Additions

Figure 6.6 The effect on ecosystem processes of adding species in an ascending or descending fashion. Additions in an ascending fashion indicates that the rarest species in the intact system is added first, followed by the next in the rank. Additions in a descending fashion indicates that the most abundant species is added first

Figure 6.6 The effect on ecosystem processes of adding species in an ascending or descending fashion. Additions in an ascending fashion indicates that the rarest species in the intact system is added first, followed by the next in the rank. Additions in a descending fashion indicates that the most abundant species is added first will depend upon the order in which different species arc added (Figure 6.6). Beyond the species richness threshold, further increases result in the partial or total replacement of one species by a new one, but processes remain at a constant level. Finally, we can envision cases in which introduction of a new species will decrease ecosystem processes.

The model developed here describes the relationship between ecosystem function and diversity within a trophic level. The same model is appropriate to describe the diversity-ecosystem function relationship within any trophic level, but different trophic levels cannot be combined. The model assists us in predicting the differential ecosystem effects of removing one plant species versus another plants species, or removing one herbivore species versus another herbivore species, but does not allow us to compare the effects of removing one plant species against removing one herbivore species.

This analysis of the effects of biodiversity on ecosystem function has focused on the species level and has only evaluated the effects of changes in species richness. We suggest that the framework developed at the species level is equally applicable at lower and higher levels of organization, and that the changes in the diversity of populations, functional groups, communities and landscapes affect functioning in the same manner that species richness affect ecosystem functioning.

The definition of the relevant ecosystem processes changes across scales as the definition of biodiversity changes across scales. Some processes are meaningful only at one scale, while others retain importance at broader levels of organization. For example, we can analyze transpiration at the population, functional group and community levels, but we can only study évapotranspiration at the community level or at larger scalcs. This is because bare soil evaporation is largely dependent on cover, an attribute which emerges only at the community or larger scales. As rank dominance curves describe the distribution of species within communities, we can construct rank-dominance curves in a similar way for individuals within populations, and communities within landscapes.

Our contention is that the model described in Figures 6.4 and 6.6 depict the overall relationship between biodiversity and function across a broad spectrum of scales. The effccts of adding or deleting individuals, species, communities or landscape units upon processes such as transpiration, évapotranspiration, watershed dynamics, production, nutrient mineralization, airshed dynamics, etc. follow the general model (Figure 6.4) and depend on the sequence in which species, communities or landscapes are deleted or added. If we start by deleting the landscape units which account for the smallest fraction of the relevant processes, no changes will be observed at the landscape level until several of these units are deleted. From that point forward, deletions will result in a steady decrease in function. Conversely, if the deletion starts with the most important units, the landscape will show rapid functional changes followed by a plateau where further changes in landscape diversity are not reflected in functional changes.

Up to this point we analyzed the effects of reducing or increasing species richness upon ecosystem function. We will now consider the opposite relationship: that is the effcet of ecosystem function on species richness. The relationship between productivity and diversity has been explored in a number of studies. At the scale of regions, a pattern is emerging: as productivity rises, diversity first increases and then declines (Currie 1991; Rosenz-weig and Abramsky 1993; Wright el al 1993). In striving to increase productivity, human beings have manipulated resource availability through means such as fertilization and irrigation. Hence, human intervention has inadvertently led to less diverse and functionally simpler systems (Mellinger and McNaughton 1975; Berendse 1993).

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