Ellwood, M. D. Farnon

Fayle, Tom M.

Foster, William A.

Snaddon, Jake L.

Turner, Edgar C.

Yusah, Kalsum M.

English

UWE Bristol Research Repository

Bird’s nest ferns: islands ofbiodiversity in the rainforest canopyTom M. FayleM. D. Farnon EllwoodEdgar C. TurnerJake L. SnaddonKalsum Mohd YusahWilliam A. FosterAs you walk through the toweringMalaysian rainforest you are bound tonotice the epiphytes that festoon thetrunks and branches of nearly everytree. Predominant among these plantsare the ferns, with perhaps the mostabundant being the bird’s nest fern(Asplenium nidus complex). These arelitter basket epiphytes, with a rosetteof simple fronds which efficientlyintercepts and retains falling leaflitter, giving them a nest-likeappearance. The litter thendecomposes providing the fern with asource of nutrients. On this diet theferns can grow to be huge, up to 200kg wet weight (Ellwood & Foster2004, Fig 1). If you cut open a bird’snest fern you discover a world of ants,termites, cockroaches, centipedes andmany other arthropods, sheltering inthe cool, humid interior of the fern,away from the hot, dry conditions ofthe canopy. Our group has beenstudying the diverse invertebratefauna of these epiphytes for over tenyears at the Royal Society runDanum Valley Field Centre in Sabah,Malaysia (Fig 2). Initially workfocussed on the importance of thefern as a previously under-sampledhabitat for invertebrate biodiversity.Further work has used the ferns asmicrocosms to investigate questionsrelating to ecosystem function andspecies assembly rules. At thelandscape scale we have investigatedthe importance of bird’s nest ferns asreservoirs of biodiversity in loggedforest and oil palm plantation.Bird’s nest ferns are found from theeast coast of Africa through India andSoutheast Asia to Japan, northernAustralia and many islands in thewestern Pacific (Holttum 1976).They are found at all heights in thecanopy, from ground level to the topsof emergent trees. Up to 15 m in thecanopy, ferns are found at densities of90, 53 and 117 ferns per hectare inprimary forest, logged forest and oilpalm plantation respectively (Turner2005). In primary forest, wheresurveys over the whole height of thecanopy have been made, ferns are Insect Ecology Group,University Museum of Zoology,Cambridge, CB2 3EJ.Figure 1. Bird’s nest ferns grow throughout the rainforest canopy at Danum Valley. Theyare found at heights of up to 60 m, and can weigh as much as 200 kg.ARTICLEfound at densities of 180 per hectare(Fayle, in prep). In addition to beingconvenient, discrete units forstudying invertebrates, the ferns canbe moved around the canopy withoutany apparent adverse effect on theirsurvival, making them ideal forexperimental studies (Ellwood 2002).The Asplenium nidus complex offerns has very few morphologicalcharacters on which to base speciesrecognition, and molecular evidencesuggests that a number of crypticspecies may co-occur at a singlelocation (Yatabe et al. 2001).Molecular and morphologicalinvestigations into the taxonomy ofthe ferns at Danum Valley areongoing.In order to assess the importance ofthe ferns as a habitat for animals inthe canopy, we compared the biomassof arthropods within these large andabundant epiphytes with that foundfrom fogging tree crowns. It wasdiscovered that the ferns may supportup to half of the estimated biomass inthe crowns of emergent trees(Ellwood & Foster 2004). Using datafrom Dial et al. (2006), who foggedover the whole height of the canopy,we find that the proportion of animalbiomass of the canopy as a wholewhich is held in the ferns issomewhat lower, although still 14%of the total. The majority ofinvertebrates in the ferns are eitherants, or as one would expect in anaerial compost heap, termites,millipedes and cockroaches (Ellwoodet al. 2002; Ellwood & Foster 2004).Indeed the arthropod community ofbird’s nest ferns is more similar tothat of the leaf litter from the forestfloor than that found in the rest ofthe canopy (Ellwood 2002; Turner2005). The canopy is a relativelydifficult habitat to survey and as aresult some of the animals found inthe ferns are new to science. Forexample, Disney & Ellwood (2001)describe a new genus of scuttle fly(Phoridae) inhabiting ferns in thehigh canopy.In addition to describing the ferns’invertebrate fauna and their role insupporting canopy biodiversity, thegroup has also used the ferns asmicrocosms for studying moregeneral ecological questions. As fernsare small contained habitats whichact as a discrete arena for speciesinteractions they are an ideal naturalmicrocosm (Srivastava et al. 2004).One line of investigation has been therole of biodiversity in themaintenance of ecosystem function.Do we need the full complement ofspecies that usually exists in a habitatfor ecosystem function to beunimpaired? The ferns, as isolatedsites of decomposition in the canopy,are ideal microcosms with which toinvestigate this question in relation todecomposition. A range of differentleaf types fall into each fern and thisdiversity can be manipulated and itseffect both on insect diversity and onthe rate of litter decomposition canbe assessed. We found that when theleaves are in monocultures the rate ofdecomposition does not differbetween different species of leaf.However, when the leaves are addedin mixtures some species decomposemore slowly than they do inmonoculture (Snaddon, in prep). Thismay be the result of a “choosingeffect” whereby more palatable leavesare favoured by decomposers when ina mixture. The responses andfunctions of the decomposersthemselves are still underinvestigation. Such processes mayhave effects beyond the immediatevicinity of the fern. For example, theconcentration of nutrients instemflow water increases on passingthrough a fern, which may alter thestructure of the trunk-dwelling plantand animal community below(Turner et al. in press).Figure 2. Danum Valley Field Centre in Sabah Malaysia is located on the edge of Danum Valley Conservation Area, an area of primarylowland rainforest. The centre is jointly run by the Royal Society and Yayasan Sabah.The ferns can also be used as amicrocosm to investigate speciesassembly rules. Approximately threequarters of the individuals in any onefern are ants, and multiple speciesbeing able to coexist in a single fern(Ellwood et al. 2002; Turner 2005, Fig3). Due to their colonial nature it ispossible to determine whether or notan ant species is living in a fern bynoting the presence of brood andreproductive individuals. We havefound that the assembly of antspecies in the ferns is partiallydetermined by the physicalenvironment. Larger ferns, with lowersurface area to volume ratios, shouldbe better at buffering microclimatethan smaller ones. This is reflected bythe fact that large ferns contain thesame assemblage of ants regardless ofheight, but small ferns containdifferent assemblages at differentheights. Presumably the hot, dryenvironment of a small fern in thehigh canopy is only a suitable habitatfor a certain set of species. Bioticfactors also play a role in theassembly of ant species in the ferns.Larger ferns contain more colonies ofants and also different species of ant.Smaller ferns are more likely tocontain ruderal genera such asParatrechina, while larger (andtherefore presumably older) ferns aremore likely to contain larger bodiedgenera such as Pachycondyla,suggesting the presence of acompetition-colonisation trade-off.Colonies from the same species neverco-occur and colonies from the samegenus co-occur less frequently thanone would expect (Fayle, in prep).This may be due to aggressiveinteractions between colonies that aresimilar taxonomically and similar interms of their niches. We are alsoinvestigating the assembly rulesgoverning another important group inthe ferns; the decomposers. There is asuccession of species that inhabit theferns over long time periods, but thecomposition of any one fern is drawnat random from the pool of speciespresent in ferns of that age (Ellwood,in prep).The ferns may also be important indetermining the structure of the antcommunity of the whole of therainforest canopy. Two or moredominant ant species may occur inthe same habitat, but exclude eachother on a local basis, forming what isknown as an ant mosaic. Ant mosaicscan be of great importance, as theidentity of the dominant speciesdetermines the nature of the rest ofthe ant community. Such mosaicshave been described from cocoafarms (Room 1971) and various otheragricultural systems, but theirprevalence in natural systems remainscontentious. Floren & Linsenmair(2000), failed to find an ant mosaic inthe rainforest understorey, but it isnot known if a mosaic exists at higherlevels in the canopy. One of the mainfactors regulating ant populations isnest site availability (Kaspari 2000).As ferns provide an important nestsite for ants in the canopy it is likely Figure 3. Two ants of vastly differing sizes survey one another. At least 35 species of ant inhabit the ferns, with up to 12 species inhabitingany one fern. Body size may play an important role in the way that ant communities are structured.that they are of great importance indetermining the existence and natureof the ant mosaic. We are in theprocess of investigating thisexperimentally.Southeast Asia has the highestrelative rate of deforestation of anytropical region, the majority of whichresults from logging and conversion offorest to oil palm plantation (Sodhi etal. 2004). Arthropods from the areaare also relatively poorly known(Chung et al. 2000). Given that theferns are important in structuringinvertebrate communities in naturalrainforest, and that they persist indegraded habitats (Fig 4), they mayplay an important role amelioratingthe effects of habitat change. Ourwork has shown that arthropodabundance and biomass decrease inthe canopy and leaf litter in transitionfrom forest to oil palm plantation. Asimilar pattern is seen if one considersindividual ferns – the biomass andnumbers of arthropods also decrease.However, while there are fewerarthropods per fern in oil palmplantations, the density and averagesize of ferns is actually greater than inthe other habitat types, meaning thatthe total fern arthropod abundanceand biomass remains unchanged.Ferns are also able to buffer theeffects of habitat conversion onarthropod community composition.Although the relative abundances ofarthropod orders in the leaf litter andcanopy are altered by habitat change,those in the fern remain unchanged(Turner 2005), possibly as a result ofthe ferns’ ability to buffermicroclimate (Turner & Foster 2006).In this way the ferns act as reservoirsof biodiversity in degraded habitats.These previously under-sampledepiphytes harbour a large proportionof rainforest canopy biodiversity byvirtue of their enormous size and highabundance. Using these isolated,highly diverse habitats we areinvestigating general ecologicalquestions relating to biodiversityecosystem function and speciesassembly rules. With increasing levelsof human impact on the natural worldthe diversity contained within theseepiphytes may provide an importantbuffer against such changes. We arenow continuing our research intothese fascinating islands ofbiodiversity in the rainforest canopy.ReferencesChung, A. Y. C., Eggleton, P., Speight, M. R., Hammond, P. M. & Chey, V. K. -(2000). The diversityof beetle assemblages in different habitat types in Sabah, Malaysia. Bulletin of EntomologicalResearch 90: 475-496.Dial, R., Ellwood, M. D. F., Turner, E. C. & Foster, W. A. (2006). Arthropod Abundance, CanopyStructure, and Microclimate in a Bornean Lowland Tropical Rain Forest. Biotropica: 643-652.Disney, R. H. L. & Ellwood, M. D. F.- (2001). An intriguing new genus of scuttle fly (Diptera:Phoridae) from Borneo. Fragmenta Faunistica 44: 319-328.Ellwood, M. D. F. (2002). The ecology and diversity of the animal communities of a rainforestcanopy epiphyte. PhD. University of Cambridge, Cambridge.Ellwood, M. D. F. & Foster, W. A. (2004). Doubling the estimate of invertebrate biomass in arainforest canopy. Nature 429: 549-551.Ellwood, M. D. F. Jones, D. T. & Foster, W. A. (2002). Canopy Ferns in Lowland Dipterocarp ForestSupport a Prolific Abundance of Ants, Termites and Other Invertebrates. Biotropica 34: 575-583.Floren, A. & Linsenmair, E. (2000). Do ant mosaics exist in pristine lowland rain forests? Oecologia123: 129-137.Holttum, R. E. (1976). Asplenium Linn., sect. Thamnopteris Presl. Gardens’ Bulletin, Singapore 27:143-154.Kaspari, M. (2000). A primer on ant ecology. in D. Agosti, J. D. Majer, L. E. Alonso, and T. R.Schultz, editors. Ants, standard methods for measuring and monitoring biodiversity. SmithsonianInstitution Press, Washington and London.Room, P. M. (1971) The Relative Distributions of Ant Species in Ghana’s Cocoa Farms. The Journalof Animal Ecology 40: 735-751.Sodhi, N. S., Koh, L. P., Brook, B. W. and Ng., P. K. L. (2004). Southeast Asian biodiversity: animpending disaster. Trends in Ecology & Evolution 19: 654-660.Srivastava, D. S., Kolasa, J., Bengtsson, J., Gonzalez, A., Lawler, S. P., Miller, T. E., Munguia, P.,Romanuk, T., Schneider, D. C. & Trzcinski, M. K. (2004). Are natural microcosms useful modelsystems for ecology? Trends in Ecology & Evolution 19: 379-384.Turner, E. C. (2005). The ecology of the Bird’s Nest Fern (Asplenium spp.) in unlogged andmanaged habitats in Sabah, Malaysia. PhD. University of Cambridge, Cambridge.Turner, E. C. & Foster, W.A. (2006). Assessing the Influence of Bird’s Nest Ferns (Asplenium spp.) onthe Local Microclimate across a Range of Habitat Disturbances in Sabah, Malaysia. Selbyana 27:195-200.Turner, E. C.,Snaddon, J.L., Johnson, H.R. & Foster, W.A. in press. The impact of bird’s nest ferns onstemflow nutrient concentration in a primary rain forest, Sabah, Malaysia. Journal of TropicalEcology.Yatabe, Y., Masuyama, S., Darnaedi, D. & Murakami, N. (2001). Molecular systematics of theAsplenium nidus complex from Mt. Halimun National Park, Indonesia: evidence for reproductiveisolation among three sympatric rbcL sequence types. American Journal of Botany 88: 1517-1522.AcknowledgementsWe would like to thank the NaturalEnvironment Research Council andthe Natural History Museum, Londonfor funding during these projects. TheDanum Valley Field Centre staff,RSSEARP research assistants andsenior scientist Glen Reynolds haveall made field work at Danum apleasure. The Danum ValleyManagement Committee, YayasanSabah, the Economic Planning UnitPutrajaya and the Economic PlanningUnit Sabah have been most helpful inallowing research to be carried out inDanum. All work has been carriedout as part of the Royal SocietySouth East Asia Rainforest ResearchProgramme. Our local collaboratorsArthur Chung and Chey Vun Khenhave been of great assistance in thecourse of this research. We aregrateful to Paul Eggleton of theNatural History Museum for his helpin the role as the CASE supervisor fortwo of us (JLS & TMF).Figure 4. Bird’s nest ferns are highlyabundant in oil palm plantations, wherethey provide an important reservoir ofarthropod biodiversity.

Bird's nest ferns: Islands of biodiversity in the rainforest canopy

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Bird's nest ferns: Islands of biodiversity in the rainforest canopy

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