DOI: 10.5061/dryad.t1g1jwt0m
title: Madagascan highlands: originally woodland and forest containing endemic grasses, not grazing-adapted grassland

description: Long considered a consequence of anthropogenic agropastoralism, the origin
 of Madagascar’s central highland grassland is hotly disputed. Arguments
 that ancient endemic grasses formed grassland maintained by extinct
 grazers and fire have been persuasive. Consequent calls to repeal
 fire-suppression legislation, burn protected areas, and accept pastoralism
 as the ‘salvation’ of endemic grasses mount, even as the IUCN declares 98%
 of lemurs face extinction through fire-driven deforestation. By analysing
 grass data from contemporary studies, and assessing endemic vertebrate
 habitat and feeding guilds, we find that although the grassland
 potentially dates from the Miocene, it is inhospitable to endemic
 vertebrates, and lacks obligate grazers. Endemic grasses are absent from
 dominant grassland assemblages, yet not from woodland and forest
 assemblages. There is compelling evidence that humans entered a highland
 dominated by woodland and forest, and burned it; by 1000 CE grass pollens
 eclipsed tree pollens, reminiscent of prevailing fire-induced
 transformation of African miombo woodland to grassland. Endemic grasses
 are survivors from vanished woody habitats where grassy patches were
 likely small and ephemeral, precluding adaptive radiation by endemic
 vertebrates to form grazing guilds. Today forests, relic tapia woodland
 and outcompeted endemic grasses progressively retreat in a burning
 grassland dominated by non-endemic, grazing-adapted grasses and cattle.
description: <i>            Vertebrate taxa adapted to
 grassland</i><i> </i> Using the
 ‘Habitat and ecology in detail’ data field from IUCN species assessments,
 we compiled a database pairing 839 taxa with habitat (forest or grassland
 adaptation, electronic supplementary material, S1). We included every
 endemic Madagascan mammal radiating from a single colonisation event
 (i.e., lemurs, euplerids, tenrecs and rodents), and all reptiles and
 birds. We did the same for Australian endemic mammals.  We then compared
 number of species in these groups adapted to either grassland, forest or
 either habitat, between the two countries, designating species as either
 “grassland” (grassland-limited) or “not” (forest or either habitat), using
 a Wilcoxon rank sum test with continuity correction. Regional endemics
 (co-occurring on the Mascarenes, Tasmania and New Guinea) were excluded,
 as were predominantly aquatic birds, reptiles, and monotremes, groups
 neither forest nor grassland-adapted. <i>Comparing Madagascan and
 East Gondwanan herbivore guilds</i>
 Given
 Africa separated from East Gondwana 155 mya, India from Australia 130
 mya, and Madagascar from India 88 mya, we compiled a database
 for the three established East Gondwana tropical grasslands, Australia,
 East Africa, and the Indian Terai-Duar, and compared herbivore guilds with
 a Madagascan suite comprising the only extant large herbivore
 (<i>Lemur catta</i>), and subfossils present in the highlands
 when humans arrived (25 species, ca. 2500 bp [4]; electronic supplementary
 material, S2). Guilds were divided into predominant browsers and
 folivores, obligate grazers, and mixed feeders, informed by a literature
 review of subfossil isotopic δ<sup>13</sup>C content [1,17], which differentiates C<sub>4</sub> grass diets from C<sub>3</sub> woody diets. To assess for an obligate Madagascan grazer-guild, we compared grazers to non-grazers across guilds and regions using a χ2 test, with Fisher’s exact test. <i>          Relative representation of endemic grass species under fire and grazing</i> We assessed and interpreted results from contemporary studies [7,9] to highlight the percentage of narrow endemic grass species in the highland relative to other ecoregions, and evaluated proportion of endemic grasses in dominant suites across highland vegetation types (electronic supplementary material, S3<i>a</i>,<i>b</i>). Next, using grasses from a fire-adapted functional group established by Solofondranohatra <i>et al</i>. [10], we compared number of sites occupied by endemic and non-endemic grasses to asses relative representation of endemics amongst dominant assemblages (electronic supplementary material, S3<i>c</i>). Lastly, we analysed data from 16 highland sites collected in December 2013 by Vorontsova <i>et al.</i> [7] to asses impact of fire, grazing and altitude on proportion of endemic grass species within sites. Grazing and fire represent the prevailing highland disturbance, impacting grasses in tandem, so we combined these two components to establish a variable that encapsulates the disturbance regime, by adding grazing (presence/absence) as recorded by Vorontsova <i>et al.</i> [7] and the frequency of fire over the three years preceding the data collection (2011-2013), using NASA Moderate Resolution Imaging Spectroradiometer data [19] (electronic supplementary material, S3<i>d</i>). If endemic grasses evolved in an ecosystem influenced by fire and grazing, they should be tolerant of these disturbances, and well-represented relative to non-endemics. We used a generalised linear model with number of endemic grass species versus non-endemics per site as the response variable, and altitude and disturbance regime (i.e., grazing index + fire frequency) as explanatory variables, binomial family with probit link. We validated the model visually, assessing residual fit for homogeneity, normality, and influential outliers [20].