Warwick Bullen, of Whanganui, asks :-
Our local ornamental lakes have not seen frogs for some years. Why?
Phil Bishop, a zoologist at the University of Otago, responded.
Disappearing frog populations have been documented around the world since the early 1990s and we now realise that this is a real problem rather than simply fluctuating populations. Frog populations in New Zealand suffered a severe decline in the mid-1990s (unknown cause) and then again around 2005-6. It is suspected that this last population crash was caused by a frog disease caused by a fungus (the amphibian chytrid fungus) which was identified in a pond in Christchurch and has now been documented to have travelled throughout the whole of New Zealand.
However, although many people in the North and South Islands reported frog declines in the last ten years, several populations seem to be in the recovery phase, and this year I have received many reports from as far as Hamilton to Roxburgh of frogs turning up in people’s gardens. It appears that these frogs are resistant to the local strain of amphibian chytrid fungus and their numbers are bouncing back. If you reduce the amount of pesticide used in your garden and you have a healthy pond with good emergent vegetation (and no fish) then you will create an environment that encourages the establishment of an introduced frog population.
There are other reasons that may explain why the frogs initially disappeared. Many mammalian predators (e.g. feral or domestic cats, hedgehogs, ferrets) like to eat (or play with) the 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 introduced 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 populations will slowly re-establish themselves.
For more information see www.nzfrogs.org.