Geographic range, spread and potential distribution of the painted reed frog Hyperolius marmoratus in the Western Cape Province, South Africa

Davies, Sarah J. (2014-12)

Thesis (PhD)--Stellenbosch University, 2014.

Thesis

ENGLISH ABSTRACT: Amphibian populations are among the most seriously threatened by global environmental change. Nonetheless, a few species have expanded their ranges to become globally invasive. In southern Africa, several anuran species are undergoing extra-limital range expansion on a regional scale and one of them, the painted reed frog (Hyperolius marmoratus Rapp.), is now invasive across the south-western Cape of South Africa. To answer the question of how this tropical, summer-breeding anuran has made such a successful transition into the temperate, winter rainfall region, I investigated several important aspects of the invasion process using a range of approaches from range ecology, physiology and niche modelling. Reconstruction of the painted reed frog’s invasion history allowed the date of introduction to be identified as 1997 or early 1998. The novel range was defined as extending from the Tsitsikamma Forest in the east to the Cape Peninsula in the west. Patches and gaps in the range structure and disparate rates of spread indicated that human-assisted jump dispersal and diffusion-based dispersal dominate in different parts of the novel range. A significant gap in the novel range distribution is formed by the Riviersonderend Mountains, a section of the Cape fold mountain range, that acts as a barrier to spread. To identify physiological range limiters, I investigated the plasticity of key physiological traits that influence thermoregulation, energetics and evaporative water loss. After thermal acclimation at three temperatures commonly encountered in their historical and novel ranges, frogs exhibited a broad thermal tolerance range and higher plasticity in CTmax than in CTmin. Resting metabolic rates were lowest in cold-acclimated animals, partially supporting the ‘colder is better’ hypothesis over beneficial acclimation. Active metabolic rates were lowest in warm-acclimated frogs, suggesting compensation for energy conservation. Notably, evaporative water loss was not significantly altered by acclimation in resting or active frogs, demonstrating a lack of plasticity in this trait. Plasticity of thermal tolerance and metabolic rate suggests that painted reed frogs efficiently conserve energy in a range of thermal environments and can withstand seasonal cooling by minimising the costs of resting metabolism. These characteristics could play a beneficial role in the novel range, which has a temporally and spatially variable climate. Together with their significant warming tolerance, they may facilitate spread into more extreme thermal environments north of their current range. On the other hand, the lack of plasticity in water loss rates, combined with reliance on the water-conserving posture to limit evaporative water loss could constrain further expansion to new sites. To form an integrated picture of the forces facilitating and constraining painted reed frog spread, reciprocal niche modelling was used to investigate the relationship between range shift and niche shift. I tested whether the niche of the painted reed frog has been conserved during recent range expansion or whether spread has been facilitated by a niche shift by using a modelling framework comprising both climatic and landscape variables. Niche models trained in the historical and novel ranges and projected in the reciprocal range revealed that H. marmoratus has undergone a niche shift between its historical range in the northern and eastern coastal regions and its novel range in the Mediterranean ecosystems of the south-western Cape. The niche shift corresponds with a climatic gradient towards higher summer aridity and generally lower precipitation in the novel range than the historical range, but is likely mediated by landscape transformation in the form of artificial water bodies that provide additional buffered habitats. I conclude that the niche shift accompanying range expansion has allowed painted reed frogs to occupy drier and more variable habitats in the novel range, while on a finer scale, access to permanent water bodies in the landscape is limiting. Artificial water bodies provide a key resource supplement for these invasive tropical frogs, which can be recognised as ‘urban exploiters’. Unspecialised habitat requirements, rapid spread and significant phenotypic plasticity suggest that they could continue to spread further within the matrix of suitable habitat available to them. In summary, this study found that availability of water in the landscape and the physiological capabilities of the frogs in relation to water loss are key determinants of the distribution and niche of painted reed frogs in their novel range. The work highlights the importance of broadscale climatic variables, landscape transformation in the form of artificial water bodies, and synergistic interactions between physiology and behaviour in determining invasion success. Om die beperkings op fisiologiese verspreiding te bepaal, is navorsing onderneem oor die plastisiteit in die vernaamste fisiologiese kenmerke wat termoregulering, energetiek en waterverlies deur verdamping beïnvloed. Ná termiese akklimatisasie by drie temperature wat algemeen in die historiese en nuwe verspreidingsgebiede van die spesie voorkom, het die paddas ’n groot termiese toleransiebestek en hoër plastisiteit by CTmax as by CTmin getoon. Rustende metaboliese tempo’s was die laagste by diere wat by lae temperature geakklimatiseer is, wat die ‘kouer is beter’-hipotese eerder as voordelige akklimatisasie ondersteun. Aktiewe metaboliese tempo’s was die laagste by die paddas wat by hoë temperature geakklimatiseer is, wat weer op kompensasie vir energiebehoud dui. Akklimatisasie het geen beduidende verskil aan waterverlies deur verdamping by rustende of aktiewe paddas gemaak nie, wat ’n gebrek aan plastisiteit in hierdie kenmerk aandui. Plastisiteit in termiese toleransie en metaboliese tempo gee te kenne dat die gestreepte rietpadda in ’n verskeidenheid termiese omgewings energie kan behou, en seisoenale afkoeling kan weerstaan deur die eise van rustende metabolisme te beperk. Hierdie kenmerke kan voordelig wees in die nuwe verspreidingsgebied, wat oor ’n temporeel en ruimtelik veranderlike klimaat beskik. Tesame met ’n beduidende toleransie vir hitte, kan hierdie

AFRIKAANSE OPSOMMING: Amfibiese populasies is onder die diergroepe wat die ergste deur wêreldwye omgewingsverandering geraak word. Nietemin het ’n paar spesies hul verspreidingsgebiede uitgebrei om wêreldwyd indringers te word. In Suider-Afrika ondergaan verskeie Anura-spesies buitegebiedsuitbreiding op ’n streekskaal. Een van dié spesies, die gestreepte rietpadda (Hyperolius marmoratus Rapp.), is nou amptelik ’n indringer deur die hele Suidwes-Kaap-streek van Suid-Afrika. Om te bepaal hoe hierdie tropiese Anura, wat in die somer aanteel, so ’n suksesvolle oorgang na ’n matige winterreënvalstreek kon maak, het hierdie studie ondersoek ingestel na verskillende belangrike aspekte van die indringingsproses deur van verskeie benaderinge, onder meer gebiedsekologie, fisiologie en nismodellering, gebruik te maak. Deur die indringingsgeskiedenis van die gestreepte rietpadda te rekonstrueer, is die eerste aanwesigheid van die spesie in die Suidwes-Kaap-streek tot 1997 of die begin van 1998 teruggevoer. Die nuwe verspreidingsgebied strek van die Tsitsikamma-woud in die ooste tot by die Kaapse Skiereiland in die weste. Kolle en leemtes in die verspreidingstruktuur sowel as ongelyke verspreidingstempo’s toon dat menslik gesteunde sprongverspreiding en diffusiegegronde verspreiding die dominante verspreidingsmetodes in verskillende dele van die nuwe gebied was. ’n Beduidende leemte in die nuwe verspreidingsgebied is die Riviersonderendberge, ’n gedeelte van die Kaapse plooiingsgebergte, wat as ’n versperring vir verspreiding dien. eienskappe van die spesie ook verspreiding na meer ekstreme termiese omgewings noord van hul huidige verspreidingsgebied moontlik maak. Tog kan die spesie se gebrek aan plastisiteit in waterverliestempo’s, tesame met afhanklikheid van die waterbehoudpostuur om waterverlies deur verdamping te beperk, verdere verspreiding na nuwe terreine bemoeilik. Om ’n geïntegreerde indruk te vorm van die kragte wat die verspreiding van die gestreepte rietpadda hetsy vergemaklik of beperk, is wederkerige nismodellering onderneem om ondersoek in te stel na die verwantskap tussen gebieds- en nisverskuiwing. Met behulp van ’n modelleringsraamwerk wat uit sowel klimaats- as landskapveranderlikes bestaan, is daar bepaal of die nis van die gestreepte rietpadda gedurende onlangse gebiedsuitbreiding behou is, en of verspreiding eerder deur ’n nisverskuiwing gefasiliteer is. Nismodelle wat aan die hand van die historiese en nuwe verspreidingsgebiede ontwikkel en wederkerig geprojekteer is, bring aan die lig dat H. marmoratus ’n nisverskuiwing van die historiese verspreidingsgebied in die noordelike en oostelike kusstreke na die nuwe verspreidingsgebied in die Mediterreense ekosisteme van die Suidwes-Kaap ondergaan het. Die nisverskuiwing val saam met ’n klimaatsneiging na hoër somerdroogheid en ’n algemeen laer reënval in die nuwe verspreidingsgebied in vergelyking met die historiese gebied, maar word waarskynlik bemiddel deur landskaptransformasie in die vorm van kunsmatige watermassas, wat bykomende bufferhabitats voorsien. Die studie kom tot die gevolgtrekking dat die nisverskuiwing wat met die uitbreiding in verspreidingsgebied gepaardgegaan het, die gestreepte rietpadda in staat stel om in die nuwe gebied in droër en meer veranderlike habitats te oorleef, waar toegang tot permanente watermassas in die landskap op ’n fyner skaal beperk is. Kunsmatige watermassas bied ’n belangrike aanvullende hulpbron vir hierdie tropiese indringerpadda, wat as ’n ‘stedelike uitbuiters’ bestempel kan word. Ongespesialiseerde habitatvereistes, snelle verspreiding en beduidende fenotipiese plastisiteit gee te kenne dat die spesie, binne die matriks van geskikte habitat tot hul beskikking, selfs verder kan uitbrei. Ter samevatting bevind die studie dat die beskikbaarheid van water in die landskap en die fisiologiese vermoëns van die padda met betrekking tot waterverlies belangrike bepalers is van die voorkoms en nis van die gestreepte rietpadda in sy nuwe verspreidingsgebied. Die navorsing beklemtoon die belang van klimaatsveranderlikes oor ’n wye skaal, landskaptransformasie in die vorm van kunsmatige watermassas, sowel as sinergistiese wisselwerking tussen fisiologie en gedrag in die bepaling van indringingsukses.

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