Restoring a critically endangered grassland orchid by co-planting to improve pollination and selecting sites based on pollinator availability

In many geographic regions grasslands have been heavily cleared and degraded, which represents a challenge for translocating threatened flora back into these landscapes. As most plant species require animals for pollination, pollinators are potentially a key limitation for re-establishing populations. For the Critically Endangered orchid <i>Diuris fragrantissima</i>, we identify the pollinator(s), survey for pollinators at candidate translocation sites, test if remnant size affects bee species richness, and test if pollination rates can be enhanced through co-planting with rewarding plant species. We found that <i>D. fragrantissima</i> is visited by ten species of bee but is only effectively pollinated by two native species, <i>Lipotriches (Austronomia)</i> sp. (Halictidae) and <i>Lasioglossum (Chilalictus) orbatum</i> (Halictidae), and the introduced honeybee <i>Apis mellifera</i> (Apidae). Interestingly, <i>A. mellifera</i> was responsible for the greatest number of pollinia removals and depositions. Pollinators of <i>D. fragrantissima</i> were not detected at some candidate translocation sites, with bee species richness and overall abundance significantly increasing with grassland remnant size. The pollination of <i>D. fragrantissima</i> was significantly enhanced through the presence of <i>Wahlenbergia stricta</i> (Campanulaceae) within 30 cm of plants, but not <i>Arthropodium strictum</i> (Asparagaceae) or <i>Dianella reflexa</i> (Asphodelaceae). We recommend that prior to conservation translocations of <i>Diuris</i> that pollinator surveys are undertaken, with preference given to larger grassland remnants. <i>Apis mellifera</i> may serve to buffer <i>D. fragrantissima</i> against loss of native pollinators from remnant grassland but could have adverse effects on other native species. We show that co-planting with rewarding species may be an effective approach for improving pollination success of threatened orchids.