Mutualism between co-occurring plant species in South Africa's Mediterranean climate heathland is mediated by birds

Interactions among plant species via pollinators vary from competitive to mutualistic and can influence the probability of stable coexistence of plant species. We aimed to determine the nature of the interaction via flower visitors between Leucospermum conocarpodendron and Mimetes fimbriifolius, two shrubs in the Proteaceae that share many ecological traits and coexist on the Cape Peninsula, South Africa. To assess the extent of pollinator sharing we analysed nectar properties and recorded the pollinator fauna, their behaviour and contribution to seed set. To test for competition via interspecific pollen transfer, we recorded the movement patterns of pollinators and quantified pollen loads. To determine the effect of co-flowering on visitation rates we recorded visits in stands that varied in the density of the two species. We found that the species produce similar rewards and share pollinating Cape Sugarbirds (Promerops cafer). Interspecific pollen transfer is avoided by placing pollen on different parts of the bird. Both species are visited by nectar-thieving Orange-breasted Sunbirds (Anthobaphes violacea). Insects and autonomous self-pollination contributed little to seed set. Pollinator visits increased with conspecific density in both species, and the slope of the increase was steepest in the presence of high densities of the co-occurring plant species. Nectar thief visits also increased with conspecific density in both species, but the slope declined with increasing density of the co-occurring species. Co-occurrence enhanced pollinator visits and alleviated nectar robbing in both plant species, consistent with mutualisms. Mutualism within a trophic level is unusual, but may help to explain the stable coexistence of ecologically similar species.