Acoustic Spatial Capture-Recapture (aSCR) and the Cryptic Cape Peninsula Moss Frog Arthroleptella lightfooti

Louw, Marike (2018-12)

Thesis (MSc)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: Quantitative measurements of wildlife populations, such as population density, are quintessential for management and conservation. Acoustic Spatial Capture-recapture (aSCR) is a technique that is used to estimate the densities of acoustically active animals. It is advantageous to use when animals defy the use of traditional methods of population estimation by being very visually cryptic, but remaining acoustically active. Arthroleptella lightfooti is a visually cryptic moss frog with an average snout to vent length of 14.5 mm. The males call during the austral winter from seepages within a restricted range across the Cape Peninsula. The species has an IUCN status of Near Threatened. Here, the population densities of the endemic A. lightfooti are estimated across their range on the Cape Peninsula for the first time using aSCR. Multiple microphones, termed “acoustic arrays”, are deployed in the field to record the calls of the frogs. I assess the use of aSCR in terms of reliability of the density estimates by examining the standard errors as coefficients of variation (CVs) of the density estimates. A density estimate with a CV above 30% was considered unreliable. Recording calls for the aSCR analyses involved visiting more than 200 sites during 2016 and 2017, and deploying acoustic arrays at a total of 149 sites, of which a subset of 85 sampling sites was used. I examined the influence of different variables on the size of the CV, namely: the average number of calls received by the acoustic array per minute, the array formation, and the detector frequencies (the combination of different numbers of microphones across which calls were heard). In addition, I made use of an output from aSCR that is an aerial view of the estimated calling locations of frogs relative to the acoustic arrays. I overlaid this output with aerial images taken at three different sites using a drone, and I examined the microhabitat features that relate most significantly to the presence of calling A. lightfooti. When there were less than 111call.min-1 received by the array, density estimates had CVs that exceeded 30% and were therefore considered unreliable. Above this threshold, 91% of density estimates were acceptable. When calls were heard on mostly one microphone, and decreasingly heard across two, three, four, five and six microphones, the density estimates were more reliable. However, when calls were mostly picked up across a combination of one and six microphones, density estimates became less reliable. This suggests that array formations should have the microphones spaced in such a way that not all calls are detected across all the microphones or only one microphone. The presence of calling frogs was significantly related to the presence of wet, seepy patches in the microhabitat and to the absence of standing water. This is consistent with observations in the field and reflects the biological needs of the species: it has no life stages in water but needs moist areas for eggs and tadpoles to develop. The successful application of aSCR to A. lightfooti is promising in the field of population studies on cryptic species, as it can be used to evaluate the populations of other calling taxa, which holds important implications for conservation and management.

AFRIKAANSE OPSOMMING: Kwalitatiewe metings van diere-bevolkings soos byvoorbeeld bevolkingsdigtheid, is belangrik vir die bestuur en bewaring van natuurlike fauna. Akoestiese Ruimtelike Opname-Heropname (ARO) is ‘n tegniek wat gebruik word om die digtheid van dié bevolkings wat akoesties aktief is, en dus maklik hoorbaar is, te skat. Dit is veral voordelig om te gebruik wanneer die diere moeilik is om op te spoor en raak te sien en tradisionele metodes van bevolkingsberaming nie gebruik kan word nie. Arthroleptella lightfooti is n paddatjie met ‘n gemiddelde lengte van nét 14,5mm. Die mannetjies roep tydens die wintermaande vanuit vogtige gebied binne beperkte areas van die Kaapse Skiereiland. Die spesie is dus endemies aan die Kaapse Skiereiland en het ‘n IUCN status van ‘n Amper-Bedreigde-Spesie. Hier word die bevolkingsdigtheid van die endemiese A. lightfooti vir die eerste keer oor hul volle Kaapse Skiereiland habitat bepaal. ‘n Groep mikrofone, bekend as ‘n “akoestiese ARO-stel”, is in die veld opgestel om die roepgeluide van die paddas in die bepaalde area aan te teken. Om die betroubaarheid van die digtheidskattings van die ARO te evalueer, ondersoek ek die standaardfout as koëffisient van variasie (KV’s) van die digtheidskatting. ‘n Digtheidsberaming met ‘n KV van bo 30% , word as onbetroubaar beskou. Daar is meer as 200 geskikte areas tydens 2016 en 2017 besoek, en hiervan is 149 areas met die Akoestiese ARO-stelsel gedoen. 85 van hierdie opnames is gebruik vir hierdie tesis. Ek het die aspekte wat moontlik die KV van die digtheidsberaming kan beivloed, naamlik die gemiddelde aantal roepe per minuut ontvang deur die ARO-stelsel, die formasie waarin die mikrofone opgestel is in die veld, en die detektorfrekwensie (verskillende aantal mikrofone waarop die paddaroepe gehoor is), ondersoek en evalueer. Ek maak ook gebruik van die ARO-stelsel se funksie, wat ‘n uitsig vanuit die lug van die roepgebied is. Hieroor oorvleuel ek n ‘n lugfoto van die area, geneem vanuit n hommeltuig. Drie areas is so ondersoek om die eienskappe van die mikro-omgewing wat die paddas se roepgebiede die meeste beindvloed, te bepaal. Wanneer daar minder as 111 roepe.min-1 deur die akoestiese ARO stelsel ontvang was, het digtheidskattings KVs gehad wat groter as 30% was, en is dus as onbetroubaar beskou. 91% Data bo hierdie drempel van digtheidskatting was wel aanvaarbaar. Wanneer roepgeluide meestal op een mikrofoon gehoor is, en dit geleidelik minder op die twee, drie, vier, vyf en ses mikrofone opgetel was, is die digtheidskatting betroubaar. Die digtheidskatting het egter minder betroubaar geword wanneer die roepgeluide op een sowel as ses mikrofone gekombineerd opgetel is, en die skatting was dus minder betroubaar. Dit dui daarop dat die formasie waarop die die mikrofone opgestel word belangrik is, sodat al die roepgeluide nie slegs of één mikrofoon, of gelyktydig op al die mikrofone opgeneem word nie. Die teenwoordigheid van manlike paddas wat roep, korreleer aansienlik met mikro leefgebiede wat klam en baie vogtig is, maar nie waar staande water voorkom nie. Dit is in ooreenstemming met veldwaarnemings en weerspieël die spesie se behoeftes: A lightfooti het geen lewensfases in water nie. Hulle benodig net vogtige areas vir eiers en die paddavissies om te ontwikkel. Die gebruik van ARO met A. lightfooti is belowend in die veld van bevolkingsdigtheid navorsing en kan gebruik word om ander roepende diere te bestudeer, wat belangrik kan wees vir die bestuur en bewaring van wild.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105151
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