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.

Warmer seed environments increase germination fractions in Australian winter annual plant species

“…Furthermore, autonomous self‐pollination may be a common pollination strategy for many of the annuals in our study system (Loy et al. ), so occasional pollen exchange among fragments may be crucial to prevent excessive inbreeding (Hobbs and Yates ), regardless of future climate conditions.…”

Plant interaction networks reveal the limits of our understanding of diversity maintenance

“…Moreover, as interaction strengths often switched sign from one environmental category to the next, the majority of common species in our system experienced some degree of facilitation at some point along the naturally occurring shade gradient. Potential mechanisms driving facilitation in this system are numerous and include micro‐environmental modification (Holmgren et al., 1997), plant–soil feedbacks (Ke & Wan, 2020) and pollinator attraction (Loy et al., 2015). Whilst these are currently being investigated, disentangling their relative contributions is complicated by the diversity, context‐dependency and covariations of such processes (Callaway, 1995).…”

Patterns of frequency and density dependence are highly variable in diverse annual flowering plant communities

“…In addition to being common and abundant in the reserve, these four species vary in relatedness (the Trachymene are sister species in Araliaceae, Goodenia rosea is in Goodeniaceae, and Arctotheca calendula is in Asteraceae) and indigenous/introduced status ( A. calendula is introduced, and the others are endemic species). Previous research suggested these species vary with respect to their selfing ability (Grace Lu, personal communication ) and the pollinators on which they rely for reproduction (Loy et al, 2015; Staples et al, 2016).…”

“…Insects—predominantly bees (e.g., Apis mellifera L. [Apidae], Neopasiphaae mirabilis Perkins [Colletidae], and species in the genus Leioproctus [Colletidae]) and, to a lesser extent, beetles (e.g., family Melyridae and genus Phlogistus [Cleridae]) and flies (e.g., genus Comptosia [Bombyliidae])—serve as pollinators in this system (Loy et al, 2015; Reynolds et al, 2022). We recognize insect pollinators can travel long distances between plants when foraging.…”