A review of Madagascar's derived grasslands: Low palatability following anthropogenic fires may threaten food security

Abstract

Societal Impact StatementThe 26th United Nations Climate Change Conference emphasised the need to modify practices that negatively impact biodiversity and food security in the context of global change. Following Madagascar's drought‐induced famine, our systematic review supports the theory that grasslands of the Malagasy Central Highland that are subjected to human‐lit fires are anthropogenically derived. Furthermore, these overly frequent fires that characterise much of the Malagasy Central Highland grasslands select poorly palatable grasses. Given the reliance on pastoralism as insurance against crop failure in Madagascar, fire‐dependent practices that degrade rangeland emerge as a threat to food security and biodiversity. Education can mitigate against future humanitarian crises.SummaryFood insecurity is greatest in countries where impacts of global change are predicted to be severe. Many, like Madagascar, rely on livestock‐based pastoralism (and consequently palatable rangelands) for insurance against natural disasters and crop failure. It is recognised that derived grasslands can impact climate and biodiversity. Furthermore, the well‐established palatability‐flammability trade‐off predicts that overly‐frequent fires select increasingly unpalatable, fire‐adapted grassland. The drought‐induced Malagasy famine of 2021 highlights the need to identify factors that threaten food security. Given the ubiquitous practice of rangeland preparation through annual, landscape‐scale human‐lit fires, we evaluate whether Malagasy grasslands are derived and then test for fire‐driven selection of increasingly degraded and unpalatable rangelands across Madagascar's largest grassland system, the Malagasy Central Highland (MCH). We conducted a systematic literature review, evaluating for a palatability‐flammability trade‐off, by determining dominant Malagasy grass species, and then applying functional traits, and palatability ratings to these species. Data were extracted using a suite of relevant search terms, and of 1977 studies identified, 145 were directly relevant to the questions posed. Evidence from the review is compelling for much of the Malagasy highland grassland being derived. Furthermore, Malagasy dominant grass species are fire‐adapted with poor forage‐value, suggesting current burning practices negatively impact both biodiversity and pastoralism. Decreasing rangeland palatability caused by human‐lit fires in a society suffering food insecurity emphasises the need to re‐evaluate pastoralist burning practices. Identifying optimal fire frequencies can avert breaching fire‐induced tipping points to rangeland palatability and the humanitarian crises that may follow.

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

“…If the first hypothesis were correct, extinct Malagasy hippos would have fed in open grasslands dominated by C 4 grasses (and have higher δ 13 C values), and browsing of C 3 woody vegetation in woodland and forested habitats would have been uncommon and atypical. Isotope data now clearly find that Malagasy hippos were mostly C 3 feeders, never predominant grazers, lived mostly in forest and browsed in moist and wooded habitats, even in regions that today are vast grasslands (Crowley et al, 2023), corroborating earlier studies that showed there never were pure C 4 grazing hippos (Crowley et al, 2021;Godfrey & Crowley, 2016;Hansford & Turvey, 2022;Joseph et al, 2024a). As Madagascar's hippos lived predominantly in forests and wetlands, their feeding strategy has been identified by various authors as similar to that of forest-limited African pygmy hippopotamus, Choeropsis liberiensis (Crowley et al, 2023;Hansford & Turvey, 2022), which does not form obligate C 4 grazing lawns .…”

“…Subfossil dietary confirmation of wooded habitat where today there is treeless grassland is further supported by the domination of the largest grasslands by pan-tropical, fire-adapted grasses and multidisciplinary evidence reveals that much of these grasslands are derived anthropogenic rangelands, degraded by human-lit fire (Joseph et al, 2024a). Malagasy grasslands are burned annually (Kull, 2003;Kull, 2004) and are grazed by introduced cattle, which are obligate C 4 grazers (Crowley et al, 2023;Joseph & Seymour, 2023a).…”

“…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).…”

“…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, 2024a, 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; Joseph & Seymour, 2022, 2023a), and thus, their feeding ecology would not select obligate grazing lawns of the type described by Solofondranohatra et al (2020).…”