Asteraceae invaders have limited impacts on the pollination of common native annual species in SW Western Australia’s open woodland wildflower communities

Abstract

The York gum–jam woodlands of southwest Western Australia support diverse annual wildflower communities despite extensive habitat fragmentation, remnant isolation and the invasion of many exotic annual plant species. Few studies have explored the pollinator–plant relationships maintaining these persistently species-rich ‘novel’ communities. We examine the pollination ecology of five native species common to York gum–jam woodland annual communities to determine whether native pollinators may be mediating impacts of exotic annual plants on native wildflower species. We determined the pollination requirements of native focal species and the diversity and frequency of pollinator visitation to these focal plant species across invasion gradients. We also recorded the pollinator community of a dominant exotic herb in this system: Arctotheca calendula (cape weed). Only two of the five native species examined had significant seed set benefits attributable to insect pollination. One native plant species, Podotheca gnaphalioides, had pollinator assemblages that overlapped significantly with exotic A. calendula, with some reduction in pollinator visitation evident. One species, Waitzia acuminata, was found to benefit from insect pollination only in the larger of two surveyed remnants, which may reflect emerging reproductive polymorphism among geographically isolated populations. We highlight two mechanisms in this system that may buffer pollinator-mediated impacts of exotic species on native species: autonomous seed production, which may be increasingly prevalent in isolated populations, and segregation of pollinator resources among species. Our findings illustrate the ways that pollinator-mediated interactions can affect seed set within plant communities persisting in highly fragmented and invaded agricultural landscapes.