Fire mosaics and habitat choice in nomadic foragers.

In the mid-1950s Western Desert of Australia, Aboriginal populations were in decline as families left for ration depots, cattle stations, and mission settlements. In the context of reduced population density, an ideal free-distribution model predicts landscape use should contract to the most productive habitats, and people should avoid areas that show more signs of extensive prior use. However, ecological or social facilitation due to Allee effects (positive density dependence) would predict that the intensity of past habitat use should correlate positively with habitat use. We analyzed fire footprints and fire mosaics from the accumulation of several years of landscape use visible on a 35,300-km2 mosaic of aerial photographs covering much of contemporary Indigenous Martu Native Title Lands imaged between May and August 1953. Structural equation modeling revealed that, consistent with an Allee ideal free distribution, there was a positive relationship between the extent of fire mosaics and the intensity of recent use, and this was consistent across habitats regardless of their quality. Fire mosaics build up in regions with low cost of access to water, high intrinsic food availability, and good access to trade opportunities; these mosaics (constrained by water access during the winter) then draw people back in subsequent years or seasons, largely independent of intrinsic habitat quality. Our results suggest that the positive feedback effects of landscape burning can substantially change the way people value landscapes, affecting mobility and settlement by increasing sedentism and local population density.

Climate, landscape diversity, and food sovereignty in arid Australia: The firestick farming hypothesis.

OBJECTIVE: Climate change has long been recognized as a significant driver of dietary diversity and dietary quality. An often overlooked aspect of climate change are shifts in fire regimes, which have the potential to drastically affect landscape diversity, species distributions, and ultimately, human diets. Here, we investigate whether the fire regimes shaped by Indigenous Australians change landscape diversity in ways that improve dietary quality, considering both the diversity and the quantity of traditional foods in the diet. METHODS: We use structural equation modeling to explore two causal models of dietary quality, one focused on the direct effects of climate change and resource depression, the other incorporating the dietary effects of landscape diversity, itself a product of fire-created patchiness. We draw on a focal camp dataset covering 10 years of observations of Martu foraging income in the Western Desert of Australia. RESULTS: We find strong support for the hypothesis that fire-created patchiness improves diet quality. Climate change (cumulative 2-year rainfall) has only an indirect effect on dietary quality; the availability of traditional foods is mediated primarily through the landscape diversity shaped by fire. CONCLUSIONS: Our model suggests that the loss of the indigenous fire mosaic may lead to worsening availability of traditional foods, measured as both caloric intake and diet diversity. Because the effects of rainfall are mediated through landscape diversity, increased rainfall may not compensate for the recent changes in fire regimes resulting from the loss of Aboriginal fire from the landscape.

Variability in the organization and size of hunter-gatherer groups: Foragers do not live in small-scale societies.

Mobile hunter-gatherers are often characterized as living in small communities where mobility and group size are products of the environmentally determined distribution of resources, and where social organization is multi-scalar: groups of co-residents are nested within small communities that are, in turn, nested within small-scale societies. Such organization is often assumed to be reflective of the human past, emerging as human cognition and communication evolved through earlier fission-fusion social processes, typical of many primate social systems. We review the history of this assumption in light of recent empirical data of co-residence and social networks among contemporary hunter-gatherers. We suggest that while residential and foraging groups are often small, there is little evidence that these groups are drawn from small communities nested within small-scale societies. Most mobile hunter-gatherers live in groups dominated by links between non-relatives, where residential group membership is fluid and supports large-scale social networks of interaction. We investigate these dynamics with fine-grained observational data on Martu foraging groups and social organization in Australia's Western Desert. The composition of Martu foraging groups is distinct from that of residential groups, although both are dominated by ties between individuals who have no close biological relationships. The number of individuals in a foraging group varies with habitat quality, but in a dynamic way, as group size is shaped by ecological legacies of land use. The flexible size and composition of foraging groups link individuals across their "estates": spatially explicit storehouses of ritual and relational wealth, inherited across generations through maintaining expansive networks of social interaction in a large and complex society. We propose that human cognition is tied to development of such expansive social relationships and co-evolved with dynamic socio-ecological interactions expressed in large-scale networks of relational wealth.

Pyrodiversity and the anthropocene: the role of fire in the broad spectrum revolution.

The Anthropocene colloquially refers to a global regime of human-caused environmental modification of earth systems associated with profound changes in patterns of human mobility, as well as settlement and resource use compared with prior eras. Some have argued that the processes generating the Anthropocene are mainly associated with population growth and technological innovation, and thus began only in the late Holocene under conditions of dense sedentism and industrial agriculture.(1) However, it now seems clear that the roots of the Anthropocene lie in complex processes of intensification that significantly predate transitions to agriculture.(2,3) What intensification is remains less clear. For some it is increasing economic productivity that increases carrying capacity, the drivers of which may be too diverse and too local to generalize.(4,5) For others using Boserup's ideas about agrarian intensification, increasing density in hunter-gatherer populations can produce declines in subsistence efficiency that increase incentives for investing labor to boost yield per unit area, which then elevates Malthusian limits on carrying capacity.(6-8) As Morgan(9) demonstrates in a comprehensive review, the legacy of such Boserupian intensification is alive, well, and controversial in hunter-gatherer archeology. This is a result of its potential for illuminating processes involved in transformations of forager socio-political and economic systems, including those dominated by harvesting more immediate-return resources and high residential mobility as well as those characterized by more delayed-return material economies with reduced residential mobility, a broader spectrum of resources, degrees of storage, and greater social stratification. Here we detail hypotheses about the processes involved in such transitions and explore the way that anthropogenic disturbance of ecosystems, especially the use of landscape fire, could be fundamentally entangled with many broad-spectrum revolutions associated with intensified foraging systems.

Aboriginal hunting buffers climate-driven fire-size variability in Australia's spinifex grasslands.

Across diverse ecosystems, greater climatic variability tends to increase wildfire size, particularly in Australia, where alternating wet-dry cycles increase vegetation growth, only to leave a dry overgrown landscape highly susceptible to fire spread. Aboriginal Australian hunting fires have been hypothesized to buffer such variability, mitigating mortality on small-mammal populations, which have suffered declines and extinctions in the arid zone coincident with Aboriginal depopulation. We test the hypothesis that the relationship between climate and fire size is buffered through the maintenance of an anthropogenic, fine-grained fire regime by comparing the effect of climatic variability on landscapes dominated by Martu Aboriginal hunting fires with those dominated by lightning fires. We show that Aboriginal fires are smaller, more tightly clustered, and remain small even when climate variation causes huge fires in the lightning region. As these effects likely benefit threatened small-mammal species, Aboriginal hunters should be considered trophic facilitators, and policies aimed at reducing the risk of large fires should promote land-management strategies consistent with Aboriginal burning regimes.

Positive and negative effects of a threatened parrotfish on reef ecosystems.

Species that are strong interactors play disproportionately important roles in the dynamics of natural ecosystems. It has been proposed that their presence is necessary for positively shaping the structure and functioning of ecosystems. We evaluated this hypothesis using the case of the world's largest parrotfish (Bolbometopon muricatum), a globally imperiled species. We used direct observation, animal tracking, and computer simulations to examine the diverse routes through which B. muricatum affects the diversity, dispersal, relative abundance, and survival of the corals that comprise the foundation of reef ecosystems. Our results suggest that this species can influence reef building corals in both positive and negative ways. Field observation and simulation outputs indicated that B. muricatum reduced the abundance of macroalgae that can outcompete corals, but they also feed directly on corals, decreasing coral abundance, diversity, and colony size. B. muricatum appeared to facilitate coral advancement by mechanically dispersing coral fragments and opening up bare space for coral settlement, but they also damaged adult corals and remobilized a large volume of potentially stressful carbonate sediment. The impacts this species has on reefs appears to be regulated in part by its abundance-the effects of B. muricatum were more intense in simulation scenarios populated with high densities of these fish. Observations conducted in regions with high and low predator (e.g., sharks) abundance generated results that are consistent with the hypothesis that these predators of B. muricatum may play a role in governing their abundance; thus, predation may modulate the intensity of the effects they have on reef dynamics. Overall our results illustrate that functionally unique and threatened species may not have universally positive impacts on ecosystems and that it may be necessary for environmental managers to consider the diverse effects of such species and the forces that mediate the strength of their influence.

Bush tucker, shop tucker: production, consumption, and diet at an Aboriginal outstation.

Foraging models have rarely been used to address how behavior is altered by the presence of non-foraged foods. Here, choices of store-bought and hunted foods in one Aboriginal community are analyzed. Hunting occurs frequently, but community residents also purchase food from the shop. Increases in the frequency of hunting certain large and small prey are associated with reduced access to food in the shop. Higher-variance hunt types are not associated with shop purchases, but continue to be acquired due to their cultural significance. The variation in these results highlights the complexity of dietary behavior in a mixed economy.

To kill a kangaroo: understanding the decision to pursue high-risk/high-gain resources.

In this paper, we attempt to understand hunter-gatherer foraging decisions about prey that vary in both the mean and variance of energy return using an expected utility framework. We show that for skewed distributions of energetic returns, the standard linear variance discounting (LVD) model for risk-sensitive foraging can produce quite misleading results. In addition to creating difficulties for the LVD model, the skewed distributions characteristic of hunting returns create challenges for estimating probability distribution functions required for expected utility. We present a solution using a two-component finite mixture model for foraging returns. We then use detailed foraging returns data based on focal follows of individual hunters in Western Australia hunting for high-risk/high-gain (hill kangaroo) and relatively low-risk/low-gain (sand monitor) prey. Using probability densities for the two resources estimated from the mixture models, combined with theoretically sensible utility curves characterized by diminishing marginal utility for the highest returns, we find that the expected utility of the sand monitors greatly exceeds that of kangaroos despite the fact that the mean energy return for kangaroos is nearly twice as large as that for sand monitors. We conclude that the decision to hunt hill kangaroos does not arise simply as part of an energetic utility-maximization strategy and that additional social, political or symbolic benefits must accrue to hunters of this highly variable prey.

Niche construction and Dreaming logic: aboriginal patch mosaic burning and varanid lizards (Varanus gouldii) in Australia.

Anthropogenic fire is a form of ecosystem engineering that creates greater landscape patchiness at small spatial scales: such rescaling of patch diversity through mosaic burning has been argued to be a form of niche construction, the loss of which may have precipitated the decline and extinction of many endemic species in the Western Desert of Australia. We find evidence to support this hypothesis relative to one keystone species, the sand monitor lizard (Varanus gouldii). Paradoxically, V. gouldii populations are higher where Aboriginal hunting is most intense. This effect is driven by an increase in V. gouldii densities near successional edges, which is higher in landscapes that experience extensive human burning. Over time, the positive effects of patch mosaic burning while hunting overwhelm the negative effects of predation in recently burned areas to produce overall positive impacts on lizard populations. These results offer critical insights into the maintenance of animal communities in the desert, supporting the hypothesis that the current high rate of endemic species decline among small animals may be linked to the interaction between invasive species and mid-century removal of Aboriginal niche construction through hunting and patch mosaic burning.

Provisioning offspring and others: risk-energy trade-offs and gender differences in hunter-gatherer foraging strategies.

Offspring provisioning is commonly referenced as the most important influence on men's and women's foraging decisions. However, the provisioning of other adults may be equally important in determining gender differences in resource choice, particularly when the goals of provisioning offspring versus others cannot be met with the acquisition of the same resources. Here, we examine how resources vary in their expected daily energetic returns and in the variance or risk around those returns. We predict that when available resources impose no trade-off between risk and energy, the targets of men's and women's foraging will converge on high-energy, low-risk resources that allow for the simultaneous provisioning of offspring and others. However, when minimizing risk and maximizing energy trade-off with one another, we expect men's foraging to focus on provisioning others through the unreliable acquisition of large harvests, while women focus on reliably acquiring smaller harvests to feed offspring. We test these predictions with foraging data from three populations (Aché, Martu and Meriam). The results uphold the predictions, suggesting that men's and women's foraging interests converge when high-energy resources can be reliably acquired, but diverge when higher-energy resources are associated with higher levels of risk. Social factors, particularly the availability of alloparental support, may also play a major role.

What Explains Differences in Men's and Women's Production? : Determinants of Gendered Foraging Inequalities among Martu.

Researchers commonly use long-term average production inequalities to characterize cross-cultural patterns in foraging divisions of labor, but little is known about how the strategies of individuals shape such inequalities. Here, we explore the factors that lead to daily variation in how much men produce relative to women among Martu, contemporary foragers of the Western Desert of Australia. We analyze variation in foraging decisions on temporary foraging camps and find that the percentage of total camp production provided by each gender varies primarily as a function of men's average bout successes with large, mobile prey. When men target large prey, either their success leads to a large proportional contribution to the daily harvest, or their failure results in no contribution. When both men and women target small reliable prey, production inequalities by gender are minimized. These results suggest that production inequalities among Martu emerge from stochastic variation in men's foraging success on large prey measured against the backdrop of women's consistent production of small, low-variance resources.

Human behaviour as a long-term ecological driver of non-human evolution.

Due to our intensive subsistence and habitat-modification strategies-including broad-spectrum harvesting and predation, widespread landscape burning, settlement construction, and translocation of other species-humans have major roles as ecological actors who influence fundamental trophic interactions. Here we review how the long-term history of human-environment interaction has shaped the evolutionary biology of diverse non-human, non-domesticated species. Clear examples of anthropogenic effects on non-human morphological evolution have been documented in modern studies of substantial changes to body size or other major traits in terrestrial and aquatic vertebrates, invertebrates, and plants in response to selective human harvesting, urbanized habitats, and human-mediated translocation. Meanwhile, archaeological records of harvested marine invertebrates and terrestrial vertebrates suggest that similar processes extend considerably into prehistory, perhaps to 50,000 yr BP or earlier. These results are consistent with palaeoenvironmental and other records that demonstrate long-term human habitat modification and intensive harvesting practices. Thus, while considerable attention has been focused on recent human impacts on 'natural' habitats, integrated evidence from modern biology and archaeology suggests a deep history of human entanglement with our ecosystems including substantial effects on the evolutionary biology of non-human taxa. The number and magnitude of such effects will probably increase given the continued intensification of anthropogenic activities and ecosystem impacts, including climate change and direct genetic modification.

People, El Niño southern oscillation and fire in Australia: fire regimes and climate controls in hummock grasslands.

While evidence mounts that indigenous burning has a significant role in shaping pyrodiversity, the processes explaining its variation across local and external biophysical systems remain limited. This is especially the case with studies of climate-fire interactions, which only recognize an effect of humans on the fire regime when they act independently of climate. In this paper, we test the hypothesis that an anthropogenic fire regime (fire incidence, size and extent) does not covary with climate. In the lightning regime, positive El Niño southern oscillation (ENSO) values increase lightning fire incidence, whereas La Niña (and associated increases in prior rainfall) increase fire size. ENSO has the opposite effect in the Martu regime, decreasing ignitions in El Niño conditions without affecting fire size. Anthropogenic ignition rates covary positively with high antecedent rainfall, whereas fire size varies only with high temperatures and unpredictable winds, which may reduce control over fire spread. However, total area burned is similarly predicted by antecedent rainfall in both regimes, but is driven by increases in fire size in the lightning regime, and fire number in the anthropogenic regime. We conclude that anthropogenic regimes covary with climatic variation, but detecting the human-climate-fire interaction requires multiple measures of both fire regime and climate.This article is part of the themed issue 'The interaction of fire and mankind'.

Constraints of knowing or constraints of growing? : Fishing and collecting by the children of mer.

Recent theoretical models suggest that the difference between human and nonhuman primate life-history patterns may be due to a reliance on complex foraging strategies requiring extensive learning. These models predict that children should reach adult levels of efficiency faster when foraging is cognitively simple. We test this prediction with data on Meriam fishing, spearfishing, and shellfishing efficiency. For fishing and spearfishing, which are cognitively difficult, we can find no significant amount of variability in return rates because of experiential factors correlated with age. However, for shellfish collecting, which is relatively easy to learn, there are strong age-related effects on efficiency. Children reach adult efficiency more quickly in fishing as compared to shellfish collecting, probably owing to the size and strength constraints of the latter.

Children on the reef : Slow learning or strategic foraging?

Meriam children are active reef-flat collectors. We demonstrate that while foraging on the reef, children are significantly less selective than adults. This difference and the precise nature of children's selectivity while reef-flat collecting are consistent with a hypothesis that both children and adults attempt to maximize their rate of return while foraging, but in so doing they face different constraints relative to differences in walking speeds while searching. Implications of these results for general arguments about factors that shape differences between child and adult behavior and human life-histories are explored.

Why women hunt: risk and contemporary foraging in a Western Desert aboriginal community.

An old anthropological theory ascribes gender differences in hunter-gatherer subsistence to an economy of scale in household economic production: women pursue child-care-compatible tasks and men, of necessity, provision wives and offspring with hunted meat. This theory explains little about the division of labor among the Australian Martu, where women hunt extensively and gendered asymmetry in foraging decisions is linked to men's and women's different social strategies. Women hunt primarily small, predictable game (lizards) to provision small kin networks, to feed children, and to maintain their cooperative relationships with other women. They trade off large harvests against greater certainty. Men hunt as a political strategy, using a form of "competitive magnanimity" to rise in the ritual hierarchy and demonstrate their capacity to keep sacred knowledge. Resources that can provision the most people with the most meat best fit this strategy, resulting in an emphasis on kangaroo. Men trade off reliable consumption benefits to the hunter's family for more unpredictable benefits in social standing for the individual hunter. Gender differences in the costs and benefits of engaging in competitive magnanimity structure men's more risk-prone and women's more risk-averse foraging decisions.