Washington, Nov 13: Biologists have determined that the evidence of global warming causing the worldwide declines of amphibians may not be as conclusive as previously thought.

Studies suggest that more than 32 percent of amphibian species are threatened and more than 43 percent face a steep decline in numbers.

Much of the massive declines associated with amphibians appear to be centered in places such as Central America and Australia,

According to Peter Hudson, the co-author of the research study, the massive declines appear to be linked to a chytrid fungus - Batrachochytrium dendrobatidis (Bd).

There are currently two theories on the extinctions.

The first, known as the ‘chytrid-thermal-optimum hypothesis’, suggests that the declines were triggered by global warming which pushed daytime and nighttime temperatures to converge to levels that are optimal for the growth of the chytrid fungus.

But according to a second theory, known as the ‘spatiotemporal-spread hypothesis’, amphibian declines were simply driven by the introduction and subsequent spread of the fungus from certain locations.

“Our models suggest that both these theories are slightly wrong,” said Hudson. “Neither of them fit available data,” he added.

While the researchers do not completely discount the role of global warming in amphibian declines, they believe that evidence linking it with the declines is weak.

Jason Rohr, lead author and assistant professor of biology at University of South Florida, and his University of South Florida colleagues Thomas R. Raffel and John M. Romansic, along with Hudson and Hamish McCallum, professor of wildlife research, University of Tasmania, tested the competing theories by re-analyzing the same data used in conceiving the two ideas.

The scientists checked the first hypothesis to see whether climatic factors such as the percentage of cloud cover, narrowing difference between the lowest average daily temperature and the highest average daily temperature, and the predicted growth rate of the fungus under certain temperatures, could accurately predict extinctions.

Their statistical analysis revealed no such narrowing of temperature spans in the 1980s, when extinctions were increasing.

When the difference in average daily temperatures did narrow in the 1990s, amphibian extinctions were decreasing.

Further, while the chytrid-thermal-optimum hypothesis showed high elevations as having the highest proportion of amphibian declines and the second highest proportion of amphibian extinctions, statistical analysis showed that growth rates for the fungus and cloud cover to be lowest at the highest elevation.

“While there is evidence to suggest that the chytrid fungus is killing the frogs, further research is needed before we can conclude that climate change is accelerating the spread,” said Rohr.

“The bottom line is that there doesn’t seem to be one single explanation for the massive amphibian declines. It could be a mix of other factors,” explained Hudson. (ANI)