Introduction

Agroecosystems are ecosystems in which humans have exerted a deliberate selectivity on the composition of the biota, i.e. the crops and livestock maintained by the farmer, replacing to a greater or lesser degree the natural ñora and fauna of the site. The establishment and management of a modified and simplified plant community, often including exotic specics, influences the composition and activities of the associated herbivore, predator, symbiont and decomposer sub-communities (Figure 11.1; Swift and Anderson 1993). The composition, diversity, system structure and dynamics of agroecosystems may thus differ in many respects from those of the natural ecosystems of the adjacent landscape.

A common perception of agroecosystems is that the diversity and complexity are lower than in natural ecosystems and the structure and function impaired. An extrapolation of this view is that the relationship between biodiversity and function is thence of little significance in agroecosystems. We believe this to be a limited concept of agroecosystem structure and function, biased by familiarity with the most intensive form of agricultural management characteristic of large areas of the northern hemisphere. In fact there exist a wide range of agroecosystems with biodiversity comparable to that of natural ecosystems and occasionally exceeding it. Furthermore, farmers in many parts of the world utilise biodiversity as a management tool.

The relationship between agroecosystem biodiversity and function is nonetheless determined to a significant extent by the intervention of the human species. We argue in this chapter that biodiversity is indeed an important regulator of agroecosystem function, not only in the strictly biological sense of impact on production and other ecosystem processes, but also in satisfying a variety of needs of the farmer and society at large. Further-

Funciionul Roles of Biodiversity: A Global Perspective

Edited by H.A. Mooncy, J.H.'Cushman, E. Medina. O.K. Sala and E.-D. Schulze (few © 19% SCOPE Published in 1996 by John Wiley & Sons Ltd unep

Figure 11.1 The impact of agricultural intensification on agroecosystem biodiversity. The most direct effects of agricultural management are those associated with the reduction, by selection, of the plants in the system. The indirect effects (on resource utilisation, pesticide use and other management practices) combine with low plant diversity to significantly reduce the total biodiversity (from Swift and Anderson 1993)

Figure 11.1 The impact of agricultural intensification on agroecosystem biodiversity. The most direct effects of agricultural management are those associated with the reduction, by selection, of the plants in the system. The indirect effects (on resource utilisation, pesticide use and other management practices) combine with low plant diversity to significantly reduce the total biodiversity (from Swift and Anderson 1993)

more, we are in agreement with Gomez-Pompa and Kaus (1992) and Pimentel et al. (1992) that appropriate agroecosystem design provides a major opportunity for conservation of biodiversity across the landscape.

Recent reviews of the role of biodiversity in ecosystem function have emphasised the influence of species richness and the role of different functional, including trophic, groups and keystone species in ecosystem functions and properties such as production, decomposition, nutrient cycling and population dynamics, including such aspects as stability and resilience

(Solbrig 1992; Schutee and Mooney 1993). Biological process studies carried out in agroecosystems provide a significant contribution to this body of evidence, some of which we review in the main body of the chapter.

11.1.1 Agroeeosystem function

Agricultural ecosystems possess a range of functional attributes which distinguish them from so-called natural ecosystems. Agroeeosystcms are designed and managed for a purpose - that of producing certain goods to serve human needs. The outcome of this activity is often defined simply in terms of its biological productivity, i.e. of the biomass yield of the desired product(s). The products of agroecosystems are also given values beyond their biomass which may be computed in monetary terms, in terms of quality (e.g. nutritional value), or in the less easily defined social and cultural benefits associated with a particular type or process of production. Agroecosystems may thus be said to possess a range of socioeconomic functions in addition to the biological functions that they share with natural ecosystems. In the following account we assess the relationship of biodiversity to the conventional ecosystem functions of biomass productivity, nutrient cycling and population dynamics, and also comment on the relationship to these broader socioeconomic functions where relevant.

J 1.2 AGROECOSYSTEMS, AGRICULTURAL INTENSIFICATION AND BIODIVERSITY

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