The contemporary distribution of grasses in Australia: a process of immigration, dispersal and shifting dominance

Aim: Little 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. Location: Australia. Taxon: Grasses. Methods: We 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’. Results: Overall, 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 Conclusions: The 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.