The previous section presented a conceptual model of the effects of changes in biodiversity on ecosystem function. This sections summarizes experimental evidence for this relationship. We organized this section according to different ecosystem processes, such as primary production, decomposition, water distribution, atmospheric properties, landscape structure and biotic linkages. Our use of the term ecosystem processes includes not only water, energy and nutrient cycling, but also atmospheric properties, landscape structure and biotic linkages which overlap with the major biogeochemical cycles. This partitioning allows us to deal explicitly with large-scale processes which show a large impact from human activity.
Our model suggests that a decrease in species richness, with initial deletion of the rarest species, results in no change in primary production until a threshold is reached, beyond which there is a steady and substantial decrease in production. Removal of rare species in the Serengeti grasslands resulted in full compensation of production by the remaining species (McNaughton 1983). Deletion of species of intermediate abundance resulted in only partial compensation in production. Finally, removal of dominant species which accounted for 70% of the initial biomass resulted in a significant decrease in production.
Grasslands provide several examples in which the relationship between diversity and primary production has been assessed experimentally. The sites studied are geographically diverse, and include California annual grasslands, old fields in New York and grasslands in the Serengeti (McNaughton 1993). Results are contradictory: a negative relationships was observed between productivity and diversity in the annual grasslands of California and the old fields of New York, whereas no relationship between productivity and diversity was found in the Serengeti. The effects of species diversity on produclion should be assessed with reference to which species have been deleted, and with respect to the driving forces behind the observed changes in diversity, rather than the diversity itself. In the case of the Serengeti, differences in diversity resulted from differences in grazing regime, while in the old fields in New York the diversity differences were a consequence of a succcssional process.
The diversity-stability hypothesis (McNaughton 1977) suggests that perturbations will result in a larger change in ecosystem function in simple systems than in diverse systems. There is experimental evidence to test this hypothesis in grasslands. McNaughton (1993) analyzed the response to a perturbation caused by fertilization along a diversity gradient which emerged as a result of a successional process. The experiment consisted of fertilizing with N, P and K old fields that were in different succcssional stages and therefore had different diversity. Similarly, Tilman and Downing (1994) analyzed the response to a perturbation caused by a severe drought along a diversity gradient. They created the diversity gradient by fertilizing the native prairie. Diversity was maximum in the native system and decreased as fertility increased. In both cases, the effect of perturbation on production was maximum in simple systems and minimum in the most diverse systems.
The effects of biodiversity on decomposition in grasslands can be viewed from the plant perspective or the microbial perspective. Microbial diversity is not well documented in grasslands, and its effect on decomposition is even less clearly understood. The effects of plant species diversity on decomposition result mainly from differences in litter quality among species. Several experiments have demonstrated the importance of species characteristics on total soil nutrients, nutrient availability and the rate of decomposition (e.g. Matson 1990; Wedin and Tilman 1990; Bobbie 1992). For example, abandonment from grazing or mowing usually result in losses of forbs and in the dominance of grasses which have different litter quality (Heal et al. 1978). Ter Heerdt et al. (1991) found that C/N ratios of fresh dead material increased significantly in sites with decreasing grazing intensities.
Important input and output flows which determine water balance and distribution of water change with the scale under consideration. At the ecosystem level, the major flows arc transpiration, bare soil evaporation, deep percolation, run-on, run-off and precipitation. At the plant level transpiration is the only relevant flow, but at higher levels of organization watershed variables become dominant. All the output flows of water at one scale are intimately related, and although the biotic components directly affect mainly absorption and transpiration, they indirectly affect all other components of the water balance.
Reduction of transpiration as a result of species deletions is related to species-specific characteristics that affect water dynamics. Rooting depth, phenology, maximum transpiration rate, drought resistance or avoidance are all species characteristics that affect water balance. Species with deep roots are able to absorb water located in a different portion of the soil profile than species with shallow roots. Species with different phenological patterns (early vs. late season) are able to use water available during different portions of the year. In addition, many of these characteristics are self-associated. For example, late-season phenology is associated in several systems with xerophytism or deep-root systems (Gulmon et al. 1983; Golluscio and Sala 1993).
Experiments and associated models of grassland water dynamics have shown how removal of functional groups such as perennial grasses or shrubs can result in alterations of ecosystem water balance (Knoop and Walker
1985; Paruelo and Sala 1995). Deep percolation losses can increase as a result of a decrease in the abundance of one of the functional groups, and the distribution of water in the soil profile can change as a result of deleting deep- or shallow-root functional groups. In the Patagonian steppe, only a fraction of the water freed by the removal of a functional group was used by the remaining functional group (Sala et al. 1989). Most experiments have focussed on the deletions of entire functional groups, providing no experimental evidence for the effects of deleting individual species.
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