5.1 INTRODUCTION
5.1.1 Biodiversity indicators and invertebrate studies
Biodiversity indicators are easily measured, correlating subsets of total biodiversity (Gaston & Spicer 1998) that are used to suggest the extent and patterns of diversity in the larger group of interest. Because they promote efficiency, biodiversity indicators are particularly useful in assessing invertebrate communities, which are often perceived to be too time consuming, costly and difficult to include in species inventories (Oliver & Beattie 1993). This is because (1) invertebrates are diverse and it is impossible to compile complete species lists for almost all invertebrate groups on any broad scale; (2) many species are difficult to identify and most species identifications can only be made or confirmed by specialists; (3) there is considerable taxonomic uncertainty and many invertebrates are undescribed or have been described from a single site or from very few individuals; (4) taxonomic expertise is limited and may not exist for some taxa; (5) the diversity and distributions of species are largely unknown; (6) many invertebrates require specialized collecting techniques and (7) collection and identification of invertebrates is usually time consuming and relatively expensive (New 1998; Slotow & Hamer 2000).
Clearly, biodiversity indicators offer a potential method of circumventing these problems, which would allow invertebrates to be more widely incorporated into inventories and conservation planning.
There are three main types of biodiversity indicators: (1) indicator taxa, individual species or taxa that are surveyed in place of entire faunas and are chosen to reflect the diversity of a much larger suite of taxa (Hammond 1995; Oliver & Beattie 1996a; McGeoch 1998;
Slotow & Hamer 2000); (2) higher taxa methods that use other levels of identification in place of species (Balmford et al. 1996; Gaston & Spicer 1998) and (3) morphospecies inventories that are generated by non-specialists and are used in place of formal species inventories (Oliver & Beattie 1996a). All three of these biodiversity indicator types are investigated in this chapter.
Individual taxa as indicators of species richness
Because our current knowledge of invertebrate taxonomy and distribution is so poor, better known indicator groups must be used as surrogates for other taxa or total invertebrate diversity. The multi-taxa focal group approach, used throughout this study and discussed
in Section 2.1.3, is one such example. Alternatively, a single taxon can be used (McGeoch 1998). For example, the species richness of an individual indicator taxon can be used to estimate the species richness of other closely related taxa (Gaston & Blackburn 1995).
No single species or taxon can adequately represent or indicate patterns of diversity for all other species and taxa (Pearson 1994). Therefore, individual biodiversity indicator groups must be chosen carefully. In order to maximize their generality and success, several criteria have been suggested for the selection of individual biodiversity indicators: (1) ease, reliability and cost efficiency of sampling, sorting and identification; (2) available taxonomic expertise; (3) high abundance and diversity; (4) correlation with the larger group of interest; (5) trophic level representation, functional importance and habitat specialization; (6) sufficient knowledge of biology and life history and (7) wide geographical range (Landres et al. 1988; Hammond 1995; McGeoch 1998; New 1998;
Andersen 1999; Caro & O’Doherty 1999). Each of the three classes of bioindicators has a different selection profile (Caro & O’Doherty 1999) and separate criteria have been given for environmental and ecological indicators (see Landres et al.; McGeoch 1998).
Many invertebrate taxa have been suggested as biodiversity indicators (Table 5.1), but there are alternatives to using a single invertebrate taxon. For example, invertebrates can be excluded entirely and inventorying and conservation area selection can depend entirely upon data for plants and/or vertebrates (Oliver & Beattie 1996b; Oliver et al. 1998). Plants and vertebrates are commonly used as indicators of total biodiversity, since data on these groups are easier to obtain than comparable invertebrate data (Mittermeier et al. 1998), especially on the global level. Birds, for example, are among some of the best-recorded organisms and are one of the most highly valued biotic groups (Williams et al. 1996).
Plants, specifically angiosperms, are the group most commonly recorded in biodiversity studies (Schwab et al. 2002). Myers (1988; 1990) and Mittermeier et al. (1998) used plants as indicators of total biodiversity and for the identification of global biodiversity hotspots. Scott et al. (1993) also suggests that plant species diversity is a good indicator of overall biodiversity. According to Panzer & Schwartz (1998), plants rank among the most promising of all indicator taxa examined thus far.
The principle debate in the use of biodiversity indicators is whether the presence of any single taxon can signify the presence of other taxa to the extent that it can be considered a
suitable surrogate for overall biodiversity (Margules & Pressey 2000). The underlying value of any indicator taxon depends upon its predictive ability, as determined by the relationships or responses that it demonstrates (McGeoch 1998; Andersen 1999). One way of assessing the value of potential biodiversity indicator taxa is to quantify the degree to which patterns of species richness coincide across different taxa (Prendergast et al. 1993;
Lombard 1995). Specifically, the richness of any biodiversity indicator must be positively correlated to the richness of the larger suite of taxa (Hammond 1995). In this chapter, I assess each individual invertebrate target group (millipedes, centipedes, earthworms, molluscs and spiders), birds and plants as potential indicators of ground dwelling, flightless invertebrate diversity in Limpopo Province forests. To do this, I determine if the richness of any individual invertebrate target group, birds or plants was correlated to total target invertebrate species richness. I also use correlations between individual taxon richness to assess if any group could be considered as a diversity indicator for any other group.
Higher taxa as indicators of species richness
It has been recognized that changes or differences in communities at the species level may also be evident at higher taxonomic levels such as genus or even phylum (Pik et al. 1999).
As a result, higher taxa richness has been proposed as a surrogate for species richness and recent studies have found some success with this approach in both contemporary biota and fossils (Gaston & Williams 1993; Gaston & Blackburn 1995; Balmford et al. 1996; New 1998; Pik et al. 1999). The higher taxon approach is well established in aquatic pollution assessment and monitoring studies using benthic communities (Pik et al. 1999). Studies have shown that lower taxonomic level identification does not always improve the resolution of results obtained using higher taxonomic levels (Oliver & Beattie 1996a).
The use of higher taxa surrogates has been advocated largely because it overcomes the problems associated with species identification. This is particularly important when inventorying the richness of invertebrates or other speciose and poorly known groups.
Using this approach, there are little or no costs for expert assistance in specimen identification, the challenges associated with a lack of taxonomic expertise are mostly eliminated and sorting is generally less labour intensive and time consuming (McGeoch 1998; Pik et al. 1999). The identities and distributions of higher taxa also tend to be better known than species (Gaston et al. 1995). In addition, there are no large numbers of species
to identify so a greater range of major taxa and functional relationships can be incorporated into surveys (Oliver & Beattie 1996a; McGeoch 1998).
Despite the advantages, this approach should be used with caution, since there are cases where higher taxa are not indicative of species numbers (Gaston & Spicer 1998). This surrogacy method requires good evidence that differences in species richness are mirrored at other taxonomic levels (Balmford et al. 1996). If they are not, the diversity estimates obtained may be misleading. In addition, higher levels of identification may cause a loss of information and interpretation sensitivity and decrease the comparability of results, since many families and orders are widespread and diverse (New 1998). Further, evidence justifying this approach comes mostly from temperate datasets or inventories over large areas and there are concerns about using this method in tropical areas (Balmford et al.
1996). In addition, few studies have been conducted using this method with terrestrial invertebrate assemblages (Pik et al. 1999). Prance (1994) suggests that species may be far better indicators of total diversity than higher taxa.
In this chapter, I assess the genus and family levels as indicators of target group invertebrate species richness by determining any relationship between species richness and higher taxon richness and establishing the relative accuracy of the genus and family levels as indicators of species richness.
Morphospecies as indicators of formal (true) species richness
The use of morphospecies is another recent attempt to improve the efficiency of biodiversity assessments. Instead of formal species, specimens are sorted by non- specialists to recognizable taxonomic units (RTUs) based solely on morphological characteristics (Hammond 1995; Oliver & Beattie 1996a; New 1998). In some cases, this approach has been useful and has produced richness estimates similar to those of formal species (Oliver & Beattie 1993; Oliver & Beattie 1996a; Pik et al. 1999).
Like the higher taxon method, the advantages to using morphospecies are mainly due to the avoidance of formal species identification. Because specimens do not require intense specialist taxonomic treatment, this method is less costly, is labour and time efficient and more taxa can be incorporated into surveys (Oliver & Beattie 1996a). However, there are also drawbacks to using this method. The legitimacy of morphospecies identification is
questionable, no distribution, endemism or interaction information can be extracted from the results and reliable comparisons cannot be made between communities (Slotow &
Hamer 2000). The disadvantages associated with using morphospecies are discussed in more detail in Section 2.1.2.
The accuracy of morphospecies as a surrogate depends upon how closely the RTUs conform to true species and the degree of success clearly varies according to taxonomic group and the abilities of the sorters (Hammond 1995). Sorting errors are the consequence of using non-specialists and diversity can be under- or overestimated by lumping or splitting true species into morphospecies (New 1998; Pik et al. 1999). Underestimation can result when formal species appear alike superficially and are only classified by minute or internal structures (New 1998; Slotow & Hamer 2000). Overestimation can occur with polymorphic species or when individuals of the same species appear different because of sex, age or developmental stage (New 1998; Slotow & Hamer 2000).
Formal species identification has been used throughout this study. However, in this chapter I evaluate invertebrate morphospecies as an indicator of true invertebrate species richness by comparing the total morphospecies richness to total formal species richness.
Invertebrate target groups were also evaluated individually to establish the relative accuracy of morphospecies identification for each taxon.