Birds, bats or climate? eucalypt floral traits reflect pollination over abiotic environment

Flowers and their traits vary greatly across species, influenced by biotic and abiotic environmental variation. We explore the relative effects of pollination and abiotic environment on flower size and colour in a species-rich tree clade (eucalypts: Eucalyptus, Corymbia and Angophora). Most eucalypt flowers are small and white-cream with generalised pollination systems. Though larger, more colourful (i.e., red, pink, orange, yellow or green) eucalypt flowers occur more frequently in southwest Australia, it remains unclear what environmental factors contribute to this pattern. We extracted bud size (as a proxy for flower size) and flower colour (as white-cream or colourful) for 798 eucalypt species from online floras. We assessed three measures of vertebrate pollination environment—flower-visiting bird species richness, flower-visiting marsupial presence/absence, and flower-visiting bat presence/absence—and three measures of abiotic environment—mean annual temperature, mean annual precipitation, and soil available phosphorus. We show that flower size and colour have evolved in tandem in eucalypts and are not well predicted by contemporary climate or soil environments. Instead, pollination environment, and particularly the absence of flower-visiting bats, was the strongest predictor of eucalypt flower size and colour. Larger, more colourful eucalypt flowers may have evolved to attract bird pollinators in landscapes where bats are not available to carry pollen long distances. Small, white-cream eucalypt flowers, conversely, may represent a successful generalist pollination syndrome where insects, bats, birds, and/or marsupials all contribute to pollination. Continental-scale patterns of floral trait variation thus reflect macroecological patterns in pollinator availability, revealing elements of the biotic environment that may shape plant reproductive strategies.