Charlie Horn, of North Otago, asks :-

I have a good population of Brown Tree Frog (Litoria ewingi) in three ponds at my batch adjacent to the Waianakarua river in North Otago. In the first year they bred in all three ponds towards the end of winter, but have not done so since. Following their introduction we also have 7 Southern Bell frogs and 2 young gold fish in one pond. Would this put them off breeding in this pond? The tree frogs also used to croak consistently from dusk to the early hours of the morning. This doesn't occur now. They sometimes croak during the day and just on dark.

Phil Bishop, a zoologist at the University of Otago, responded.

Unfortunately, once the Southern Bell frogs move in the populations of Litoria ewingii are destined to decline. The bell frogs actually locate the L. ewingii by locating their calls and then they eat them! And the gold fish in the pond ensure that no tadpoles or frogs eggs survive so the few that escape the voracious appetite of the bell frogs would have their spawn eaten by the tadpoles, so I’m afraid its a sad story. Occasionally L. ewingii breed in small ponds like tyre ruts on the road or ornamental ponds or even buckets of water, so the population may be able to hang on by breeding in these suboptimal habitats.

In your case it appears that L. ewingii breed mainly in late winter early spring, while the bell frogs prefer hot summer months – so there maybe some temporal differentiation with their breeding seasons that would reduce the amount of contact they have. Perhaps with only two goldfish they have not been able to eat all the tadpoles and a number of them have managed to survive to metamorphosis.

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