Indirect facilitation of an anuran invasion by non‐native fishes

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Abstract
Positive interactions among non‐native species could greatly exacerbate the problem of invasions, but are poorly studied and our knowledge of their occurrence is mostly limited to plant‐pollinator and dispersal interactions. We found that invasion of bullfrogs is facilitated by the presence of co‐evolved non‐native fish, which increase tadpole survival by reducing predatory macroinvertebrate densities. Native dragonfly nymphs in Oregon, USA caused zero survival of bullfrog tadpoles in a replicated field experiment unless a non‐native sunfish was present to reduce dragonfly density. This pattern was also evident in pond surveys where the best predictors of bullfrog abundance were the presence of non‐native fish and bathymetry. This is the first experimental evidence of facilitation between two non‐native vertebrates and supports the invasional meltdown hypothesis. Such positive interactions among non‐native species have the potential to disrupt ecosystems by amplifying invasions, and our study shows they can occur via indirect mechanisms.