Mosaics of fire and water: the co-emergence of anthropogenic landscapes and intensive seed exploitation in the Australian arid zone

In arid Australia, the antiquity, role and ecological contexts of ‘firestick’ farming in seed-based foraging economies remain unclear. We use Landsat imagery to analyse effects of contemporary Martu hunting fires on seed-bearing grasses and forbs. Today, Martu rarely harvest wild seeds but inadvertently foster patches of grass when they burn to hunt burrowing monitor lizards. Therefore, anthropogenic seed patches need only be by-products of fires set to achieve other goals rather than the intended crop of firestick farming. Nonetheless, sustained burning over the long-term creates and maintains closely juxtaposed mosaics of seed and small game patches. We use the marginal value theorem (MVT) to model how pre-contact foragers may have used seed patches within such mosaics in response to climate change and population growth. We show that seeds would have been reliably available to foragers in anthropogenic patches whenever small game hunting returns were low and travel distances to new hunting patches long. Such circumstances probably occurred during the middle to late Holocene when population growth filled better-watered habitats of arid Australia, and climatic variability associated with the El Niño/Southern Oscillation (ENSO) reduced the number of water sources that could support foraging. Prolonged occupation around the water sources that remained triggered the emergence of fine-grained, anthropogenic fire mosaics. If so, the late Holocene proliferation of formalised seed milling equipment closely followed the emergence of firestick farming and signalled the consequent development of seed-based foraging economies, further fuelling population growth and social complexity when more mesic climatic conditions returned. Earlier milling technology during the Pleistocene and early Holocene probably accommodated seed distributions created in fire regimes other than the mosaic burning conducted by Martu today.