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)

3.4 Identities of species in different response groups

3.4.1 Plants

Increaser and decreaser response groups were determined separately for each gradient and each plant group. Full listings of the response groups determined for all the species recorded at each gradient are given in Appendix 4. Of the total of 673 plant species identified, many (around 75%) were recorded at one gradient only. They include annual and perennial grasses and forbs, shrubs and trees, from many families. There are too many species to consider their identities individually. If they could be classified by their functional attributes, rather than their taxonomic identities, we could determine which attributes increaser and decreaser species have in common. The development of such a scheme is desirable (Sections 4.2.3 and 4.3.3), but is outside the scope of this report.

Some species were recorded at two or more gradients, however. For this subset of species it is instructive to consider the identities of those that showed the same pattern of response on different gradients (e.g those that were classified as decreasers on each gradient where they occurred) with those that showed different patterns of response on different gradients (e.g. those that were classified as decreasers on some gradients and increasers on others).

Around 130 of the 478 plant species growing in the understorey were recorded at two or more gradients. Moderate numbers were classified as decreasers on at least two gradients and were not classified as increasers at any gradient, although some were classified "not determined" at some gradients (Table 3.4.1.1). These "repeat decreasers" included forbs, grasses and shrubs from many families (Appendix 4). Some, such as the grasses Enneapogon polyphyllus and Monachather paradoxa, are known to be highly palatable and prone to decrease under sustained grazing (Appendix 1; Cunningham et al. 1992). Others, such as woollybutt grass, Eragrostis eriopoda, and purple lovegrass, E. lacunaria, are more usually considered to be hardy and persistent under grazing and some, such as the copper burrs Dissocarpus paradoxus and Sclerolaena decurrens are considered to be seldom eaten by stock. Others, such as Centipeda thespidioides, are known to be palatable to stock (Cunningham et al. 1992) but have not been reported as liable to decline under grazing. Very few of the repeat decreasers are commonly recognised "indicator species" for range assessment (e.g. none is listed in the extensive lists presented by Pringle 1994). One of the repeat decreasers was an exotic but its abundance was very low. Many other species of repeat decreasers were also locally uncommon or rare, however; whether this is a cause or a consequence of their decreaser trends is a moot point.

We identified very few species that were increasers at several gradients and decreasers at none (Table 3.4.1.2). Two of the five repeat increasers, Carrichtera annua and Schismus barbatus, are exotic weeds that have become widespread in the semi-arid regions of southern Australia (Section 2.2.3). They were also the most abundant repeat increasers (Appendix 4). One of the other repeat increasers is a common and widespread copperburr, Sclerolaena diacantha, which is considered one of the more valuable copper burrs for pastoralism because of its moderate palatability (in contrast to the reputed lower palatability of the three decreaser copper burrs), coupled with an ability to colonise eroded areas (Cunningham et al. 1992). The occurrence of Sclerolaena species in both increaser and decreaser response groups shows clearly that the characteristics associated with persistence under disturbance may be quite subtle and need not necessarily correlate very closely with taxonomic relationships.


Table 3.4.1.1: Understorey plant species that were repeat decreasers

ie., species that were classified as decreasers at two or more gradients and were not classified as increasers at any gradient.

Species                  Inc    Dec     ND     Total    Mn freq%     

*Malvastrum americanum          2       1      3        0.07         
Abutilon malvifolium            2       1      3        0.07         
Amaranthus mitchellii           2              2        0.415        
Aristida obscura                2              2        1.69         
Brachiaria gilesii              2              2        0.105        
Bulbine alata                   2              2        28.44        
Calandrinia eremaea             3       1      4        1.043        
Calotis cuneifolia              2       1      3        0.217        
Calotis inermis                 2              2        0.01         
Centipeda thespidioides         2       3      5        0.25         
Dactyloctenium radulans         3              3        0.973        
Dissocarpus paradoxus           2       1      3        6.18         
Enneapogon polyphyllus          2       2      4        2.245        
Eragrostis eriopoda             2       3      5        0.512        
Eragrostis lacunaria            2              2        0.02         
Erodium cygnorum                2              2        0.105        
Goodenia berardiana             2              2        0.21         
Goodenia cycloptera             2              2        1.875        
Lepidium oxytrichum             2              2        11.565       
Lepidium phlebopetalum          3       2      5        0.796        
Maireana villosa                2              2        0.53         
Malacocera tricornis            2              2        0.01         
Marsdenia australis             2              2        0.01         
Monachather paradoxa            2       1      3        0.23         
Portulaca filifolia             2       1      3        0.07         
Portulaca oleracea              2              2        11.875       
Rhagodia spinescens             2       1      3        0.07         
Rhodanthe maryonii              2              2        0.01         
Rhyncharrhena linearis          2              2        0.01         
Schoenia ayersii                2              2        0.105        
Sclerolaena decurrens           2              2        6.46         
Sporobolus caroli               2              2        0.23         
Triglochin calcitrapum          2              2        3.96         
Triraphis mollis                2              2        1.77         
Vittadinia eremaea              3              3        0.21         

Inc = the number of gradients where classified increasers; Dec = the number where classified decreasers; and ND = the number where classified not determined. Total is the total number of gradients where recorded. Mnfreq% is the mean frequency across all gradients. *=Exotic species.

Table 3.4.1.2: Understorey plant species that were repeat increasers

ie., species that were classified as increasers at two or more gradients and were not classified as decreasers at any gradient.

Species                   Inc    Dec    ND      Total   Mn freq%    

*Carrichtera annua        2                     2       4.01        
*Schismus barbatus        2                     2       11.04       
Osteocarpum               2                     2       0.05        
   dipterocarpum                                                       
Sclerolaena diacantha     2             1       3       2.237       
Solanum esuriale          2                     2       1.165       

Inc = the number of gradients where classified increasers; Dec = the number where classified decreasers; and ND = the number where classified not determined. Total is the total number of gradients where recorded. Mnfreq% is the mean frequency across all gradients. *=Exotic species.

Also of great significance, however, is the number of species that showed different response trends at different gradients (Table 3.4.1.3). Some of these species could have been mis-classified; this is particularly likely to occur when a species was locally rare or patchily distributed. For example, ruby saltbush, Enchylaena tomentosa, is a widespread shrub that is generally considered a decreaser species (e.g. Pringle 1994). It was classified in the decreaser group at three of the gradients where it occurred, and as an increaser at one gradient only, where it was locally rare (Appendix 4). Similarly, blue crowfoot, Erodium crinitum, a widespread herb, is generally considered highly palatable to stock (Cunningham et al. 1992) and therefore would be expected to show a decreaser response. It was classified as a decreaser at the four gradients where it was more common and an increaser at only one gradient, where it was locally rare (Appendix 4). In these instances it is possible that the apparent increaser trends in abundance may have arisen by chance or been due to responses to underlying environmental variations, rather than responses to distance from water.

For many of the species with mixed responses, however, their grouping as decreasers in one environment and increasers in another may be a real result of differences in environmental context. This is most likely to be the case where they were classified as decreasers in several gradients and increasers in several others (Table 3.4.1.3). The expression of plant traits and grazing animal preferences are both highly context-specific. For example, a grazing animal may prefer a plant species in one community (e.g. if it is the only perennial grass on offer) and avoid it in another (e.g. if it is a very fibrous grass in a community dominated by softer grasses). Similarly a plant species may be very rare and therefore vulnerable in one plant community (e.g. near the edge of its range) but very abundant and therefore more likely to persist in another. Thus just as taxonomic relationships (such as species belonging to the same genus) do not necessarily correlate with ability to persist in the face of disturbance, the ability of a species to persist in one environment does not necessarily correlate with its ability to persist in a different environment.

Table 3.4.1.3: Understorey plant species that were mixed responders

ie., species that were classified as decreasers at one or more gradients and were also classified as increasers at one or more gradients.

Species                            Inc    Dec     ND      Total   Mn freq%    

*Sonchus oleraceus                 1      2       1       4       0.005       
Abutilon otocarpum                 2      1       1       4       0.058       
Acacia aneura                      2      1       1       4       0.213       
Aristida contorta                  2      2       1       5       3.42        
Atriplex holocarpa                 1      1               2       1.875       
Atriplex vesicaria                 1      2       1       4       9.895       
Brachycome ciliaris                1      2       1       4       0.99        
Calotis hispidula                  2      3       2       7       0.654       
Calotis plumulifera                1      1               2       9.48        
Chamaesyce drummondii              1      2       3       6       7.257       
Chenopodium cristatum              3      1               4       0.163       
Chenopodium melanocarpum           2      3       1       6       0.007       
Convolvulus erubescens             1      2       1       4       0.833       
Crassula colorata                  2      1               3       4.513       
Daucus glochidiatus                1      4       2       7       0.239       
Digitaria brownii                  1      1       1       3       6.75        
Dissocarpus biflorus               1      1               2       0.125       
Enchylaena tomentosa               1      3       2       6       0.368       
Enteropogon acicularis             2      1               3       0.007       
Eragrostis dielsii                 1      2       1       4       0.053       
Eremophila gilesii                 1      1               2       0.22        
Erodium crinitum                   1      4       1       6       0.972       
Euphorbia eremophila               1      2               3       12.64       
Fimbristylis dichotoma             1      1       1       3       0.417       
Gnephosis arachnoidea              2      2       1       5       0.088       
Hyalosperma semisterile            1      1               2       1.98        
Nicotiana occidentalis             2      1               3       0.013       
Nicotiana velutina                 1      1               2       1.665       
Pimelea simplex                    1      1               2       0.335       
Plantago turrifera                 1      2       1       4       0.52        
Ptilotus gaudichaudii              1      2               3       3.487       
Ptilotus helipteroides var.        1      1               2       0.105       
   helipteroides                                                                 
Ptilotus obovatus                  2      2       1       5       0.004       
Rhodanthe floribunda               1      4       1       6       3.618       
Rhodanthe stricta                  1      2               3       0.763       
Salsola kali                       1      4       2       7       0.274       
Senna artemisioides artemisioides  2      1               3       0.277       
Sida fibulifera                    2      3               5       0.584       
Solanum ellipticum                 2      2       1       5       0.066       
Solanum quadriloculatum            1      3       1       5       0.02        
Sporobolus actinocladus            1      2               3       1.057       
Tetragonia eremaea                 1      1               2       2.51        
Thysanotus baueri                  1      1               2       0.52        
Tragus australianus                2      1               3       0.277       
Tripogon loliiformis               1      3       1       5       3.334       
Zygophyllum simile                 2      1               3       5.493       

Inc = the number of gradients where classified increasers; Dec = the number where classified decreasers; and ND = the number where classified not determined. Total is the total number of gradients where recorded. Mnfreq% is the mean frequency across all gradients. *=Exotic species.

Several of the species identified as repeat decreasers or repeat increasers in the seedbank showed the same pattern as understorey plants growing in the field (Tables 3.4.1.4-5). For example, the grasses Enneapogon polyphyllus and Eragrostis eriopoda were identified as decreasers at more than one gradient in both seedbank and field (Tables 3.4.1.4 & 1). Similarly the exotic weed Carrichtera annua was consistently identified as an increaser in the seedbank and field at a number of gradients (Tables 3.4.1.5 & 2).

Table 3.4.1.4 Seedbank plant species that were repeat decreasers

ie., species that were classified as decreasers at two or more gradients and were not classified as increasers at any gradient.

Species                     Inc     Dec     ND     Total    No. of indiv..  

*Rostraria pumila                   2       1      3        42              
Aristida contorta                   4              4        74              
Aristida sp1620                     2       1      3        2               
Calandrinia balonensis              2              2        1               
Calandrinia polyandra               2              2        8               
Calandrinia sp2712                  2              2        1               
Calocephalus ?knappi                2              2        38              
UCalotis inermis                    3              3        2               
Crassula sieberiana                 3              3        449             
Crassula spC7                       2       1      3        9               
Cyperus sp2199                      2       1      3        4               
Digitaria brownii                   2              2        19              
UEnneapogon polyphyllus             2              2        263             
UEragrostis eriopoda                3       1      4        5               
Mollugo cerviana                    2              2        16              
Paspalidium rarum                   2              2        1               
Pseudognaphalium                    2       1      3        7               
   luteoalbum                                                                  
Rhodanthe floribunda                2       1      3        13              
Sclerolaena sp1891                  2              2        11              
unidentified sp2011                 2              2        9               
Wahlenbergia gracilenta             2              2        87              
Zygophyllum spC2                    2              2        2               

Column headings as above, except that abundance is shown by average number of individuals per gradient. U indicates species that were also classified as repeat decreasers growing in the understorey.

Several of the species identified as repeat decreasers in the seedbank (e.g. Calandrinia balonensis, C. polyandra) are reported to be very palatable to livestock (Cunningham et al. 1992) and were not detected growing in the field. Others, such as Digitaria brownii, were classified as mixed responders in the field (Table 3.4.1.3) but are recorded as highly palatable and liable to decrease in degraded areas (Cunningham et al. 1992); thus their inclusion as decreasers in the seedbank is consistent with what is known of their persistence under grazing. Rhodanthe floribunda, however, has previously been reported as becoming abundant in heavily grazed areas (Cunningham et al. 1992), although our surveys suggest otherwise: it was classified as a decreaser in the understorey of four gradients and an increaser at only one gradient (Table 3.4.1.3); and was classified as a repeat decreaser in the seedbank (Table 3.4.1.4).

Table 3.4.1.5: Seedbank plant species that were repeat increasers

ie., species that were classified as increasers at two or more gradients and were not classified as decreasers at any gradient.

Species                    Inc     Dec     ND     Total    No. of Indiv..  

*UCarrichtera annua        2                      2        1               
Bergia trimera             2                      2        1113            
Calotis hispidula          3               2      5        35              
Centipeda thespidioides    2                      2        5               
Chenopodium cristatum      3               1      4        1               
Chenopodium melanocarpum   5                      5        2               
Sclerolaena patenticuspis  2                      2        35              
Synaptantha tillaeacea     2               1      3        1               

Inc = the number of gradients where classified increasers; Dec = the number where classified decreasers; and ND = the number where classified not determined. Total is the total number of gradients where recorded. Mnfreq% is the mean frequency across all gradients. *=Exotic species. U indicates species that were also classified as repeat increasers growing in the understorey.

Several of the seedbank repeat increasers (e.g. Chenopodium cristatum, C. melanocarpum, Sclerolaena patenticuspis) are reported to be unpalatable to stock and/or indicators of severely grazed sites (Appendix 1; Cunningham et al. 1992); their identification as increasers in the seedbank is consistent with this description. Bergia trimera, one of the species identified as an increaser in our seedbank studies, has also been identified as increasing in the seedbank at sites close to water in another grazing study (Navie et al. 1996). The classification of Centipeda thespidioides as a repeat increaser in the seedbank is puzzling, however, since it was classified as a repeat decreaser when growing in the understorey (Table 3.4.1.1). Its close relative C. cunninghamii is reported to be unpalatable to stock much of the time, but utilised to a moderate extent in dry times (Cunningham et al. 1992). Perhaps this is also the case for C. thespidioides. If so, the pattern of decreasing abundance in the field might reflect recent grazing patterns resulting from depletion and drying off of more preferred species, while its pattern of increasing abundance in the seedbank might reflect more long-term grazing patterns.

Only one species of shrub was classified in the same response group at different gradients (Appendix 4). It was Pimelea microcephala, Shrubby Rice-flower, and was classified as an increaser at the two gradients where it occurred. Although not generally considered as prone to increase it is known to be unpalatable to stock and very hardy. It has also been suspected of poisoning stock (Cunningham et al. 1992).

3.4.2 Birds

Among bird species, the Galah, Crested Pigeon, Yellow-throated Miner, and corvids (crows and ravens) were the most consistently identified increaser species, in both our own and previous studies (Table 3.4.2.1; Appendix 4; Curry and Hacker 1990; Saunders and Curry 1990), and were often very abundant within only 1 km of water. We also identified as increasers, a number of other species that have been reported as increasing in abundance in heavily grazed rangelands in Western Australia. Examples include Bourke's Parrot and Zebra Finch (Saunders and Curry 1990). However, Bourke's Parrot has also been identified as having undergone a pronounced decrease in abundance in New South Wales (Smith and Smith 1994) and responded as a decreaser at one of our Queensland gradients (Appendix 4). The Spiny-cheeked Honeyeater was another decreaser identified at this gradient, that has apparently increased in abundance and/or range in Western Australia (Saunders and Curry 1990). These and some other species that were only found on a few gradients, or where classed as increasers for some of the gradients on which they occurred (e.g., Australian Ringneck, Blue Bonnet, Common Bronzewing) are likely to be abundant close to water due to their dependence on drinking water.

Many of the species identified as having increased in abundance and/or geographic range in the arid zone since European occupation (Reid and Fleming 1992) were recorded during our surveys (Table 3.4.2.1). Most of these species were recorded as increasers or not determined, however this was not always consistent when the species occurred on a number of gradients. The occasional recording of one of these species in a decreaser group along one gradient may be a chance statistical occurrence, or an anomaly of the particular habitat of the gradient or the time of sampling.

Table 3.4.2.1: Birds seen during the surveys that have been identified as having increased in range or abundance since European occupation of the arid zone.

Species               Number of  Sites   Abundance     Response groups       
                      gradients                                              

Australian Kestrel        4      1-6     Common        EX on 2; ND on 2      
Banded Lapwing            2      1-2     Common        INC                   
Common Bronzewing         5      1-6     Common        INC on 3; ND on 1; DEC on 1              
Crested Pigeon            6      1-6     Abundant      INC on 5; ND on 1     
Bourke's Parrot           2      2-6     Common        DEC                   
Galah                     7      1-6     Abundant      INC on 5; ND on 2     
Spiny-cheeked             6      1-6     Abundant      INC on 2; ND on 4     
   Honeyeater                                                                   
Yellow-throated           5      1-6     Abundant      INC on 4; ND on 1     
   Miner                                                                        
White-plumed              1      1,2,6   Common        INC                   
   Honeyeater                                                                   
Yellow-rumped             5      2-6     Common        ND on 3; DEC on 2     
   Thornbill                                                                    
Southern Whiteface        6      1-6     Abundant      ND on 4; INC on 1; DEC on 1              
Willie Wagtail            6      1-6     Common        INC on 3; DEC on 2; ND on 1               
Black-faced               7      1-6     Abundant      ND on 5; DEC on 2     
   Woodswallow                                                                  
Australian Raven          4      1-6     Common        EX on 2; DEC on 1; ND on 1               
Little Crow               3      1-5     Common        INC on 2; ND on 1     
Torresian Crow            1      1,3,4,6 Rare          ND                    
Richard's Pipit           4      1-6     Common        ND on 3; INC on 1; DEC on 1              
Brown Songlark            3      1,3,5,6 Common        INC on 1; DEC on 1; ND on 1               

EX = excluded from analyses INC = increaser; DEC = decreaser; ND = not determined. Rare = 1-10; Common = 11-100; Abundant >100.

Source: Table 8 in Reid and Fleming 1992.

Species of birds detected during the study that have been recognised as having declined or being at risk (Reid and Fleming 1992) are shown in Table 3.4.2.2. Many of these species were rare and so their abundance profile along the gradient could not be determined. Where the species were classified into a response group, they were mostly identified as decreasers. For those species that could not be classified, they were usually sighted on sites away from water (ie sites 4-6). Comparison of Tables 3.4.2.1 and 3.4.2.2 shows that birds that have been identified as having increased in abundance or geographic range generally occurred on more gradients and were more abundant than species that have been identified as rare or threatened, or having declined in abundance, or being at risk.

Table 3.4.2.2: Birds seen during the surveys that have been identified as rare or threatened

(Garnett 1992), or having declined in abundance or being at risk (from Table 6 in Reid and Fleming 1992).

Species                 Number of   Sites    Abundance   Response groups     
                        gradients                                            

Little Button-quail         3       3-6      Rare        EX on 2, ND on 1    
Pink Cockatoo               1       5        Rare        ND                  
Scarlet-chested             1       3-4      Rare        ND                  
   Parrot                                                                       
White-winged                4       1-6      Abundant    ND on 3, DEC on 1   
   Fairy-wren                                                                   
Grey Honeyeater             1       6 only   Rare        DEC                 
Pied Honeyeater             1       4        Rare        DEC                 
Redthroat                   3       1-6      Common      ND on 2, DEC on 1   
Rufous Fieldwren            2       1-6      Common      ND                  
Red-browed Pardalote        2       2-3      Rare        ND and INC          
Chiming Wedgebill           2       1-6      Common      ND and DEC          
Hall's Babbler              2       4-6      Common      ND                  
Cinnamon Quail-thrush       2       1-6      Common      ND and DEC          

EX = excluded from analyses; INC = increaser; DEC = decreaser; ND = not determined. Rare = 1-10; Common = 11-100; Abundant >100.

It is not surprising that species dependent on water were classed consistently as increasers because their current distribution and abundance throughout the arid and semi-arid zones is a direct reflection of the provision of artificial water. In contrast, species that are not dependent on regular drinks occurred throughout these landscapes before the advent of pastoralism and the provision of water. The factors influencing the distribution and abundance of species that do not depend on water are likely to be more complex. Hence, in these species we see less consistency among gradients in their classification in relation to distance from water.

Some of the factors that may influence the distribution and abundance of species in relation to a piosphere are: (1) the presence of aggressively-dominant bird species near a water point; (2) disturbance and opening-up of the habitat under heavy grazing; (3) changes to vegetation composition and structure; and (4) changes to food resources. No patterns of species displacement by known aggressively-dominant bird species (e.g., Yellow-throated miners – Grey 1996) were obvious in our gradient data, although displacement of some species possibly occurs. There was a suggestive anomaly in the bird data from the NSW mulga gradient. At this gradient the reference site was lightly grazed because it was a long way from the nearest water accessible to large grazing animals, but for birds, water was readily available across the dingo-proof border fence (Section 1.3.4). Yellow-throated Miners, which were abundant all along this gradient were particularly prominent at the reference site. Presumably this was in direct response to the availability of water, rather than as an indirect consequence of grazing impact. No other bird species were common at the reference; therefore it is possible that other species may have been displaced by the aggressively-dominant Miners.

The second and third factors incorporate a range of changes in the vegetation and ground surface and no common response to these is apparent (or expected). Ground-dwelling species have been suggested as one group at high risk from modification of their habitat because of grazing (Reid and Fleming 1992) and our data show that ground-feeders were a larger proportion of the decreaser species than they were for increaser species (Table 3.4.2.3) In contrast, canopy and shrub feeders were a larger proportion of the increaser species than they were for decreaser species. There was no predominance of taxa such as bush birds among the decreasers or pigeons and parrots among the increasers, nor is there any relationship apparent between the structure of the dominant vegetation on a gradient (woodland vs. shrubland) and the proportion of decreaser bird species identified there.

Table 3.4.2.3: Proportions of increaser and decreaser bird species in guilds that predominantly forage at different levels.

Foraging level    % of decreasers  % of            
                                   increasers      

Aerial            8                2               
Canopy and shrub  32               47              
Ground            61               52              
Total no.         59               60              
species

3.4.3 Reptiles

As for birds, attributes pre-disposing some species of reptiles to decline are not readily apparent. In general, the clarity of the response groups for reptiles was not as obvious as it was for other plant groups and animal taxa. That is, many species were relatively uniformly spread along gradients, suggesting that they may be relatively insensitive to piosphere effects. James et al. (1995b) suggested that diurnal species closely associated with shrub leaf-litter, such as Morethia boulengeri and Ctenotus regius, may be disadvantaged by heavy grazing if the leaf-litter is disrupted. However, we could only detect a decline in leaf litter close to water at two gradients (NSW and Qld mulga gradients; Section 3.1) and neither species was identified among the decreaser species there or elsewhere (Appendix 4). There were no obvious associations between particular guilds (nocturnal vs. diurnal) or families of reptiles and membership of different response groups.

3.4.4 Ants

Many species of ants occurred on more than one gradient (Appendix 4), but only a few of these were repeatedly classified as increasers (or decreasers) at each of the gradients where they occurred (Table 3.4.4.1). Of the 11 species that were repeatedly classified in the same response group, eight were species that showed a decreaser pattern. In addition to the three species that were repeatedly classified as increasers (and never classified as decreasers; Table 3.4.4.1) several other species were predominantly classified as increasers, ie. they occurred on more than two gradients and were classified as increasers more often than not. They were Camponotus ephippium, C. spAF and C. spAH, Iridomyrmex agilis, I. galabanus, and I. spCJ, Rhytidoponera spAA, R. spAE, and Tapinoma spAA. Several other species of ants were predominantly classified as decreasers; they included Calomyrmex spAA, Cerapachys spAB and Leptogenys centralis.

Table 3.4.4.1: Species of ants that occurred on more than one gradient, their relative abundance, and response group if they were classified in the same response group when they occurred on more than gradient.

Table 3.4.4.1: Species of ants that occurred on more than one gradient, their relative abundance, and response group if they were classified in the same response group when they occurred on more than gradient.

Functional groupings of ants have been discussed in the literature on a number of occasions (Greenslade and Greenslade 1985; Andersen 1990, 1992, 1995a&b) and provide a convenient method of collapsing large numbers of species into a few genus-based groups from which behavioural and ecological inference can be drawn. Increaser and decreaser groups in mulga habitats were mostly composed of hot-climate specialists (Melophorus and Meranoplus) sub-dominant camponotini (Camponotus, Opisthopsis and Polyrachis) and generalised myrmicines (Monomorium, Pheidole and Crematogaster) (Table 3.4.4.2). In chenopod habitats the increaser and decreaser groups were dominated by different functional groups: hot-climate specialists and generalised myrmicines made up most of the species of increasers, and opportunists (Rhytidoponera, Tetramorium, and Odontomachus), generalised myrmicines and dominant dolichoderines (Iridomyrmex) made up most of the species of decreasers.

Table 3.4.4.2: Average percentages of species of ants in different functional groups and response groups across the four gradients in each major habitat type.

Percentages are calculated as the number of species in a functional group over the total number of species in the whole response group, and averaged across gradients. Functional group names are from Andersen (1992).

                                  Mulga                 Chen                 
                            -----------------     -----------------
                            INC    DEC     ND     INC    DEC     ND   

Dominant dolichoderines     13%     9%    14%     12%    21%    15%   
(1)                                                                   

Sub-dominant componotini    22%    24%    18%     7%      6%    10%   
(2)                                                                   

Hot climate specialist      36%    34%    37%     33%    18%    29%   
(3a)                                                                  

Cold climate specialist     0%      2%     0%     4%      0%     0%   
(3b)                                                                  

Cryptic and sub-cryptic     7%      4%     8%     9%      9%     5%   
(4a, b)                                                               

Opportunists (5)            7%     18%    20%     16%    27%    17%   

Generalised myrmicines      16%    12%    18%     20%    26%    24%   
(6)                                                                   

Solitary/ specialist        10%     4%     0%     5%      5%     3%   
predators (7)                                                         

Based on functional groups, we may expect a number of patterns in the distribution of ant species along a gradient of disturbance by grazing. First, that dominant dolichoderines (Iridomyrmex spp) may be dominant in disturbed habitats (ie near water points) and exclude most other ants except the sub-dominant camponotini and some generalised myrmicines (Andersen 1995a; Scougal et al. 1993; Bestelmyer and Wiens 1996). This dominance does not appear to be manifested in higher species richness of Iridomyrmex in increaser groups (Table 3.4.4.2) but may be manifested in higher biomass of Iridomyrmex which has not been examined in this study.

Second, there may be a trend for more species of hot-climate specialists in increaser groups because of the more open habitat near water points. They were not more speciose in increaser groups in mulga habitats but were so in chenopod habitats. This is consistent with the changes in cover that occurred at these gradients: shrub cover tended to increase close to water at mulga gradients, but to decline close to water at chenopod gradients (see Section 3.1).

Third, opportunists are poor competitors and generally predominate in disturbed locations where there is structural simplification (Greenslade 1978) so they may be more speciose in habitats near water points. Our data contradict this prediction because they show that opportunists were more speciose in decreaser groups compared to increaser groups in both habitats. The presence of larger numbers of species of opportunists in decreaser faunas suggests that groups such as dominant dolichoderines, sub-dominant camponotini, and generalised myrmicines may exclude them from areas closer to water.

Finally, solitary and specialist predators have been found to be more speciose in less-disturbed sites (Bestelmyer and Wiens 1996). In our study they were more speciose in increaser groups than decreaser groups but only in mulga habitats.

Although many of the predictions for functional groups seem to be contradicted by our data, this analysis only looks at the relative proportion of species in each functional group and does not take into account the abundance of the species. It is not clear whether species richness or biomass is the main factor that responds to a gradient of disturbance or to interactions with other ants. It is likely that both species composition and the abundance of species are key structuring components and understanding how assemblages of ants respond to grazing disturbance will require further data collection (e.g., refining taxonomic and behavioural information) and analysis that is beyond the scope of this report. However, the data collected during this project will form the foundation of future analyses aimed at functional classification of ants to incorporate functional variation within genera, particularly for those species that repeatedly show the same responses to disturbance.