Exploring the invasion of the guttural toad Sclerophrys gutturalis in Cape Town through a multidisciplinary approach

[摘要] ENGLISH ABSTRACT: Invasive populations of amphibians may have considerable ecological and socio-economic impacts; reconstructing their invasion dynamics is essential to perform adaptive management. Investigating these populations is also an opportunity to address eco-evolutionary questions; it helps to improve our comprehension of biological systems and define in greater detail invasion potential. This study explores the invasion of the guttural toad Sclerophrys gutturalis in Cape Town through a multidisciplinary approach that integrates physiology, evolutionary biology, ecological modelling and environmental economics.The species is domestic exotic in South Africa, being native in most of the country but not in Cape Town, where an invasive population established in 2000. Although an extirpation program (started in 2010) removed some thousand adults, tadpoles and eggs until 2016, the population is still spreading. Invasion dynamics emerging from traits of the invader and characteristics of the invaded landscape are unknown. Additionally, efficacy and efficiency of the current mode of removal as well as the possibility to implement more effective extirpation strategies have not been investigated. Since the winter rainfall environment of Cape Town is drier and colder than that of the source population (Durban), especially during the summer breeding season, the species' ability to spread is remarkable. Currently it is not clear how the abiotic conditions of Cape Town constrain this species and whether invasive toads adaptively respond to reduce phenotypic mismatch in the novel environment.Firstly, I built an age structured model that can be utilized to simulate population dynamics of invasive pond-breeding anurans. The model follows a metapopulation approach and simulates change in survival and dispersal behaviour as a function of age. It also integrates dispersal with landscape complexity through least cost path modelling to depict functional connectivity across the pond network. Then I applied the model to my case study; parameterization was conducted through field and laboratory surveys, a literature review and data collected during the extirpation. I found a lag phase in both demographic and spatial dynamics. Also, I found that the spatial spread fits an accelerating trend that causes the complete invasion of the network in six years. Such dynamics match field observations and confirmed patterns previously detected in other invaders characterized by high dispersal abilities.The age structured model was further employed to explore efficacy and efficiency of the current management. I investigated how a scenario incorporating the demographic effects of the current removal differs from a no-extirpation scenario. I also asked which limitations might impede the management from being successful and whether alternative strategies may determine better results. I found that the current management does not sufficiently take into consideration non-linear population dynamics and it reduces the efficiency; moreover the removal started during the spread phase of the invasion. Spatial limitations linked to the social dimension of the landscape severely reduce efficacy of the current removal; other management countermeasures such as control or containment should thus be considered.To explore how the species phenotype is constrained in the invaded environment during the breeding period and whether invasive toads underwent any adaptive response, I performed a comparison between the invasive population of Cape Town and the native source population of Durban. Field data and physiological traits such as evaporative water loss, water uptake, sensitivity of locomotion to desiccation and critical thermal minimum were collected. In accordance to the more desiccating and colder environment of Cape Town, invasive guttural toads responded physiologically and behaviourally on short time scale (less than two decades) to reduce sensitivity to lower conditions of hydration and temperature. The species is still constrained in the novel environment but its invasion potential is higher than I could infer from the source population.To confirm that the colder environment of Cape Town constrains invasive toads also during the non-breeding season, I investigated post-breeding energy storage in populations from Cape Town (high latitude), Durban (intermediate latitude), Mauritius and Reunion (low latitude) where the species is also invasive. Although post-breeding energy storage should be high (capital breeding strategy) at high latitudes and low (income breeding strategy) at low latitudes, guttural toads unexpectedly shifted energy storage strategy from capital to income breeding when introduced from lower to higher latitude. The invaded environment is therefore less severe during the non-breeding season; winter rainfall promotes, and does not reduce, toads' activity.In summary, I showed that the invasion success of the guttural toad in Cape Town may be attributable to several factors such as initial lag that delayed management, accelerating spread, rapid adaptive response and less severe non-breeding season. The spatial dimension of the invaded landscape strongly limited the efficacy of the current management program. My work has relevant management implications; it shows that the invasion potential of the species is already higher than that I could infer from the source population and only tackling social limitations could have promoted effective extirpation.

[发布机构] Stellenbosch University