Relationships Between Biodiversity And Function In Agroecosystems

11.3.1 The issue of scale and functional domains

The preceding sections provide a contextual framework within which the role of biodiversity in agroecosystems can be evaluated. There has been very little controlled experimentation in agroecosystems in which the effect of species number (or other features of biodiversity) on a specific function or set of functions has been directly examined. We therefore explore the nature of the relationships by means of brief descriptions of a number of selected examples. The examples cover clfects over a range of scales in space and time, in relation to different components of the agroecosystem community.

A major problem in linking the diversity of species functions to ecosystem processes is the disparity between the scales at which different organisms operate, and the scales at which measurements of their effects are made. Processes have no inherent dimensions; we impose operational scales of space and time, often as a matter of methodological convenience or convention, which are not necessarily related to the scale at which biological processes operate. With increasing sample size, or duration of process measurements, the "signal" of single-species cffects may be difficult to detect against the "noise" of all the other biotic and abiotic factors regulating system function. Process measurements made at any particular scale are therefore the net effects of organisms operating over many orders of magnitude in space or time. To isolate the signal of species populations or functional groups, sampling must be carried out al the ecological scale, or domain, at which the effects of those organisms are expressed.

The domain of a species is the zone of influence determined by the magnitude of local effects created by the individual within the spatial and temporal patterns of the species population in the system. The functional properties of individual species are therefore determined by the body size and activity of individuals, and the density and aggregative characteristics of the population. In the case of soil organism communities, for example, eollembola or mite species, feeding on fungi and bacteria at a scale of millimetres or less, form aggregations affecting microbial processes within a volumetric decimeter or more; earthworms, forming burrows of a few millimetres in diameter, have aggregative effects on hydrologic processes in patches of several metres; fungal hyphae, active at the cellular level, can be a component of a single genetic individual dominating wood decomposition over many hectares with a biomass of tonnes; littcr-feeding termites (Macro-terminae) determine soil physical and chemical processes from the plot to landscape scale (Anderson 1994b). Hence the activities of other microorganisms or invertebrates operating in smaller domains are expressed against the background of effects from the domain of a larger organism.

The functional properties of the domains of species or functional groups can therefore be considered as a hierarchy of nested interacting systems. Successive levels in this hierarchy may accommodate the same processes, but with slower dynamics over a larger area. As these scales increase, the correlates between species and processes also shift from proximate factors, which may be related to species activities, through functional groups, to distal environmental factors.

In agricultural systems the links between species diversity and function may be more evident than in more diverse natural systems because the controls over functional domains may be more obvious as a result of simplification of the system as a whole. In the transition from traditional agricultural systems to intensive cash-crop systems, processes over large areas become dominated by the activities of a few dominant plant or animal species. Furthermore, the same transition is marked by the increasing dominance of above- and below-ground processes by even-aged species cohorts operating on rotation times from months (herbaceous crops) to decades (tree crops). These cohorts of plants synchronise the populations of herbivores associated with them and, by means of their inputs to the soil, those of the decomposers. This tight synchronisation of ecosystem functions at the plot scale contrasts with the situation in natural ecosystems, where overlapping cohorts and large resource pools may buffer the specific effccts. The features producing this transparency is the relationship between scale and functional domain.

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