Trophic interactions of the Guttural Toad (Sclerophrys gutturalis, Power 1927) along an invasion gradient

Thesis (MSc)--Stellenbosch University, 2022.

ENGLISH ABSTRACT: Invasive amphibians have caused widespread negative ecological and socio- economic impacts globally, yet most of the research investigating this topic is focused on a small handful of species. The Guttural Toad (Sclerophrys gutturalis) is one such understudied species that has a long invasion history on Mauritius (~100 years) and Reunion (~95 years), and a recent domestic invasion in Cape Town (~22 years). It is a successful invasive species and has been implicated in the imperilment of native taxa, however, little is known about their specific diet or how their feeding habits have facilitated their invasion success. In Mauritius, S. gutturalis has been suggested to be a contributing factor to declines of several species of endemic invertebrates, while their invasion in Cape Town has raised concerns about the potential for congeneric competition with the IUCN endangered Western Leopard Toad (Sclerophrys pantherina). The overarching goal of this thesis was to examine the diets, feeding habits, and trophic position of S. gutturalis along its invasion route from native-natural to native-urban (KwaZulu-Natal), to invasive- urban (Cape Town and Mauritius), and invasive-natural populations (Mauritius). Specifically, I undertook a comprehensive investigation of their dietary niche breadth, diet composition, and prey preferences using stomach content analysis. Moreover, I complimented stomach content analysis with stable isotope analysis to examine the toad’s trophic position, isotopic niche breadth, and competitive interaction with S. pantherina. Stomach content data demonstrated that S. gutturalis is a highly adaptable and flexible feeder, with a diet typically comprising of terrestrial invertebrates and a limited proportion of small-bodied vertebrates. Native-natural populations contracted their dietary niche breadth as they transitioned into native-urban habitats due to reduced prey diversity in urban habitats. The expansion of dietary niche breadth in invasive-natural populations may be linked with a shift towards diurnal feeding activity, allowing for increased access to resources within their invaded range. Across populations prey preference varied, however, all populations preferred invasive prey – an indication that the presence of other invasive taxa may promote the toad’s invasion success. Compared to rural populations, urban populations prefer ingesting insects. This is indicative of the influence of garden plants and artificial lighting that help centralize food sources, as well as human activities that may further boost S. gutturalis invasions. Isotopic data demonstrated that, on average, invasive populations experienced a drop in trophic position. However, urban populations typically had elevated trophic positions – an indication of consuming nitrogen-rich prey that may be anthropogenically subsidised within urban spaces. The expansion and contraction of isotopic niche breadth reflected varying prey availability and isotopic baselines. The broad isotopic niche breadth in native populations indicated consumption of isotopically distinct prey. The moderate isotopic niche overlap between S. gutturalis and S. pantherina demonstrated the potential for competitive interactions, but the extent of this overlap varies greatly between these species. This research provides valuable dietary information that generates insight into how these toads have adapted to both urban and invaded habitats. This work contributes to both our knowledge of native S. gutturalis in southern Africa and informs future conservation actions and the management of invasive populations.

AFRIKAANSE OPSOMMING: Indringer amfibieë het wydverspreide ekologiese en sosio-ekonomiese impakte wêreldwyd veroorsaak, maar meeste van die navorsing wat hierdie onderwerp ondersoek, is gefokus op 'n klein handjievol spesies. Die Gorrelskurwepadda (Sclerophrys gutturalis) is een so 'n onderbestudeerde spesie wat 'n lang indringingsgeskiedenis in Mauritius (~100 jaar) en Réunion (~95 jaar) het, en 'n onlangse huishoudelike indringing in Kaapstad (~22 jaar). Dit is 'n suksesvolle indringerspesie en is geïmpliseer in die bedreiging van inheemse taksa, maar min is bekend oor hul spesifieke dieet of hoe hul voedingsgewoontes hul indringer sukses vergemaklik het. In Mauritius is voorgestel dat S. gutturalis 'n bydraende faktor is tot die afname van verskeie endemiese ongewerwelde spesies, terwyl hul indringing in Kaapstad kommer laat ontstaan het oor die potensiaal vir kongeneriese mededinging met die IUCN-bedreigde Westelike Luiperdskurwepadda (Sclerophrys pantherina). Die oorkoepelende doel van hierdie tesis was om die diëte, voedingsgewoontes en trofiese posisie van S. gutturalis langs sy indringingsroete te ondersoek van: inheems-natuurlik tot inheems-stedelike (KwaZulu-Natal), tot indringer-stedelike (Kaapstad en Mauritius), en indringer-natuurlike bevolkings (Mauritius). Ek het spesifiek 'n omvattende dieet ondersoek onderneem om hul dieet-nisbreedte, dieet samestelling en prooi-voorkeure te ondersoek deur van maaginhoudanalise gebruik te maak. Daarbenewens het ek maaginhoud-analise met stabiele isotoop-analise gekomplimenteer om die padda se trofiese posisie, isotopiese nisbreedte en mededingende interaksie met S. pantherina te ondersoek. Maaginhouddata het getoon dat S. gutturalis 'n hoogs aanpasbare en buigsame voeder is, met 'n dieet wat tipies bestaan uit land-ongewerweldes en 'n beperkte proporsie van klein-liggaam gewerweldes. Inheems-natuurlike bevolkings het hul dieet nisbreedte verklein soos hulle oorgegaan het na inheems-stedelike habitatte as gevolg van verminderde prooi diversiteit in stedelike habitatte. Die uitbreiding van dieet nisbreedte in indringer-natuurlike bevolkings kan gekoppel word aan 'n verskuiwing na dag-aktiewe voedingsaktiwiteit, wat voorsiening maak vir verhoogde toegang tot hulpbronne binne hul ingedringde gebied. Regoor bevolkings het prooi voorkeur gewissel, maar alle bevolkings het indringerprooi verkies – 'n aanduiding dat die teenwoordigheid van ander indringer taksa die padda se indringingsukses kan bevorder. Stedelike bevolkings het insekte sterk verkies – ‘n aanduiding van die invloed van tuinplante en kunsmatige lig wat prooi sentraliseer as menslike aktiwiteite wat ook S. gutturalis indringings kan versterk. Isotopiese data het getoon dat indringerbevolkings, gemiddeld, 'n daling in trofiese posisie ervaar het. Stedelike bevolkings het egter tipies verhoogde trofiese posisies gehad – 'n aanduiding van die inname van stikstofryke prooi-items wat antropogenies binne stedelike ruimtes gesubsidieer kan word. Die uitbreiding en inkrimping van isotopiese nisbreedte het wisselende prooibeskikbaarheid en isotopiese basislyne weerspieël. Die wye isotopiese nisbreedte in inheemse bevolkings het die verbruik van isotopies spesifieke prooi aangedui. Die matige isotopiese nis oorvleueling tussen S. gutturalis en S. pantherina het die potensiaal vir mededingende interaksies getoon, maar die omvang van hierdie oorvleueling wissel baie tussen hierdie spesies. Hierdie navorsing bied waardevolle dieet inligting wat insig genereer in hoe hierdie paddas aangepas het by beide stedelike en ingedringde habitatte. Daarbenewens verhoog hierdie werk nie net ons kennis van inheemse S. gutturalis in Suider-Afrika nie, maar dit kan ook toekomstige bewaringsaksies inlig en die bestuur van hul indringerbevolkings.

Masters