Ecology of Agriculture monocultures in which pesticides are used Recent pest management programs in Southeast Asian paddy rice have taken deliberate advantage of this diversity and drastically reduced pesticide inputs without sacrificing yields In traditionally managed rice fields, the predators are likely to include fish and amphibians, which contribute to pest regulation and also provide additional nutritional resources for farm families Unplanned Diversity and Soil Processes Despite some significant advances in our understanding of soil processes, we know relatively little about the soil biota and their impact on agricultural productivity In natural ecosystems, decomposition and soil nutrient cycling are regulated by a diverse community of invertebrates and microorganisms, such as termites, earthworms, nematodes, fungi, and bacteria The composition, abundance, and activity levels of the soil biota in agricultural systems are markedly different from those in surrounding natural ecosystems For example, the diversity and abundance of soil insects and earthworms in tropical agroecosystems are typically significantly reduced compared to those of a wide range of undisturbed tropical ecosystems In cases in which abundance remains high in agricultural systems, the soil communities are often dominated by a single or small number of species highly adapted to the modified environment The changes in the soil community under agriculture result from a variety of perturbations to the soil environment The initial conversion from undisturbed ecosystems typically involves the removal and/or burning of plant biomass followed by tillage – activities which have drastic impacts on soil structure and soil chemistry The physical changes at the soil surface amplify diurnal and seasonal fluctuations in temperature and moisture In addition, organic inputs to the soil are significantly reduced as a result of plant biomass removal, and the chemical composition of organic inputs is altered These extreme modifications of the soil environment can result in the elimination of some soil organisms and, at a minimum, are likely to change the competitive balance among species To the extent that agricultural systems minimize these perturbations (e.g., by reducing tillage or burning), the impacts on the soil community may be less severe Despite these well-known effects of agricultural conversion on the soil community, the link between loss of soil biodiversity and various ecological processes has not been extensively studied Decomposition and nutrient mineralization, for example, are controlled by the activities of a diverse community of organisms It is not clear that the loss of some species will result in significant changes to these functions, but the degree of functional redundancy among different species is still controversial However, some processes such as nitrogen fixation are carried out by very specific organisms, whose loss might substantially affect nitrogen cycling Given the importance of soil biota for decomposition and nutrient cycling, it is essential that the link between this component of unplanned diversity and ecological processes be explored more fully 13 Agriculture and Diversity in the Landscape Agroecosystems typically sit in a landscape of multiple land uses Human-modified landscapes fall along a continuum from relatively homogeneous landscapes consisting of a small number of cropping systems to heterogeneous mosaics of diverse cropping systems within a matrix of natural ecosystems Agricultural activities can have important effects on natural ecosystems through the movement of organisms, pesticides, nutrients, and soil At the same time, the landscape context can strongly influence ecological processes that occur within agroecosystems, such as pollination and pest regulation The spatial structure of the landscape, including the size, shape, and connectedness of different ecosystem types can have dramatic effects on these ecological processes Consequences of Agricultural Activities Ecological concepts have often been utilized to address the consequences of agricultural perturbations for natural ecosystems in agricultural landscapes Agricultural intensification can diminish ecosystem services such as the provision of clean water due to increased nutrients, agrochemicals, and dissolved salts entering groundwater and surface water Significant sedimentation of waterways and dams results from the loss of riparian vegetation that often accompanies intensification The impacts of nutrient inputs and pesticides for downstream aquatic and terrestrial ecosystems have been well studied Much of the pesticide applied to crops fails to reach the target pests and instead moves into adjacent ecosystems, where it can have significant impacts on the diversity and abundance of nontarget species Studies have shown that pesticides have strong effects on trophic dynamics and a range of ecosystem processes in complex natural ecosystems, both aquatic and terrestrial Similarly, movement of nitrogen from agroecosystems to natural ecosystems through leaching or deposition may lead to significant impacts at several trophic levels Increased nitrogen loading may reduce plant species richness while increasing the biomass of dominant species In response to these changes, both pathogens and herbivorous insects are likely to exhibit lower species richness but higher population abundance, just as they in agricultural systems Changes in pathogen and herbivore populations may in turn influence the productivity and fitness of their host plants Even in the absence of changes in herbivore population pressure, research has shown that the effects of herbivory on plant growth and reproduction may be more severe at high levels of nutrient availability As with the complex effects of pesticides, these impacts of inadvertent fertilization are likely to impact trophic dynamics and ecosystem processes Spillover of Organisms from Agroecosystems In addition to the movement of soil and agricultural inputs, a variety of organisms move from agroecosystems to natural ecosystems in agricultural landscapes There is abundant evidence that pathogens move from domestic animals to wild animals (and humans), along with more limited evidence for movement of plant pathogens from crops to wild plants