Community Composition and Ecosystem Function

Intact biological assemblages with a diverse mix of species are expected to carry out various ecosystem functions including primary production, organic matter decomposition, nutrient cycling, and secondary production of harvestable species at their natural and presumably optimal levels. As species are lost from ecosystems due to the relentless pace of human activities (Chapter 13), the extent to which system function and resilience depend on the number and characteristics of species present becomes an issue of considerable concern (Covich et al. 2004).

The expectations that biodiversity matters to ecosystem function and also that high biodiversity serves as a buffer against the consequences of species loss have theoretical and empirical support (Loreau et al. 2001), primarily from terrestrial ecosystems and at relatively low levels of diversity. Several mechanisms potentially are responsible (Giller et al. 2004). When species have complementary (overlapping but not identical) roles, the rate and efficiency of a process should increase when multiple species are present, and especially whenever the activities of one species facilitate those of a second. Thus, a species loss is expected to lower the efficiency of the process in question. When species have redundant roles, the loss of one species may not immediately result in a decline in the rate or efficiency of some process, but the presence of multiple spe cies provides insurance against a breakdown in ecosystem function should one or more species be adversely affected by environmental change. Complementarity, facilitation, and redundancy are mechanisms linking biodiversity to ecosystem function. On the other hand, the particular suite of traits and functional role of individual species also must be carefully considered. Individual species can have unique roles in ecosystems, be disproportionately abundant, dominate energy fluxes (Figure 10.10), and strongly influence other members of the assemblage (Figure 9.11). In some instances, individual species have been shown to play such a strong and unique role that species identity rather than overall diversity is of primary importance. This raises the possibility that a positive relationship between biodiversity and ecosystem function may be an artifact of sampling, because these species with unique traits and roles are more likely to be encountered when more of the species pool is included in an experimental study.

Facilitation of feeding efficiency was demonstrated by the enhanced particle capture of a mixed assemblage of three species of filterfeeding caddis larvae versus capture rates by single species in a laboratory stream (Cardinale et al. 2002a). "Current shading'' was reduced at higher species diversity, with the consequence that diverse assemblages were able to capture a higher fraction of suspended particles than could any monoculture (Figure 10.11). This hydrody-namic facilitation was due to the alteration of near-bed flows by capture nets, which resulted in increased bed roughness, higher near-bed velocities, and thus higher capture rates.

Leaf litter breakdown is a key ecosystem process in detritus-based stream ecosystems (Figure 7.3), serving as a basal energy resource to numerous detritivores and ultimately converting organic matter inputs to CO2. The breakdown of organic matter by physical abrasion, microbes, and shredders is a key ecosystem process that is well studied and relatively easy to manipulate. Using laboratory microcosms, Jonsson

Community Structure Ecology

FIGURE 10.11 Per capita consumption of suspended particulate matter (cubic millimeters per larva) in single-species and mixed-species assemblages of filterfeeding caddisfly larvae in laboratory streams. Interspecific facilitation is indicated by the higher per capita capture rates in mixed assemblages. (Reproduced from Cardinale et al. 2002.)

FIGURE 10.11 Per capita consumption of suspended particulate matter (cubic millimeters per larva) in single-species and mixed-species assemblages of filterfeeding caddisfly larvae in laboratory streams. Interspecific facilitation is indicated by the higher per capita capture rates in mixed assemblages. (Reproduced from Cardinale et al. 2002.)

and Malmqvist (2000) demonstrated that leaf mass loss increased with number of shredder species present, due either to differences in mode of feeding, facilitation of feeding efficiency, or both. By assembling a data set of litter breakdown studies from 36 streams of northern Sweden and northeastern France, Dangles and Malmqvist (2004) were able to evaluate the influence of species richness versus relative abundance components of diversity (Figure 10.12). The litter decomposition rate increased with the number of species present, but at a lower rate at sites with low dominance. In other words, detrital processing was more rapid in streams that were strongly dominated by one or a few species. Other studies have shown that the presence or absence of a single species, Gammarus fossorum, has a disproportionately strong influence on litter breakdown, and it is noteworthy that the highest breakdown rate in

Figure 10.12 occurred in a stream where this amphipod was the sole shredder present.

In contrast to the demonstrated influence of shredder diversity on leaf breakdown, fungal diversity apparently had no effect of leaf mass loss or fungal spore production (a measure of micro-bial production) in stream microcosms (Dang et al. 2005). This was true with oak and alder leaves, at high and low nutrient levels, and across a range of 1-8 fungal species. There was some suggestion of greater variability in fungal activity at low fungal diversity, consistent with a portfolio effect (the averaging and dampening out of the influence of extreme species as richness increases, just as a diverse stock portfolio is expected to guard against swings in a single stock's value). Although Barlocher and Corkum (2003) reported a positive effect of fungal diversity on leaf decomposition rates, their result is less convincing because those authors relied on initial inoculates rather than realized communities, which typically have greater unevenness.

If species loss is expected to result in reductions in ecosystem function, then highly diverse communities may be better buffered by their presumed greater levels of complementarity and redundancy, and therefore less affected by the loss of a single species. Thus, it is intriguing that a particularly strong example of species identity has been reported from a hyperdiverse tropical stream. During the dry season, the flannel-mouth characin Prochilodus mariae migrates from floodplain locations into headwater streams in the foothills of the Venezuelan Andes, where it feeds on organic-rich sediments on the streambed, creating visible feeding scars and enhancing sediment transport. As Taylor et al. (2006) document with comparisons between open and Prochilodus-restricted stream sections, this one species of detritus-feeding fish uniquely influences C flow and ecosystem metabolism. In its absence, the amount of particulate organic carbon (POC) on the stream-bed was higher, the downstream flux of POC declined due to reduced bioturbation

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