The trouble with frogs

Forest and Bird, Issue 295, 1 February 2000, Page 14

The trouble

with frogs

A recent population crash of Archey’s frogs in the central Coromandel is causing concern. SHAUN BARNETT reports.

en Bell, from the School of Biological Sciences at Victoria University, has a special interest in native frogs, having studied them for over 25 years. During December 1996 he was carrying out some routine surveying for Archey’s frogs Leiopelma archeyi at a site on the Coromandel Ranges near Tapu, and made the alarming discovery that he could hardly find any. Subsequent searching did reveal a few, mainly larger females, but in nowhere near the numbers which had been recorded during most previous years. Two years later, in November 1998, a similar population crash became apparent at the other nearby site Ben Bell was monitoring, at Tokatea behind Coromandel town, 30 kilometres north. After analyzing the figures, results showed an approximately 90 percent reduction in numbers of Archey’s frog at the two study sites in the central Coromandel. Fortunately, no such decline seemed to be occurring for Hochstetter’s frogs Leiopelma hochstetteri in nearby streams at both sites. Frog populations are cyclical by nature, and like any species, fluctuate up and down according to complex interactions between population dynamics (such as breeding success) and environmental factors (such as climate, predation, and habitat change). But such a rapid population crash was of real concern, and Ben

Bell says it constitutes ‘the first systematically documented decline of a frog species in New Zealand’. Late in 1998, an emergency meeting of the Frog Recovery Group, which includes Department of Conservation staff and ‘outside’ academics, met to discuss the apparent decline in the frog population. DoC decided to step up its annual monitoring of other frog populations to threemonthly checks. These checks indicated substantial populations still of Archey’s frog at Whareorino (in the Western King Country), and Moehau (at the tip of the Coromandel range). The decline in the central Coromandel remains a concern, however, and with the recovery group’s support, DoC has given Dr Bell a $25,000 study grant over the next three years to investigate ‘the factors influencing the status and decline of Archey’s frog in the Coromandel Ranges’. s a rule, amphibians are good AN cositosmena indicators of ecosystem health. Because they absorb air and moisture through their skin, amphibians are vulnerable to pollutants and chemicals. They are also highly sensitive to ultra-violet radiation. As amphibians require damp locations, they are easily affected by climate changes that may affect their habitat. Bruce Waldman, a senior lecturer at the University of Canterbury, says it is generally accepted in the scientific community that amphibians world wide are in major decline. In Australia alone some 15 species of frog have disappeared from the eastern parts of the country since the

early 1980s. Still others are in serious decline. Disturbingly, some of the declining species inhabit near-pristine rainforests in remote parts of Queensland, where pollution and habitat destruction are not obvious factors. One species of frog, the golden bell frog Litoria aurea was once so common in its native Australia that it was collected by the bucketful for dissection classes by Zoology students. In New South Wales it has disappeared from 90 percent of its range, and is currently considered threatened. The causes of decline (aside from collection!) are not understood. This same frog species was introduced to New Zealand earlier in the century, and there is some anecdotal evidence to suggest declines here too. But any declines have been patchy, and in other areas the species has recovered or even appeared at sites from where it was not previously known. One obvious threat to the Australian frogs here is the. mosquito. . fish, Gambusia, introduced to control mosquitoes. Although smaller than guppies, the aggressive fish attack their much larger prey by tearing strips of skin and flesh off. Bruce Waldman believes mosquito fish are a major threat, not only to Litoria frogs, but also to native fish and invertebrates. (See also Forest and _ Bird, November 1998.) Bruce Waldman and a colleague at Otago University, Phil Bishop, have established a database on the three Litoria species present in New Zealand

which, with their own research and information from the public, aims to find out about distribution trends. Monitoring the introduced Australian species may provide important clues about possible threats to our native species. The only previous mapping survey was one coordinated in New Zealand by Ben Bell in the 1970s. Bruce Waldman says finding the causes of decline may be no easy task. In some areas in both Central America and Queensland, dead frogs have been found with lesions and scars, possibly caused by a Chytrid fungus. The fungus has, as yet, not been found in New Zealand. While at least one researcher has speculated this fungus may be a major cause of frog decline, Waldman cautions that causal factors are likely to be more complicated. For instance, in North America large declines in frog populations have occurred before, from an Aeromonas bacterium, which is normally harmless, but under certain conditions causes symptoms of a lethal, highly contagious frog disease known as ‘red leg’. All sorts of factors may be interacting to cause decline of certain species: for example, climatic stress may render a frog species more susceptible to disease — even one which is not normally problematic. Similarly, when suitable habitat becomes restricted, conditions will be more crowded for frogs at remaining sites, thereby rendering them more vulnerable to predators and disease. Identifying causes of decline may

involve some cunning detective work, and Bruce Waldman says the whole issue has highlighted the importance of longterm studies where normal frog population dynamics are well documented. Without long-term studies it would be difficult even to know if a decline has occurred. Ben Bell’s long-term work in the central Coromandel Ranges is a perfect example — without it, we might not have known about the crash there, at all. Ben Bell thinks climate change is one factor that may have possibly been responsible in central Coromandel. Other possible causes of decline include habitat disturbance, illegal collection, predation, and disease. Native New Zealand frogs are very sensitive to habitat disturbances, and Ben Bell wanted to know if his own sampling technique might have been responsible for the decline at the survey sites. To test this, he re-surveyed a line that had not been monitored since 1983, and was therefore unlikely to have suffered much research disturbance. But the line came up with the same crash results as his regular sites, and searches by DoC staff in other nearby locations indicated similar results. While 1080 poison has been used to control possums in areas of Coromandel in recent years, declines of Archey’s frog have occurred both in areas where 1080 has and has not been used, strongly suggesting that the toxin is not responsible for recent frog declines. Archey’s frog populations, carefully monitored at Tapu on the Coromandel by Alison Perfect and Ben Bell over the months before and after a 1080 drop in June 1995, showed no decline in numbers. Although native frogs are fully protected (you need a DoC permit even to look for them), illegal collection is a possible cause of decline that needs consideration, Ben Bell says. ‘But, he cautions, "You would expect a decline from collection to be more patchy than what is evident. Predation is another possible factor. At Whareorino, predation of Archey’s frogs by rats has been documented, but this seemed to affect mainly the larger frogs, especially females (which are larger than males). In the central Coromandel Ranges, it is mostly the larger females

which seem to have survived, and Ben Bell suspects predation is a less likely cause of the decline, though more investigation is needed. ‘The fact that larger frogs are remaining does suggest climate may be a factor, as larger frogs would be less prone to dehydration than smaller frogs, he notes. ‘1995 to 1998 was a time of exceptionally dry periods on the Coromandel. This same drought may not have affected the higher altitude Moehau population because of its frequently cloudy (and hence damp) summit.’ However, Ben Bell says that over the thousands of years of evolution in New Zealand, Leiopelma frogs would have been exposed to periods of extreme climatic conditions, and in recent times the frogs on the Coromandel have survived kauri logging, bush fires, and mining within their range. As yet the cause of the decline at central Coromandel is a matter of speculation, and it is not known if the population there will recover. The crash is ‘..not only an ecological tragedy, it’s also a research tragedy as most of the frogs which have been monitored for so long are no longer there to study. One frog in the study population was known to have lived for 17 years. Fortunately, for the short term, the decline does not seem to be continuing. But distressingly, Ben Bell has seen no indication of brooding (which is done by the males) at sites where frogs were previously known to breed. A PhD student will shortly begin a three-year study of Archey’s frogs on Moehau. It is hoped this study will reveal more about frog habitat requirements, and particularly about their altitudinal distribution over the mountain. The project also aims to identify possible causes of decline and the conservation measures to combat them. Until then DoC can only monitor and hope the central Coromandel crash is a localized and temporary one. It would be a biodiversity tragedy for the world if we were to lose any more of our unique native frogs.

SHAUN BARNETT is a photographer and writer specializing in the natural world. He is based in Wellington.