Gill Caradoc-Davies, of Portobella, asks :-
The frogs in my neighbour's pond vanished three years ago. My neighbour said a heron and kingfish ate them all. Could there be other causes and is the pond likely to get repopulated?
Phil Bishop, a zoologist at Otago University, responded.
Disappearing frog populations have been documented since the early 1990s and we now realise that this is a real problem rather than simply fluctuating populations. In 2000 a frog disease caused by a fungus (a chytrid fungus) was identified in a pond in Christchurch and this has been suspected to have caused many populations to disappear around New Zealand, including native frogs on the North Island. The southern most point that the disease has been positively identified is Cave, so it may not have travelled as far south as Dunedin yet.
However, many people in the Otago region have reported frog declines in the last ten years, and now many populations seem to be in the recovery phase, so it is possible that the disease-front moved through the Dunedin area and we have not yet identified it from that region.
There are other reasons that may explain why the frogs disappeared. Many mammalian predators (e.g. feral or domestic cats, hedgehogs, ferrets) like to eat (or play with) the brown tree frogs and herons and kingfishers include them in their diet. Other diseases such as those caused by other fungi or Rana virus may attack frog populations that are under stress from a changing environment and cause the local populations to become extinct. Ponds act as localised sources of frogs and there will be many others in the surrounding vegetation, so if the pond-dwelling frogs are wiped out by predators or disease then it is likely that the population will slowly re-establish itself.
2008 was recognised as the Global Year of the Frog to raise awareness of the frog extinction crisis and to get people to help. For more information see www.nzfrogs.org.