Can rewilding with giant tortoises increase woody habitat and limit fire across Madagascar's grasslands?

Abstract

Societal Impact StatementTo better understand the impact of human settlement on vegetation distribution and disturbance regimes in Madagascar, the 2021 Malagasy Grassy Biomes Workshop called for an examination of the role of extinct megafauna in determining the historic distribution of habitat mosaics across regions that are today dominated by vast treeless grasslands. A systematic literature review facilitated the reconstruction and evaluation of giant tortoise habitat, feeding habitats, diets and Aldabrachelys tortoise impacts on disturbance regimes. In conclusion, reintroduction of mixed‐feeding Aldabrachelys (the diets of which were often dominated by C3 woody material, but never by C4 grasses, across its range), will likely impact plant species composition, select a tree‐rich habitat mosaic, reduce fire frequency, re‐establish lost ecological functions and improve landscape productivity at a range of scales, including rangeland palatability, benefitting both biodiversity and human wellbeing.SummaryGrasslands with little tree cover today comprise 80% of Madagascar's habitat. Determining their extent at human settlement can guide ecological restoration and enhance human well‐being, so the 2021 Malagasy Grassy Biomes Workshop identified the role of extinct megafauna in determining habitat as a critical knowledge gap. Using a systematic literature review, combined with extracted datasets, we address this, examining anticipated habitat selection by giant tortoises following reintroduction to Madagascar (where the Aldabran giant tortoise, Aldabrachelys gigantea, provides ecological functions lost when A. abrupta and A. grandidieri went extinct). When comparing current and historical tortoise selection of habitat across the Mascarenes and Aldabra with contemporary Malagasy habitat, areas in Madagascar where giant tortoises historically ranged, today have a significantly different habitat composition to the forested habitat that supported giant tortoises on other islands. Dietary 13C isotope ratios show that Malagasy Aldabrachelys and Mascarene tortoises were mixed feeders, with diets often dominated by C3 woody intake, but never by C4 grasses. Across systems, giant tortoises required and selected, tree‐rich habitat mosaics, different to current pastoralist fire‐selected Malagasy grasslands characterized by sparse tree cover. Furthermore, Aldabran Aldabrachelys tortoise turf, restricted to small areas (large tracts of unshaded vegetation present physiological challenges to Aldabrachelys' survival), is compositionally different to Malagasy and African obligate C4 grazing lawns. Ecological, palaeoecological, geomorphological and molecular evidence support a lost Malagasy habitat mosaic where hippo and tortoise diets were C3‐dominated, because they inhabited closed‐canopy systems, with abutting open‐canopy areas harbouring endemic‐rich, C4 grassy understories and limited grasslands. The review suggests that rewilding with A. gigantea will help restore ecological functions, productivity and landscape‐scale degradation lost through cattle‐based pastoralism, re‐establish tree‐rich habitat mosaics, and mitigate against frequent bushfires, benefiting biodiversity and humans at multiple scales.