Biodiversity publications archive

The effects of artificial sources of water on rangeland biodiversity

Final report
Jill Landsberg, Craig D. James, Stephen R. Morton, Trevor J. Hobbs, Jacqui Stol, Alex Drew and Helen Tongway
CSIRO Division of Wildlife and Ecology
Biodiversity Convention and Strategy Section of the Biodiversity Group, Environment Australia, January 1997
ISBN 0 6422 7010 4

3. Analysis of biodiversity change along gradients (continued)

3.3 Change in species composition along gradients

3.3.1 Species response groups in diverse, abundant groups and taxa

Correspondence analyses of the more diverse and abundant plant groups and animal taxa revealed consistent patterns of variation in abundance of individual species with distance from water, for most of the groups and taxa at most of the gradients. On the correspondence graphs (Appendix 3), the distance between sites is a measure of their similarity. Site 1 was often plotted a long way from the other sites, indicating that the species profile for that site often differed from the species profiles for the other sites. Site 6, also, was often plotted separately from all other sites. Sites 1 and 6 were not always isolated, but when they were not, they nearly always grouped with their neighbours. For example, sites 1 and 2 were sometimes grouped together, and occasionally grouped with site 3. Site 6 was often grouped with site 5, particularly for the NT mulga gradient, where both sites were more than 9 km from water. The middle sites (site 3 particularly, but often with site 4 and sometimes with sites 2 and/or 5) were not grouped so consistently; sometimes they clumped together indicating a similarity in their species profiles and sometimes they were moderately distant from each other, as well as being distant from the sites at either end of the gradient. This shows that the middle sites shared similar species profiles for some groups and taxa at some gradients, but for others each of the middle sites had its own distinctive profile.

The consistent grouping of sites among eight gradients indicates two things: (1) that the biotic assemblages do vary along gradients away from water; and (2) that this variation is a consistent result across the range of environments sampled.

For the species corresponding to these site groups, there were consistent trends in the ways their abundance varied with distance from water (Figure 3.3.1.1; Tables 3.3.1.1-6). Regression analyses revealed consistent, significant, log-linear relationships between species abundance and distance from water for the site groups at both ends of the gradients. The species associated with sites closest to water (usually site 1, sometimes sites 1 and 2) consistently showed an "Increaser" pattern of response to the disturbance associated with water, in that their abundance significantly increased with increasing proximity to water (Figure 3.3.1.1). The species associated with sites remote from water consistently showed a "Decreaser" pattern of response (ie opposite to the "Increaser" pattern).

The general linear regressions showing these trends were invariably highly significant (P usually < 0.001) and generally explained high proportions of the variation in species abundance (%Var in Tables 3.3.1.1-6). Occasionally, all the species in a response group were present at similar levels of abundance and showed the same trend with water (ie. only the distance term was significant; Tables 3.3.1.1-6). More frequently, however, the species in each response group had different average levels of abundance (from locally abundant to locally rare; Appendix 4) but shared the same log-linear trend of changing abundance with distance from water (Tables 3.3.1.1-6).

The species corresponding to the middle sites were more variable in their responses. They were frequently present at different average levels of abundance (ie. the "species" term was often significant in Tables 3.3.1.1-6) but their relationship with distance from water was more variable. Some species probably showed flat linear trends comparable to the "neutral species" in Figure 3.3.1.1. Others, however, may have shown non-linear trends (e.g. quadratic relationships with distance from water). Species in this more variable response group have been designated "Not Determined" in the analysis summary (Tables 3.3.1.1-6)

Figure 3.3.1.1: Relationships between abundance and distance from water for the grazing response groups identified by regression analysis of correspondence groups.

Figure 3.3.1.1: Relationships between abundance and distance from water for the grazing response groups identified by regression analysis of correspondence groups.

Increaser and decreaser response groups were generally most apparent and most consistently associated with opposite ends of the gradients for plants growing in the understorey (Table 3.3.1.1) and detected in the seedbank (Table 3.3.1.3). There was a moderate degree of overlap in the species represented in these two plant groups (Section 2.2.2) and some of the increaser and decreaser species identified growing in the understorey were also identified as increasers or decreasers in the seedbank (Section 3.4.1). This indicates that the increaser/decreaser patterns apparent among plants growing in the understorey are likely to be perpetuated in future generations of understorey plants, via the seedbank.

Increaser and decreaser response groups were also consistently apparent for birds along most gradients, with the exception of WA chenopod, where no response groups were detected (Table 3.3.1.4). Abundance and diversity of birds were low at both chenopod gradients (Section 2.2.1) and livestock grazing is a much more recent development on the WA chenopod gradient than on any of the others (31 years, compared with at least 100 for the other gradients; Section 1.2.1). This combination may explain why trends in the composition of bird species with distance from water were less apparent here. Response groups at the NSW mulga gradient were only marginally significant (Table 3.3.1.4; 0.05<P<0.10) but birds had access to drinking water from the reference at this gradient, although large grazing animals did not (Sections 1.3.2.2 and 1.3.4). Thus at this gradient there were two potential confounding influences on birds: changes in habitat caused by grazing, and access to drinking water independently of any habitat change.

Increaser and decreaser response groups were also readily apparent among ant species at most gradients (Table 3.3.1.6). Trends were weakest at the chenopod gradients, with no decreaser response group apparent at the SA chenopod gradient and no increaser response group apparent at the WA chenopod gradient.

Trends were generally weakest for overstorey plants (Table 3.3.1.2) and for reptiles (Table 3.3.1.5) which were the least diverse and/or abundant groups and taxa analysed in this way. Increaser and decreaser response groups were evident among reptiles at three of the gradients. Only at one gradient (Qld mulga) were there no response groups detected. The abundance and diversity of reptiles were low at this gradient, but not exceptionally so (Section 2.1).

No increaser or decreaser groups were detected among overstorey plant species at four of the gradients (Table 3.3.1.2). Two of these were chenopod gradients where there were very few species of overstorey plants (Section 2.2.1). Generally trends were most apparent at the gradients that had highest diversity of overstorey plants, with the exception of the Qld gidgee/chenopod gradient where no response groups were apparent despite a moderate diversity of species. In any case, overstorey plants are all perennial shrubs or trees which have much longer life-spans than most species detected in the understorey. For some overstorey plant species (e.g. western myall; Lange and Purdie 1976) the time since the advent of pastoralism is little more than one generation (where generation time is the average age of reproductive adults). It is not surprising that these long-lived plants show fewer signs of change than species that have passed through many generations since pastoralism was introduced.

Tables 3.3.1.1-6 Correspondence and regression relationships for species response groups in diverse, abundant plant groups and animal taxa

Summary of results of correspondence and regression analyses used to identify species response groups for the more diverse and abundant groups and taxa at each gradient. Correspondence graphs (Appendix 3) were used to identify associations between groups of species and sites. The resulting groups of species were analysed by regressions of the form log(species abundance) = ai + bi (distance) where ai represents the intercept and bi represents the slope of the regression for the ith species. For plants growing in the field, abundance was measured as frequency of occurrence of species in quadrats. For seedbank plants and animals it represents the total number of individuals counted.

Explanatory notes for headings in summary tables:
  1. Terms that were significant in the best-fit model. D=Distance of site from water; S=species, D.S = the interaction between them. D was tested first, to determine whether there was a significant trend in abundance with distance, regardless of any differences among species. If D was significant, it was then tested in combination with S, to test whether each of the species had significantly different abundances (different intercepts) but the same trend with distance (same slope). If D was not significant, S was fitted on its own, to see whether the abundances of the different species were significantly different from each other. If D and S were both significant, they were tested in combination with D.S, to test whether each of the species had significantly different abundances (different intercepts) and significantly different trends with distance from water (different slopes).
  2. Regression slope (± standard error) of the best-fit model, for the relationship between (log) abundance of species and distance from water. Slopes are only recorded where there was a common slope for all species (ie. where the best-fit model terms were D, or D and S).
  3. Summary of best-fit model: Prob. = probability that the slope is significant; d.f. = degrees of freedom (model & residual) and %Var = percentage of variance accounted for by the model.
  4. Response groups are determined from the slope of the best-fit model: INCREASERS have negative regression slopes (indicating increasing abundance with increasing proximity to water) and decreasers have positive regression slopes (indicating the reverse). Species response groups are shown as "nd" (NOT DETERMINED) if none of the regressions was significant, or if S was the only significant term, or if the interaction D.S was significant and indicated a mixture of positive and negative trends with distance.

    The probability of the regression slope differing from zero is considered marginal at 0.10>P>0.05; where this occurs the response group is shown in lower case.

Table 3.3.1.1: Species response groups for plants growing in the understorey

Table 3.3.1.1: Species response groups for plants growing in the understorey

Notes: See above

Table 3.3.1.2: Species response groups for plants growing in the overstorey

Table 3.3.1.2: Species response groups for plants growing in the overstorey

Notes: See above

Table 3.3.1.3: Species response groups for plants in the soil seedbank

Table 3.3.1.3: Species response groups for plants in the soil seedbank

Notes: See above

Table 3.3.1.4: Species response groups for birds

Table 3.3.1.4: Species response groups for birds

Notes: See above

Table 3.3.1.5: Species response groups for reptiles

Table 3.3.1.5: Species response groups for reptiles

Notes: See above

Table 3.3.1.6: Species response groups for ants

Table 3.3.1.6: Species response groups for ants

Notes: See above

3.3.2 Proportions of species in different response groups

The proportions of each gradient's species in the different response groups were moderately consistent for most of the plant groups and animal taxa surveyed at most of the gradients (Tables 3.3.2.1-6). From 36-75% of species did not show any linear trend in abundance with distance from water (those in the "not determined" response group). Overstorey plants were the most prominent group in this category (Table 3.3.2.2). The presence of mature individuals may not be a very good indication of trend in overstorey plants, however, because they tend to be long-lived (see section 3.3.1). Therefore they may not be doing as well as the high proportions of apparently unaffected species indicate.

The abundance of the remaining species in all groups and taxa was strongly influenced by proximity to water. Many were decreasers; species whose abundance decreases with proximity to water. Across all gradients this averaged around 15% of the overstorey plant species at each gradient, around 22% of reptiles, 23% of plants in the seedbank, 23% of birds, 26% of ants and 38% of understorey plants (Tables 3.3.2.1-6). It is worth noting that the trend in abundance shown by species in these groups is log-linear (Section 1.5.3.1). That is, their abundance drops logarithmically with proximity to water. Conversely, as distance from water increases the abundance of decreaser species rises logarithmically. There is no evidence that their abundance has begun to stabilise even at the reference sites, which are 8-15 km from water (Table 1.3.1.1). Presumably their optimal habitat lies even further away from water.

Other species were increasers, whose abundance is favoured by proximity to water. On average, the proportions of increaser species in the different groups and taxa were very similar to the proportions of decreasers, ranging from 10% for overstorey plants to 33% for plants in the seedbank. (Paired two-tailed t-tests comparing increasers and decreasers in each plant group and animal taxon showed no significant differences at P<0.10). Although there was considerable variation between individual gradients, the highest average proportion of increaser species was found in the seedbank (Table 3.3.2.2). The prominence of increaser species in this plant group may be partly an artefact of the way it was assessed. The glasshouse trials detected only the readily germinable component of the soil seedbank; that is, only those species that germinated when water and temperature were not limiting and did not have any complex dormancy-breaking requirements (Section 1.4.2.5). Since large seedbanks and relative ease of germination are common attributes of colonising species (Thompson 1992) it is likely that our estimation of seedbank species was biased toward those species most likely to be favoured by disturbance. These are also the species most likely to be increasers.

Whatever the reason, both increaser species and those for which we could determine no response to distance from water, appear able to cope with the changes associated with water provision. Therefore, around 75% of species at each gradient (from 62% for understorey plants to 85% for overstorey plants) appear able to cope. Even if overstorey plants are not included, an average of around 73% of species at each gradient appear not to be disadvantaged by the provision of water for pastoralism.

These are not the species of greatest conservation concern, however. Water points are now so widespread across the pastoral rangelands that most of the chenopod and acacia rangelands now lie within 10 km of an artificial source of water (Landsberg and Gillieson 1996; Appendix 2). Therefore most of the pastoral rangelands now provide habitat that is potentially suitable for species that are advantaged or unaffected by the provision of water. In contrast, habitat likely to be suitable for the persistence of decreaser species has been reduced to a very small fraction of its former extent. Perhaps as little as 3-8% of pastoral rangelands are now potentially remote from water (calculated from Appendix 2; assuming that only one third of water points are named, and the remainder have similar patterns of distribution). Therefore the area of potentially suitable habitat remaining for the 15-38% of species showing decreaser trends may be as little as 3-8% of its original extent.

Tables 3.3.2.1-6 Proportion of each gradient's total species in each response group, for the more diverse and abundant plant groups and animal taxa.

See Tables 3.3.1.1-6 for an explanation of column headings. Only response groups where the trend is significant at P<0.05 are included.

Table 3.3.2.1: Proportion of species in response groups: plants growing in the understorey

Gradient              Total No. Spp.   %Increasers     %nd     %Decreasers   

NT mulga                    54            16.7        53.7         29.6      
NSW mulga                   55            18.2        49.1         32.7      
Qld mulga                   127           29.9        16.5         53.5      
Qld gidgee/chenopod         113           15.9         0.0         84.1      
WA chenopod/acacias         120           33.3        48.3         18.3      
SA chenopod/myall           121           18.2        43.0         38.8      
SA chenopod                 71            50.7        19.7         29.6      
WA chenopod                 63            22.2        57.1         20.6      
mean ± std error          90 ± 11        26 ± 4      36 ± 7       38 ± 8     

Table 3.3.2.2: Proportion of species in response groups: plants growing in the overstorey

Gradient                Total No.     %Increasers     %nd      %Decreasers   
                           Spp.                                              

NT mulga                    19            0.0        100.0         0.0       
NSW mulga                   19           42.1         36.8         21.1      
Qld mulga                   24            0.0         41.7         58.3      
Qld gidgee/chenopod         28            0.0        100.0         0.0       
WA chenopod/acacias         50           18.0         56.0         26.0      
SA chenopod/myall           25           16.0         68.0         16.0      
SA chenopod                 6             0.0        100.0         0.0       
WA chenopod                 13            0.0        100.0         0.0       
mean ± std error          23 ± 5        10 ± 5      75 ± 10       15 ± 7     

Table 3.3.2.3: Proportion of species in response groups: plants in the soil seedbank

Gradient              Total No. Spp.   %Increasers     %nd     %Decreasers   

NT mulga                    69            34.8        65.2         0.0       
NSW mulga                   102           36.3        41.2         22.5      
WA chenopod/acacias         83            43.4        21.7         34.9      
SA chenopod/myall           82            19.5        42.7         37.8      
WA chenopod                 51            33.3        49.0         17.6      
mean ± std error          77 ± 8         33 ± 4      44 ± 7       23 ± 7     

Table 3.3.2.4: Proportion of species in response groups: birds

Gradient                Total No.     %Increasers     %nd      %Decreasers   
                           Spp.                                              

NT mulga                    26           15.4         26.9         57.7      
NSW mulga                   28            0.0        100.0         0.0       
Qld mulga                   44           59.1         25.0         15.9      
Qld gidgee/chenopod         46            0.0         43.5         56.5      
WA chenopod/acacias         18           11.1         77.8         11.1      
SA chenopod/myall           47           21.3         57.4         21.3      
SA chenopod                 16           31.2         43.8         25.0      
WA chenopod                 15            0.0        100.0         0.0       
mean ± std error          30 ± 5        17 ± 7      59 ± 11       23 ± 8     

Table 3.3.2.5: Proportion of species in response groups: reptiles

Gradient                Total No.     %Increasers     %nd      %Decreasers   
                           Spp.                                              

NT mulga                    14           14.3         28.6         57.1      
NSW mulga                   14            0.0         85.7         14.3      
Qld mulga                   15            0.0        100.0         0.0       
Qld gidgee/chenopod         14            0.0         85.7         14.3      
WA chenopod/acacias         20           45.0         35.0         20.0      
SA chenopod/myall           23            0.0         56.5         43.5      
SA chenopod                 13           61.5         15.4         23.1      
WA chenopod                 13           23.1         76.9         0.0       
mean ± std error          16 ± 1        18 ± 8      60 ± 11       22 ± 7     

Table 3.3.2.6: Proportion of species in response groups: ants

Gradient              Total No. Spp.   %Increasers     %nd     %Decreasers   

NT mulga                    79            51.9        19.0         29.1      
NSW mulga                   100            0.0        40.0         60.0      
Qld mulga                   92            21.7        28.3         50.0      
Qld gidgee/chenopod         69             0.0        89.9         10.1      
WA chenopod/acacias         96            44.8        32.3         22.9      
SA chenopod/myall           87            28.7        58.6         12.6      
SA chenopod                 34            47.1        52.9         0.0       
WA chenopod                 50             0.0        76.0         24.0      
mean ± std error          76 ± 8         24 ± 8      50 ± 9       26 ± 7     

3.3.3 Contribution of exotics to response groups

The statistical determination of response groups did not differentiate between exotic and indigenous species, since both contribute to patterns of changing abundance. Exotics were only apparent in response groups for understorey plants and seedbank plants, where they were most prominent as increasers at some chenopod gradients (Tables 3.3.3.1-2). The highest proportion of increaser exotics was found growing in the understorey at the WA chenopod gradient. Exotics comprised half of the understorey plant species in the increaser group at this gradient (11% of all species; Table 3.3.3.1). Increaser exotics were a smaller proportion of the seedbank flora at this gradient (4%) but were still apparent (Table 3.3.3.2). Exotics were also moderately prominent among the increaser plants at the SA chenopod/myall gradient, where increaser exotics constituted 5% of both understorey and seedbank plants. Exotic species were also found in other response groups, but generally as a minor component. The highest proportions of exotic decreasers were identified in the seedbank at the NSW mulga gradient and the WA chenopod/acacias gradient, where they constituted 1-2% of species detected (Table 3.3.3.2). No exotic decreasers were detected growing in the understorey during the field survey of these gradients (Table 3.3.3.1) but their presence in the seedbank is of concern, since it suggests that exotics have the potential to establish at these gradients in little disturbed sites a long way from water. The detection of exotics among the decreasers growing in the understoreys of four of the gradients supports these concerns, although the exotics currently contribute only minor proportions of the flora (Table 3.3.3.1).

Table 3.3.3.1: Contribution of exotic species to response groups for plants growing in the understorey

Gradient       Total No.    % Increaser         % Not          %Decreaser    
                                              Determined                     

              of species   Native   Exotic   Native   Exotic   Native   Exotic  

NT mulga          54        16.7     0.0      53.7     0.0      29.6     0.0    
NSW mulga         55        18.2     0.0      47.3     1.8      32.7     0.0    
Qld mulga         127       29.1     0.8      16.5     0.0      52.0     1.6    
Qld gidg/         113       15.9     0.0      0.0      0.0      83.2     0.9    
   chen                                                                            
WA chen/          120       30.8     2.5      14.2     2.5      18.3     0.0    
   acac                                                                            
SA chen/          121       13.2     5.0      42.1     0.8      38.0     0.8    
   myall                                                                           
SA chenopod       71        47.9     2.8      19.7     0.0      29.6     0.0    
WA chenopod       63        11.1     11.1     54.0     3.2      19.0     1.6    
Mean             90.5       22.9     2.8      30.9     1.0      37.8     0.6    
Standard         10.7       4.1      1.3      6.9      0.4      7.0      0.2    
   Error                                                                           

Table 3.3.3.2: Contribution of exotic species to response groups for plants in the seedbank

Gradient       Total No.    % Increaser         % Not          %Decreaser    
                                              Determined                     

              of species   Native   Exotic   Native   Exotic   Native   Exotic  

NT mulga          70        34.3     0.0      15.7     0.0      0.0      0.0    
NSW mulga         102       34.3     2.0      16.7     0.0      20.6     2.0    
WA chen/          83        41.0     2.4      19.3     2.4      33.7     1.2    
   acac                                                                            
SA chen/          82        14.6     4.9      41.5     1.2      37.8     0.0    
   myall                                                                           
WA chenopod       51        29.4     3.9      47.1     2.0      17.6     0.0    
Mean             77.6       30.7     2.6      28.1     1.1      21.9     0.6    
Standard          7.5       4.0      0.8      6.0      0.4      6.0      0.4    
   Error                                                                           

3.3.4 Effect of seasonal conditions on response groups

Fortuitously, the four gradients sampled to represent two main vegetation types (acacia woodland and chenopod shrubland) also represented contrasting seasonal conditions in terms of rainfall: for each vegetation type two of the gradients were sampled after "below average" seasonal conditions while the other two were surveyed after "average" seasonal conditions (Section 1.3.5). This allowed us to investigate whether there were any consistent trends suggesting potential seasonal differences in the proportion of species in different response groups. We were particularly interested to see whether the proportion of decreaser species appeared to drop when seasonal conditions were relatively good ("average"), compared with relatively bad ("below average"). If so, it could indicate that populations of some decreaser species may be capable of recovering to their full potential abundance in very favourable seasons.

Unfortunately, our data provided no clear indication of consistent seasonal trends in proportions of decreaser species at the gradients. This may have been because of the very small number of replicates (only two vegetation types per season) and partly because seasonal effects were only readily apparent among species of understorey plants and birds at the acacia woodland gradients (Section 2.2.1). For birds there was no evidence of a consistent seasonal trend at these gradients: both high and low proportions of decreaser species being apparent on different gradients, regardless of season (Table 3.3.4.1). For understorey plants, however, the proportion of decreaser species was much higher on the two woodland gradients surveyed after average seasons than on the two woodland gradients surveyed after below average seasons. Thus for understorey plants, rather than there being any indication of a seasonal recovery in decreaser trends, higher proportions of decreasers were apparent in better seasons.

Table 3.3.4.1: Proportions of species identified as decreasers in the plant groups and animal taxa that showed most evidence of seasonal variation.

(See Table 2.2.1.1 for derivation of seasonal variation data, but note that some bird species were excluded from statistical analyses and are not included below.)

Group or taxon     Season          Gradient           No.      % decreasers  
                                                    species                  

understorey        below average   NT mulga            54          29.6      
plants                                                                       

                                   NSW mulga           55          32.7      

                   average         Qld mulga          127          53.5      

                                   Qld gidg/          113          84.1      
                                   chen                                      

birds              below average   NT mulga           26           57.7      

                                   NSW mulga          28            0.0      

                   average         Qld mulga          44           15.9      

                                   Qld gidg/          46           56.5      
                                   chen                                      

3.3.5 Response types for individual species in less diverse or abundant taxa

The numbers of species of small mammals and springtails detected were so low (Table 2.1.1) that correspondence analysis was not appropriate for these taxa. Nor was it appropriate for any other of the invertebrate taxa identified to species, apart from ants. Species numbers for beetles and grasshoppers tended to be higher than for springtails, but the numbers of animals per species were still very low (Table 2.1.2), and high counts were restricted to very few species. Since there were so few species with moderately high counts among these taxa, regression analyses were undertaken individually for each species with a total count of > 5 animals per gradient (Section 1.5.3.2).

However, few of the regressions were significant, and these did not tend to be consistent. For small mammals the two different response types that were significant were only marginally so, and of doubtful biological significance (Table 3.3.5.1). Except for unidentified immature springtails, only three species of springtails showed significant trends: one showed a decreaser trend at one gradient, another an intermediate (hump-shaped) trend at another gradient, and a third showed an increaser trend at a third gradient (Table 3.3.5.2). Drepanura cinquilineata was one of the most abundant and widespread springtails (occurring at three of the four gradients assessed) but its abundance did not generally vary significantly with distance from water, apart from a weak decreaser trend at one gradient.

Similarly, trends shown by individual species of beetles were not significant or consistent (Table 3.3.5.3). Of the eleven species for which regressions were tested only three showed any significant trends and they were all different. The most abundant and widespread beetle, Corticaria subtilissima, did not show any significant trends with distance from water. Twelve species of grasshoppers and crickets were sufficiently abundant to analyse (Table 3.3.5.4). Two showed significant and consistent trends of increasing in abundance close to water, but ten, including the most widespread and abundant species, Endocusta spA, showed no significant relationships with distance from water.

Table 3.3.5.1-4 Regression relationships for species in less well represented taxa

Summary of results of regression analyses for individual species in animal taxa with low species diversity and/or low abundance. The regressions tested were of the form log(count) = a + b1 (distance) + b2 (distance)². Regressions were only tested for those species that had a total count of > 5 animals across a gradient.

Explanatory notes for headings in summary tables:
  1. Species with total counts > 5 animals in the gradient
  2. Significant terms and their coefficients (regression slope ± standard error) for the most significant relationship between (log) count and distance from water.
  3. Summary of best-fit model: Prob. = probability that the slope is significant; d.f. = degrees of freedom (model & residual) and %Var = percentage of variance accounted for by the model.
  4. Response types are recorded as INC (increaser) when log (count) = a - b1 (distance); DEC (decreaser) when log (count) = a + b1 (distance); INT (intermediate) when log (count) = a + b1 (distance) - b2 (distance)² and INV (inverted) when log (count) = a - b1 (distance) + b2 (distance)².

    When regressions were significant at P<0.05 response types are shown in upper case; when regressions were only marginally significant (0.10>P>0.05) response types are shown in lower case.

Table 3.3.5.1: Small mammals: Regressions for species with a total count of > 5 animals across a gradient. (*=Exotic species)

Table 3.3.5.1: Small mammals: Regressions for species with a total count of greater than 5 animals across a gradient.

Notes: See above

Table 3.3.5.2: Springtails (Collembola): Regressions for species with a total count of > 5 animals across a gradient

Table 3.3.5.2: Springtails (Collembola): Regressions for species with a total count of greater than 5 animals across a gradient

Notes: See above

Table 3.3.5.3: Beetles (Coleoptera): Regressions for species with a total count of > 5 animals across a gradient

Table 3.3.5.3: Beetles (Coleoptera): Regressions for species with a total count of greater than 5 animals across a gradient

Notes: See above

Table 3.3.5.4: Grasshoppers and crickets (Orthoptera): Regressions for species with a total count of > 5 animals across a gradient

Table 3.3.5.4: Grasshoppers and crickets (Orthoptera): Regressions for species with a total count of greater than 5 animals across a gradient

Notes: See above

3.3.6 Species found only at reference sites

Many of the species identified as decreasers were locally rare (Appendix 4) and moderately high proportions of species were found only at the reference sites (Tables 3.3.6.1-6) which were 8-15 km from water (Table 1.3.1.1). If the distance from water is the primary reason for the local rarity of these species it is a matter of great conservation concern, because there are very few areas left this far from water in the more productive rangelands (Appendix 2).

Table 3.3.6.1: Understorey plant species found only at the reference sites

Gradient            Total number of   Number found only   % found only at   
                        species          at reference        reference      

NT mulga                   54                 5                 9.3         
NSW mulga                  55                 6                 10.9        
Qld mulga                 127                 9                 7.1         
Qld gidg/chen             113                 10                8.8         
WA chen/acacias           120                 9                 7.5         
SA chen/myall             121                 16                13.2        
SA chenopod                71                 5                 7.0         
WA chenopod                63                 3                 4.8         
mean ± std error         91±11               8±1                9±1         

Table 3.3.6.2: Overstorey plant species found only at the reference sites

Gradient            Total number of   Number found only   % found only at   
                        species          at reference        reference      

NT mulga                   19                 0                 0.0         
NSW mulga                  19                 4                 21.1        
Qld mulga                  24                 4                 16.7        
Qld gidg/chen              28                 0                 0.0         
WA chen/acacias            50                 3                 6.0         
SA chen/myall              25                 1                 4.0         
SA chenopod                6                  0                 0.0         
WA chenopod                13                 1                 7.7         
mean ± std error          23±5               2±1                7±3         

Table 3.3.6.3: Plant species in the soil seedbank found only at the reference sites

Gradient           Total number of    Number found only   % found only at   
                       species          at reference         reference      

NT mulga                  71                  3                 4.2         
NSW mulga                103                  7                 6.8         
WA chen/acacias           89                  6                 6.7         
SA chen/myall             83                  2                 2.4         
WA chenopod               56                  1                 1.8         
mean ± std error         80±8                4±1                4±1         

Table 3.3.6.4: Bird species found only at the reference sites

Gradient           Total number of    Number found only   % found only at   
                       species          at reference         reference      

NT mulga                 21                   3                 14.3        
NSW mulga                28                   3                 10.7        
Qld mulga                44                   0                 0.0         
Qld gidg/chen            44                   4                 9.1         
WA chen/acacias          18                   1                 5.6         

SA chen/myall            47                   4                 8.5         
SA chenopod              16                   1                 6.2         
WA chenopod              15                   0                 0.0         
mean ± std error        29±5                 2±1                4±1         

Table 3.3.6.5: Reptile species found only at the reference sites

Gradient            Total number of   Number found only   % found only at   
                        species          at reference        reference      

NT mulga                   14                 1                 7.1         
NSW mulga                  14                 2                 14.3        
Qld mulga                  15                 2                 13.3        
Qld gidg/chen              14                 0                 0.0         
WA chen/acacias            20                 1                 5.0         
SA chen/myall              23                 3                 13.0        
SA chenopod                13                 3                 23.1        
WA chenopod                13                 0                 0.0         
mean ± std error          16±1              2±0.5               9±3         

Table 3.3.6.6: Ant species found only at the reference sites

Gradient            Total number of   Number found only   % found only at   
                        species          at reference        reference      

NT mulga                   80                 4                 5.0         
NSW mulga                  99                 8                 8.1         
Qld mulga                  92                 5                 5.4         
Qld gidg/chen              69                 3                 4.3         
WA chen/acacias            96                 2                 2.1         
SA chen/myall              87                 4                 4.6         
SA chenopod                34                 2                 5.9         
WA chenopod                50                 2                 4.0         
mean ± std error          76±8               4±1                5±1         

Unfortunately, it is not possible to determine from the gradient surveys the reasons why some species were found only at the reference sites. One possibility is that the number of species was not related to grazing intensity, but was merely the result of natural variation in the distribution and abundances of some species. Study sites were not arrayed at uniform distances along the gradients; instead the spacing between the reference sites and their nearest neighbour was greater than the spacing between the sites closest to water (Table 1.3.1.1). Thus it is possible that the reference sites had high numbers of species not found at other sites because they were relatively isolated, while the sites close to water may have had more species in common because they were closer together.

One way to test this is to consider those species found only at a subset of equidistant sites. For all the gradients except NT mulga, sites 2, 4, 5 and 6 were approximately 2.5 km apart. For these gradients, a subset was formed by excluding the other two sites (sites 1 and 3) and the species found only at them. For most plant groups and animal taxa (the exception was understorey plants) there were no significant differences among any of these four sites in the numbers of species found at only one of them (Tables 3.3.6.8 -13). That is, for most groups and taxa the number of species found only at a reference site was comparable to the number of species found only at any one of the other three equidistant sites along a gradient, regardless of distance from water. For the understorey plants, however, the number of species found only at site 6 was significantly higher than the number of species found only at site 2 or site 4 or site 5 (Table 3.3.6.7 and 3.3.6.13; 0.01<P<0.05).

Table 3.3.6.7: Plants growing in the understorey found only at one of sites 2, 4, 5 or 6

Gradient               Site 2   Site 4    Site 5   Site 6  

NSW mulga                3         6        4        7     
Qld mulga                4         5        10       13    
Qld gidgee/chenopod      5         8        7        14    
WA chenopod/acacias      10        8        8        10    
SA chenopod/myall        9        11        6        20    
SA chenopod              3        11        4        5     
WA chenopod              6         4        1        3     

Table 3.3.6.8: Plants growing in the overstorey found only at one of sites 2, 4, 5 or 6

Gradient               Site 2   Site 4    Site 5   Site 6  

NSW mulga                2         1        0        4     
Qld mulga                3         0        1        5     
Qld gidgee/chenopod      0         6        3        3     
WA chenopod/acacias      7         3        4        3     
SA chenopod/myall        4         0        1        2     
SA chenopod              0         1        2        0     
WA chenopod              1         0        3        2     

Table 3.3.6.9: Plants in the soil seedbank found only at one of sites 2, 4, 5 or 6

Gradient               Site 2   Site 4    Site 5   Site 6  

NSW mulga                9        13        5        11    
WA chenopod/acacias      9         7        7        13    
SA chenopod/myall        6         8        10       2     
WA chenopod              4         2        4        3     

Table 3.3.6.10: Bird species found only at one of sites 2, 4, 5 or 6

Gradient              Site 2   Site 4    Site 5   Site 6  

NSW mulga               5         1        3        3     
Qld mulga               7         1        5        3     
Qld gidgee/chenopod     1         3        2        4     

WA chenopod/acacias     2         2        0        1     

SA chenopod myall       1         5        5        6     
SA chenopod             4         1        0        1     
WA chenopod             0         1        2        0     

Table 3.3.6.11: Reptile species found only at one of sites 2, 4, 5 or 6

Gradient              Site 2   Site 4    Site 5   Site 6  

NSW mulga               0         0        2        3     
Qld mulga               3         1        1        2     
Qld gidgee/chenopod     0         4        3        1     

WA chenopo/acacia       2         1        4        1     

SA chenopod myal l      2         4        2        5     
SA chenopod             1         0        1        3     
WA chenopod             0         0        2        1     

Table 3.3.6.12: Ant species found only at one of sites 2, 4, 5 or 6

Gradient              Site 2   Site 4    Site 5   Site 6  

NSW mulga               8         7        9        12    
Qld mulga               5         6        11       7     
Qld gidgee/chenopod     6         4        14       4     

WA chenopod/acacia      13        4        9        4     

SA chenopod myall       8        12        4        6     
SA chenopod             8         5        1        2     
WA chenopod             3         5        2        2     

Table 3.3.6.13: Statistical analysis of differences among the four equidistant sites in the number of species found only at one of them.

(See Section 1.5.4 for analytical details.)

Group or taxon       d.f.   deviance ratio    Prob.   

Understoreyplants   3, 18        3.76         <0.05   

Overstorey plants   3, 18        0.46         >0.10   
Seedbank plants      3,9         0.07         >0.10   
Birds               3, 18        0.24         >0.10   
Reptiles            3, 18        0.69         >0.10   
Ants                3, 18        0.54         >0.10   

Apparently for most plant groups and animal taxa, some species were so uncommon along the gradients that they were detected at only one of the four equidistant sites. They may have been locally rare along a gradient because the gradient's environment was only marginally suitable for them. If this was the case they were probably more common in other parts of the landscape (e.g. upslope or downslope of our sites), or other landscape types (e.g. dunefields or floodplains), or other climatic zones (e.g. more humid or more arid). For species for which a gradient's environment was only marginal, occurrence and detection at any one site might reflect nothing more than chance.

This possibility does not negate the concern about species found only at the reference sites, however. The total number of species found at each of the sites along a gradient was usually rather constant (Section 3.2), but this was a composite result masking considerable variation in underlying composition. The total number of species at each site represented a changing balance between increaser species predominating near water and decreaser species predominating far from water (Figure 3.3.1.1). It is possible that some of the species found at only one site reflect this underlying compositional shift. For understorey plants, the number of species occurring only at one site was significantly higher at the reference sites than at any of the other equidistant sites. Understorey plants constituted the most species-rich group or taxon sampled and are also most directly affected by the elevated grazing that occurs around water points. Thus it is possible that this group might be the most sensitive indicator of species in decline.

A more regional picture of the distribution and abundance of species is needed before it will be possible to determine whether the species found only at the reference sites are more abundant elsewhere in the region, independently of proximity to water. Alternatively, systematic regional surveys may identify species that are everywhere restricted to sites remote from water; for most of the productive rangelands water is now so widespread that any such species may be at risk of regional extinction.