Challenges of a novel range: Water balance, stress, and immunity in an invasive toad

Abstract

Species introduced by human activities can alter the normal functioning of ecosystems promoting negative impacts on native biodiversity, as they can rapidly expand their population size, demonstrating phenotypic plasticity and possible adaptive capacity to novel environments. Twenty years ago, the guttural toad, Sclerophrys gutturalis, was introduced to a peri-urban area of Cape Town, with cooler and drier climatic characteristics than its native source population, Durban, South Africa. Our goal was to understand the phenotypic changes, in terms of physiology and immunity, of populations in native and novel environments. We evaluated body index (BI), field hydration level, plasma corticosterone levels (CORT), proportion of neutrophils: lymphocytes (N: L), plasma bacterial killing ability (BKA), and hematocrit (HTC) in the field, and after standardized stressors (dehydration and movement restriction) in males from the native and invasive populations. Toads from the invasive population presented lower BI and tended to show a lower field hydration state, which is consistent with living in the drier environmental conditions of Cape Town. Additionally, invasive toads also showed higher BKA and N:L ratio under field conditions. After exposure to stressors, invasive animals presented higher BKA than the natives. Individuals from both populations showed increased CORT after dehydration, an intense stressor for these animals. The highest BKA and N:L ratio in the field and after submission to stressors in the laboratory shows that the invasive population has a phenotype that might increase their fitness, leading to adaptive responses in the novel environment and, thus, favoring successful dispersion and population increase.

No evidence for innate differences in tadpole behavior between natural, urbanized, and invasive populations

“…In the case of the invasive population near Cape Town, eggs and tadpoles are presumed to have been unintentionally introduced via a consignment of aquatic ornamental plants in 1998 (De Villiers 2006 ; Measey et al 2017 ). This urban-invasive Cape Town population has since shown rapid adaptation to the drier climate in the Cape region in the adult stage, altering their behavior, physiology, and reproductive strategy (Vimercati et al 2018 , 2019 ; Madelaire et al 2020 ; Barsotti et al 2021 ).…”

Habitat split as a driver of disease in amphibians

“…In addition, corticosterone in part mediates the physiological response to dehydration stress (Uchiyama & Konno, 2006) and may also play a role in water‐seeking behaviour (Madelaire et al ., 2020). Mounting an appropriate corticosterone response appears vital for survival and limiting water loss in two toad species invading arid habitats (Jessop et al ., 2013; Barsotti et al ., 2021). However, whether persistent exposure to dehydration stress affects immunity still remains largely unexplored.…”

Introduction to the special issue Amphibian immunity: stress, disease and ecoimmunology

“…One physiological aspect that has been recently observed in some outbreaks is the increase in amphibian plasma glucocorticoid levels, signalling a stress response. In the last decades, studies have been showing that habitat alterations and fragmentation, changes in temperature, and dehydration are associated with increased corticosterone levels, the main glucocorticoid in amphibians [8][9][10][15][16][17]. In addition, increased pathogenicity of the fungus Batrachochytrium dendrobatidis (Bd) and ranavirus has been positively correlated with corticosterone levels in some amphibian species [11,[18][19][20][21][22], pointing to a possible glucocorticoid influence on the stress-related augmented mortality in diseased individuals.…”