Abstract
The relative importance of climate shifts and human activities on Madagascar's Holocene ecological transitions is highly contentious. Increasing evidence supports a scenario of negligible impact of climate changes on megafauna extinctions. However, a comprehensive picture can only be obtained by overcoming a dichotomic view and weighing the contributions of diverse disciplines in an integrated framework. Multidisciplinary approaches implementing genetic data reveal scenarios of higher complexity during the human colonization of the island, sometimes unevenly related to climate-driven landscape transformations. Reconstructions of Holocene demographics from the genomes of living and sub-fossil species show signs of a closer association between past population size contractions and paleo-climatic shifts. From this perspective, the main causes of the megafaunal demise likely trace back to the mutual reinforcement between the ecological shifts that followed the settlement of new immigrants and those induced by millennial climate trends.
Highlights
The genomes of living and subfossil animals retain signs of demographic fluctuations that may be interpreted under model-free and model-based parameters to infer either the human impact on wild fauna (Frantz et al, 2016; Pujolar et al, 2017) or the effect of climate changes on population size and structure (Kozma et al, 2018; Miller et al, 2021; Song et al, 2021)
In one of its most widely used forms, it infers the size of an idealized population (Wright–Fisher) which, through inbreeding and/or genetic drift, underwent the same loss of genetic diversity observed in the population under study
Other studies detected recent bottlenecks in species from different regions over the island with a population decrease of approximately two orders of magnitude that occurred in the last millenium both in small nocturnal lemurs (Lepilemur edwardsi: Craul et al, 2008; Microcebus ravelobensis: Olivieri et al, 2007; Microcebus murinus and Microcebus ravelobensis: Teixeira et al, 2021) and larger diurnal lemurs (Lemur catta: Parga et al, 2012; Propithecus verreauxi: Lawler, 2008; Propithecus perrieri and Propithecus tattersalli, Salmona et al, 2017)
Summary
The extinction of the megafauna in Madagascar and surrounding archipelagos (Seychelles, Comoro, and Mascarene islands) has been associated with evidence of ecological transformations, explained either by the increase of human activities (Hixon et al, 2018, 2021; Douglass et al, 2019; Godfrey et al, 2019; Railsback et al, 2020) or hydroclimatic shifts (Virah-Sawmy et al, 2009; Quéméré et al, 2012) or a combination of both (Salmona et al, 2017; Li et al, 2020; Teixeira et al, 2021). Investigations of stalagmites at Anjohibe Cave in northwestern Madagascar are eloquent (Burns et al, 2016; Wang et al, 2019; Railsback et al, 2020) They suggest a rapid increase in δ13C values not correlated to a simultaneous growth in δ18O values from by 1,300 cal yBP. Other lines of evidence support a scenario of a highly fluctuating landscape with a mosaic of grassy biomes and forested habitats at different altitudes and a diversity of endemic grass species spanning millennia (Bond et al, 2008; Vorontsova et al, 2016; Yoder et al, 2016; Samonds et al, 2019; Solofondranohatra et al, 2020; Crowley et al, 2021) This questions the dichotomy between natural and anthropogenic transformation of modern grasslands and the extent of the indirect role of humans (use of fire, introduction of domesticated species) in triggering megaherbivore decline. A reliable scenario should not disregard the long-standing relationships among droughts, plant communities, natural fires, and mega-herbivores (elephant birds, giant lemurs, giant tortoises, and hippopotami) that pre-dated human arrival (Samonds et al, 2019)
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