Coexistence of succulent tree aloes: partitioning of bird pollinators by floral traits and flowering phenology

Coexistence among species that lack genetic barriers to hybridization usually depends on pre-mating isolating barriers. It has been difficult to explain coexistence among African Aloe species because they readily hybridize, often flower simultaneously and are mostly bird-pollinated. Here we show that co-flowering aloes in a succulent thicket community in South Africa partition the fauna of flower-visiting birds. Aloe species with small amounts of concentrated nectar in long corolla tubes were pollinated primarily by long-billed sunbirds. These species co-flowered with species with large amounts of dilute nectar in short corolla tubes which were pollinated primarily by short-billed, generalist nectarivores. Aloe species which share pollinators tend to have divergent flowering times and differences in pollen placement on birds. Without these isolating barriers, genetic dissolution of sympatric Aloe species would be likely.