The cane toad (Rhinella marina) has become invasive in much of its introduced range, impacting significantly on biodiversity in these regions. Not only does the cane toad prey upon and compete with native species, it also produces a potent toxin that can be deadly to would-be predators. Nowhere is the impact of the cane toad more apparent than in Australia and a recent study indicates that edge-of-range populations there are evolving to become even better invaders.
Following the toads’ intentional release in Queensland, first in 1935 and in higher numbers in 1936, R. marina spread, moving into the Northern Territory and New South Wales, consuming and poisoning an array of native animals along the way. More recently the toad has been detected in Western Australia, though significant endeavour and finance is being invested in attempts to prevent the toad from gaining a foothold in the region. Those charged with stalling the advancing front of the cane toad invasion may, however, be facing the cane toad invasive elite. A recent study by Phillips et al. (2010) in the Journal of Evolutionary Biology has described how the increasing rate of spread of the cane toad at the edge of its range in Australia correlates with the evolution of heritable traits in edge-of-range populations that enable higher dispersal rates in these toads as compared with populations at the core of the cane toad range. In short, frontier cane toads continue to evolve traits that enable them to disperse at a higher rate, propelling what is a highly successful invader to new levels of invasiveness.
You may be surprised by the choice of the cane toad as the focus of a blog by a biocontrol practitioner, however, I think it is worthwhile tackling head on the ugly truth of the cane toad and to try to provide a realistic view of biocontrol as it is carried out in this day and age; a stark contrast to the process that resulted in the introduction of this devastating amphibian. The cane toad was introduced to Australia as a biological control agent in the 1930s. The intended targets were the cane beetles Dermolepida albohirtum and Lepidiota frenchi – pests of sugar cane – however, the cane toad had little impact on these pests and instead fed upon most other organisms that would fit into its mouth whilst also poisoning would-be predators, including a range of native endemic species. The reason for the introduction was based on the perceived success of the cane toad in controlling ‘agricultural pests’ in other regions such as Puerto Rico and several Pacific Islands (where the cane toad has also gone on to become an invasive pest), but it was not subject to host-range testing, pest risk assessment or any real scientific process, at a time when there was no real regulation or safety analysis for this type of action. At the time some scientists recommended against the introduction of the cane toad, but their advice was not heeded. Modern biological control is a heavily regulated field with the focus on proving the safety and specificity of a potential biocontrol organism through extensive testing before introduction to a new region is even considered. The topic of the cane toad and the field of biocontrol is assessed comprehensively in the article “Beyond the Cane Toad” by Rob Tanner and Dick Shaw which I would thoroughly recommend reading. In short, the idea of introducing a vertebrate for pest control is abhorrent to today’s biocontrol practitioners.
Invasive Species Research Scientist
Llewelyn, J., Phillips, B.L., Alford, R.A., Schwarzkopf, L. & Shine, R. (2010) Locomotor performance in an invasive species: cane toads from the invasion front have greater endurance, but not speed, compared to conspecifics from a long-colonised area. Oecologia. 162: 343-348.
Phillips, B.L., Brown, G.P. & Shine, R. (2010) Evolutionarily accelerated invasions: the rate of dispersal evolves upwards during the range advance of cane toads. Journal of Evolutionary Biology. 23: 2595-2601.