Orchid conservation: from theory to practice

Abstract

Background: Given the exceptional diversity of orchids (26 000+ species), improving strategies for the conservation of orchids will benefit a vast number of taxa. Furthermore, with rapidly increasing numbers of endangered orchids and low success rates in orchid conservation translocation programmes worldwide, it is evident that our progress in understanding the biology of orchids is not yet translating into widespread effective conservation.

Scope: We highlight unusual aspects of the reproductive biology of orchids that can have important consequences for conservation programmes, such as specialization of pollination systems, low fruit set but high seed production, and the potential for long-distance seed dispersal. Further, we discuss the importance of their reliance on mycorrhizal fungi for germination, including quantifying the incidence of specialized versus generalized mycorrhizal associations in orchids. In light of leading conservation theory and the biology of orchids, we provide recommendations for improving population management and translocation programmes.

Conclusions: Major gains in orchid conservation can be achieved by incorporating knowledge of ecological interactions, for both generalist and specialist species. For example, habitat management can be tailored to maintain pollinator populations and conservation translocation sites selected based on confirmed availability of pollinators. Similarly, use of efficacious mycorrhizal fungi in propagation will increase the value of ex situ collections and likely increase the success of conservation translocations. Given the low genetic differentiation between populations of many orchids, experimental genetic mixing is an option to increase fitness of small populations, although caution is needed where cytotypes or floral ecotypes are present. Combining demographic data and field experiments will provide knowledge to enhance management and translocation success. Finally, high per-fruit fecundity means that orchids offer powerful but overlooked opportunities to propagate plants for experiments aimed at improving conservation outcomes. Given the predictions of ongoing environmental change, experimental approaches also offer effective ways to build more resilient populations.

Keywords: Orchid; conservation; conservation translocations; demography; genetics; mycorrhiza; pollination; reintroduction; restoration.

Fig. 1.

Steps in the currently successful conservation translocation programme of Caladenia colorata, a threatened species from south-eastern Australia. (A) Petri dishes of seedlings germinated symbiotically using a specific species of Serendipita mycorrhizal fungi. (B) Plants grown through to adulthood in glasshouse conditions. (C) Translocation to wild sites that were selected based on a detailed assessment of the vegetation community and confirmation of the presence of the primary pollinator species (Reiter et al., 2018a). (D) Wild recruits around the adult orchids that were originally planted. A total of 883 plants were introduced between 2013 and 2017. As of September 2018 there were 593 (67 %) of these plants surviving plus an additional 580 recruits, an increase of 65 % in the population beyond those initially planted and 97.8 % beyond those that survived translocation. Monitoring will now be conducted to either confirm long-term viability of established populations or alert managers to life cycle stages that are limiting the maintenance of positive population growth. Photographs by Noushka Reiter.

Fig. 2.

The life cycle of orchids. (A) Flowering. (B) Fruit formation. (C) Seed dispersal. (D) Germination through association with mycorrhizal fungi. (E) Recruitment to adulthood. Features associated with these life cycle stages are elaborated upon in Table 1.

Fig. 3.

Examples of pollination strategies in the orchids. (A) Pollination by fragrance-collecting euglossine bees: a species of Euglossa bee pollinating a species of Gongora. The bees are attracted by fragrances that they collect and use in courtship bouquets (Ramirez et al., 2011). Photograph: Santiago Ramirez. (B) Pollination by sexual deception: Caladenia crebra is pollinated by sexual deception of the thynnine wasp Campylothynnus flavopictus (Phillips et al., 2017). Long-distance attraction is by mimicry of a blend of (methylthio)-phenol sex pheromones (Bohman et al., 2017). Photograph: Rod Peakall. (C) Pollination by oil-collecting bees: Corycium nigrescens is pollinated by the melittid bee Rediviva brunnea, which collects oil to provision its brood. Photograph: Michael Whitehead. (D) Pollination by sexual deception: Drakaea glyptodon is pollinated by sexual deception of the thynnine wasp Zaspilothynnus trilobatus (Peakall, 1990). Long-distance attraction is by mimicry of a blend of pyrazine sex pheromones (Bohman et al., 2014). Here the flower is shown along side the female wasp. Photograph: Rod Peakall. (E) Pollination by nectar foraging hawkmoths: Disa crassicornis is pollinated by hawkmoths that feed on nectar produced at the end of the long nectar spur. Photograph: Michael Whitehead. (F) Pollination by brood site mimicry: Gastrodia similis mimics forest fruits using chemical cues, which attract pollinating Scaptodrosophila flies searching for an oviposition site (Martos et al., 2015). Photograph: David Caron.

Fig. 4.

Summary of the number of fungal operational taxonomic units (OTUs) that orchid species associate with based on the literature summarized in Supplementary Data Table S1. Studies were included if they presented data on the ITS sequence locus, 15 or more orchid individuals were sampled, and orchids were sampled from two or more sites. For full methodology see Supplementary Data Table S1. (A) Number of orchid species that exhibit varying levels of specialization in mycorrhizal association, subdivided into species that are photosynthetic terrestrials, leafless terrestrials and epiphytes. (B) Mean number (± s.e.) of fungal OTUs associated with photosynthetic terrestrial orchids in Europe compared with Australia.

Fig. 5.

Example of the potential benefits of genetic rescue in orchids. (A) The endangered Thelymitra mackibbinii, as of 2017 known from 40 wild plants across three populations (15, 22 and 1 plant per population) in Victoria, Australia. (B) Thelymitra mackibbinii plants grown from seed generated from the remaining wild plants via hand cross-pollination. Using the two largest remaining populations, plants on the left and right are from cross-pollination within populations, while the plants in the centre exhibiting the most robust growth are from cross-pollination between populations. All seedlings shown belong to the F₁ generation. Photographs and cultivation by Noushka Reiter.