5.4 DISCUSSION
5.4.1 Individual taxa as indicators of species richness
were similar to those of Lawton et al. (1998) who assessed butterflies, flying beetles, canopy beetles, canopy ants, leaf-litter ants, termites, soil nematodes and birds as diversity indictors in a tropical Cameroon forest and found that no single group was a good indicator of total richness. Many other authors have questioned the use of indicator species to predict wholescale species richness (Majer 1983; Prendergast et al. 1993; Ehrlich 1994;
Lombard 1995; Oliver & Beattie 1996a; Kerr 1997; Prendergast & Eversham 1997). In fact, even if a single taxon had been found in the current study to correlate significantly with total target invertebrate richness in Limpopo Province forests, it could only be suggested as a good indicator of the groups examined. Its value as an indicator of total invertebrate biodiversity would remain untested.
Although it was not significant, the relatively strong relationship between bird richness and total target invertebrate richness was surprising. The richness of any single invertebrate taxon was expected to correlate more closely to the total target invertebrate richness than the more distantly related birds. The reason for this pattern is not evident.
With the exception of millipedes and centipedes, none of the individual taxa investigated in the present study could reliably indicate the richness of any other group (Table 5.3).
Several recent studies that included similar analyses have also found few correlations in richness across taxa, although results have been mixed and have yielded few generalizations (Table 5.7). Howard et al. (1998) showed that there was low congruence between the species richness of moths, butterflies, birds and small mammals in Ugandan forests. Prendergast et al. (1993) found little coincidence of richness hotspots for birds, butterflies, dragonflies, liverworts and aquatic angiosperms in Britain. According to Gaston (1996) most known relationships between the species richness of different taxa are positive, but statistically strong correlations are rare.
Table 5.7: Summary of significant correlations in taxon richness across sites in various habitats (Significant correlations: P < 0.05), listed in order of decreasing proportion of significant positive correlations. Only recent (1996+) studies that have included invertebrate taxa are listed. Proportion of Proportion of significant HabitatPlaceTaxa investigatedsignificant correlationspositive correlationsReference RainforestsQueensland, AustraliaPlants, insects, snails, vertebrates6/6 (100%)6/6 (100%)Moritzet al. 2001 Hay meadowsNorth-eastern SwitzerlandAngiosperms, spiders, true bugs3/3 (100%)3/3 (100%)Schwab et al. 2002 ForestsUgandaWoody plants, large moths, butterflies, birds,3/10 (30%)3/10 (30%)Howardet al. 1998 small mammals Temperate forestsIndian Garhwal HimalayaMacrolichens, mosses, liverworts, woody plants,7/10 (70%)3/10 (30%)Negi & Gadgil 2002 ants Afromontane forestKwaZulu-Natal, South AfricaSpiders, ground beetles, rove beetles, ants1/6 (16%)1/6 (16%)Kotze & Samways 1999b Semi-natural grasslandsSouth-central SwedenPlants, ground beetles, dung beetles, butterflies,7/45 (15%)6/45 (13%)Vessbyet al. 2002 bumblebees, birds Natural and plantation forestsSouth-central CameroonButterflies, flying beetles, canopy beetles, canopy4/36 (11%)3/36 (8%)Lawtonet al. 1998 ants, leaf-litter ants, termites, soil nematodes, birds Multiple-use forestEastern AustraliaPlants, ants, beetles, mammals, birds, frogs, reptiles8/42 (19%)3/42 (7%)Oliver et al. 1998 Indigenous forestsLimpopo Province, South AfricaPlants, millipedes, centipedes, earthworms, 1/21 (5%)1/21 (5%)This study terrerstrial molluscs, spiders, birds ForestsNew South Wales, AustraliaAnts, beetles, spiders1/3 (33%)0/3 (0%)Oliver & Beattie 1996a Meadows, aspen forests, Colorado, USAButterflies and moths0/1 (0%)0/1 (0%)Rickettset al. 2002 conifer forests
According to Landres et al. (1988), the implicit assumption in the use of indicators is that they provide an assessment of habitat quality and that if the habitat is favourable for the indicator, conditions will be suitable for other species. Therefore, positive correlation is most likely to occur among closely related taxa or groups in the same guild, which includes taxa that use and respond to habitat in similar ways, exploit the same class of resources and have similar breeding characteristics, foraging behaviours, diet and habitat requirements.
The unique significant positive correlation between millipede and centipede richness in the present study (Figure 5.3) was somewhat anticipated, since they are the most similar of the target taxa investigated in terms of mobility and body size range. Although they have different functional roles, millipedes and centipedes have similar microhabitat requirements and could be expected to respond to environmental conditions in similar ways. The single significant positive correlation in species richness found by Kotze and Samways (1999b) was between closely related invertebrate groups with similar requirements: ground beetles (Carabidae) and rove beetles (Staphylinidae). However, Ricketts et al. (2002) found no correlation between moth and butterfly diversity in Colorado, USA, although these two groups are relatively closely related. In some cases, behavioural traits, such as nocturnality, can overwhelm phylogeny and other ecological similarities in determining correlations in diversity (Ricketts et al. 2002). Clearly, phylogenetic relatedness, although intuitive, is not a reliable criterion for selecting appropriate indicator taxa (Holl 1996; Ricketts et al. 2002).
The significant correlation between millipede and centipede richness in Limpopo Province forests does have a possible practical application. There were fewer centipede species (7) and individuals (3247) than millipede species (24) and individuals (3372) sampled in the study area, so sorting and identification of centipedes would be more efficient than millipedes. Therefore, centipedes alone could be sampled and identified and the ratio of centipedes to millipedes determined in the present study could be used to predict millipede richness in these forests with some degree of accuracy. This would effectively reduce the costs and time associated with surveying both myriapod groups. However, this application is of limited use in conservation since it only provides insight into a small proportion of the total invertebrate diversity of Limpopo Province forests and may be applicable only on a regional or habitat scale.
Overall, these results support the assertion made by Lawton et al. (1998) that attempts to assess the richness of many taxa using one or a limited number of indicator taxa may be highly misleading. Because neither conceptual nor empirical considerations support the use of indicator species as surrogates for other species, this approach should be avoided (Landres et al. 1988). The results presented in this chapter suggest that without confirmatory research, it is inappropriate to use any single taxon as an indicator of total invertebrate biodiversity in Limpopo Province forests. Given these results, all of the information presented in the current study should be examined with caution, since only a limited number of target groups were sampled during this study and they cannot be assumed to represent all invertebrate diversity in Limpopo Province forests. Additional invertebrate taxa with higher mobility (i.e. flying insects) and different life histories (i.e.
complete metamorphosis, social insects), functional roles (i.e. pollinators) and body sizes should also be investigated in Limpopo Province forests before any generalizations regarding all invertebrates can be accurately made.