% transect−1 (mean ± standard error = 26 ± 4%) and decreasers ranged from 18% to 84% (38 ± 8%; Landsberg et al. 1999a). It is not valid to make direct comparisons, because survey designs and methods of classification differed between the two studies. However, the size of the differences suggests that the results from the current study region may be unusual in the relatively small proportion of its species apparently affected by proximity to water and grazing.
The classifications produced at different levels illustrate a common outcome of aggregating between hierarchical scales: some higher-level properties are an amalgamation of lower-level systems and many are not (O’Neill, Johnson & King 1989; Fox 1992). Hierarchical integration is illustrated by the set of species showing trends with proximity to water across the combined paddock transects (Table 7) compared with the larger set of species showing trends along individual transects (Table 8): the higher-level set is essentially a subset of the lower level. This also indicates that one of our original hypotheses is likely to be true: some localized effects of grazing (i.e. trends detected in single paddocks) appear to be less severe when considered in a regional context (i.e. fewer trends were significant across several paddocks). Whether this is due to different grazing histories is unclear, partly because there are no accurate paddock-based stocking figures to help interpret trends. The lowest proportions of increaser and decreaser species were detected along the transect in the Digitalis paddock (Table 8), which is also the paddock with the lightest stocking history (Table 1). There was no obvious relationship between stocking history and trend groups among the other three paddocks, but then stocking figures were far less certain for these paddocks.
Hierarchical integration is not apparent in the set of species identified at the regional development level. The classification of species affected by regional development per se (Table 6) could not be predicted from the classifications at lower spatial scales: the species identified and the predominant trends were quite different. The classification of effects of pastoral development on species at this scale therefore provides an insight into regional processes that is potentially powerful, because it is substantial but not anticipated. How robust is it?
There are few comparable studies in the literature, largely because rangeland areas that have not been exposed to livestock are extremely rare (Fleischner 1994; Landsberg & Gillieson 1996). Even in this study there was only a limited area of undeveloped land that was biophysically comparable to the land types that had been developed into paddocks. This severely constrained the number of transects we could survey in the undeveloped lands, resulting in an unbalanced survey design with two transects in the undeveloped lands compared with four transects in paddocks. However, this lack of balance was conservative in terms of the inferences we drew, because it meant that we were statistically more likely to assign occurrences on undeveloped transects to chance. For example, some species found on only one of the undeveloped transects (e.g. Zygophyllum aurantiacum) were not identified as being significantly more abundant in the undeveloped lands, even though they only occurred there. Thus it is likely to be biologically significant that, in the comparison between undeveloped and developed transects, many more species were identified as decreasers than increasers, when on the basis of the statistical design the reverse was to be expected.
influence of pastoral development on species richness, occurrence and abundance
The nearly 200 species recorded were evidence of a moderately rich regional flora but there was little evidence of species richness being affected by pastoral development. The number of species per site-habitat tended to increase with proximity to water in paddocks, but only for those species that were relatively common (i.e. recorded in quadrats). No trends were apparent for the total number of species per site-habitat (i.e. including those species that were locally rare), nor for the regional comparison of species richness in paddocks compared with undeveloped lands.
The very patchy distributions of most species made it statistically difficult to detect regional trends in abundance of individual species, because few species occurred at enough sites to allow robust detection of regional patterns. Nevertheless Monte Carlo tests showed that abundances of moderate numbers of species were probably affected by development per se and/or the proximity of sites to water in developed paddocks, at both regional and more localized scales.
The nature of these effects varied considerably, depending on the scale of inquiry. At the level of regional development the predominant impact on the abundance of species was negative: 15 species were less abundant in developed paddocks overall, and only one species was more abundant. Within the developed paddocks, the predominant impact was more positive, with more species appearing to be advantaged as the pressures associated with development increased with proximity to watering points. Many of these species were short-lived forbs, which may be palatable to stock but are capable of colonizing bare soil and eroded areas (Cunningham et al. 1992). Thus their tendency to increase in abundance near water in paddocks may relate more to environmental changes associated with grazing (e.g. opening of canopy gaps, trampling or erosion of soils) than to the direct impact of herbivory (Harrington et al. 1984a; Nash et al. 1999).
Conversely, the changes in abundance shown by many species in paddocks compared with areas that have never been developed might relate more directly to herbivory. All of the four upper layer species identified as showing regional patterns of decreasing abundance, Enchyleana tomentosa, Eremophila latrobei, Atriplex vesicaria and Eremophila longifolia, are widely recognized as palatable perennial species prone to decrease under grazing (Cunningham et al. 1992; Mitchell & Wilcox 1994; Pringle 1994). Much less is known about the ground layer species, particularly the short-lived or uncommon forbs. Several of the longer-lived ground layer species that decreased in regional abundance, e.g. Ptilotus gaudichaudii var. gaudichaudii, have been recorded as grazing-sensitive in other regions (Cunningham et al. 1992). Also, little is known of the ecology of Solanum cleistogamum, the one species that was more abundant in paddocks than undeveloped lands. Because other related species are known to be unpalatable or toxic (Cunningham et al. 1992), its regional increase in abundance might reflect selective avoidance by stock.
Overall, the study results are consistent with a general pattern whereby pastoral development enhances richness of plant species at a local scale by providing opportunities for increaser species to establish, but has the potential to decrease it at a regional scale, by removing the most grazing-sensitive decreaser species from the regional species pool. This interplay of spatial scales is one of the avenues that Olff & Ritchie (1998) and Chaneton & Facelli (1991) have suggested may give rise to apparent discrepancies in the effects of herbivores on grassland plant diversity.
However, the influence of the scale of measurement is not a sufficient explanation for one particularly puzzling aspect of our results. Those species that were substantially less abundant in paddocks than in the undeveloped lands were presumably the most sensitive to grazing overall. Why then were they apparently insensitive to the intensity of grazing within paddocks, as evidenced by absence of trend along paddock transects?
One potential explanation may lie in the dichotomy that exists between the selective pressures associated with the actual process of grazing, mainly to do with animal preferences and the ability of plants to deter, escape or tolerate herbivory, and those associated with the environmental changes that occur as a consequence of grazing. These latter are mainly to do with gap creation and soil changes, and the ability of plants to compete, establish and grow in a grazed environment (Landsberg, Lavorel & Stol 1999b). Increaser species tend to be those with attributes that are favoured by the environmental consequences of grazing. Decreaser species may be disadvantaged by either (or both) the process of grazing or the consequent environmental change.
For most of the time in rangelands such as these, location of the water supply is of such dominant importance in determining the grazing range of livestock that the environmental changes associated with grazing are strongly patterned around watering points (Lange 1969; James, Landsberg & Morton 1999). Thus it is not surprising that most increaser species should be closely associated with proximity to water, where the environmental consequences of grazing are most profound. During extended dry periods, when only perennial plants persist and when grazing animals are most severely constrained by the location of their water supply, the plants most likely to sustain the greatest grazing pressure are palatable drought-hardy perennials within grazing range of the water supply. Thus the species most likely to show trends of declining abundance with proximity to water are likely to be those same palatable drought-hardy perennials. The species we identified as showing regional trends of decline with proximity to water fit this pattern (Urban 1990; Cunningham et al. 1992; Mitchell & Wilcox 1994). So too does the trend we found for a significant proportion of upper layer species, which are mostly perennial shrubs, to occur at fewer locations in paddocks than in undeveloped lands.
However, domestic livestock exhibit the greatest selectivity in diet during particularly good seasons, when the usual suite of perennial species is supplemented with a wide array of more ephemeral plants; and in these circumstances it is usual for the more ephemeral plants to be actively selected (Wilson & Harrington 1984). In these circumstances, too, the spatial distribution of grazing is least constrained by the location of permanent watering points, because the water content of feed is high, and ephemeral surface waters are widely available. Thus, during particularly good seasons animals are both most selective and most able to forage away from their usual source of water. It follows that grazing pressure may be most severe on highly preferred, usually short-lived, plant species during those rare good seasons when water supply is not limiting. If the species are locally rare and/or particularly sensitive to grazing, this may also be the time when subpopulations are at greatest risk of local extinction (Fischer & Stocklin 1997), regardless of their proximity to watering points. Although there is no direct evidence, this scenario provides a plausible explanation of why many of the species that had reduced populations in paddocks, but were not apparently affected by proximity to water, were also short lived and/or locally uncommon.
relevance for regional planning
Our results indicate that few native plant species may be seriously disadvantaged by the livestock grazing associated with proximity to water in paddocks in our study region. Of more immediate concern are the species that appear to be considerably disadvantaged by livestock grazing anywhere in paddocks, regardless of proximity to water. Few of these species appear at imminent risk of regional extinction, however. The risk is probably highest for the three species that were found only in the undeveloped lands, but each may be more secure in other regions. They have all been recorded occurring in many other regions of South Australia (Jessop & Toelken 1986) and one, Danthonia caespitosa (common wallaby grass), often dominates the herb layer of less arid chenopod and grassland communities in south-eastern Australia (Cunningham et al. 1992).
However, our results also show indications of changing abundances that may be an early warning of problems to come, unless provision is made to safeguard those plant populations that are in decline. These indications include: a reduction in regional populations of a moderate number of naturally uncommon and/or short lived species; a decline in the abundance of some palatable perennial plants with proximity to water in paddocks; increasingly localized occurrences of a moderate proportion of upper layer (shrub and tree) species in paddocks; and changes near water favouring encroachment by a subset of opportunist increaser species, to the possible future detriment of less opportunistic decreasers.
What do these results mean for planning regional conservation networks not only in the study region, but more generally in the Australian rangelands? First, it is notable that our study region has experienced a lighter grazing history than most Australian rangelands. This was an almost inevitable consequence of our requirement for a region adjacent to similar lands that had never been developed for pastoralism. What it means, however, is that our study region probably illustrates the conservation status of native flora under a best-case scenario for Australian arid rangelands. In more heavily grazed regions there are likely to be many more species in decline, as our earlier study indicated (Landsberg et al. 1999a).
Secondly, a striking feature of our results is the very patchy distribution of many species. This inherent spatial patchiness, coupled with a high degree of temporal and spatial fluctuation in seasonal conditions (Morton et al. 1995), exacerbates the difficulties of achieving a network of conserved areas likely to ensure the persistence of all species. It also highlights the importance of including a large number of protected areas in any reserve network, in order to increase the probability of species at risk being present in at least some areas. For plants at least, number and dispersal of protected areas may be more important than the extent of each individual area. There may be an important proviso, however, if fencing is used to protect individual areas from domestic livestock but not wild herbivores. Our data suggest that, in this case, larger areas may be needed to reduce fence-proximity effects of wild herbivores.
Thirdly, the possibility that there may be two fundamentally different mechanisms whereby species decline in abundance under grazing means that there may need to be a mix of strategies for protecting areas in any regional reserve network. Morton et al. (1995) envisaged a hierarchy of reserve units ranging in size from national parks, through smaller but more numerous, and usually fenced, ‘excised management units’, to ‘restricted use units’ that may require special protection at critical times (e.g. temporary swamps at times when they provide important breeding habitat) but be available for pastoral production at other times. More recently, the financial costs of different strategies for achieving regional conservation networks have also been estimated (Biograze 2000; James et al. 2000). James et al. (2000) explored the economic costs that might be associated with three different options for protecting small conservation units within a pastoral matrix. They were (i) removing whole paddocks permanently from production; (ii) fencing off water-remote corners of paddocks; and (iii) ensuring that some corners of paddocks currently remote from water remain that way. The first option had a small one-off cost but a substantial on-going loss of production. The other options shared modest on-going costs in terms of lost opportunity to develop water points, with option (iii) saving the one-off establishment cost of fencing but option (ii) offering more reliable long-term protection.
In terms of the mechanisms whereby plant species may decline under grazing, all three options should afford similar levels of protection for those species most disadvantaged by the long-term grazing pressure that accumulates around watering points. However, the moderate number of species that appear to be affected by grazing anywhere within a paddock are more likely to require total exclusion of stock, for example by permanent fencing off, to protect at least a subset of the areas where they occur. Given that many of these species are also locally rare and patchily distributed, identifying which areas are most appropriate will be a challenge.