Fire incongruities can explain widespread landscape degradation in Madagascar's forests and grasslands

Abstract

Societal Impact StatementThe relationship between rainfall, fire and habitat can display incongruencies. The 2021 Malagasy Grassy Biomes Workshop identified understanding fire regimes as a knowledge gap. This study pinpoints regions where anthropogenic fire has the potential to transform or has transformed habitat to treeless‐grassland, by identifying landscape‐scale, island‐wide fire anomalies across half of Madagascar. Its eastern forests burn like savannas, and central‐western grasslands burn frequently and intensely despite receiving rainfall usually associated with forest and fire‐absence. Recognising the incongruity and better understanding its drivers can mitigate against landscape‐scale degradation, improving ecological function, and human well‐being.Summary Data show that since 1953, human‐lit fires on Madagascar have transformed clear‐cut forest to treeless‐grasslands. To address the extent of Malagasy treeless‐grasslands at human settlement, the 2021 Malagasy Grassy Biomes Workshop identified the role of fire as a critical knowledge‐gap for understanding ecological function. The relationship between mean annual precipitation (MAP), fire and habitat is well established across mesic systems. Anthropogenically transformed habitats often deviate from expected ecological patterns, so we tested for landscape‐scale, island‐wide MAP‐related fire and habitat anomalies. We collated Malagasy fire, habitat and MAP datasets, identifying location and scale of incongruities relative to global fire‐habitat‐MAP expectations. Next, we tested for mismatches in fire regimes (frequency, timing, extent and intensity of fires) between Malagasy and equivalent global biomes, using global, comprehensive landscape‐scale fire regime data. Across half of Madagascar, fire frequency and habitat are decoupled from MAP, and fire regimes across Malagasy ecoregions differ significantly from those in shared biomes elsewhere in the world. Landscape‐scale incongruities span Malagasy eastern forests (which burn like savanna systems) and central‐western treeless‐grasslands, which burn frequently and intensely despite receiving MAP typical of forest presence and fire‐absence, globally. Fire‐MAP incongruities identify potentially transformed areas, or those undergoing transformation by fire, and establish a platform for investigating the nuanced social, political and ecological dynamics that may contribute to and perpetuate these anomalies. Incongruities also highlight the anthropogenic landscape degradation associated with fire anomalies. Addressing these impacts can facilitate restoration of ecological function, productivity and food security, benefiting biodiversity and humans at multiple scales.

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

“…Indeed, rangeland degradation in the Malagasy highlands has now been linked to pasture preparation for zebu cattle (Joseph et al, 2024a), as has landscape-scale habitat degradation of both forest and grassland, as a consequence of human-associated fire anomalies that now impact half the surface area of Madagascar (Joseph et al, 2024b). savanna systems, and fire-dominated, treeless grasslands abound in regions where rainfall in other systems would support forest, and limit fires (Joseph et al, 2024b). In Madagascar, natural fire is rare (frequent anthropogenic fires diminish available substrate; Archibald et al, 2010;Bloesch, 1999).…”

“…This is critical for Aldabrachelys, and all Malagasy biota, which evolved without human-lit fire and obligate C 4 grazing ungulates. Given that half of Madagascar burns anomalously (Joseph et al, 2024b), anthropogenic fire has been identified as a central threat to the Critically Endangered Malagasy ploughshare tortoise (Astrochelys yniphora), with models supporting extinction within 50 years, even with 10-yearly fire intervals (Pedrono et al, 2004;Pedrono & Clausen, 2018). Even in fireadapted systems like the Mediterranean, tortoises cannot tolerate fire frequencies exceeding 30-year cycles (Sanz-Aguilar et al, 2011).…”

“…In the context of grasslands depauperate in tree cover, and annual landscape-scale fires degrading both extant grasslands and forests over vast areas (Joseph et al, 2024b), introduction of A. gigantea will likely have multiple effects. This is particularly relevant given that endemic trees are overall poorly adapted to fire: of 2900 endemic tree species, only 16 (0.5%; and this includes those found on forest boundaries) are thought to be fire adapted in some way, and only 5 (0.17%) are considered highly fire-adapted (see table 2 in Silander et al, 2023).…”

“…F I G U R E 4 Reintroduction of Aldabrachelys into a fire-driven pastoralist system can be expected to influence frequency of fire, composition of grass assemblages and density of trees (Joseph et al, 2024a;Joseph et al, 2024b). (a) At Anjajavy Reserve (Madagascar), Aldabrachelys mixed-feeding selects favoured grasses and reduces leaf litter, mitigating against fire.…”

“…This rapid shift in grass dominance with novel disturbance (Linder et al, 2018;Styger et al, 2007) is central in counterargument to the first hypothesis. The alternative hypothesis acknowledges that Madagascar's C 4 grass species date to the Miocene but contends that the vast treeless expanses seen today arose circa 1 ka and formed when woodland, forest, ericoids and smaller grassy patches characterized by endemic grasses were transformed through anthropogenic clearance of original habitat, human-lit fires, and introduced cattle (Crowley et al, 2021;Gade, 1996;Godfrey & Crowley, 2016;Joseph et al, 2024aJoseph et al, , 2024b. According to this hypothesis, there is no ecological relationship between cattle-selected grazing lawns and extinct endemic herbivores, because extinct herbivores were neither dedicated obligate C 4 grazers nor even predominant C 4 grazers (Crowley et al, 2021;Godfrey & Crowley, 2016;, 2023a, and thus, their feeding ecology would not select obligate grazing lawns of the type described by Solofondranohatra et al (2020).…”