Ah, orchids, those rare, delicate blossoms. Well – not always so delicate. An increasing number of orchids (over 90 species now) are documented as being invasive or weed-like – but why some species and not others?
Apart from their beauty, another feature of orchids is their reliance on highly specialized symbiotic mycorrhiza (Gk. mykós, “fungus” and riza, “roots), an association between a fungus and the roots of a vascular plant. Orchids are well known for their specialization to certain habitats, pollinators, and their mycorrhizal fungi. However, an increasing number of orchids are exhibiting weed-like attributes, e.g. they are disturbance-tolerant and readily proliferate in novel environments. Do weed-like orchids form novel mycorrhizal associations, or do they utilize a diverse range of fungi that are widespread and/or have broad growth properties? Although evidence to support these possibilities is still limited, recent analyses have shown that weed-like orchids form mycorrhizas with widespread fungi or may exploit a wide spectrum of fungi.
Over the last 30 years, Microtis media has been appearing opportunistically in terrestrial orchid collections, horticultural beds, disturbed roadsides and bushlands in south-western Australia and is now established in New Zealand, south-east Asia, Japan and China. New research in Annals of Botany asks: what role(s) might mycorrhizal specificity play in the distribution of the weed-like orchid M. media? By combining molecular and experimental studies, the authors showed that:
- Adult M. media plants formed mycorrhizal associations with a broad phylogenetic range of fungi
- A high proportion of these fungi were capable of initiating germination in seed of M. media but not sympatric terrestrial orchid species
- Mycorrhizal fungi isolated from M. media from natural habitats had higher germination efficacy than fungi from M. media in disturbed environments
- M. media–mycorrhizal associations display an unusual breadth of habitat tolerance
The findings from support the hypothesis that invasive species are possibly governed by more general rather than idiosyncratic patterns, including high phylogenetic breadth of associating mycorrhizal fungi, capacity for germination with a broad range of fungi and broad habitat tolerance. With its clear potential to act as an invasive species (rapid maturation of seedlings; clonal habit; self pollination; mycorrhizal generalist), M. media may impact populations of rare or threatened orchids through a resource competition process whereby it will eventually replace existing populations given its remarkable ability to utilize mycorrhizal and nutrient resources. Alternatively, M. media may exploit previously unutilized resources such as ecologically underutilized or unused mycobionts through niche differentiation at a much faster rate than the existing native species. Future research should test the competitive advantage of invasive orchid species and their role as a threat to other rare or ecologically constrained orchids.
Mycorrhizal preference promotes habitat invasion by a native Australian orchid: Microtis media. (2013) Annals of Botany 111(3): 409-18. doi: 10.1093/aob/mcs294
Mycorrhizal specialization has been shown to limit recruitment capacity in orchids, but an increasing number of orchids are being documented as invasive or weed-like. The reasons for this proliferation were examined by investigating mycorrhizal fungi and edaphic correlates of Microtis media, an Australian terrestrial orchid that is an aggressive ecosystem and horticultural weed. Molecular identification of fungi cultivated from M. media pelotons, symbiotic in vitro M. media seed germination assays, ex situ fungal baiting of M. media and co-occurring orchid taxa (Caladenia arenicola, Pterostylis sanguinea and Diuris magnifica) and soil physical and chemical analyses were undertaken. It was found that: (1) M. media associates with a broad taxonomic spectrum of mycobionts including Piriformospora indica, Sebacina vermifera, Tulasnella calospora and Ceratobasidium sp.; (2) germination efficacy of mycorrhizal isolates was greater for fungi isolated from plants in disturbed than in natural habitats; (3) a higher percentage of M. media seeds germinate than D. magnifica, P. sanguinea or C. arenicola seeds when incubated with soil from M. media roots; and (4) M. media-mycorrhizal fungal associations show an unusual breadth of habitat tolerance, especially for soil phosphorus (P) fertility.
The findings in M. media support the idea that invasive terrestrial orchids may associate with a diversity of fungi that are widespread and common, enhance seed germination in the host plant but not co-occurring orchid species and tolerate a range of habitats. These traits may provide the weedy orchid with a competitive advantage over co-occurring orchid species. If so, invasive orchids are likely to become more broadly distributed and increasingly colonize novel habitats.
- Ackerman JD. Invasive orchids: weeds we hate to love? Lankesteriana 2007; 7: 19-21.
- Cohen IM, Ackerman JD. Oeceoclades maculata, an alien tropical orchid in a Caribbean rainforest. Annals of Botany 2009;104: 557-563.
- Huynh TT, Thomson R, McLean CB, Lawrie AC. Functional and genetic diversity of mycorrhizal fungi from single plants of Caladenia formosa (Orchidaceae). Annals of Botany 2009;104: 757-765.