Haycock et al. 1993). Peak outflows of materials and organisms are associated with extremes of this pattern of wide variation in activities and abundances. For example, high-yield monocultural cropping systems receive large inputs of mobile nutrients to meet a single period of intense nutrient demand by the developing crop (Robertson 1997). At other times, crop nutrient uptake is modest, and uptake by other plant species is limited by stringent weed management and the absence of plant cover during fallow periods (Swift and Anderson 1993, Robertson 1997). Typically, a significant fraction of total nutrient inputs is not used by the crop, and this residual nutrient pool becomes available for outflow to surrounding systems (Robertson 1997). Outflow commonly occurs by wind or water vectors, and resources also can be carried by organisms, sometimes resulting in highly focused or long-distance transport of materials. For example, in New Mexico, geese feeding on agricultural fields vector large quantities of nitrogen into their roosting areas, which are managed wetlands (Jefferies 2000).
When agroecosystems discharge large flows of resources into surrounding systems, strong ecosystem effects can result. For example, outflows of nitrogen can cause major changes in plant communities, as species that were adapted to low nitrogen are replaced by a less diverse group of species (often mainly composed of exotic invasive species) adapted to higher nitrogen levels (Jefferies and Maron 1997, Vitousek et al. 1997). Plant community changes and resultant changes in landscape-level biodiversity, caused in large part by nitrogen deposition from intensified agriculture, have been particularly dramatic in the Netherlands (Aerts and Berendse 1988), where nitrogen deposition rates are among the highest observed rates globally (Vitousek et al. 1997). High levels of nitrogen deposition have been associated with serious degradation in forest ecosystems, including high rates of tree mortality and problematic soil acidification (Draaijers et al. 1989). Such nitrogen-related reductions in plant diversity appear to reduce ecosystem resilience. In experiments on temperate North American grasslands, nitrogen enrichment caused species loss, and grassland plant communities that had lost species were much less resilient to rainfall and climate variation than more diverse communities that were not nitrogen-enriched (Tilman and Downing 1994, Tilman 1996). Nitrogen deposition also reduces soil quality and fertility in surrounding ecosystems and causes freshwater acidification and coastal zone eutrophication (Jefferies and Maron 1997, Vitousek et al. 1997).