Abstract
Recently expanded estimates for when humans arrived on Madagascar (up to approximately 10 000 years ago) highlight questions about the causes of the island's relatively late megafaunal extinctions (approximately 2000–500 years ago). Introduced domesticated animals could have contributed to extinctions, but the arrival times and past diets of exotic animals are poorly known. To conduct the first explicit test of the potential for competition between introduced livestock and extinct endemic megafauna in southern and western Madagascar, we generated new radiocarbon and stable carbon and nitrogen isotope data from the bone collagen of introduced ungulates (zebu cattle, ovicaprids and bushpigs, n = 66) and endemic megafauna (pygmy hippopotamuses, giant tortoises and elephant birds, n = 68), and combined these data with existing data from endemic megafauna (n = 282, including giant lemurs). Radiocarbon dates confirm that introduced and endemic herbivores briefly overlapped chronologically in this region between 1000 and 800 calibrated years before present (cal BP). Moreover, stable isotope data suggest that goats, tortoises and hippos had broadly similar diets or exploited similar habitats. These data support the potential for both direct and indirect forms of competition between introduced and endemic herbivores. We argue that competition with introduced herbivores, mediated by opportunistic hunting by humans and exacerbated by environmental change, contributed to the late extinction of endemic megafauna on Madagascar.
Highlights
Until quite recently, Madagascar’s diverse endemic fauna included gorilla-sized lemurs (Archaeoindris fontoynontii), giant tortoises (Aldabrachelys spp.), threemetre-tall elephant birds (Aepyornis maximus) and one-metre-tall pygmy hippos (Hippopotamus spp.)
Brief temporal overlap between introduced livestock and endemic megafauna is consistent with a key component of the ‘subsistence shift’ hypothesis for megafaunal extinction [12], and isotopic niche overlap among coastal goats, hippos and giant tortoises indicates that direct forms of competition could have existed in some cases between introduced and endemic herbivores
Regardless of when humans first arrived on the island, multiple lines of evidence suggest that the spread of pastoralism in southwest Madagascar contributed to megafaunal extinction
Summary
Madagascar’s diverse endemic fauna included gorilla-sized lemurs (Archaeoindris fontoynontii), giant tortoises (Aldabrachelys spp.), threemetre-tall elephant birds (Aepyornis maximus) and one-metre-tall pygmy hippos (Hippopotamus spp.). We assume that people were on the island 2000–1600 years ago based on a recent review of radiocarbon (14C) data associated with traces of human activity [5] It was not until approximately 1000 years ago that most populations of endemic megafauna crashed [6]. The idea that island-wide aridification drove extinctions is inconsistent with (i) the persistence of diverse Malagasy megafauna during relatively severe Pleistocene climate fluctuations, (ii) megafaunal bone stable isotope records that suggest few directional changes in habitat aridity [9,10] and (iii) palaeoclimate records that reveal asynchronous changes in regional climate during the late Holocene [8,11]. Direct competition between introduced and endemic animals resulting from overlap in diet or habitat use can be partially inferred through stable isotope analysis of consumer tissue. Non-overlapping δ15N and δ13C values suggest distinct diets and habitat use and leave little potential for direct competition (figure 1)
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