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- Materials and methods
The determinants of local species richness remain the subject of ongoing debate. Although it is now widely appreciated that regional scale processes determine the potential membership of local assemblages (Ricklefs 1987; Gaston 2000; Godfray & Lawton 2001), how membership of an assemblage is finally determined remains contentious. Membership may be a consequence of highly stochastic processes such as random colonization (see Gaston 2000), or it may be the outcome of more deterministic processes such as the operation of a set of assembly rules (Weiher & Keddy 1999). Depending on the taxon and system concerned, interspecific competition may be an important local process structuring the assemblage, may be negligible relative to other processes, or may simply not occur.
The regulation of small-scale diversity by dominant ants (regulation of ‘momentary’ diversity or the numbers of species in attendance at a bait at a given time) has been found in several baiting studies, including those in tropical savannas of Australia and Africa (Andersen 1992; Andersen & Patel 1994; Parr et al. 2005), in tropical French Polynesia (Morrison 1996), in the boreal taiga biome (Savolainen & Vepsäläinen 1988), and in tropical agro-ecosystems (Perfecto & Vandermeer 1996). At baits, it has been shown that the full relationship between species richness and dominance is unimodal: species richness is low at very low abundances of dominant ants, and as the abundance of dominant ants increases, species richness also increases until a point is reached after which species richness declines as dominance increases (Andersen 1992; Parr et al. 2005). While the ascending portion of the curve is thought to correspond to increasing habitat favourability and reduced stress for ants, as well as constraint imposed by abundance frequency distributions, the descending part of the dominance–richness relationship is due to an increase in the abundance of dominant ants, to such an extent that they reduce species richness via competitive exclusion (Andersen 1992; Parr et al. 2005).
However, the use of baits to emphasize the importance of competition in structuring ant assemblages is problematic because it represents only one scale of the dominance–species richness relationship, and exclusion from baits does not necessarily imply that competitive exclusion by dominants is taking place at the population level, and therefore that competition should be considered a key mechanism structuring assemblages (see Hairston 1981; Andersen & Patel 1994; Sanders & Gordon 2000; Ribas & Schoereder 2002; Gibb & Hochuli 2003).
Although competitive exclusion and the regulation of richness at baits appear to be the rule (e.g. Fellers 1987; Morrison 1996; Bestelmeyer 2000), to-date no studies have demonstrated that this also translates to control of resources and the regulation of richness by dominant ants at the assemblage level. Indeed, the available evidence suggests that this does not occur (Andersen & Patel 1994; Ribas & Schoereder 2002; Gibb & Hochuli 2003). Dominant species may control bait resources, but because at a broader scale, resources may either not be limited or species in the community use different resources, it may not be possible to predict with confidence whether competition is also the main structuring force for the local assemblage. Other factors including predators (Gotelli 1996), parasitoids (LeBrun & Feener 2002), environmental fluctuations (Cerdá, Retana & Cros 1997) and patchy distributions may also prevent dominant species from regulating assemblage structure.
This study investigates whether competitive exclusion at baits scales up to competition at the assemblage level by comparing dominance–species-richness patterns found with baits and pitfall traps (i.e. do temporal relationships at baits also hold true at the assemblage level?). Baits represent localized richness or small-scale diversity as a result of interactions at a resource (‘momentary diversity’, Andersen 1992), while pitfall traps represent general foraging activity and richness at the assemblage level. Thus, the study explores the extent to which competition and dominant ants play a role in regulating assemblage level richness.
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- Materials and methods
The relationship between dominance and species richness both at baits (‘momentary diversity’) and pitfall traps (assemblage level) revealed strong convergence in pattern with data from baiting and from pitfall traps both producing the full unimodal relationship, although clearly the richness and dominance values are rather different (Fig. 1). Importantly, this is the first study to show that the full unimodal dominance–richness relationship found at baits (Fig 1a, see also Andersen 1992; Parr et al. 2005) can also be found with pitfall trap data representing the broader assemblage level pattern (Fig. 1b). This finding suggests that at the assemblage level, different parts of the dominance–richness relationship face the same constraints as the relationship at baits, with the ascending part due to both environmental stress and particular abundance frequency distributions, while the descending part is due to interspecific competition (see Parr et al. 2005). This is most noteworthy because it demonstrates that ant behaviour at baits and at the assemblage level can produce a similar outcome, irrespective of scale. The importance of competition as the causal mechanism reducing species richness is further suggested at Satara where, under a favourable temperature regime, the proportion of ant species at baits relative to the species pool decreases as the abundance of dominant ant species increases (Fig. 2b).
Alternative explanations and mechanisms to that of competitive exclusion resulting in reduced assemblage richness when abundances of dominant ants are very high are hard to reconcile. Top-down control by predation is not a satisfactory alternative explanation: not only are there few known predators of ants in this savanna system, but of those that are known, dominant ants with conspicuous nests are the favoured prey, rather than subordinate species [e.g. A. custodiens constitutes an important part of the pangolin's (Manis temminckii Smuts) diet, Swart, Richardson & Ferguson 1999]. The finding that the dominance–richness pattern at baits scales up to the assemblage level, indicates that in this savanna system patterns of diversity and behavioural dominance at baits are likely to reflect broader patterns of foraging activity and changes at the population level. This therefore underscores the importance of competition in structuring local ant assemblages.
When looking at the full unimodal dominance–richness relationship, species richness is influenced both by environmental stress (the ascending portion of the curve) and competition (descending portion) (Andersen 1992). Importantly, with bait data, the wider range of dominance values at each site compared with assemblage level patterns is driven primarily by temperature influencing momentary diversity. Ants are considered thermophilic (Hölldobler & Wilson 1990; Kaspari 2001), with temperature an important influence affecting foraging behaviour (Cerdáet al. 1997). When ground surface temperatures are considered physiologically high for ants (i.e. above 36 °C), typically at midday, the majority of ant species reduce foraging activity (C. L. Parr, personal observation). Given baiting was conducted at different times throughout the day (including midday) to cover a range of temperatures, it is not therefore surprising that a negative linear relationship between abundance of dominant ants and percentage of species at baits relative to the species was not found at Satara, and instead a unimodal relationship is apparent (Fig 2). High temperature acts as a significant stress factor reducing the abundance of all ants at baits (including dominant species), and as a result, species richness declines (Fig. 2b). Under more favourable temperature regimes, dominant species can control species richness through competitive exclusion at baits (descending part of relationship), as is evidenced by the data from Satara (Fig. 2b). At Mopane and Pretoriuskop, weather conditions during baiting meant air temperatures, and consequently ground surface temperatures were slightly lower than Satara. This resulted in few baiting periods with stressful conditions for ants, and consequently the full unimodal relationship was not apparent (Figs 1 and 2).
At the assemblage level however, the dominance–richness pattern was slightly different with the different savanna sites making up different parts of the unimodal dominance–richness pattern: low and ascending at Mopane (indicative of a stressful habitat, see Andersen 1992; Parr et al. 2005), ascending at Pretoriuskop, and descending at Satara (Fig. 1b). The importance of competition in structuring assemblage richness within each site therefore varied considerably. At the assemblage level, control of species richness by dominant ants was restricted to the Satara Acacia savanna where very high abundances of dominant ants resulted in low assemblage richness (Figs 1b and 2b). In this regard, co-occurrence analyses indicating nonrandom co-occurrence patterns in Satara provide further support for competition as a key mechanism structuring ant assemblages in this savanna habitat (see Gotelli & Ellison 2002) (Table 3). It should be noted that previous work at the same study sites (Parr et al. 2004) demonstrated that the nonrandom co-occurrence patterns found at the plot level in Satara cannot be attributed to differences in fire history between the plots (the plots represent several different burning treatments as part of a long-term fire experiment with several burning treatments).
In contrast to Satara, the low dominance levels at Mopane are likely to be related to local environmental stresses, such as reduced food resources and limited nest availability due to heavy clay soils (maximum 23% clay content, mean > 18% clay; Mills & Fey 2004) which reduce ant abundance (see Andersen 2000); the reduced importance of competition was further evidenced through random co-occurrence patterns (Table 3). In less favourable sites, competition therefore does not appear to be an important factor in structuring ants at the assemblage level, and patterns at baits do not scale up.
Because the importance of competition in structuring assemblages varies with habitat type, it should not, therefore, automatically be assumed that the dominance–richness relationship found here with assemblage level data occurs in other locations. In order to detect the full unimodal relationship between dominant ants and species richness, it is likely that a wide range of environments need to be surveyed (e.g. from low- to high-productivity sites, from stressful to more favourable sites), and partial sampling may only produce part of the full relationship (e.g. the ascending portion of the curve). Indeed, limited sampling (spatially or temporally) may explain why in some previous studies, the regulation of richness by dominant ants has been found at baits but not at the assemblage level. For example, Andersen & Patel (1994) showed that northern meat ants, Iridomyrmex sanguineus Forel, reduced ant abundance and species richness at baits but similar patterns could not be detected at the assemblage level, possibly because of the short-term nature of the study (< 2 months). Likewise, where exclusion experiments remove only one dominant ant species, when there may be several dominant species in a system, the effects of competition at the assemblage level may be masked (e.g. Gibb & Hochuli 2004).
Further work is needed to elucidate the factors that determine under what conditions dominant ants are able to play a key role in determining species richness and assemblage structure, and those conditions where other factors play a more important role regardless of the abundance of dominant ants. For example, although theory and field experiments suggest that dominant species increase where resources are abundant and resource quality is high (Davidson 1997; Palmer 2003; MacNally & Timewell 2005), do the resultant high numbers of dominant ants always exert a competitive influence? Finally, given that both natural and anthropogenic disturbance can promote dominant ant species (Gibb & Hochuli 2003; Hoffmann & Andersen 2003), what might the implications of this be for processes such as ecological restoration?