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

[摘要] ENGLISH ABSTRACT: Amphibian populations are among the most seriously threatened by global environmentalchange. Nonetheless, a few species have expanded their ranges to become globally invasive. Insouthern Africa, several anuran species are undergoing extra-limital range expansion on aregional scale and one of them, the painted reed frog (Hyperolius marmoratus Rapp.), is nowinvasive across the south-western Cape of South Africa. To answer the question of how thistropical, 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 arange of approaches from range ecology, physiology and niche modelling. Reconstruction of thepainted reed frog's invasion history allowed the date of introduction to be identified as 1997 orearly 1998. The novel range was defined as extending from the Tsitsikamma Forest in the east tothe Cape Peninsula in the west. Patches and gaps in the range structure and disparate rates ofspread indicated that human-assisted jump dispersal and diffusion-based dispersal dominate indifferent parts of the novel range. A significant gap in the novel range distribution is formed bythe Riviersonderend Mountains, a section of the Cape fold mountain range, that acts as a barrierto spread.To identify physiological range limiters, I investigated the plasticity of key physiological traitsthat influence thermoregulation, energetics and evaporative water loss. After thermal acclimationat three temperatures commonly encountered in their historical and novel ranges, frogs exhibiteda broad thermal tolerance range and higher plasticity in CTmax than in CTmin. Resting metabolicrates were lowest in cold-acclimated animals, partially supporting the 'colder is better'hypothesis over beneficial acclimation. Active metabolic rates were lowest in warm-acclimatedfrogs, suggesting compensation for energy conservation. Notably, evaporative water loss was notsignificantly altered by acclimation in resting or active frogs, demonstrating a lack of plasticityin this trait. Plasticity of thermal tolerance and metabolic rate suggests that painted reed frogsefficiently conserve energy in a range of thermal environments and can withstand seasonalcooling by minimising the costs of resting metabolism. These characteristics could play abeneficial 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 extremethermal environments north of their current range. On the other hand, the lack of plasticity inwater loss rates, combined with reliance on the water-conserving posture to limit evaporativewater 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 nicheshift. I tested whether the niche of the painted reed frog has been conserved during recent rangeexpansion or whether spread has been facilitated by a niche shift by using a modellingframework comprising both climatic and landscape variables. Niche models trained in thehistorical and novel ranges and projected in the reciprocal range revealed that H. marmoratus hasundergone a niche shift between its historical range in the northern and eastern coastal regionsand its novel range in the Mediterranean ecosystems of the south-western Cape. The niche shiftcorresponds with a climatic gradient towards higher summer aridity and generally lowerprecipitation in the novel range than the historical range, but is likely mediated by landscapetransformation 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 tooccupy drier and more variable habitats in the novel range, while on a finer scale, access topermanent water bodies in the landscape is limiting. Artificial water bodies provide a keyresource supplement for these invasive tropical frogs, which can be recognised as 'urbanexploiters'. Unspecialised habitat requirements, rapid spread and significant phenotypicplasticity suggest that they could continue to spread further within the matrix of suitable habitatavailable to them.In summary, this study found that availability of water in the landscape and the physiologicalcapabilities of the frogs in relation to water loss are key determinants of the distribution andniche of painted reed frogs in their novel range. The work highlights the importance of broadscaleclimatic variables, landscape transformation in the form of artificial water bodies, andsynergistic interactions between physiology and behaviour in determining invasion success.Om die beperkings op fisiologiese verspreiding te bepaal, is navorsing onderneem oor dieplastisiteit in die vernaamste fisiologiese kenmerke wat termoregulering, energetiek enwaterverlies deur verdamping beïnvloed. Ná termiese akklimatisasie by drie temperature watalgemeen 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. Rustendemetaboliese 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 metaboliesetempo's was die laagste by die paddas wat by hoë temperature geakklimatiseer is, wat weer opkompensasie vir energiebehoud dui. Akklimatisasie het geen beduidende verskil aan waterverliesdeur verdamping by rustende of aktiewe paddas gemaak nie, wat 'n gebrek aan plastisiteit inhierdie kenmerk aandui. Plastisiteit in termiese toleransie en metaboliese tempo gee te kenne datdie gestreepte rietpadda in 'n verskeidenheid termiese omgewings energie kan behou, enseisoenale afkoeling kan weerstaan deur die eise van rustende metabolisme te beperk. Hierdiekenmerke kan voordelig wees in die nuwe verspreidingsgebied, wat oor 'n temporeel enruimtelik veranderlike klimaat beskik. Tesame met 'n beduidende toleransie vir hitte, kan hierdie

[发布机构] Stellenbosch University