Orchid conservation: making the links

Abstract

Orchidaceae, one of the largest families of flowering plants, present particular challenges for conservation, due in great part to their often complex interactions with mycorrhizal fungi, pollinators and host trees. In this Highlight, we present seven papers focusing on orchids and their interactions and other factors relating to their conservation.

Much has been written about human-induced changes to environments around the world, including deforestation, urbanization, agricultural change and, more recently, climate change, and the effects that these changes have on species of plants and other organisms. For Orchidaceae (possibly the largest family of flowering plants; Chase et al., 2015), the influence of these extrinsic factors is complicated by the fact that many orchid species rely on a complex set of interactions with other organisms for their survival (Fay and Chase, 2009; Bronstein et al., 2014; and references therein) and as a result they may be particularly sensitive to environmental change, either directly or indirectly through the organisms with which they interact (Swarts and Dixon, 2009). These interactions involve host trees for the majority of orchids (69 % of orchid species are epiphytes; Zotz, 2013), fungi (orchid species have seeds that cannot germinate without forming a parasitic relationship with one or more species of fungus; e.g. Phillips et al., 2011) and a diverse array of pollinators (most orchids depend on a pollinator to achieve seed production; Micheneau et al., 2009). Although some orchids provide a reward, the reliance on pollinators is further complicated by the high frequency of deceipt found in orchids, with pollinators being duped into visiting flowers in the hope of receiving a reward including food (e.g. Peter and Johnson, 2013; Ren et al., 2014) or even a sexual partner (e.g. Peakall and Whitehead, 2014; Phillips et al., 2014). In addition to survival, distributions of orchids and interacting organisms are also influenced by climatic factors (e.g. Devey et al., 2009; Swarts and Dixon, 2009); as a result, climate change in general (e.g. Fay, 2015) or specific changes, for example in prevailing wind patterns (Chalk, 2014) and levels and quality of light (Abeli et al., 2013; Falara et al., 2013; Kindlmann et al., 2014), can influence the distribution and survival of orchids.

Managing orchid populations following disturbance and creating new populations are important facets of orchid conservation (Swarts and Dixon, 2009, and references therein). Providing an additional tool for this purpose, Tremblay et al. (2015, this issue) investigate the use of population projection matrices in populations that have not yet reached stable-stage distribution, concluding that the method will be useful for designing rapid interventions after disturbance and for establishing new populations.

The complex seed germination requirements of orchids present many challenges. Relationships with fungi can affect the ability of an orchid to become established and invade new areas (De Long et al., 2013) and potentially explain why some orchid species are common and others are rare (Nurfadilah et al., 2013). Following germination, the relationships with fungi can change during the lifespan of the orchid, with different fungi playing important roles at different life stages in at least some species (e.g. Bonfante and Selosse, 2009). Following her review of orchid seedling mycorrhizas (Rasmussen and Rasmussen, 2014), Rasmussen et al. (2015, this issue) discuss the importance of conditions in germination sites, identifying the need for a new experimental approach to studies, especially for epiphytic species. Also in this issue, Lee et al. (2015a) study the role of abscisic acid in seed dormancy in the lady’s slipper orchid Cypripedium formosanum, demonstrating that it is a key inhibitor of germination.

Davis et al. (2015, this issue) investigate an Australian orchid that occurs on both sides of the continent, despite high mycorrhizal specificity, demonstrating that the fungal partner is the same in eastern and western populations. Instead, they hypothesize that specific edaphic requirements and pollination by sexual deception may explain biogeographic patterns in southern Australian orchids.

To date, most information about the relationship between orchids and fungal mycorrhizas relates to temperate terrestrial orchids, but papers on subtropical and tropical orchids, both terrestrial and epiphytic, are now beginning to accumulate (e.g. Leake and Cameron, 2012; Martos et al., 2012). Adding to this literature, Lee et al. (2015b) have studied fungal associations of fully mycoheterotrophic orchids, revealing that saprophytic non-Rhizoctonia fungi are more important than previously thought.

Although, as observed by Darwin (1862), some species of orchids are capable of self-pollination (e.g. Suetsugu, 2013; Gamisch et al., 2014), the diverse pollination syndromes of orchids have fascinated scientists since Darwin wrote his seminal book on the subject (Darwin, 1862; van der Pijl and Dodson, 1966; Micheneau et al., 2009; van der Niet et al., 2014). The ongoing interest in these syndromes is reflected in the large number of papers that are still being published on orchid pollination. Many groups of organisms act as orchid pollinators, including birds (e.g. Micheneau et al., 2006; van der Niet et al., 2015), bees (e.g. Davies et al., 2013; Peter and Johnson, 2013; Sugiura, 2013; Vale et al., 2013; Pellegrino and Bellusci, 2014), wasps (e.g. Peakall and Whitehead, 2014; Menz et al., 2015), butterflies (e.g. Sun et al., 2014), moths (e.g. Brzosko and Wróblewska, 2013; Boberg et al., 2014), flies (e.g. van der Niet et al., 2011), beetles (e.g. Peter and Johnson, 2014) and even a cricket (Micheneau et al., 2010), but, despite the intensity of study for many decades, new pollination syndromes are still being discovered (e.g. Micheneau et al., 2010). Another new syndrome is reported by Karremans et al. (2015, this issue), with a group of Neotropical orchids being shown to attract Drosophila fruit flies by producing chemicals that act as aggregation pheromones. Also involved in seed production, the evolution of reproductive isolation has been implicated as a factor in speciation, and Pinheiro et al. (2015, this issue) present the first broad study of transitions between self-compatibility and self-incompatibility in the evolution of genetic barriers in a tropical orchid group (the genus Dendrobium).

Due to the factors discussed above, conserving orchids presents huge challenges as not all of the interactions will be affected in the same way by global change, habitat degradation and other environmental perturbations. This has led to much active research within the subject, reflected in meetings such as the regular Orchid Conservation Congresses, the most recent of which (the fifth) was held in St Denis, La Réunion, in December 2013. The theme of this meeting was ‘Orchid conservation – making the links’, and this has also been chosen as the title for the group of seven papers presented in this Highlight, which focus on several aspects of orchid biology, populations dynamic and interactions with other organisms