An empirical test of species range limit theory using the world’s largest amphibian invasion, the cane toad (Rhinella marina) in Australia
Invasive species are one of the key drivers behind the global loss of biodiversity. The cane toad (Rhinella marina) is one of only three amphibians on the 100 World’s Worst Invasive Alien Species list maintained by the International Union for Conservation of Nature. This large, Neotropical toad has invaded over 40 countries worldwide, including their largest invasion site in Australia. There they are detrimentally impacting native species as novel toxic prey items, predators, and competitors. Although unfortunate, species invasions also provide unique “natural experiments” to investigate the ecological and evolutionary processes underlying species’ geographic range limits. Much theoretical work has been devoted to species range limits, but empirical testing of the models has lagged behind. We are using population and landscape genetic/genomic techniques to investigate factors affecting cane toad dispersal and gene flow in core versus edge regions of their invaded Australian range. This work tests two major species range limit hypotheses: (a) whether asymmetric, maladaptive gene flow from large core populations to smaller edge populations swamps local adaptation, or (b) whether small, isolated edge populations lack the genetic diversity to locally adapt to edge conditions and expand their geographic range.
We conducted population-level sampling from 89 localities across the invaded range of the cane toad, organized into 9 broad-scale, core-to-edge transects. Individuals were initially genotyped using four published microsatellite loci. Preliminary population structuring results confirmed high dispersal rates observed in toads, with a broad geographic break between the actively expanding populations in the northwest and more stable populations in the southeast. In the southern portion of the range where sampling was densest, we tested landscape-gene flow relationships using 9 environmental variables related to climate, landcover, and water bodies. Temperature, precipitation, and heat load index were the best predictors of gene flow, supporting our hypotheses that cold temperatures and dry conditions limit toads along southern latitudinal and inland aridity gradients, respectively. With regard to species range limit hypotheses, we found evidence of reduced genetic diversity along two core-to-edge transects in the south, and no evidence of asymmetric gene flow on any transect. However, more loci are needed to increase power. We are currently using restriction site associated DNA (RAD) sequencing to find single nucleotide polymorphisms (SNPs) across the genome. Preliminary RAD sequencing of 24 individuals across the range yielded 10,422 SNP loci, to be used for landscape genomic analyses.