Australia's flora and fauna have long been considered unique, but whether this applies to its grasses is less known. This study characterises the Australasian grass flora biogeographically. We investigate the distribution of C3 and C4 grass genera across four continents and construct broad profiles of their grass flora. We use endemism to examine global patterns of specialisation, and inter‐continental distributions as indicators of dispersal, using databases constructed over twenty years. We examined Australasian patterns with regard to endemicity and shared groups and categorised all of the region's genera into four age classes, from Australia's separation from Gondwana to the present. Globally, each continent presented a unique profile and C4 grasses were more widely shared than C3. Australasia's grasses equally comprise C3 and C4 genera; it shared two thirds of its C4 types with other continents, whereas C3 types split evenly between shared and endemic. Australasia shared relatively few genera with just one neighbour (7% C3, 13% C4), primarily with EurAsia. Australasian grass genera and species were either endemic or globally widespread, and 88% of C3 and 93% of C4 species were derived from lineages that originated elsewhere. We conclude Southeast Asia was the gateway for dispersal into Australasia, akin to rainforest taxa exchanges which increased from c12 Ma, with about 65% of Australasia's grass genera arriving in the past 3.5 Ma. The strong presence of C4 grasses in Australasia implies they have infiltrated a wide range of ecosystems, many probably occupied by ancient taxa with which they had not co‐evolved.

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AimLittle is known about the distribution of grasses throughout Australia. Using endemism as a basis for understanding biogeographical distributions, we hypothesised that contemporary species richness would be the result of environmental factors and dynamic ecological interactions spanning more than 25 Ma.LocationAustralia.TaxonGrasses.MethodsWe mapped the distribution of all Australian grass species and modelled climatic and landscape correlates according to photosynthetic type (C3 or C4), endemism, age in Australia, phylogenetic lineage and traits linked to dominance, using height as a proxy. Age classes comprised ‘Ancient’ (Gondwanan), and three others related to migration during the Sunda‐Sahul Interchange (SSI): Early, Mid or Recent. In some analyses, ‘Ancient’, ‘Early SSI’ and ‘Mid SSI’ were combined into ‘Pre‐Recent SSI’.ResultsOverall, species richness of C4 grasses increased with warmer mean annual temperatures, while richness of C3 grasses was higher in cooler areas. Recent SSI species had strong associations with summer rains and were dominant in the continent's northeast, with Pre‐Recent SSI species concentrated in the southeast, a pattern largely reflecting photosynthetic type (C4 and C3 respectively). Endemic and shared species distribution patterns support a migration sequence in which most C3 Pooideae and Panicoideae genera arrived in Australia before the Pliocene aridifications, followed by C4 Chloridoideae as aridification increased, with C4 Andropogoneae immigrating most recently across Lake Carpentaria's open habitats in the later Pleistocene. Recent SSI shared species were significantly taller than Pre‐Recent SSI endemic grasses.Main ConclusionsThe few grasses present in Australia before the Pliocene grew in cooler areas. The influx of taller Recent SSI grasses contributed to dramatic environmental changes—including creation of the northern savannas—with repercussions for resident taxa. Contemporary methods of fuel management could be promoting invasion by grass, thereby jeopardising the conditions suited to ancient taxa and threatening the region's evolutionary history.

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