Fire and grazing determined grasslands of central Madagascar represent ancient assemblages

The ecology of Madagascar’s grasslands is under-investigated and the
dearth of ecological understanding of how disturbance by fire and grazing
shapes these grasslands stems from a perception that disturbance shaped
Malagasy grasslands only after human arrival. However, worldwide, fire and
grazing shape tropical grasslands over ecological and evolutionary
timescales, and it is curious Madagascar should be a global anomaly. We
examined the functional and community ecology of Madagascar’s grasslands
across 71 communities in the Central Highlands. Combining multivariate
abundance models of community composition and clustering of grass
functional traits, we identified distinct grass assemblages each shaped by
fire or grazing. The fire-maintained assemblage is primarily composed of
tall caespitose species with narrow leaves and low bulk density. In
contrast, the grazer-maintained assemblage is characterized by
mat-forming, high bulk density grasses with wide leaves. Within each
assemblage, levels of endemism, diversity and grass ages support these as
ancient assemblages. Grazer-dependent grasses can only have co-evolved
with a now-extinct megafauna. Ironically, the human introduction of cattle
likely introduced a megafaunal substitute facilitating modern day
persistence of a grazer-maintained grass assemblage in an otherwise
defaunated landscape, where these landscapes now support the livelihoods
of millions of people.

马达加斯加草原的生态学研究尚未得到充分开展，学界对火与放牧干扰如何塑造该类草原的生态学认知匮乏，这一现状源于一种普遍观点：即马达加斯加草原的干扰塑造过程仅始于人类抵达之后。然而在全球范围内，火与放牧干扰在生态及演化时间尺度上均持续塑造着热带草原生态系统，马达加斯加却成为全球反常的特例，这令人倍感疑惑。本研究针对马达加斯加中央高地的71个草原群落，开展了草原功能生态学与群落生态学调查。通过结合群落组成的多元多度模型与草本植物功能性状聚类分析，我们鉴定出两类截然不同的草本群落组合，分别由火干扰与放牧干扰主导塑造。火维持型群落主要由植株高大、叶片狭长的丛生草本物种构成，且具有较低的植株干密度；与之相对，放牧维持型群落则以铺散垫状、叶片宽大且植株干密度较高的草本物种为典型特征。两类群落的特有性水平、物种多样性以及草本演化年代均证实，它们均为古老的群落组合。依赖放牧的草本植物，仅能与现已灭绝的巨型动物群协同演化。颇具讽刺意味的是，人类引入家牛的行为相当于填补了该区域巨型动物的生态位空缺，助力放牧维持型草本群落得以在如今动物区系匮乏的景观中存续至今；而这类草原景观目前正支撑着数百万民众的生计。