Biogeography of agricultural environments.

Abstract

Agrarian culture developed independently in seven regions across the world between 4,000 to 10,000 years ago, changing dramatically the nature of both human society and global ecology. According to Food and Agricultural Organization of the United Nations (FAO), the land devoted to crop and livestock production today occupies nearly 40% of the world's land area and its impact on the planet's ecology is therefore enormous. The change from hunter-gatherer to farmer lifestyles was perhaps the greatest single cultural shift our species has experienced (Boyd and Richerson, 2005), and it has had equally substantial effects on other species, yet it has taken place in our very recent evolutionary history. In this chapter I describe this change in human ecology in the context of the wider environment and consider in some detail the impacts on today's environment and species, the processes involved in present-day agricultural ecosystems, and the implications for the future.
Previous chapters have described a range of environments outside the area that is managed directly by humans. As described by Jentsch and Beierkuhnlein (this volume), natural disturbances play an important role in ecosystem functioning, even in forests and wetlands, and even in the absence of human impacts. Fire, tornadoes, drought, tree fall, animal burrowing, flooding, erosion, and sedimentation are all processes of disturbance that influence plant and animal distributions at different scales. Enright (this volume) describes how, in exploiting fire as a management tool, humans are often accelerating a naturally occurring phenomenon. The pre-agricultural use of fire by Native Americans to encourage grazing for wild herbivores, and the regeneration of Camassia quamash, a culturally, economically, and nutritionally important bulb plant, provides a typical and once widespread example (Stripen and DeWeerdt, 2002). Such management prevented the regeneration of oak forest while stimulating the spread of (now rare) species such as the Willamette daisy (Erigeron decumbens) and the Oregon silverspot butterfly (Speyeriza zerene hippolyta). Disturbance of 'natural' ecosystems is, and has always been, central to the distribution of plant and animal species, especially when such disturbance is associated with agriculture. Even in early Neolithic Europe, the landscape was not one of continuous forest but was of a heterogeneous nature with open areas of varying scales, maintained by large herbivores such as aurochs (Bos primigenius) (Hodder et al., 2005). Vaughan et al., (2005) describe the evolution and dispersal of rice (Oryza spp.), aided initially by birds and large herbivores associated with wetland habitat and, in its more recent evolutionary history, with human domestication. Countless species are therefore adapted to disturbed environments and it is these species that have accompanied us through our cultural development as managers of plant and animal species for food and fiber.
Plant species richness under disturbed conditions tends to be relatively high (e.g., Harrison et aI., 2001). Small- and dormant-seeded species tend to predominate, but large- and non-dormant seeded species coexist with these. However, Harrison et al., (2001) have shown how types of disturbance and soil type interact in their effects on plant communities, and therefore on the animals associated with them. In their study, the effects of grazing and fire differed between serpentine and non-serpentine soils. The ecological consequences of disturbance therefore vary between regions. Some species of disturbed environments themselves provide food for humans; others compete with plant or animal species managed for food, and others have taken on a cultural significance independent of any perceived practical value. Papaver rhoeas has become a symbol of remembrance in Britain since the large-scale appearance of this species on the First World War battlefields, and Guiera senegalensis, a shrubby species of farmland across West Africa, is valued as a talisman for safe journeys, including that into the afterlife (Stoate et al., 2001a).