HARMONIC RADAR - A METHOD USING INEXPENSIVE TAGS TO STUDY .
1LÖVEI etL.al.:GÁBORLÖVEIHARMONIC, IAN A.N.RADARSTRINGER, CHRIS D. DEVINE and MARC CARTELLIERI187Department of Ecology, Massey University, Private Bag 11222, Palmerston North, New Zealand1Corresponding author. Present address: HortResearch, Private Bag 11030, Palmerston North, New Zealand. E-mail:glovei@hort.cri.nzSHORT COMMUNICATIONHARMONIC RADAR - A METHOD USING INEXPENSIVE TAGSTOSTUDY INVERTEBRATE MOVEMENT ON LANDSummary: We describe the use of harmonic radar in the field with simple, inexpensive tags with extendedlifespan. The effects of aerial size and shape, and the detection range of several types of diodes underdifferent conditions are described. Examples are provided of tracking short-term movement of a groundbeetle, Plocamosthetus planiusculus, and long-term movement of a snail, Paryphanta busbyi watti. Thepotential and limitations of the method are discussed.Keywords: Harmonic radar; Carabidae; Pulmonata; land snails; movement; tracking.IntroductionSpatial behaviour of individuals is a key componentto understanding the population dynamics oforganisms (Turchin, 1991). Many animals do noteasily lend themselves to such studies and observingthem without disturbing their natural behaviour andhabitat is sometimes almost impossible. Moreover,many organisms are cryptic, sensitive, or too rare tostudy directly. Capture-recapture methods aresuitable for using with a wide range of organisms(Southwood, 1978) but their resolution levels inspace and time are often not fine enough. Toovercome this, many types of tracking and remotesensing methods have been applied to studies ofmigration and behaviour (Riley, 1989; Pride andSwift, 1992).The application of these methods to the study ofsmall organisms has been problematic. Followingindividuals usually requires some form ofradiotracking, i.e. locating an individual carrying asmall radio transmitter. Two important technicallimitations to overcome are the size of thetransmitter and its battery, and the limited lifetime ofthe energy source. Thanks to recent technologicaldevelopments, miniature, lightweight transmittersare now available, and have been used for trackinginvertebrates (Riecken and Raths, 1996). Their cost,however, puts them beyond many research budgets.Moreover, even a miniature transmitter needs anenergy source, and this limits its useful life.Technical failures can be frequent (Riecken andRaths, 1996) and this contributes to the cost of usingthese devices.An alternative is to use a passive reflector thatdoes not depend on an attached energy source. If aconductor with nonlinear characteristics, a diode, isilluminated by radar waves, it can re-radiate anharmonic of the original radar signal. This harmonicsignal can be detected and used to locate thereflector together with anything attached to it. Theenergy to operate the reflector is delivered by theilluminating radar. The harmonic radar is such adevice. It is a hand-held emitter which generates acontinuous, unmodulated wave. Diodes are availablethat reflect the signal at double the originalwavelength. The harmonic radar unit also detects thereflected signal and transforms it into an audiblesignal. The harmonic radar was originally developedto locate avalanche victims and it was first used fortracking invertebrate movements around 1985(Mascanzoni and Wallin, 1986; Hockmann et al.,1989; Wallin, 1991). The reflected signal is notspecific, so individual markings have to be appliedto the animals if they are to be identified once theyare found.In this communication we call attention to thepotential usefulness of this equipment for trackinginvertebrates in New Zealand. We present detectiondistance data for several types of diodes which canbe used as transponders, and provide examples of itsuse for tracking the short term movements of aground beetle and long term movements of anendangered snail.MethodsWe used a portable transmitter-receiver designed byRecco (Recco Rescue Systems, Lidingö, Sweden),which weighs about 8 kg. It consists of a battery, ahand-held Yagi aerial which is both the transmitterNew Zealand Journal of Ecology (1997) 21(2): 187-193 New Zealand Ecological Society
188NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 21, NO. 2, 1997Table 1: Maximum detectable distance for different diodes, orientations and aerial shapes tested. The aerial length was 12cm in all cases.Aerial shape/orientationRecco S2HI 489.822.402.2013.203.604.000.650.607.703.00Maximum detectable distance (m)HP 280C1N 343C1Z 3040 Z 32321N 60HP 2835BAT 851Elongated parallelElongated perpendicularCircularRecco parallelRecco 1.902.1010.306.00not testednot testednot tested1.200.001The BAT 85 diode responded poorly in initial tests so was not tested in all combinations.and the receiver, and earphones. The transmitteremits a 1.7 W continuous microwave frequency of917 MHz.The tag on the target organism reflects thisenergy at double the frequency (1,834 MHz). Thedetection range depends on the type of diode used toconstruct the tag, and the shape and size of the aerialconnected to the diode.We tested the following Schottkey type diodes:Recco ‚ S2 (provided by Recco, Lidingö, Sweden),HI 48, Hewlett-Packard HP 280C 3C1 (5082-2800),HP 2835, Z 3040 (Dick Smith Electronics,equivalent to 1N 60 specifications), Z 3232 (DickSmith Electronics), 1N 34 and 1N 60 (which areequivalents), and BAT 85.Transponders were made by soldering an aerialof copper wire (0.5 mm diameter) to a diode to forma closed loop. Each transponder was tested ten timeswith the wire loop in two configurations, a circle andan elongated oval. The maximum detection distancewas measured with the harmonic radar. Twoorientations of the oval aerial were tested, with thelong axis either parallel or perpendicular to themicrowave beam.Following testing with closed-loops, each diodewas connected to the rectangular Recco ‚ aerial andthe maximum detection range measured with theaerial parallel and perpendicular to the microwavebeam.The three diodes which performed best weretested again with a single length of wire attached tothe cathode end of each diode. This was the samedesign as used by Mascanzoni and Wallin (1986).The length of the copper wire varied between0 - 20 cm. Parallel and perpendicular readings weremade.Tests were conducted in an open field. Eachtransponder was placed on a plastic plate to isolate itfrom the ground to control for environmental factorssuch as damp grass that alter detectable range.Readings were taken with the radar held 1.2 m abovethe ground. Care was taken to keep the polarity ofthe diodes the same in relation to the direction of thebeam. The diode that proved most effective in thefirst part of the experiment was then used to testdifferent aerial shapes on the maximum detectionrange.Transponders were attached to five groundbeetles (Plocamosthetus planiusculus White,Coleoptera: Carabidae), and 37 snails (Paryphantabusbyi watti (Powell), Pulmonata: Rhytididae), tostudy their spatial behaviour and habitat preference.Tagged organisms were relocated at periodicintervals by systematic searching. The periodbetween relocations was short (15-30 min) forground beetles, but was up to several months forParyphanta snails.Table 2: Maximum detectable distance (m) for transponders manufactured from Z 3040 diode - copper sheet combinations.Orientation / 705.5010.7010.603.8010.90Aerial shapeContinuouscircular/diode 106.201.904.60ParallelPerpendicularBehind treeUnder dry litter1.501.500.001.50
LÖVEI et al.: HARMONIC RADARFigure 1: Maximum detection range of the diodes Z 3040,Z 3232, and Recco S2 with different aerial lengths inparallel (solid line) and perpendicular (broken line)orientation to the radar beam. The aerial was alwaysattached to the cathode end of the diode.ResultsThe effects of diode type and aerial designon detection distancesThe maximum detection distances varied from lessthan 2 m to 13 m, depending upon diode type,position, aerial length and shape (Tables 1, 2).Transponders were always detected from furtheraway when parallel to the radar beam rather thanperpendicular to it. The aerial length allowingmaximum detection distance was 12 cm for bothlinear or oval aerials, orientations and all types ofdiodes tested (Fig. 1; data for oval aerials notshown).The Recco aerial was superior to the simple wireloop aerials (Table 1), possibly because of its largersurface area. Diodes with copper sheet aerials were189Figure 2: Two invertebrates fitted with a Schottkey-typediode and appropriate aerial for harmonic radar studies.Top: a female carabid beetle, Plocamosthetus planiusculus.Bottom: the snail Paryphanta busbyi watti. Note thedifferent aerial shapes. The thick arrow points to the diode,the narrow one to the aerial.always superior to the wire loop aerials of similarsize and were sometimes better than wire loopaerials regardless of loop size (Devine, 1997). The‘open circuit’ design (with the aerial attached to onlyone end of the diode) used by Mascanzoni andWallin (1986) was detected from greater distancesthan the Recco design, especially when the aerialswere parallel to the microwave beam (data notshown).Diode attachment and movementsof tagged animalsEach tag, consisting of a diode with an appropriatelyshaped aerial, was glued to the elytra of the groundbeetle or to the shell of the snail (Fig. 2). We foundthat quick-drying adhesive (Ados1 or Uhu brand)was suitable for ground beetles whereas either
190NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 21, NO. 2, 1997Figure 3: Movements of a male carabid beetle, Plocamostethus planiusculus, in Keeble’s Bush, Manawatu, North Island,New Zealand, during the night of 23-24 February 1997. The beetle was relocated every 15 min.5-minute Araldite or 'Liquid nails' (Selleys) were themost suitable for snails. The periostracum of thelatter was lightly buffed first with fine carborundumpaper to effect good attachment. Note the differentshape of the aerial for the two organisms. The longaerial simply trails behind the beetle. The flat,circular copper aerial was moulded to the shape ofthe snail shell so that it did not impede its movementthrough litter.We were able to relocate ground beetles overseveral one-night sessions. Typically, movementwas only a few metres per night. We relocated thebeetles every 15 or 30 min. No long term directionalmovement was detected (Fig. 3) but the beetlesshowed signs of ‘typical’ invertebrate searchingbehaviour. This included movement within a smallarea with frequent turns, followed by longer walks instraight lines.One snail was relocated at intervals of 6-16weeks over 30 months. It showed a tendency tooccupy a localised area of ecotone between kanukascrub (Kunzia ericoides (A.Rich)) and a grassclearing although it often showed periodicdisplacements of up to 25 m from this (Fig. 4). Onone occasion another snail was relocated under arock 25 cm thick and other snails were frequentlydetected under concrete slabs up to 10 cm inthickness.
LÖVEI et al.: HARMONIC RADAR191Figure 4: Movement pattern of an individual snail, Paryphanta busbyi watti, over a 30 month period from August 1994 toJanuary 1997 at Te Paki Farm Park, Northland, New Zealand.DiscussionWe have shown that harmonic radar is suitable forconducting detailed studies of spatial behaviour overboth short and long intervals providing the animalshave localised ranges. It also has the advantage ofallowing cryptic invertebrates to be relocated withminimal disturbance to the habitat. This wasespecially applicable to working with the snailP. busby watti. Earlier research on this depended onhand-searching through leaf-litter which causedconsiderable habitat disturbance, was very timeconsuming and was relatively inefficient. The use ofan harmonic radar unit allowed the snails to bequickly and easily relocated and habitat disturbancewas minimised to less than 0.25 m2 of forest flooraround each snail.The normal movement speed of the studyorganism should be considered when deciding on thetime intervals between relocations. Ground beetlescan move fast and far enough (up to 30 m withinan hour, Wallin, 1991) to make relocation verydifficult in dense forest habitats so common in NewZealand. For such organisms, relocation every 10 -15min is recommended, at least during the initial phaseof the study.Mascanzoni and Wallin (1986) found thatvirtually no spurious signals were generated by the
192NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 21, NO. 2, 1997harmonic radar. In contrast, we experienced avariable amount of background noise, caused byincidental metal objects such as refuse, metal fencesor rubbish tins. Operator experience helped withdistinguishing background noise from thetransponder signal at extreme range.Water (also humidity) attenuated the signal, andother, unexplained interference sometimes reducedthe detection range to a few metres. In ourexperience, the clarity of the signal improved but themaximum detection distance did not change whenthe target was above the ground on vegetation.The orientation of a transponder with respect tothe microwave beam had a marked effect on themaximum detectable distance: when the long axis ofthe transponder was perpendicular to the beam, thedetectable distance was much smaller. Mascanzoniand Wallin (1986) did not discuss orientation, butprobably made their measurements with thetransponder in the parallel position. The only diodecommon to both studies (the HP 2835) gave similarreadings to theirs in this position only. It should benoted that Mascanzoni and Wallin (1986) used anolder version of the harmonic radar unit with adifferent type of antenna.It is also worth noting that some organisms mayspontaneously generate false signals. When trackingground beetles, we were misled several times by afalse signal, whose origin was always a tree weta(Hemideina crassidens (Blanchard), Orthoptera:Stenopelmatidae).A circular transponder was chosen forattachment to P. busbyi watti to avoid the lowperpendicular detection distance. This circulardesign was not as good as long rectangulartransponders measured parallel but it wasconsistently good from all directions. This designweighed more than the thin wire aerial. Terrestrialsnails can carry up to 50 times their body weight(Croizer and Federighi, 1925, cited by Jones, 1975)but in this case the transponder weight (0.7 - 1.25 g)was approximately 10 % of body weight of thesnails.While the weight of the aerial was notsignificant for the carabid, the necessary length ofthe aerial could hamper normal movement. Tominimize this, a very fine and flexible wire ispreferred that can freely trail behind the beetle.We believe that the harmonic radar can be usedfor tracking a wide range of invertebrates andvertebrates that are expected to undertake smallscale movements. Many such animals are in need ofprotection and management, but our lack ofknowledge of their spatial behaviour and habitatpreferences often limits the effectiveness of ouractions. We suggest that harmonic radar can befruitfully used to describe habitat use of suchanimals as geckoes, tuatara, weta, as well as largeflightless beetles. The inability to identify the targetindividual is not necessarily an obstacle because weare often interested in microhabitat use, and/or directmeasurements (size, body mass), so the organismhas to be handled anyway. Even lighter tag designsnow exist for smaller organisms that are activeabove ground level, so bees (Riley et al., 1996),caterpillars, butterflies and parasitic flies (Roland etal., 1996) can now be tagged and relocated. Thisextends the applicability of the method to an evenwider range of organisms.AcknowledgementsThe equipment was purchased with the help of aLottery Science Grant no. SR17964 to GLL. Fieldwork was supported by the New ZealandDepartment of Conservation (grant to IANS,Investigation Number 1939), the EntomologicalSociety of New Zealand (21st Anniversary ResearchFund, to GLL), and the Studienstiftung desdeutschen Volkes (MC). We thank G. Sherley forsupport and discussions, D. M. Lambert forlogistical help, E.A. Grant for drawing the figures,L. Doel and P. Spring for the photographs.Comments by K. Clapperton and two anonymousreviewers have improved the manuscript. Work byCDD was in partial fulfilment of the requirementsof a MSc Thesis at Massey University. GLL thanksK. Clapperton for taking over the editorialresponsibilities for this manuscript.ReferencesDevine, C. D. 1997. (Unpublished). Some aspects ofbehaviour and ecology in the land snailPowelliphanta traversi traversi Powell(Rhytididae: Rytidinae). MSc thesis, MasseyUniversity, Palmerston North, N.Z 142 pp.Hockmann, P.; Schlomberg, P.; Wallin, H.; Weber,F. 1989. Bewegungsmuster und Orientierungdes Laufkäfers Carabus auronitens in einemwestfälischen Eichen-Hahnbuchen-Wald(Radarbeobachtungen undRückfangexperimente). AbhandlungenWestfälisches Museum für Naturkunde 51:1-71.Jones, H. D. 1975. Locomotion. In: Fretter, V. andPeake, J. (Editors), Pulmonates Volume 1.Academic Press, London, U.K. 417 pp.
LÖVEI et al.: HARMONIC RADARMascanzoni, D.; Wallin, H. 1986. The harmonicradar: a new method of tracing insects in thefield. Ecological Entomology 11: 387-390.Pride, I.G.; Swift, S.M. (Editors) 1992. Wildlifetelemetry. Remote monitoring and tracking ofanimals. Horwood, New York, U.S.A. 542 pp.Riecken, U.; Raths, U. 1996. Use of radio telemetryfor studying dispersal and habitat use ofCarabus coriaceus L. Annales ZoologiciFennici 33: 109-116.Riley, J.R. 1989. Remote sensing in entomology.Annual Review of Entomology 34: 247-271.Riley, J.R.; Smith, A.D.; Reynolds, D.R.; Edwards,A.S.; Osborne, J.L.; Williams, I.H.; Carreck,193N.L.; Poppy, G.M. 1996. Tracking bees withharmonic radar. Nature 379: 29-30.Roland, J.; McKinnon, G.; Backhouse, C.; Taylor,P.D. 1996. Even smaller radar tags on insects.Nature 381: 120.Southwood, T.R.E. 1978. Ecological methods. 2nded. Chapman & Hall, London, U.K. 524 pp.Turchin, P. 1991. Translating foraging movementsin heterogeneous environments into thespatial distribution of foragers. Ecology 72:1253-1266.Wallin, H. 1991. Movement patterns and foragingtactics of a caterpillar hunter inhabiting alfalfafields. Functional Ecology 5: 740-749.
LÖVEI et al.: HARMONIC RADAR 191 Figure 4: Movement pattern of an individual snail, Paryphanta busbyi watti, over a 30 month period from August 1994 to January 1997 at Te Paki Farm Park, Northland, New Zealand. Discussion We have shown that harmonic radar is suitable for conducting detailed studies of spatial behaviour over
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