Phenotypic evolution in the invasive cane toad (Rhinella marina): adaptations for dispersal

Abstract

Phenotypic evolution in the invasive cane toad (Rhinella marina): adaptations for dispersal

Abstract Invasive species provide a rare opportunity to study how organisms adapt when colonising novel environments. Despite the negative ecological impacts caused by the introduction of alien species, biological invasions act as natural experiments that we can exploit for ecological and evolutionary research. This is particularly true for introductions with precise geographic and historical records, such as the cane toad (Rhinella marina) introduction to Australia. Arguably the most successful invasive anuran worldwide, the cane toad was first introduced to the Hawai’ian Islands in 1932, and subsequently to northeastern Queensland in 1935. Over the past 82 years, R. marina has spread rapidly and at an accelerating pace throughout northern and eastern Australia, causing massive ecological disturbances in its wake. This acceleration of dispersal capability is a well-documented phenomenon in invasive organisms that can occur via a combination of natural selection, and spatial sorting. Although the increasing rate of cane toad dispersal has been extensively researched, my thesis aims to address questions about the evolutionary changes that have taken place throughout the process. Namely, what morphological, behavioural, or physiological shifts in cane toad phenotypes are associated with increased dispersal ability? I began by taking gross morphological measurements from toads across a transect through their northern Australian range that included long-colonised and invasion-front populations. Additional populations from a prior invasion (Hawai’i), and the native range (French Guiana) were later added to compare the morphology of individuals from the source populations. I focused on the components of each limb (hand, radioulna, humerus [forelimb]; femur, tibiofibula, foot [hindlimb]), as limb morphology is strongly linked to locomotor ability, but also collected data on mass, snout-vent length, head width, and parotoid gland shape. A subset of Australian individuals were collected from the edges of the range, representing the oldest and newest toad populations to be used in a common-garden breeding experiment. These individuals, and their resulting offspring were held in captivity and subjected to a series of performance trials. A separate group of individuals from invasion-front and range-core populations was used for Computerised X-ray Tomography (CT) scanning for precise geometric morphometric comparison of skeletal structure. Captive breeding of cane toads from the invasion-front (in Western Australia) and long-colonised areas (in Queensland) allowed me to control for the influence of rearing environment on common-garden F1 individuals. This also enabled analyses on heritability of morphology and performance traits by comparing offspring to their parents, and siblings to each other. Common-garden F1 offspring were raised over a period of 27 months, and measured repeatedly throughout ontogeny. Performance trials consisted of climbing trials (where toads would have to escape from a mesh tube by climbing vertically) and anti-predator raceway trials (where toads were encouraged to hop down a raceway by prodding). These trials were conducted on wild and captive toads. Within this thesis I document reproductive differences between invasion-front and long-colonised populations, significant geographic variation in locomotor performance, regional changes to skeletal structure, shifts in sexual dimorphism with time since colonisation, and heritability of behavioural and morphological traits; and I demonstrate that these changes have arisen via the rapid evolution of a high-dispersal phenotype during the invasion process.