Fire mosaics and habitat choice in nomadic foragers Rebecca Bliege Birda,1, Chloe McGuirea, Douglas W. Birda, Michael H. Priceb, David Zeanahc, and Dale G. Nimmod aDepartment of Anthropology, Pennsylvania State University, University Park, PA 16801; bSanta Fe Institute, Santa Fe, NM 87501; cDepartment of Anthropology, California State University, Sacramento, CA 95819; and dInstitute for Land, Water and Society, Charles Sturt University, Albury, NSW 2640, Australia Edited by Donald K. Grayson, University of Washington, Seattle, WA, and approved April 20, 2020 (received for review December 11, 2019) In the mid-1950s Western Desert of Australia, Aboriginal popula- influenced by Allee effects: the presence of positive-density de- tions were in decline as families left for ration depots, cattle pendence (14). Under positive-density dependence, individuals stations, and mission settlements. In the context of reduced pop- in small populations or who are living at low densities may actually ulation density, an ideal free-distribution model predicts landscape have lower fitness, despite the release from resource competition, use should contract to the most productive habitats, and people because they have lost some of the benefits conspecifics provide. should avoid areas that show more signs of extensive prior use. For dispersing, obligate cooperators like humans, Allee effects However, ecological or social facilitation due to Allee effects (pos- can be quite robust and significantly alter an IFD at low pop- itive density dependence) would predict that the intensity of past ulation densities (15–18). At higher population densities nearing habitat use should correlate positively with habitat use. We ana- the equilibrium condition, or under situations where individuals lyzed fire footprints and fire mosaics from the accumulation of are spreading into unoccupied landscapes, predictions of the several years of landscape use visible on a 35,300-km2 mosaic of IFD with and without Allee effects are similar: The first arrivals aerial photographs covering much of contemporary Indigenous should preferentially occupy the highest ranked habitats, and at Martu Native Title Lands imaged between May and August equilibrium, population densities should correlate with resource 1953. Structural equation modeling revealed that, consistent with availability. If Allee effects are strong, however, the population an Allee ideal free distribution, there was a positive relationship density at equilibrium might be higher than expected for a given between the extent of fire mosaics and the intensity of recent use, habitat quality (19, 20). When populations are thinly distributed and this was consistent across habitats regardless of their quality. across a landscape—a circumstance that could arise during range Fire mosaics build up in regions with low cost of access to water, expansions, colonization episodes, or recovery from stochastic high intrinsic food availability, and good access to trade opportu- population declines—Allee effects can change habitat choice de- nities; these mosaics (constrained by water access during the win- cisions dramatically. Individuals may be drawn to lower-quality ter) then draw people back in subsequent years or seasons, largely habitats if others are already present, ignoring unoccupied high- independent of intrinsic habitat quality. Our results suggest that quality habitats in favor of those already occupied by conspecifics. the positive feedback effects of landscape burning can substan- Allee effects can arise in a number of different ways: Some tially change the way people value landscapes, affecting mobility through passive facilitation, like the dilution effect of predator and settlement by increasing sedentism and local population avoidance (21), the increased costs of finding mates in low-density density. populations (22), or increases in resource availability through habitat modification (23); others through active cooperation, such ideal free distribution | positive density dependence | niche construction | as reproductive enhancement through communal caretaking (24) historical ecology | hunter-gatherer mobility and the sharing of information, food, or resource defense (25, 26). Allee effects can be present at the level of the group, or at the he ideal free distribution (IFD) is a game theoretic model Tcommonly used to model habitat choice at landscape scales Significance (1, 2). Increasingly applied to questions of human mobility and settlement systems, it has proven useful in explaining the pat- Models of human habitat choice and landscape use assume terns of Mormon land settlement (3), sustainability in an open- that people have negative effects on resource availability, access pastoral system in Cameroon (4), dispersal and settlement which causes them to avoid regions that are already occupied in Arctic Alaska (5), the distribution of indigenous language or that show signs of extensive past use in favor of regions of groups in California (6), and the spread of populations in Oce- higher quality. We show that when people engage in activities ania (7), California’s Channel Islands (8, 9), and the West Indies that increase resource productivity, like burning, there is the (10). The logic behind the IFD is simple: Given two empty potential for these improvements to change habitat prefer- habitats that differ in quality, the richer one should be utilized ences in favor of places that have been previously modified first; then, as population density increases, resource competi- and occupied by people. This process changes the way we tion causes patch quality to decline. Eventually, the payoffs to think about intensification (and the origins of broad-spectrum remaining in the highest quality habitat are equal to those of the economies), which may arise not from the negative effects of next most suitable (but empty) habitat, at which point it should people on resources, but from the positive (and often un- become attractive to new residents. At equilibrium, population intentional) feedbacks between people and their environments. density is proportional to resource availability. Once all habitats are filled—a condition known as habitat or population packing Author contributions: R.B.B. and D.W.B. designed research; R.B.B., C.M., D.W.B., D.Z., and (11)—constraints on mobility and increases in resource compe- D.G.N. performed research; R.B.B., M.H.P., and D.G.N. analyzed data; and R.B.B., C.M., tition are argued to lead to the conditions that favor more in- D.W.B., M.H.P., D.Z., and D.G.N. wrote the paper. tensified economies (12, 13). The authors declare no competing interest. In most applications to human mobility decisions, the IFD is This article is a PNAS Direct Submission. assumed to be structured by negative-density dependence, in Published under the PNAS license. which per capita fitness declines with population growth as more See online for related content such as Commentaries. conspecifics compete for the same limited set of resources. In 1To whom correspondence may be addressed. Email: [email protected]. sparsely occupied landscapes, however, where individuals may This article contains supporting information online at https://www.pnas.org/lookup/suppl/ have limited access to mates and there are benefits to coopera- doi:10.1073/pnas.1921709117/-/DCSupplemental. tion that are facilitated through congregation, fitness may be First published May 27, 2020. 12904–12914 | PNAS | June 9, 2020 | vol. 117 | no. 23 www.pnas.org/cgi/doi/10.1073/pnas.1921709117 Downloaded by guest on October 1, 2021 population as a whole; they need not require individuals to live in and Commonwealth expeditions from 1954 to 1957 estimate that social groups, only to live at higher density. Allee effects are no more than 250 nomadic individuals remained in the region commonly thought of mainly as a form of social facilitation, in during this time, the majority of whom were observed along the which the benefits of communal caretaking or information sharing CSR (34). Based on the proportion of surviving elders we cause individuals to preferentially choose to join a group (in- interviewed who remained in the desert until the mid/late 1960s, creasing group size) regardless of the quality of the habitat asso- when the final groups were cleared by government patrols in ciated with that group (27). However, one often overlooked type preparation for the Blue Streak intercontinental ballistic missile of Allee effect arises through ecological facilitation due to per- tests (30), we estimate that this represents less than half of the sistent niche construction, which may not affect the propensity of original occupants of the region (35). While economic “pull” individuals to move into an area that is currently occupied, but factors undoubtedly played a large role in desert migration, rather may cause individuals to prefer habitats that have been “push” factors, negative affordances to remaining in the desert, previously occupied by other individuals. That is, Allee effects are may also have been important. Those who remained in the 1950s felt at the level of the population. These modified habitats may felt the impacts associated with the emptying desert so much so increase the size of the residential group but they may also reduce that they recall traveling long distances just to meet up with the rate at which resources decline, causing land use to intensify by others, following the smoke of distant fires to come together in supporting longer residence times