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The cane toad invasion
Phillips, B. L., G. P. Brown, J. Webb, and R. Shine. 2006. Invasion and the evolution of speed in toads. Nature 439:803.
In the first few decades after toads were released in Queensland, they expanded their range at about 10 km per year. That rate then began to increase, and now averages about 55 km per year. We show that toads with relatively long legs (compared to their body length) move faster and further than shorter-legged toads of the same body size; and that the invasion front is composed of very mobile, long-legged toads. Thus, toads have changed dramatically, through rapid evolution, in the course of only about 50 generations in Australia.
Brown, G. P., B. L. Phillips, J. K. Webb, and R. Shine. 2006. Toad on the road: use of roads as dispersal corridors by cane toads (Bufo marinus) at an invasion front in tropical Australia. Biological Conservation 133:88-94.
As noted above, invasion-front toads are in a real hurry to move, and so don’t waste their time struggling through thick vegetation. Instead, they take advantage of the roads (and cattle paths, etc) that we humans have so thoughtfully provided for them. Most of our radio-tracked toads zipped along the highway rather than moving through dense vegetation. When they reached crossroads, their paths of movement often took a right-angle turn because they would rather follow roads, even if the direction isn't exactly perfect, than try bashing through the scrub. By the way, this also means that people tend to overestimate the abundance of toads - because they prefer open spaces, they are much easier to see than are native frogs (that are smaller, and hide in vegetation most of the time).
Phillips, B. L., G. P. Brown, M. Greenlees, J. K. Webb, and R. Shine. 2007. Rapid expansion of the cane toad (Bufo marinus) invasion front in tropical Australia. Austral Ecology 32:169-176.
We radio-tracked toads at the invasion front (on the Adelaide River floodplain), by putting small waistbands on the toads and attaching miniature transmitters. Then, Greg was able to go out and locate the toads every day. This is a really useful method, because otherwise the toads are pretty much invisible within the landscape unless they gather beside a pool to start breeding. The tracking showed that the toads move quite amazing distances, and in incredibly consistent directions. We also mapped the advance of the toad front through Kakadu and towards Darwin, and found very similar rates of progress of the front as estimated by the radio-tracking. This is probably one of the most astonishing results from our fieldwork: the invasion-front toads move hundreds of metres every night, an incredible achievement for a small amphibian. Of course, this makes the front almost impossible to stop - if you miss even a single toad, it will be many kilometres past you within a few weeks or perhaps, in a few days.
Urban, M., B. L. Phillips, D. K. Skelly, and R. Shine. 2008. A toad more traveled: the heterogeneous invasion dynamics of cane toads in Australia. American Naturalist 171:E134-E148.
How fast have cane toads traveled through Australia since their release in 1935, and why have they moved more quickly through some areas than others? We have incredibly detailed information about the rate that toads have spread, because people really notice toads and so, have been able to record when they turn up. We put all this information together with data on climates and landforms etc, to try to work out what factors affect toad invasion speeds. The biggest factor turns out to be the gradual acceleration of the toad front through time - they have evolved to disperse more rapidly (with longer legs and different behaviours) and thus move westwards about five times faster than in the early years of toad invasion.
Phillips, B. L., G. P. Brown, J. M. J. Travis, and R. Shine. 2008. Reid's paradox revisited: the evolution of dispersal kernels during range expansion. American Naturalist 172:S34-S48.
There is lots of recent interest in the effects of climate change, and one interesting issue is how quickly animals and plants could move across the landscape to colonise a newly-available area. This happened after the last Ice Age, for example, as glaciers retreated - how quickly could plants and animals move into the new space? Many years ago, Reid noticed a puzzle - species seemed to move much faster than we would expect from the rates that we measure animals and plants dispersing under today's conditions. We show, with some mathematical modeling and studies on movements of radio-tracked toads, that the answer lies in the amount of variation in movement distances. Toads at the invasion front have more variable movement distances per day, and this allows some of them to move much further than we would expect from the "average" values.
Alford, R. A., G. P. Brown, L. Schwarzkopf, B. Phillips, and R. Shine. 2009. Comparisons through time and space suggest rapid evolution of dispersal behaviour in an invasive species. Wildlife Research 36:23-28.
In previous work, we showed that cane toads are moving faster and faster through tropical Australia, and that one of the reasons for that acceleration is the evolution of longer legs (that enable toads to move quicker). But of course, how fast a cane toad can disperse depends on its behaviour as well as its running speed. Fortunately, Ross Alford and Lin Schwarzkopf had radio-tracked cane toads in Queensland several years ago, so we were able to combine our results with theirs to see what behaviours have changed over the course of the toads' Australian invasion. As we expected, the invasion-front toads move a lot further every day, and tend to move whenever conditions are suitable - whereas the Queensland toads end up staying a lot closer to home.
Shilton, C. M., G. P. Brown, S. Benedict, and R. Shine. 2008. Spinal arthropathy associated with Ochrobactrum anthropi in free-ranging cane toads (Chaunus [Bufo] marinus) in Australia. Veterinary Pathology 45:85-94.
In other work, we showed that the cane toad invasion front has moved faster and faster across tropical Australia, due to rapid evolutionary changes. Our radio-tracking has confirmed incredible rates of dispersal in invasion-front toads - often more than a kilometer in a single night. How can a toad move so far? Wouldn't this put extreme pressure on their bodies - after all, toads look to be built for comfort not for speed, and no other frog species worldwide moves so far and so fast. Sure enough, the invasion-front toads are paying a price - many of them develop severe spinal arthritis, reflecting their inability to stop a common bacteria from infecting their spinal joints.
White, A. W., and R. Shine. 2009. The extra-limital spread of an invasive species via "stowaway"dispersal: toad to nowhere? Animal Conservation 12:38-45.
Working closely with a community group in Sydney, we have examined records of cane toads that have found their way outside the main range. It turns out that most of these toads have hitched a ride with building and landscaping materials - and that the numbers of adult toads reaching Sydney are very high. Clearly, we need to be doing something about cutting down the numbers of hitch-hikers, to prevent toads expanding their range even more rapidly than they are already achieving.
Shine, R., G. P. Brown, and B. L. Phillips. 2011. An evolutionary process that assembles phenotypes through space rather than time. Proceedings of the National Academy of Sciences USA 108:5708-5711 (selected by ‘Faculty of 1000’).
Shine, R., G. P. Brown, and B. L. Phillips. 2011. Spatial sorting, assortative mating, and natural selection. Proceedings of the National Academy of Sciences USA 108:E348.
These two papers explore the process that has resulted in cane toads evolving to move faster and faster across tropical Australia. Unlike conventional evolution, that is driven by natural selection, we think that the accelerated toad invasion results from “spatial sorting”. Genes for moving faster and further become concentrated at the increasingly fast-moving invasion front.
Shine, R. 2012. Invasive species as drivers of evolutionary change: cane toads in tropical Australia. Evolutionary Applications 5:107-116.
This paper reviews the evidence that we have gathered about how cane toads in Australia have caused rapid evolutionary change. The changes have occurred in toads themselves, in their parasites, and in the native species with which the toads interact.
Llewellyn, D., M. B. Thompson, B. L. Phillips, G. P. Brown, and R. Shine. 2012. Reduced investment in immune function in invasion-front populations of the cane toad (Rhinella marina) in Australia. Biological Invasions 14:999-1008.
Following on from earlier work where we found that invasion-front toads appear to have weakened immune systems, David actually measured the energy investment into the immune response in cane toads whose parents were collected from a range of areas – some from long-colonised regions, some from the invasion front. As predicted, the invasion-front toads invested less energy into the immune response.
Kelehear, C., G. P. Brown, and R. Shine. 2012. Rapid evolution of parasite life-history traits on an expanding range-edge. Ecology Letters 15:329-337.
Its not just toads and their predators that have evolved – the toads’ parasites also have changed. Crystal showed that parasites from invasion-front toads have larger body sizes in the stages that live outside the host – probably because low numbers of toads at the invasion front make it hard for parasites to find a new host, so they have to live longer in the soil.
Brown, G., C. Kelehear, and R. Shine. 2013. The early toad gets the worm: cane toads at an invasion front benefit from higher prey availability. Journal of Animal Ecology 82:854-862.
Why has the cane toad invasion front accelerated over the time since toads arrived in Australia? One possible explanation is that there has been an evolutionary advantage to the toads that are the first to arrive in an area – perhaps because there is less competition from other toads, and hence more food. Our studies at Fogg Dam confirm that the first toads to arrive did indeed eat more, and grow faster, than the toads that arrived in the same area a few years later.
Lindstrom, T., G. P. Brown, S. A. Sisson, B. L. Phillips, and R. Shine. 2013. Rapid shifts in dispersal behavior on an expanding range edge. Proceedings of the National Academy of Science (USA): in press.
In earlier studies, we showed that cane toads have invaded faster and faster since they were first released into Australia; and that those changes are due to evolution (that is, offspring inherit their parent’s dispersal speed). But until now, we hadn’t looked at what happens to the dispersal rate of toads as the invasion front passes through. We radio-tracked toads at Fogg Dam for several years after they first arrived, and our analyses reveal that toads were moving a lot further in the first couple of years (at the invasion front) than they have subsequently. The general message from this study is that if we want to understand how quickly a species can spread into new territory, we need to actually measure what happens at the expanding range edge – animals there will probably move a lot quicker and further than we expect from studies of the same species at the range core
Cane toad research photo credits: Christa Beckmann, Haley Bowcock, Greg Brown, Elisa Cabrera-Guzman, Travis Child, Michael Crossland, Matthew Greenlees, Mattias Hagman, Crystal Kelehear, John Llewelyn, David Nelson, Stephanie O'Donnell, Ben Phillips, Ligia Pizzatto, Sam Price-Rees, Cathy Shilton, Ruchira Somaweera, Peter Street, Georgia Ward-Fear, Jonathan Webb