CSIRO PUBLISHING

www.publish.csiro.au/journals/ajb Australian Journal of Botany, 2007, 55, 293–307

Scientific approaches to Australian temperate terrestrialorchid conservation

Mark C. Brundrett

Terrestrial Ecosystems Branch, Policy and Coordination Division, EPA Service Unit, Department ofEnvironment, PO Box K822, Perth, WA 6000, Australia and Faculty of Natural and Agricultural Sciences,School of Plant Biology, The University of Western Australia, Crawley, WA 6009, Australia.Email: brundrm@cyllene.uwa.edu.au

Abstract. This review summarises scientific knowledge concerning the mycorrhizal associations, pollination,demographics, genetics and evolution of Australian terrestrial orchids relevant to conservation. The orchid family ishighly diverse in Western Australia (WA), with over 400 recognised taxa of which 76 are Declared Rare or PriorityFlora. Major threats to rare orchids in WA include habitat loss, salinity, feral animals and drought. These threatsrequire science-based recovery actions resulting from collaborations between universities, government agencies andcommunity groups.

Fungal identification by DNA-based methods in combination with compatibility testing by germination assays hasrevealed a complex picture of orchid–fungus diversity and specificity. The majority of rare and common WA orchidsstudied have highly specific mycorrhizal associations with fungi in the Rhizoctonia alliance, but some associate with awider diversity of fungi. These fungi may be a key factor influencing the distribution of orchids and their presence canbe tested by orchid seed bait bioassays. These bioassays show that mycorrhizal fungi are concentrated in coarse organicmatter that may be depleted in some habitats (e.g. by frequent fire). Mycorrhizal fungi also allow efficient propagation ofterrestrial orchids for reintroduction into natural habitats and for bioassays to test habitat quality.

Four categories of WA orchids are defined by the following pollination strategies: (i) nectar-producing flowers withdiverse pollinators, (ii) non-rewarding flowers that mimic other plants, (iii) winter-flowering orchids that attract fungus-feeding insects and (iv) sexually deceptive orchids with relatively specific pollinators. An exceptionally high proportion ofWA orchids have specific insect pollinators. Bioassays testing orchid-pollinator specificity can define habitats and separateclosely related species. Other research has revealed the chemical basis for insect attraction to orchids and the ecologicalconsequences of deceptive pollination. Genetic studies have revealed that the structure of orchid populations is influencedby pollination, seed dispersal, reproductive isolation and hybridisation. Long-term demographic studies determine theviability of orchid populations, estimate rates of transition between seedling, flowering, non-flowering and dormant statesand reveal factors, such as grazing and competition, that result in declining populations.

It is difficult to define potential new habitats for rare orchids because of their specific relationships with fungi andinsects. An understanding of all three dimensions of orchid habitat requirements can be provided by bioassays with seedbaits for fungi, flowers for insects and transplanted seedlings for orchid demography. The majority of both rare and commonWA orchids have highly specific associations with pollinating insects and mycorrhizal fungi, suggesting that evolution hasfavoured increasing specificity in these relationships in the ancient landscapes of WA.

Introduction

The South-west Australian Floristic Region (SWAFR) ofWestern Australia (WA) has a mediterranean climate andpossesses an exceptionally high plant species richness andendemism and ranks as one of the world’s 25 hotspots ofconservation priority (Myers et al. 2000; Hopper and Gioia2004). The high plant diversity in the SWAFR is thought to resultfrom a combination of factors, including a long period sincemajor tectonic or glacial disturbance, highly infertile soils andperiodic minor disturbances such as drought (Pate and Hopper1994; Hopper and Gioia 2004). Recent human impacts haveresulted in south-western WA becoming one of the most stressedregions in Australia as a result of over-clearing, fragmentation

and declining quality of vegetation (Commonwealth ofAustralia 2002).

The SWAFR is a terrestrial orchid biodiversity hotspot ofworldwide significance, with an estimated 400 orchid taxa,most of which are endemic (florabase.calm.wa.gov.au). Of these,36 taxa are Declared Rare Flora as defined by WA legislation(DRF), a further 39 taxa are Priority Flora, many of which requirefurther assessment (florabase.calm.wa.gov.au). However, evencommon species are declining in urban and rural areas owingto habitat loss, salinity, altered fire regimes, weed invasion andland degradation.

Most WA orchids occur in the SWAFR, which has aMediterranean climate with cool, wet winters, followed by

© CSIRO 2007 10.1071/BT06131 0067-1924/07/030293

294 Australian Journal of Botany M. C. Brundrett

5–8 months of summer drought when most orchids aestivate asdormant tubers. Unfortunately, this region has sustained some ofthe highest levels of land clearing and fragmentation in Australia(Brown et al. 1998; Commonwealth of Australia 2002).The SWAFR is a living biological laboratory where it ispossible to contrast closely related orchids that are commonor rare, widespread or highly localised, have general orhighly specific habitat requirements or have geographicallydisjunct populations.

Objectives of this review are to summarise knowledge ofmycorrhizal associations, pollination, demographics, geneticsand evolution relevant to orchid conservation and identifykey questions that must be answered to manage rare orchids.The impacts of highly specialised pollination and fungalassociations on orchid rarity and evolution are discussed.Although the focus of this review is on SWAFR terrestrialorchids, the information on temperate terrestrial orchidssummarised is of worldwide scope.

Mycorrhizal associations of orchids

Mycorrhizas are symbiotic associations between specialised soilfungi and plants, and are primarily responsible for nutrienttransfer (Brundrett 2004). Most plants in natural ecosystemshave mycorrhizal associations, but little is known about theecology of their associated fungi (Brundrett 1991). The majorityof WA plants have these associations and the rest have alternativenutritional strategies such as cluster roots (Brundrett and Abbott1991). Orchid mycorrhizal associations have a higher degreeof plant–fungus specificity than those of most other plants. Forexample, there is no evidence of plant–fungus specificity forvesicular-arbuscular mycorrhizas, the commonest mycorrhizalassociation of Australian plants (Brundrett and Abbott 1991).

Orchid mycorrhizas consist of coils of hyphae in corticalcells of the root, stem or protocorms (germinating seedlings)of orchids (Rasmussen 1995; Peterson et al. 1998). Thesesymbiotic associations are considered essential to germinate thetiny seeds of orchids and for the nutrition of adult terrestrialorchids (Rasmussen 1995; Batty et al. 2002). These associationsdiffer from the mutualistic mycorrhizas of other green plants,as the fungus can provide both the energy and mineral nutrientsrequired by their hosts, but in most cases the fungi seem to receivevery little or no benefit in return (Julou et al. 2005). Evidence thatorchid mycorrhizas are not mutualistic includes the following:(i) the loss of chlorophyll because of myco-heterotrophy(Molvray et al. 2000; Brundrett 2004), (ii) the distribution ofmycorrhizal fungi being independent of orchids (Brundrett et al.2003), (iii) periods of subterranean dormancy lasting one ormore years when orchids may recover from stress (Rasmussen1995; Shefferson et al. 2005a) and (iv) the survival of albinomutants of orchids (Julou et al. 2005). However, the natureof orchid mycorrhizas is complex, and probably includes bothexploitative and mutualistic phases. For example, Cameron et al.(2006) demonstrated substantial plant-to-fungus carbon transferin an experimental system, whereas Julou et al. (2005) foundan orchid received almost 50% of its carbon from fungi whengrowing in situ. Regardless of whether or not they obtain partof their carbon from fungi, there can be no doubt that mostWA orchids are extremely dependent on mycorrhizal fungi

for mineral nutrition, because they have highly reduced ornon-existent roots that would be useless at directly obtainingsoil nutrients.

Most mycorrhizal fungi of green orchids are basidiomycetesassigned to the polyphyletic and very diverse form genusRhizoctonia (Table 1). These fungi are identified by asexualstate names like Epulorhiza and Ceratorhiza, or sexual statenames such as Tulasnella, Thanatephorus and Sebacina (Currahet al. 1996; Roberts 1999; Taylor et al. 2002). Most studiesin Table 1 used sequences of the highly variable rDNA ITSregion to assign fungi to lineages and genera by phylogeneticcomparisons with other fungi in the rapidly expanding genedatabases (e.g. GenBank, www.ncbi.nlm.nih.gov). We currentlylack clear concepts of what constitutes species or genera in theRhizoctonia complex and know very little about the ecologyof these fungi. The Rhizoctonia alliance includes pathogensthat infect many crop and horticultural species, but thesefungi also have endophytic competence and the capacity tolive independently in soils as saprophytes (Sivasithamparam1993; Roberts 1999; Rasmussen 2002; Brundrett 2006). Fungiin Table 1 include members of the Sebacina clade, a groupconsidered to be predominantly ectomycorrhizal (Weiss et al.2004), the Ceratobasidiales clade, which includes many plantpathogens (Gonzales et al. 2001; Pope and Carter 2001), and theEpulorhiza clade, many of which are only known from orchids.Knowledge gained about the biology of orchid fungi in naturalecosystems is also applicable to understanding and controllingrhizoctonias parasitic on plants.

Four categories of orchids based on specificity, defined bythe diversity of compatible mycorrhizal fungi, are recognisedin Table 1. This table summarises studies of terrestrial orchidsin Australia and includes some recent studies from otherregions for comparison. Although relatively few species ofAustralian orchids have been studied, consistency within orchidgenera allows some trends to be established. The majorityof Australian orchids in Table 1 have highly specific or veryhighly specific fungal associations, as is also the case formany Northern Hemisphere orchids. However, there also areorchids such as Microtis spp. which are capable of formingassociations with a wide diversity of fungi. The diversity offungi in myco-heterotrophic orchids is summarised elsewhere(see tables in Batty et al. 2002; Rasmussen 2002). In contrastwith green orchids, the majority of achlorophyllous myco-heterotrophic orchids have associations with fungi outside theRhizoctonia alliance. Most myco-heterotrophs examined fromNorth America and Europe associate with ectomycorrhizalfungi that provide links to forest trees, whereas those fromAsia often associate with wood-rotting saprophytic fungi(Batty et al. 2002).

Although many DNA-based studies of Northern Hemisphereorchid roots found them to associate with narrowly definedgroups of fungi in the Rhizoctonia alliance (Table 1), otherstudies have reported that a wider diversity of fungi, or fungifrom other genera, occur in orchids. (e.g. Bidartondo et al.2004; Selosse et al. 2004; Julou et al. 2005). These contraryresults require cautious interpretation in cases where fungiwere not isolated and compatibility was not tested, becauseendophytic fungi (which are neither harmful nor beneficial) oftenoccur in orchids (Bayerman and Otero 2006; Brundrett 2006).

Scientific approaches to Australian temperate terrestrial orchid conservation Australian Journal of Botany 295

Table 1. The diversity of fungi that associate with temperate photosynthetic terrestrial orchidsInformation presented is primarily from Australian orchids. DNA, fungi identified by DNA sequences; Germ, Koch’s Postulate tested by symbiotic germinationassays in sterile conditions; AG, anastomosis groups. Typical levels of mycorrhizal-fungus specificity assigned to genera: 1 = low, several broad groups of

fungi; 2 = medium, one broad group of fungi; 3 = high, a narrow group of fungi; 4 = extremely high, a single fungus

Location Orchid Specificity Method Fungus Reference

Australia Thelymitra spp. Low–medium(1–2)

Germ Epulorhiza Warcup (1973)

Western Australia Pterostylis, 18 spp. High–very high(3–4)

Germ Rhizoctonia – 20AGs

Ramsay et al. (1987)

New South Wales, Australia P. acuminata High (3) Germ Rhizoctonia solani– 2 AGs

Perkins and McGee(1995); Pope andCarter (2001)

New South Wales, Australia Microtis spp. Low (1) Germ Epulorhiza andSebacinaisolates

Milligan and Williams(1988); Warcup (1988);Perkins et al. (1995)

Western Australia Diuris spp. Medium–high(2–3)

Germ Rhizoctonia S.L. L. Quay, pers. comm.

Queensland Acianthus, Caladenia,Pterostylis sp.

High (3) DNA, germ Rhizoctonia –different groups

Bougoure et al. (2005)

Western Australia Caladenia 5 spp.,Drakaea 4 spp.

High–very high(3–4)

Germ Not identified Hollick (2004)

Western Australia Disa bracteata, M. media Low (1) DNA, germ Most Epulorhiza Bonnardeaux et al. (2007)Western Australia Pyrorchis Low (1) DNA Most Epulorhiza,

some SebacinaBonnardeaux et al. (2007)

Australia Caladenia formosa High (3) Germ 2 fungusmorphotypes

Huynh et al. (2004)

Malaysia Neuwiedia veratrifolia Medium–high(2–3)

DNA Most Epulorhiza Kristiansen et al. (2004)

USA Liparis lilifolia, Goodyerapubescens, Tipulariadiscolor

High (3) Germ, DNA Narrow, clades inEpulorhiza

McCormick et al. (2004)

Turkey Orchis palustris, Serpiasvomeraceae

High (3) Germ Rhizoctonia solaniisolates

Esitken et al. (2005)

Northern Hemisphere Cypripedium species High–very high(3–4)

DNA Narrow clades inEpulorhiza

Shefferson et al. (2005b)

Compatible orchid mycorrhizal fungi (orchid fungi) shouldbe defined as fungi isolated from mycorrhizal structures thatsuccessfully germinate seed of the same orchid to an advancedseedling stage (Batty et al. 2001a; Bonnardeaux et al. 2007).This narrow definition is required because some orchidswill germinate into protocorms with a wider diversity offungi than can support semi-autotrophic seedlings (Masuharaand Katsuya 1994; Zelmer et al. 1996; Esitken et al. 2005;Bonnardeaux et al. 2007).

Since orchids can grow only in locations where a compatiblemycorrhizal fungus is active, the capacity to detect these fungiin soils and understand their separate habitat requirements is anessential tool for orchid conservation. This is possible by usingorchid seeds as baits for mycorrhizal fungi (Fig. 1G). Orchidseed baiting techniques utilise the following two methods:(i) in situ baiting where orchid seeds are buried in soilpackets in natural habitats, or (ii) ex situ baits where seed isplaced on a membrane filter and incubated over organic matterextracted from soil (Rasmussen and Whigham 1993; Batty et al.2001a; Brundrett et al. 2003). The ex situ method by Brundrettet al. (2003) has the following advantages over in situ seedpackets: (i) independence from climatic factors, (ii) soil canbe transported from remote locations, (iii) the time-course ofgermination can be observed and (iv) resulting protocorms can

be used to isolate mycorrhizal fungi or to propagate orchids(Brundrett et al. 2003). Orchid seed baiting can identify suitablehabitats (new areas within existing locations, or new locationsin separate patches of remnant vegetation) for transplantationof rare orchids, but other evidence of habitat suitability isalso required (see ‘Demographics and habitats’). Seed baitinghas also been used to investigate fungal substrate preferences,orchid seed bank persistence and fungal distribution patternsin soil (Rasmussen and Whigham 1993; Batty et al. 2001a;Feuerherdt et al. 2005; Whigham et al. 2006). Orchid fungiare known to vary in their capacity to utilise different organicsubstrates and occupy particular soil micro-habitats (Zelmeret al. 1996; Brundrett et al. 2003). Knowledge of how orchidfungus diversity varies between and within habitats is a keyaspect of orchid recovery actions and isolates of fungi areused for symbiotic orchid propagation, as explained in ‘Usingscientific knowledge to rescue rare orchids’.

Pollination

The unusual means by which orchids are pollinated byinsects has been the focus of scientific enquiry and debatesince the time of Darwin (1904), but there are still majorgaps in our knowledge of these associations. The complex

296 Australian Journal of Botany M. C. Brundrett

Fig. 1. Examples of threats to orchid populations of Declared RareWestern Australian orchids and recovery actions. (A) Severe disturbanceby feral pigs to habitat of Caladenia elegans (arrow) required fencing formanagement. (B) Caterpillar (Anthema sp.) grazing of the carousel spiderorchid (Caladenia arenicola) in urban bushland. (C) Competition by grassyweeds (Brizia major) has a major impact on some populations of Pterostylissp. Northampton. (D) Probable clonal spread shown by new leaves (*) ofthe lonely hammer orchid (Drakaea isolata), which is known only froma single location. (E) The hinged dragon orchid (Caladenia drakeoides) ispollinated by male wasps that are lured to flowers by sexual deception. Risingsaline groundwater has resulted in habitat loss for populations of this rareorchid, which often grows near salt lakes. (F) Suspected wasp pollinator ofthe ballerina orchid (Caladenia melanema) at the only known location forthis orchid. (G) Germinating seeds and a larger seedling of C. arenicola ona membrane filter square over soil organic matter from a natural habitat.Orchid seed bait. (H) Hand-pollination to ensure seed set of the rare granitespider orchid (Caladenia graniticola). (I) Out-planting trial to test impacts ofweed abundance in degraded urban bushland habitats on survival of orchidseedlings planted into grid squares. Seedlings were produced by symbioticgermination in the laboratory (arrows).

nature of insect–orchid interactions arises because (i) orchidsproduce thousands of seeds in each capsule, (ii) they havepollinia containing numerous pollen grains so require precisepollination mechanisms, (iii) many orchids have specific insectpollinators and (iv) both orchids and pollinators may have patchydistributions and separate habitat requirements (Benzing andAtwood 1984; Arditti and Ghani 2000; Pacini and Hesse 2002;Tremblay et al. 2005).

Orchidaceae is the only very large family of plants in WA thatis exclusively insect-pollinated (Brown et al. 1997). Despite the

difficulty of obtaining conclusive proof of pollinator specificity(see Adams and Lawson 1993), repeated observations haveshown that many Australian terrestrial orchids are pollinated bya specific insect, whereas others have more diverse pollinators(Table 2). These relationships tend to be consistent at thegeneric or subgeneric level for WA orchids. The most commonlyobserved pollinators include wasps, bees, beetles and fungusgnats. In most cases, little is known about the ecology or habitatpreferences of the insects that pollinate orchids.

Orchids that occur in the SWAFR are assigned to four groupson the basis of pollination mechanisms in Table 2. The first groupin Table 2 consists of orchids that produce nectar or other foodrewards and attract a wide diversity of insects (up to 12 types).A few orchids included in this category are capable of selfpollination as a primary or secondary means of fertilisation.Orchids that supply food rewards to insects are visited frequentlyby them, but pollen transfer may only be for short distances(Johnson et al. 2005).

The second major group of orchids in Table 2 hasintermediate pollinator specificity based on food deception.These orchids lack nectar or other food rewards and attractinsects primarily by visual deception, but may also be scented(Beardsell et al. 1986; Brown 1991; Elliott and Ladd 2002). Inthese orchids, insect specificity results because the mimickedflowers are preferred by particular pollinators, which ofteninclude several different types of insects. Detailed studies of theAustralian orchids Thelymitra antennifera and Diuris maculatafound them to have a suite of pollinators that were also attractedto co-flowering plants in other families of similar appearance,resulting in a low frequency of density-dependent seed-capsuleproduction (Beardsell et al. 1986; Dafni and Calder 1987).Deceptive means of attracting insects are much more commonin orchids than in other plants, occurring in an estimatedone-third of all orchid species (Dafni 1984; Schiestl 2005;Tremblay et al. 2005).

Schiestl (2005) and Tremblay et al. (2005) discussedecological consequences of deceptive pollination which includethe following: (i) orchid species should be less common thanmimicked species, (ii) polymorphism of floral characters isadvantageous to confuse pollinators and reduce avoidance and(iii) orchid flowers are often larger and more attractive (colouror scent) than the mimicked species. Deceptive pollinationsyndromes distract insects from profitable activities, but theimpact of orchids on pollinators is expected to be small,since orchids are usually much less common than the speciesthey mimic. Identification of orchids from pollen on insectsestablished that sympatric European deceptive orchids oftenshare pollinators (Cozzolino et al. 2005).

The third group of orchids in Table 2 is characterised by lateautumn and winter flowering in WA and narrow insect specificity.As these orchids are primarily visited by mycophagous insects,it has been proposed that they have deceptive pollination byfungus mimicry (Bernhardt 1990; Adams and Lawson 1993;Hoffman and Brown 1998). Potentially fungus-deceptive orchidsflower in the wet season (autumn or winter in the SWAFR) whenmycophagous insects are most active and cool temperatures limitthe activity of many other insects. These orchids often grow insubstrates and habitats preferred by fungi (e.g. shade, deep leaflitter or rotten wood for Corybas and some Pterostylis species).

Scientific approaches to Australian temperate terrestrial orchid conservation Australian Journal of Botany 297

Table 2. Four categories of pollination specificity of Western Australian orchidsMain references: Erickson (1965); Stoutamire (1983); Jones (1988); Peakall (1990); Bernhardt (1990); Brown (1991); Adams and Lawson (1993); Brown

et al. (1997); Hoffman and Brown (1998); Elliott and Ladd (2002); Hopper and Brown (2004)

Category Features

1. Low specificityDefinition Pollination by widely diverse insect groupsMechanisms Direct insect attraction by nectar/food reward, often scented (some are self pollinating)Consequences High rates of pollination (low outcrossing rates expected)Orchid genera Cyrtostylis, Microtis, Prasophyllum, Caladenia (some)Insect groups Beetle, bee, fly, wasp, gnat

2. Medium specificityDefinition Pollination by insects with similar food requirementsMechanisms Food deception mimicry by resembling flowering plants in other families. May be scented, but no nectar or foodConsequences Low rates of pollination (outcrossing rates expected to be higher than Category 1, but lower than 4)Orchid genera Diuris, Caladenia (∼1/3 of spp.), Cyanicula, Eriochilus, ThelymitraInsect groups Beetle, bee, fly (especially bee-fly), wasp

3. High specificityDefinition Small group of similar insects in the same functional groupMechanisms Fungus mimicry deception, or entrapment mechanisms?Consequences High rates of pollination (low outcrossing rates expected due to small range of insects and aggregation of flowers)Orchid genera Corybas, Pterostylis (most), RhizanthellaInsect groups Fungus gnat, phorid fly, mosquito

4. Very high specificityDefinition Pollination by a single insect or several similar speciesMechanisms Sexual deception where orchid mimics female insect by pheromones and shapeConsequences Low rates of pollination (high outcrossing rates, where examined)Orchid genera Caladenia (∼2/3 of spp.), Calochilus (some self-pollinate also), Cryptostylis, Drakaea, Leporella, Paracaleana,

(some Pterostylis?), SpiculaeaInsect groups Thynnid wasp, ichneumon wasp, ant

Presumably, these flowers are also coloured or scented toresemble fungi. The underground orchid (Rhizanthella gardneri)is a good example of these characteristics, as its burgundy flowersoccur with the buttons of mushrooms under leaf litter in earlywinter and phorid flies (which are attracted to fermentation) arethe most frequently observed potential pollinators of this species(George 1980; J. Bougoure, pers. comm.).

Insect-attraction mechanisms for Category 3 orchids arenot well understood, as many species have complex flowershapes that guide and entrap insects (e.g. Pterostylis spp.),others may use sexual deception and some attract other types ofinsects such as mosquitos (Table 2). Attraction of mycophagousinsects to some orchids may have evolved readily if digestionof mycorrhizal hyphae releases volatile chemicals that attractmycophagous insects. Whatever the means of attraction, itappears to be very efficient, as these orchids typically have ahigh frequency of seed set (Table 2). These orchids also benefitfrom damper conditions during the period for seed developmentthan do spring-flowering SWAFR orchids.

The fourth and largest group of WA orchids in Table 2 haveextremely specific interactions that are based on sexual deceptionwhere flowers mimic female insects, especially thynnine wasps.These orchids vary in shape from relatively unmodified spiderorchids (Fig. 1F) to dragon, flying duck and hammer orchids(Caladenia, Paracaleana and Drakaea spp.), with a hingedlabellum shaped to mimic female wasps (Fig. 1E), which aresome of the world’s most bizarre orchid flowers. The majorityof wasp-pollinated WA orchids have a single known pollinator,or several similar pollinators (Brown et al. 1997; A. Brown,

pers. comm.). Sexually deceptive orchids have complex shapesthat help to deceive and guide insects, which are thought to beprimarily attracted by pheromones (Stoutamire 1983; Schiestl2005). Electrophysiological measurements, where electricalsignals from the receptors in insect antennae are measured inparallel with chemical analysis, have revealed that pollinatorspecificity in the Australian orchid genera Chiloglottis andCryptostylis results from production of the same uniquechemical pheromone as is produced by female wasps of thepollinator species (Schiestl et al. 2004; Schiestl 2005). Insectbehaviour in the sexually deceptive orchids that have beenstudied in Australia results in low pollination success, butit is thought that this is compensated for by a relativelyhigh frequency of outcrossing (see ‘Population genetics’).Consequences of pollination strategies are discussed further in‘Consequences of the unique biology of orchids’.

The distribution patterns of some orchids may be shaped bythe distribution of pollinators. For example, hammer orchids(Drakaea spp.) flower in sand patches, which also are thehabitat of the female thynnine wasps they mimic (Peakall1990). Pollinating insects have limited home ranges andspecific requirements for food sources that determine wherethey occur. The distribution patterns of pollinators relative tothose of orchids can be tested by bioassays, where orchidflowers are transported to different locations as baits forinsects (e.g. Stoutamire 1983; Bower 1996). These bioassayshave revealed cryptic orchid species that are distinguishedby pollinators, not previously recognised by botanists (Bower1996; Mant et al. 2005b). Knowledge of orchid pollinators

298 Australian Journal of Botany M. C. Brundrett

and pollination success rates is required to effectively managepopulations of rare orchids (see ‘Using scientific knowledgeto rescue rare orchids’). For example, orchids with a singlepollinator (many of those in Category 4 in Table 2) are muchmore likely to have pollination-limited reproduction, than areorchids with less-specific pollinators (Categories 1–3).

Demographics and habitats

Long-term studies lasting a decade or more have measuredpopulation dynamics and established causes of mortality fororchids in Europe and North America (Hutchings 1987;Primack and Stacy 1998; Willems and Dorland 2000; Kery andGregg 2004; Light and MacConaill 2005; Nicole et al. 2005;Pfeifer et al. 2006). Demographic studies provide estimatesof population sizes that include long-term dormancy (whereorchids remain underground for one or more years) and transitionrates between juvenile, flowering, non-flowering and dormantstates (Brzosko 2002; Kery and Gregg 2004). Population sizesfluctuate annually, primarily because of climatic conditions(Light and MacConaill 2005; Pfeifer et al. 2006). Sheffersonet al. (2005a) suggested that long-term dormancy is a meansof buffering stress in terrestrial orchids, as defoliation resultedin increased rate of dormancy in two orchids, but not increasedmortality. In semi-arid parts of WA, orchids that are much moreabundant in wet years probably exhibit long-term dormancy(A. Brown, pers. comm.).

Long-term studies of orchids have revealed contrastingdemographic trends, which at the extremes include species thatreproduce only from seed and live for a few years (e.g. Hutchings1987; Coates et al. 2006), in contrast to orchids with clonalspread that are capable of persisting for more than 300 years(e.g. Cypripedium calceolus, Nicole et al. 2005). Clonal speciesusually persist for much longer than non-clonal species in theabsence of major disturbance to their habitats. Clonal and non-clonal orchids are equally common in WA and both categoriesinclude orchids that flower infrequently after hot fires (Dixon1991; Hoffman and Brown 1998; Bell 2001). There are bothadvantages and disadvantages to plants that spread primarily byclonal growth, as the reduced risk of local extinction is oftenbalanced by a reduced capacity for long-range spread by sexualreproduction (Honnay and Bossuyt 2005). I have observed thatsome Critically Endangered species in WA have the capacity forclonal spread but most spread by seed only (Fig. 1D–F).

Pollination, seed dispersal, seed germination and seedlingsurvival are the key limiting factors in the lifecycle of orchidsthat determine overall recruitment rates (Table 3). Rates oftransition between these stages are summarised in Fig. 2 forCaladenia arenicola, which is a relatively common orchid in theurban bushland of Perth. These rates suggest that populations ofC. arenicola will increase in habitats where recruitment rates(estimated at <1 seedling per plant per year) exceed rates ofmortality. Some of the transition rates in Fig. 2 are unknown(e.g. seed-dispersal efficiency and mortality). Others, such asin situ seed germination, change substantially from year-to-year(Hollick 2004). However, the overall estimate seems realistic onthe basis of observations of the long-term stability of populationsof orchids in urban bushland within Perth. Recruitment rates forrare orchids are expected to be much lower than for C. arenicola,

owing to the low probability of seed dispersal to a suitable habitat(in highly cleared and fragmented landscapes) or infrequentpollination, or declining habitat conditions (see ‘Using scientificknowledge to rescue rare orchids’). The wide range in values inTable 3 shows that generalisations about orchid demographicsare dangerous, so taxon-specific knowledge is essential toconserve rare species.

Major factors that influence the survival, growth andreproduction of orchids are listed in Table 3. The most frequentlyreported impacts include grazing by vertebrates, disturbance,competition by weeds and climatic factors (Fig. 1A, C). Grazingby invertebrates can also be a major factor (Fig. 1B). Petit andDickson (2005) established that the South Australian orchid(Caladenia behrii) survived best where protected from animalgrazing by the shrub Xanthorrhoea semiplana, but this alsoresulted in a lower frequency of pollination. The cost of floweringor seed production by orchids can be expressed by reducedsize or by frequency of flowering in subsequent years (Primackand Stacy 1998; Willems and Dorland 2000). Vallius (2001)determined that flowering was supported by stored reserves inthe European orchid Dactylorhiza maculata, as shade treatmentshad little effect until the following year.

Studies of European orchids have linked substantialpopulation decline for many species in the past centuries torequirement for specific habitats such as calcareous grasslandsor woodlands, rather than ecological characteristics such aspollination strategies (Jacquemyn et al. 2005a; Nicole et al.2005; Kull and Hutchings 2006). Changes to land-managementpractices, especially altered grazing, mowing, afforestationand fertilisation, which increase competition between orchidsand larger plants, are the key causes of orchid decline inthese habitats (Table 3). Rare orchids that occur in Australiangrasslands, European meadows and North American prairies,typically require particular disturbance regimes in the form offires, grazing or mowing to reduce competition (Reinhammaret al. 2002; Wotavova et al. 2004; Coates et al. 2006). SomeWA orchids are ineffective at spreading to new habitats post-disturbance, whereas others rapidly colonise these habitats,presumably because seeds and mycorrhizal fungi are morereadily available (Grant and Koch 2003; Collins et al. 2005).Unfortunately, disturbance or high fire frequency most oftenseems to result in the dominance of alien weeds at the expense oforchids and other native plants in most habitats in the SWAFR.In WA, major threats to orchids and other Declared RareFlora include (i) habitat factors including altered hydrology,salinity and drought, (ii) various types of disturbance, such asland clearing or altered fire regimes and (iii) biotic factors,such as grazing by feral animals and competition by weeds(Brown et al. 1998; Coates and Atkins 2001; and other papersin this volume).

Clark et al. (2004) found the habitat of the rare easternAustralian species Cryptostylis hunteriana was defined byclimate, soils and vegetation types. In other biomes, orchidsoften occur in narrow climatic or altitudinal zones (Linder1995; Clark et al. 2004; Jacquemyn et al. 2005b). In contrast,Bowles et al. (2005) established that the endangered NorthAmerican prairie orchid Platanthera leucophaea was primarilydetermined by soil types. Analysis of vegetation and soildata has helped identify habitats where European orchids are

Scientific approaches to Australian temperate terrestrial orchid conservation Australian Journal of Botany 299

Table 3. Major recruitment and attrition factors that determine terrestrial orchid demography

Factor Lower range Upper range Impact Reference

RecruitmentLifespan of flowers 1 day 60 days Pacini and Hesse (2002)Flowers plant−1 year−1 0 >100 Hoffman and Brown

(1998)A

Flowering frequency Annual Decades (requires fire) Hoffman and Brown(1998)A

Fruit set <10% 80% Elliott and Ladd (2002)A

Seed production (per plant) Thousands Millions Arditti and Ghani (2000)Dispersal Most fall within a few

metresRare events 100 s or

1000 s of kmArditti and Ghani (2000);

Chung et al. (2004)Seed banks <1 yearA >4 years (North America) Batty et al. (2001a);

Whigham et al. (2006)Germination in soil 1% 45% Scade et al. (2006)A

Tuber production after first season 10% 40% Scade et al. (2006)A

Underground phases None >10 years Brzosko (2002); Kery andGregg (2004)

Consequences of flowering Minor reduction in sizeand floweringfollowing year

>80% of plantsnon-flowering in nextyear

Primack and Stacy(1998); Willems andDorland (2000); Keryet al. (2005)

Natural lifespan <3 years >300 years Willems and Melser(1998); Kery andGregg (2004); Coateset al. (2006)A

AttritionGrazing by vertebrates Major cause of mortality McKendrick (1995); Kery

and Gregg (2004); Petitand Dickson (2005)A;Coates et al. (2006)A

Grazing by invertebrates Caterpillars and otherinsects

Scade et al. (2006)A

Disturbance Loss of habitat orreduction in habitatquality

Brown et al. (1998)A;Light and MacConaill(2005)

Tourism Trampling, picking,recreation

Kelly et al. (2003)A

Shade Reductions in floweringand survival, increaseddormancy

Willems et al. (2001);Shefferson et al.(2005a)

Habitat fragmentation Lower fruit set Murren (2002)Weed competition Lower survival McKendrick (1995);

Scade et al. (2006)A

Changing agricultural practices Loss of habitat byreduced mowing,fertilizers, afforestation

Wotavova et al. (2004);Myklestad andSaetersdal (2005)

Climate Rainfall and temperaturelinked to survival anddetection

Wotavova et al. (2004);Kery et al. (2005);Light and MacConaill(2005); Pfeifer et al.(2006)

AData from Australian orchids.

most likely to survive (Reinhammar et al. 2002; Kull andHutchings 2006). Most rare WA orchids are confined to relativelyspecific habitats as defined by geography, soils, hydrology andvegetation (Fig. 3), but we lack sufficient knowledge of therelative importance of these factors to explain why orchidsoccur in certain locations and are absent from others that looksimilar. Thus, a better understanding of the habitat requirementsof orchids, as well as their pollinators and fungi, is required to

conserve rare species (see ‘Using scientific knowledge to rescuerare orchids’).

Population genetics

Genetic studies have provided a valuable insight into the impactof the unique reproductive strategies of orchids and the geneticstructure of their populations. Deceptive species normally have

300 Australian Journal of Botany M. C. Brundrett

Seed germinates in 14% oflocations used for seed

baiting

30,000 seeds per capsule(up to 3 capsules per plant)

10% of seeddispersed into

topsoil?

Flo

wer

ing

aft

er 3

–10

year

s?

See

d li

fe <

1 ye

ar in

so

ilS

eed

via

bili

ty 6

0–80

%

Fir

st s

um

mer

mo

rtal

ity

(un

kno

wn

) <1% ofseedlings

form a tuber

4% o

f fl

ower

s ar

e po

llina

ted

& s

et s

eed

Seed set % offlowers

Estimated overallrecruitment rate: 0.4

seedlings plant–1 year–1

Fig. 2. Measured and estimated success rates for different stages in thelifecycle of Caladenia arenicola growing at Kings Park. Data from Battyet al. (2001a). Drawings by G. Rodrigues.

B. Habitat factorsA. Landscape factors C. Biology & Ecology

• Land clearing*• Habitat fragmentation*• Climate & rainfall*• Hydrology & salinity*• Topography

• Disturbance*• Weeds*• Grazing*• Fire*• Vegetation type

• Specialised habitats• Pollination*• Mycorrhizal fungi*• Reproduction

• Clonal spread• Seed biology

Knowledge of these factors is required to allow recovery of orchids

Location specific factors Orchid specific factors

Fig. 3. Information required for effective management of rare flora(*factors that change within locations over time).

lower proportions of flowers that set fruit than other orchids, butthis is typically compensated for by higher rates of outcrossing(Dafni 1984; Peakall 1990; Peakall and Beattie 1991, 1996;Nilsson 1992; Calvo 1993; Schiestl 2005). Studies of Australiansexually deceptive orchids such as Caladenia tentaculataand Drakaea glyptodon show that low rates of pollinationare compensated for by relatively long distances of pollentransfer compared with other orchids (Peakall 1990; Peakalland Beattie 1996). In contrast, Johnson et al. (2005) foundmost pollen dispersal was to neighbouring plants in a nectar-producing orchid (Disa cooperi). Infrequent pollination shouldbe considered a normal and healthy situation for orchids (Darwin1904; Tremblay et al. 2005). Many orchids have responded tothe low frequency of pollination by extended flowering timesthat last for several weeks (Pacini and Hesse 2002).

Hybridisation between sympatric taxa is common indeceptive orchids (Neiland and Wilcox 1998; Soliva and Widmer2003). However, Cozzolino et al. (2006) found hybrids were lesslikely to reproduce than their parents so they did not pose amajor threat to parent species. Hybrids of some rare orchidsoccur in WA, but others do not often hybridise. Gene flowbetween species owing to hybridisation is one likely cause ofindistinct boundaries between taxa which causes taxonomic

difficulties within large Australian terrestrial orchid generasuch as Caladenia, Pterostylis and Chiloglottis (Adams andLawson 1993; Mant et al. 2005a). Narrow genetic distancesbetween sexually deceptive species are also likely to result fromrapid divergence of new species (Cozzolino and Widmer 2005).Floral diversity in food-deceptive orchids is believed to resultfrom selective pressure to prevent acclimation by pollinators,which may also contribute to rapid taxonomic divergence(Tremblay et al. 2005).

Despite the fact that orchids have dust-like seeds, populationgenetics and wind-dispersal studies have revealed that mostseeds are dispersed less than 10 m; thus, long-range dispersalevents are rare but important (Murren and Ellison 1998; Machonet al. 2003; Chung et al. 2004). Consequently, gene flow isunlikely between populations of rare orchids separated by largedistances (Wallace 2002). Research on Northern Hemisphereterrestrial orchids found that restricted gene flow can belinked to rarity (Cozzolino et al. 2003; Wallace 2003; Chunget al. 2004). Evidence of inbreeding depression is provided bygrowth experiments where outcrossed seeds have higher ratesof germination or faster growth than inbreed seeds (Peakall andBeattie 1996; Wallace 2003). However, Sharma et al. (2003)found no evidence of inbreeding in a rare species of Pterostylis.Contrary to expectations, measured levels of genetic diversityof plants that spread primarily by clonal growth are oftensimilar to those of plants that reproduce by seed (Honnay andBossuyt 2005).

The examples summarised above demonstrate how geneticstudies can answer key questions about long-term viabilityof orchid populations, required for effective conservationmanagement. This knowledge includes outcrossing frequencies,the risk of hybridisation with other species, links betweenpollination strategies and population genetics and distancesbetween closely related taxa. This information is also requiredto design effective conservation reserve networks in highlyfragmented landscapes such as the SWAFR (e.g. Coates et al.and other papers in this volume for more detailed informationon the genetics of rare species in WA).

Using scientific knowledge to rescue rare orchids

The wheatbelt of WA is one of the most highly fragmentedand cleared regions in Australia (Commonwealth of Australia2002). Nine species of orchids listed as Critically Endangeredby using IUCN criteria occur in this region (A. Brown, pers.comm.). At least three of these species are at high risk ofextinction, as they occur at a single location only (Fig. 1D, F).Threatening processes that have been identified in the habitats ofthese orchids are illustrated in Figs 1 and 3. Of particular concernis the risk of rising saline groundwater, as many of these orchidsgrow in highly cleared catchments and some occur in closeproximity to salt lakes (Fig. 1D, F). It is anticipated that recoveryactions for most rare SWAFR species will focus on propagationand attempts to establish seedlings at new locations that areexpected to be more sustainable in the long term. However,it is necessary first to determine the suitability of potentialnew habitats. This approach is based on the assumption thatlimited reproductive success and dispersal, coupled with localextinctions of populations, have resulted in the situation where

Scientific approaches to Australian temperate terrestrial orchid conservation Australian Journal of Botany 301

rare WA orchids occupy only a fraction of their suitable habitats;however, this may be incorrect for some species.

The principal method of terrestrial-orchid propagation forconservation is by symbiotic germination with a compatiblemycorrhizal fungus (Zelmer and Currah 1997; Stewart andZettler 2002; Ramsay and Dixon 2003; Batty et al. 2006a,2006b). Propagation of these orchids by aseptic production,or tissue culture, is also effective, but requires more complexand expensive procedures than symbiotic germination (Ramsayand Dixon 2003). When rare orchids are to be propagated,flowers are usually hand-pollinated to ensure seed set, as shownin Fig. 1H. After collection, seeds are cleaned, dried and keptat 4◦C for short-term storage or in liquid nitrogen for long-term storage (Batty et al. 2001b). Other advances in orchid-conservation science have contributed to methods that producelarger seedlings better able to survive translocation (Batty et al.2002, 2006b; Ramsay and Dixon 2003).

It is futile to translocate orchids to habitats where they will notbe sustained, and translocation attempts with plants and animalsoften fail because of a lack of understanding of ecologicalrequirements of species (Burgman and Lindenmayer 1998).Knowledge of the factors in Fig. 3 is required to select potentialnew habitats for rare orchids. Some information concerningvegetation, soils, climate and hydrology can be found in existingspatial datasets. However, micro-scale information that willhave to be collected from potential habitats is likely to bemore important than existing broad-scale information. Micro-scale information would include variations in soil characteristicsand the distribution of fungal species and pollinators. Key soilcharacteristics include texture, soil moisture and coarse organicmatter, which are considered to be a key resource for orchidsand their fungi (Brundrett et al. 2003). The strong associationbetween orchid fungi and coarse soil organic matter and leaf litterrequires further investigations, as these resources are depletedin many WA habitats as a result of frequent fires.

The suitability of potential new habitats can be tested bybioassays to detect compatible pollinators and mycorrhizal fungi

Table 4. Proposed recovery-plan actions for Western Australian terrestrial orchids, showing tasks that involve scientists and communitygroup members

Action Scientist Community

Contribute to meetings of recovery teams and community groups + +Obtain relevant permits and permissions +Continue annual surveys that estimate population sizes + +Measure orchid mortality, seed set and recruitment in permanent monitoring plots + +Assess habitat condition and orchid habitat preferences +Select orchids that are in the greatest danger of extinction +Obtain required biological and ecological knowledge for each species to identify the most important threats + +Undertake feasible actions to ameliorate threats and promote population growth (fencing, pollination, weed control, etc.) + +Provide expertise in plant propagation and habitat management to community groups and conservation workers + +Investigate the benefits or costs of supplementary pollination +Collect seeds and isolate fungi required to propagate orchids +Use orchid seed baiting of soil samples to identify suitable new sites and locations within sites + +Propagate sufficient material for out-planting to field sites + +Introduce orchid seedlings or seed to permanent plots within chosen sites + +Monitor survival of planted or seeded orchids within plots + +Use knowledge gained to recommend future conservation activities and habitat management by updating recovery plans +Summarise results and communicate to community groups and government agencies + +Undertake other communication activities to promote awareness of rare flora and threats to biodiversity + +

(see ‘Mycorrhizal associations of orchids’ and ‘Pollination’).The presence of particular fungi and insects may also help predictrisks of hybridisation and competition with other orchid speciesthat share insects or fungi. It is also necessary to evaluate thedirect impacts of soil, vegetation and climatic factors on orchidplants (Fig. 3). Thus, the following three separate dimensionsto orchid habitat matching need to be considered: mycorrhizalfungus, insect pollinator and plant. In addition to bioassays forfungi and pollinators, the plant dimension can be tested bytransplanting orchid seedlings into sites (Fig. 1I). Short-termsurvival of outplanted orchids has been reported for severalspecies (McKendrick 1995; Ramsay and Dixon 2003; Batty et al.2006b; Scade et al. 2006). Batty et al. (2002, 2006b) describedexamples of translocation attempts for Critically EndangeredWA orchids, but long-term monitoring of survival rates for theseorchids is still required.

Threats to orchid populations listed in Tables 2 and 3 canbe addressed by conservation actions in the field. A genericlist of ‘recovery action’ objectives for Critically EndangeredWA orchids is provided in Table 4. Recovery actions requiresurveys to measure the size of known populations and attemptto locate new populations, and to allow the benefits of actionssuch as fencing to reduce grazing to be measured. Examplesof recovery plans for rare WA orchids that identify threats andpropose management options are available on the DEC website(www.dec.wa.gov.au). Table 4 acknowledges that many of theseactions require collaboration between community groups andconservation scientists, as well as landowners and fundingsources. The abundance of Declared Rare and Priority flora inWA (>2300 taxa) is the most important factor leading to gaps inthe knowledge and capacity required to design and implementrecovery plans for threatened species.

Consequences of the unique biology of orchids

Many orchids are endangered, as they are a high-profileplant family with many commercially exploited species. Thispopularity may have resulted in a greater proportion of orchids

302 Australian Journal of Botany M. C. Brundrett

in rare species lists, owing to a greater effort by taxonomists andconcern by the public than for other plant families. However,there is a highly consistent relationship between plant familysize and numbers of rare species in WA, as shown in Fig. 4,with the position of orchids in the figure showing they may beunder-represented as rare flora. This may be the result of thecurrent estimate of 400 orchids in WA including many recentlyrecognised taxa of poorly known conservation status.

The linear relationship in Fig. 4 suggests that rarity isprimarily linked to the taxonomic size of WA plant familiesand thus to speciation rates. Orchids occupy a much smallergeographic area in WA (primarily in the SWAFR) than the otherprominent families in Fig. 4. Thus, the orchids would have oneof the highest speciation rates relative to occupation area inWA. A worldwide summary of data on rare species (IUCN2006, www.iucn.org) also shows a strong linear relationshipbetween family size and rarity, with orchids as one of thehighest-ranked families in terms of both diversity and numbersof rare taxa world-wide (data not shown). Other regional studiesfound a weaker relationship between diversity and rarity in plantfamilies, owing to substantial variations between families andgeographic locations (Edwards and Westoby 2000; Schwartz andSimberloff 2001).

Figure 5, which uses data summarised in Tables 1 and 2,shows that the majority of both rare and common orchids havehighly specific symbiotic associations with both insects andfungi and that the proportion of rare WA orchids in differentcategories is similar. The majority of both rare and commonWA orchids are in rapidly speciating genera which tend to havehighly specific associations. In contrast with rare orchids thatusually grow very slowly and have a limited capacity to competewith other plants, some orchids behave like weeds. Weed-likeorchids, such as Disa bracteata an invader from South Africaand the indigenous WA orchid Microtis media, were found tobe compatible with a wider diversity of fungi than most otherorchids and are also capable of self pollination (Bonnardeauxet al. 2007). Orchids from both ends of the generalist–specialist

1

10

100

1000

10 100 1000 10000

Total taxa (log)

Rar

e &

prio

rity

taxa

(lo

g)

Poaceae

Chenopodiaceae

Cyperaceae

AmaranthaceaeEuphorbiaceae

Asteraceae

Papilionaceae

OrchidaceaeEpacridaceae

Mimosaceae

Myrtaceae

Proteaceae

GoodeniaceaeMyoporaceae

Rutaceae StylidiaceaeSterculiaceae Lamiaceae

, , ,

y = 0.2561x = 0.9083R 2

Fig. 4. The proportion of rare and priority taxa within families relative totheir size. Names are provided for the 18 of the largest families and thosewith <10 taxa are excluded.

F1F2

F3F4P1

P2P3

P4

0

20

40

60

80

100

120

140

160

Num

ber

of ta

xa

F1F2

F3F4P1

P2P3

P4

0

5

10

15

20

25

30

35

40

45

Num

ber

of ta

xa

F1F2

F3F4P1

P2P3

P4

0

5

10

15

20

25

30

35

40

Rar

e ta

xa (

%)

Fungus specificity index

Pollinator specificity

index

A

B

C

Fig. 5. The number of (A) all Western Australian (WA) orchids, (B) rareWA orchids and (C) the proportion of rare taxa relative to specificity ofassociations with mycorrhizal fungi (F) and insect pollinators (P). Scales areaverages for genera containing 10 or more taxa, using categories in Tables 1and 2.

Scientific approaches to Australian temperate terrestrial orchid conservation Australian Journal of Botany 303

continuum are common in many WA habitats, suggesting thesestrategies involve evolutionary tradeoffs where the benefits equalor exceed the risks associated with new strategies.

Dual specificity interactions with symbionts will inevitablylead to highly specific habitat requirements, as orchids canreproduce only when both of these partners are present. Forexample, most orchids with a single pollinator associate withthynnine wasps, which are parasites of a soil-dwelling beetlelarvae found under the plant on which it feeds (Risdall-Smith 1970; Peakall 1990). Unfortunately, we know very littleabout the ecology of these insects. The taxonomy, primaryroles and distribution in soil habitats of orchid fungi are alsovery poorly known. Consequently, studies of the taxonomy,distribution, ecology and biology of orchid pollinators and fungiare as important as knowledge about the orchids themselves,and should be a primary research objective for CriticallyEndangered orchids.

Orchid evolution

No discussion of the unique aspects of orchid biologyand ecology would be complete without considering theevolutionary consequences of these strategies. Orchids haveunique ecological properties, including microscopic dust-likeseeds with insufficient food reserves to grow unaided, the lackof a persistent soil seed bank, subterranean dormant phaseslasting one or more years and highly specific fungal andinsect associations relative to most other plants. Many of theseproperties are linked to their unique mycorrhizal associations(see ‘Mycorrhizal associations of orchids’). Some of thesecharacteristics are shared with myco-heterotrophic plants inother families with convergent evolution (Leake 1994).

The switch to a new type of mycorrhizal fungus associationfrom vesicular-arbuscular mycorrhizas probably was one ofthe key defining events in the evolution of the Orchidaceaemore than 100 million years ago (Brundrett 2002; Chaseet al. 2003). The evolution of orchid mycorrhizas coincidedwith the evolution of abundant microscopic seeds for moreefficient dispersal in patchy environments, and these, in turn,required mycorrhizal fungi for germination and highly efficientpollination mechanisms to fertilise thousands of ovules (Benzingand Atwood 1984; Adams and Lawson 1993; Rasmussen 1995;Brundrett 2002). It is not possible to know which of theseinterdependent selective pressures was the first to drive orchidevolution in a completely new direction, but for the past100 million years they have acted in concert to keep orchidson divergent evolutionary trajectories from most other plants.

The size of the orchid family is ultimately due to veryrapid rates of speciation and this is thought to be, in part, aconsequence of the highly specific interactions with insects andfungi necessary for their survival and reproduction (Benzing andAtwood 1984; Molvray et al. 2000; Brundrett 2002; Mant et al.2005a, 2005b; Schiestl 2005; Tremblay et al. 2005). Deceptivepollination is considered to be the primitive character state inorchids and a powerful driver of rapid evolution (Dafni andCalder 1987; Nilsson 1992; Neiland and Wilcox 1998; Schiestl2005). Low pollination rates are normal in deceptive orchids andare linked to low rates of gene flow within species and geneticdrift within populations, which are also likely to be a driving

force for high rates of diversification in orchids (Tremblayet al. 2005). Cozzolino and Widmer (2005) demonstrated thatsexually deceptive orchids seem to be diverging much fasterthan related food-deceptive orchids, as shown by the geneticdistances between species. Thus, there may be a hierarchy ofspeciation patterns between terrestrial orchids linked to differentpollination strategies. The relationship between specificity anddiversity in WA orchids in Fig. 5 further supports the hypothesisthat orchids with highly specific associations are evolvingmost rapidly.

Sexual deception in terrestrial orchids has independentorigins in Europe, Africa, South America and Australia, but isusually less common than food deception in areas other thanAustralia (Schiestl 2005; Tremblay et al. 2005). Neiland andWilcox (1998) found that orchids with deceptive pollinationare more likely to be rare than other orchids in the UK,whereas Jacquemyn et al. (2005a) found that declining orchidspecies were linked to particular habitats but not pollinationstrategies. Comprehensive studies of closely related speciesin the Australian orchid genus Chiloglottis and their wasppollinators found that the diversity of both groups of organismswas expanding in parallel (Mant et al. 2005a, 2005b; Schiestl2005). It seems inevitable that such complicated intersectingphylogenies will be a factor contributing to rarity in orchidspecies, since pollinators and orchids can have non-intersectinghabitats (C. Bower, pers. comm.). However, the primary reasonfor (abundant) rarity in orchids seems to be rapid diversificationand not specialisation, owing to the strong relationship betweenplant family size and rarity (see Fig. 4).

Otero and Flanagan (2006) suggested that orchid evolutionmay also be driven by high fungus specificity; however, there islittle evidence to support this. Evolutionary lineages of orchidsand fungi seem to be expanding in parallel for some non-photosynthetic myco-heterotrophic orchids, but switches to newlineages of fungi also occur (Taylor et al. 2003; Bidartondoet al. 2004). Relationships between green orchids and fungiare also highly complex, as recent molecular studies haveshown that different orchid genera seem to associate withparticular groups (defined as genera or clades within genera)of the Rhizoctonia complex (Table 1). This suggests that theevolution of green orchids with highly specific mycorrhizalassociations may also be linked to diversification in particulargroups of fungi. However, additional studies of orchid–fungus specificity are required to allow accurate phylogeneticcomparisons across a wider diversity of orchids. Since bothorchid pollination and mycorrhizal associations are primarilyexploitative relationships, they represent one-sided selectionpressures acting on orchids only. There are many parallelsbetween orchid–insect and orchid–fungus interactions, whichsuggest similar evolutionary processes are at work, although theyare unlikely to be acting in concert.

It seems that in orchids, evolution favours the brave, asthe WA orchid genera with the highest taxonomic diversityalso have the highest proportion of species with highly specificsymbioses with insects and fungi (Fig. 5). High speciation inthe WA flora is probably also linked to ancient landscapes rarelyinterrupted by mass extinction events, where other stresses suchas extended droughts would have caused extinctions on a smallerscale, resulting in fragmented and restricted distributions for

304 Australian Journal of Botany M. C. Brundrett

rare species (Hopper and Gioia 2004). Most of our knowledgeof orchid demographics is from habitats in Europe and NorthAmerica where highly specific orchids are more likely tohave been lost through mass extinction events (e.g. glaciation).The available evidence suggests that there are fewer orchidswith specific pollinators in Europe than there are in WA (see‘Pollination’), but there is insufficient evidence to contrast thespecificity of mycorrhizal associations of orchids in differentregions (see ‘Mycorrhizal associations of orchids’).

It is becoming increasingly clear that high rates ofdiversification (and consequently rarity) in many orchid generaare associated with complex and highly specific symbioses withboth pollinators and fungi. Consequently, the study of orchidecology requires a three-dimensional approach based on anunderstanding of the habitat requirements of organisms in threedifferent kingdoms that must all be met for orchids to thrive.The extreme complexity of these interactions has amazed andconfounded scientists since the time of Charles Darwin andwill continue to occupy conservation scientists for many yearsto come.

Acknowledgements

Andrew Brown and Steve Hopper provided valuable comments andinformation. I also particularly thank the following people: Andrew Battyand Professor Sivasithamparam at the University of Western Australia.I gratefully acknowledge the many colleagues, students and volunteers whohave collaborated with me in orchid research: Kingsley Dixon, Eric Bunn,Keran Keys, Bob Dixon and Siegy Krauss at the Botanic Gardens and ParksAuthority; postgraduate students: Nura Abdul Karim, Jeremy Bougoure,Margaret Collins, Sofi Mursidawati, Belinda Newman; honours students:Yumi Bonnardeaux, Ailsa Scade, Erin Wright, Danika Collins; and AndrewBrown, Jillian Stack, Beth Laudon and others at the Department ofConservation and Land Management. Volunteers of the West AustralianNative Orchid Study and Conservation Group, Friends of Kings Park andKings Park Master Gardeners. Funding was provided by ARC, The BotanicGardens and Parks Authority, ALCOA World Alumina Inc., The NationalHeritage Trust and Lotterywest.

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