Biodiversity provides us with a host of cultural, scientific, and economic benefits, and highly biodiverse islands such as Madagascar are the focus of many conservation efforts. History provides essential context to the current biodiversity crisis and can influence how conservation groups engage with local communities. On Madagascar, environmental history narratives since the French colonial era have emphasized past human contributions to deforestation and species extinctions. However, these inferred contributions, and even the time of initial human arrival on Madagascar, are based on limited data and subject to ongoing archaeological debate. I use chemical analyses of subfossil bone and lake sediments recovered from archaeological fieldwork and museum collections to test ideas that past animal introductions and drought contributed to the disappearance of endemic megafauna in arid SW Madagascar.
Radiocarbon data from introduced ungulates (cattle, sheep, goats, and bushpigs, n=59) and endemic megafauna (pygmy hippopotamuses, giant tortoises, elephant birds, and giant lemurs n=213) demonstrate 1) that the spread of pastoralism in SW Madagascar coincided with a pulse of megafaunal extinction ~1,000 years ago and 2) that this turnover occurred hundreds to possibly thousands of years after the first directly 14C-dated trace of human activity on the island. Directly 14C-dated butchered bone suggests that humans hunted extinct Malagasy megafauna for thousands of years, yet the spread of pastoralism likely heightened the impact of this hunting as pastoralism aided expanding human populations. Moreover, stable carbon and nitrogen isotope data suggest that sheep, goats, tortoises, and hippos had broadly similar diets or exploited similar habitats, which left potential for direct forms of competition between sheep and goats and endemic megaherbivores. Introduced domestic dogs also share a mutualism with pastoralists who occasionally hunt, and 14C-dated dog bone collected from across Madagascar suggest that dogs at least briefly co-occurred with the island’s extinct megafauna ~1,000 years ago. Dogs could have facilitated human-led hunts of forest-dwelling extinct megafauna, yet relatively high stable carbon isotope values in dog collagen suggest few individuals consumed primarily forest bushmeat. The distinct isotopic niches of dogs and the endemic forest predator known as the fosa (Cryptoprocta ferox) suggest that any negative interaction between the two follows from something other than predation and exploitation competition (e.g., interference competition and disease transmission).
Ongoing aridification is a major concern for agropastoralists in southern Madagascar, and it is unclear the degree to which introduced livestock and endemic megafauna were sensitive to drought. Geochemical analysis of a sediment core from SW Madagascar revealed a succession of dry intervals during the last ~1600 years. The driest interval coincided with the appearance of livestock and the disappearance of endemic megafauna around the lake. Coupled lake sediment and bone stable carbon isotope records suggest that pygmy hippos and cattle coped with vegetation changes associated with past aridification and forest clearance by consuming more xerophytic C4 and CAM plants during drier times. However, unlike the endemic megafauna, cattle may have been more sensitive to drought, as their bone collagen stable isotope values suggest they survived by strategically exploiting wet habitat during dry intervals of the past millennium. I worked with members of the Morombe Archaeological Project to excavate three additional coastal ponds in SW Madagascar (Tampolove, Ankatoke, and Andranobe), which revealed different lengths of co-occurrence among humans, introduced vertebrates, and extinct endemic livestock during the past six millennia. Although the modification of a bone from an extinct pygmy hippo and the deposition of human subsistence remains (e.g., cutmarked fish bones and processed mangrove whelk shells) recovered from these ponds likely span only the past millennium, a directly dated introduced bushpig tooth from ~4,000 years ago may help confirm that humans were present on the island by the mid Holocene. The disappearance of extinct megafauna (particularly giant tortoises and pygmy hippos) in the vicinity of these three sites matches regional extinction patterns in SW Madagascar. Together, these data clarify species-specific responses to past ecological and climatic stressors and suggest that the spread of pastoralism, rather than simply the presence of human hunters or occurrence of drought, contributed to past megafaunal extinction on Madagascar.
