杏吧原创

(Image: Clay Bolt and Twan Leeanders)

In March 2013, Twan Leenders and a student were hiking in the Cocobolo reserve in central Panama when they spotted a precious jewel: a delicate string of white pearls floating at the bottom of a rocky puddle at a stream鈥檚 edge. 鈥淭hat was absolutely tremendous,鈥� says Leenders. 鈥淓verything fit for Atelopus, there was nothing else it could be.鈥�

Leenders, a herpetologist at the Roger Tory Peterson Institute of Natural History in Jamestown, New York, has spent his career studying tropical frogs and toads. That had mostly meant documenting their astonishing decline. Climate change and human encroachment on the amphibians鈥� favoured habitats were a major part of the problem 鈥� as was the advance of the deadly chytrid fungus.

Since the chytrid strain Batrachochytrium dendrobatidis was identified in 1999, it has been seen in amphibian populations across Europe, Africa, Australia and Central and South America. The fungus accumulates in the outer skin layers of amphibians, overstimulating production of the protein keratin and hardening the skin. That prevents the uptake of water and essential minerals 鈥� and is normally lethal.

The tiny, colourful toads of the Atelopus genus 鈥� known colloquially as harlequin frogs 鈥� suffered badly. 鈥淗arlequin frogs were particularly hard hit by the fungus, and dozens of species disappeared rapidly in the 1980s and 90s,鈥� says Robin Moore of Global Wildlife Conservation and the Amphibian Survival Alliance, both based in Austin, Texas.

Hence the reaction of Leenders and his crew to that pearl-string of spawn. It came from a species, Atelopus limosus, thought to have been wiped out in Panama. Here was a population that hadn鈥檛 just survived, but was reproducing in the wild. A few days later, the team spotted juveniles further upstream. In the following years, more and more sightings were made around Cocobolo.

鈥淭his species hadn鈥檛 just survived 鈥� it was reproducing鈥�

A. limosus is one of a small number of 鈥淟azarus鈥� amphibian species. In Costa Rica, Moore and Leenders are watching the recovery of Atelopus varius, a species thought extinct until one was found sitting on a rock a few years ago.

We still can鈥檛 fully explain the comebacks. One theory is that long-term exposure to non-lethal levels of the fungus may confer individuals with resistance to the fungus. The Cocobolo reserve may also have been unsuitable for chytrid when it first took hold in Central America 10 to 20 years ago. At the time, it was pasture 鈥� too hot and dry for the fungus, which thrives on the cool surfaces of riverside rocks, just like the toads. A. varius, meanwhile, lives in a valley surrounded by a belt of agricultural land and plantations, which perhaps formed a protective barrier against the fungus.

The fact that the threatened amphibians have prospered in habitats that are far from pristine may point to new ways to protect them 鈥� for example, by managing farmed landscapes to create new habitats. 鈥淚 would like to work with local farmers to help them restore some of the stream corridors,鈥� says Leenders.

The amphibians aren鈥檛 out of danger yet. 鈥淎 few frogs have reappeared after massive declines and seem to be hanging on in relictual populations,鈥� says Ariadne Angulo, who co-chairs the International Union for Conservation of Nature鈥檚 Amphibian Specialist Group. 鈥淭hey are not the norm, but there are some remarkable cases.鈥� As if to underline the point, Leenders鈥檚 team is now getting reports of amphibian declines in areas just outside the Cocobolo reserve.

But Lazarus species give room for hope. 鈥淭he fact that some of these species are reappearing years or even decades after they were last seen is enormously encouraging,鈥� says Moore 鈥� and an opportunity to understand what sets these populations apart. 鈥淎fter decades of witnessing rampant declines, these glimmers of hope are much-needed morale boosters.鈥�

Out on a limb

Atelopus limosus

Panama

(Image: Clay Bolt and Twan Leeanders)

Discovery of spawn from this toad species known as 鈥渉arlequin frogs鈥� in the streams of the Cocobolo reserve indicated the revival of a species thought almost gone. The toad鈥檚 colourful skin harbours bacteria that produce a neurotoxin also found in blue-ringed octopuses and the notorious 鈥渇ugu鈥� pufferfish. Until the advent of chytrid, this was sufficient protection against most enemies.

Atelopus varius

Costa Rica

(Image: Clay Bolt and Twan Leeanders)

Before the arrival of the chytrid fungus, more than 100 populations of this harlequin species were once known in the mountains of Costa Rica and adjoining Panama. One population is now known to have survived and thrived in a single stream valley on an isolated mountain in Costa Rica鈥檚 Pacific coastal plain. What conferred it with resistance to chytrid is a mystery.

Dream stream

(Image: Clay Bolt and Twan Leeanders)

Cocobolo is a small, privately owned nature reserve in central Panama established in 2006. Photographer Clay Bolt first visited in March this year and fell in love. 鈥淓ach night we would go out into the field and discover scores of incredibly beautiful, bizarre, wonderful creatures hanging onto every limb and rock,鈥� he says.

Fungal menace

(Image: Clay Bolt and Twan Leeanders)

Many tropical amphibians had hardly been studied before the chytrid fungus kicked in, and population declines were initially put down to natural cycles. The Lazarus toads give a much-needed chance to see what factors influence survival in the wild. 鈥淭his is where Cocobolo becomes a pivotal point in the research and conservation of these most at-risk species,鈥� says Leenders.

Desirable residence

(Image: Clay Bolt and Twan Leeanders)

Leenders and his students monitor temperature, humidity, water chemistry, sun exposure and other environmental parameters in two 1-kilometre-long sections of stream in Cocobolo, one that houses Atelopus limosus and one that doesn鈥檛. By superimposing that data on a biological and physical habitat map they hope to pinpoint the qualities of an ideal toad home.

Tiny survivor

(Image: Clay Bolt and Twan Leeanders)

Harlequin species such as this Atelopus varius, pictured in Costa Rica, have unique dorsal patterns that allow individuals to be 鈥渇ingerprinted鈥� and tracked throughout their lives. Skin swabs are used to test for chytrid fungus, allowing the researchers to assess how factors such as age and habitat use affect susceptibility to the fungus 鈥� and so how best to protect these species.

This article appeared in print under the headline 鈥淟azarus鈥�