Invader impact clarifies the roles of top-down and bottom-up effects on tropical snake populations
Summary Disentangling the effects of prey limitation (bottom-up) and predation (top-down) processes on natural populations is difficult, but the perturbations introduced by an invasive species can provide pseudo-experimental evidence on this issue. In tropical Australia, keelbacks (Tropidonophis mairii) and slatey-grey snakes (Stegonotus cucullatus) experience little direct cost or benefit from the arrival of toxic invasive cane toads (Rhinella marina), because they rarely eat toads and if they do, are relatively resistant to the toads' toxins. Nonetheless, these snakes could be affected indirectly by toad-induced decreases in the availability of prey (native frogs) and/or by fatal poisoning of the snakes' predators (large varanid lizards). The former (bottom-up) effect predicts decreases in snake body condition, feeding and growth rates after toads arrive, whereas the latter (top-down) effect predicts increases in survival. Our mark-recapture studies on these snakes in tropical Australia (for 7 years before and 7 years after toad arrival) reveal bottom-up effects both on an anurophagous dietary specialist (keelbacks) and a generalist feeder (slatey-grey snakes). Top-down effects of toad arrival were seen in keelbacks but not in slatey-grey snakes, perhaps reflecting the latter's larger body size. Indirect effects coinciding with the timing of toad invasion thus were mediated through changes in food supply for both native species and in rates of predation for one species.
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