systems of food production and this requires that knowledge must be effectively delivered to the people who are in a position to take appropriate action.2. A history of agroecologyThe term agroecology was in parallel proposed by German zoologists (Friederichs, 1930), and American crop physiologists (Hanson, 1939) as a synonym for the application of ecology within agriculture. At that time, ecologists had relatively narrow foci but with a trend towards a more integrative view of ecosystems. The early population ecology school of Herny Gleason investigated plant populations seen from the organism's perspective, thereby focusing on the hierarchical levels of the organism (Fig. 1). In contrast, the community ecology school of Frederic Clements viewed plant populations from the landscape perspective, thereby also including higher hierarchical levels than the organism (O'Neill et al., 1986). However, the true roots of agroecology probably lie in the school of process ecology as typified by Tansley (1935), whose worldview included both biotic entities and their environment (Fig. 1). It was from this school of process ecology that the agroecosystem concept emerged (Harper, 1974), and the foundations for modern agroecology were laid.Fig. 1. The box symbolises the window of agroecology within food production systems. The viewpoints of the different schools of ecology are marked with eye signatures. The classical, scientific disciplines, where some are within the window of agroecology, are lined up in the right column, ordered in a hierarchy with the hard disciplines at the bottom and the 'soft' disciplines at the top (Checkland, 1999).2.1. "Hard" agroecologyAccording to Hecht (1995), the hard branch of agroecology (physical-analytical and natural science based) was initiated by works such as "Silent Spring" (Canson, 1964). "The Population Bomb" (Erlich, 1966), "Tragedy of the Commons" (Hardin, 1968) and "The Limits to Growth" (Meadows et al., 1972). The gloomy predictions of these and similar polemical writings have largely not come to pass, mainly because the speed of technological developments was underestimated. However, hard agroecology has shown that badly managed agriculture can lead to the degradation of agricu1tural land (Waldon et al., 1998), undesirable changes in semi-natural ecosystems (Lambert et al., 1990) and the depletion and pollution of natural resources (e.g. de Molenaar, 1990). Consequently, the focus of agricultural science has changed over the past 20-30 years from the maximisation of food and fibre production towards understanding the mechanisms linking costs (nutrient losses, loss of biodiversity and landscape degradation) to the benefits of agriculture (production, wealth generation and landscape maintenance). To understand these linkages required a combination of ecology, agronomy and economy (Reintjes el al., 1992) that may be considered "hard" agroecology. Such hard systems thinking, integrating various disciplines within natural sciences and economy, was significantly developed during the 1980s and 1990s, but remains the approach of an engineer or a classical economist (Checkland, 1999). This means that the resources entering and leaving agricultural systems are considered to be finite capital measured in physical or monetary units (Pearce, 1996). Furthermore, the position of the observer and scientist are thought of as external to the systems under study, which as we will see is not necessarily the case