Is Captive Breeding in the Mountain Yellow-legged Frogs Future?

In last week's post, I wrote about the captive breeding of amphibians in general, and this week I'll focus specifically on the mountain yellow-legged frog. With the ongoing decline of this species, the topic of captive breeding seems to come up at every meeting in which population recovery efforts are discussed. Although the debate about what role captive breeding can realistically play in the global amphibian decline crisis is heated and will likely continue for many years, efforts to develop a captive breeding program for mountain yellow-legged frogs have already been underway for several years.

In April 2005, the Conservation and Research for Endangered Species (CRES) group at the San Diego Zoo began a captive breeding effort with southern mountain yellow-legged frogs (Rana muscosa) obtained from the last population hanging on in the San Bernardino Mountains. The goal of the effort was to create an "integrated program of captive propagation, headstarting, and release that will ensure the long-term viability of the species in the wild". These frogs were taken into captivity following a fire that swept across the area, creating the very real threat that these last frogs could be lost during winter floods and debris flows. Ultimately, seven frogs were captured and transferred to the San Diego Zoo. Unfortunately, all seven died due to an unspecified disease.

A second chance to develop a captive frog population came along a year later. In August 2006, a U.S. Forest Service biologist observed R. muscosa tadpoles stranded in drying stream pools in the San Jacinto Mountains. No suitable habitat was present in the vicinity to which the stranded tadpoles could be transferred, so all 82 tadpoles were collected and brought to the San Diego Zoo. According to a short article published in yesterday's Los Angeles Times, 62 of the tadpoles have metamorphosed into froglets. If these animals survive into adulthood, the plan is to keep some at the zoo as part of a captive breeding program, and the remaining animals will be released back into their original habitat.

Successful reintroduction into the wild is only possible if the original cause of population decline has been mitigated. Unfortunately, for mountain yellow-legged frogs in southern California's Transverse Ranges, the cause of decline remains obscure. Introduced trout have likely played a role (see my post from May 16, 2008), but the relative importance of other factors is unknown. With fewer than 150 mountain yellow-legged frogs remaining in the Transverse Ranges, the only way find out what the population-limiting factors are may be to carefully follow the fate of frogs once they are released back into the wild.

Clearly, the path to recovering this species will be a slow and uncertain one.

Back to The Mountain Yellow-legged Frog Site.

The Amphibian Ark: Good Intentions, Uncertain Outcomes

The Amphibian Ark is a conservation effort developed to implement the ex situ (i.e., captive breeding) components of the IUCN Amphibian Conservation Action Plan (ACAP). The mission of the Amphibian Ark is to "ensure long-term survival of species in nature utilizing short-term ex situ management of amphibian taxa for which adequate protection in the wild is not currently possible". Although it is very unfortunate that adequate protection in the wild is in fact no longer possible for an increasing number of amphibian species, this is a reality of our time. The recent global emergence of the amphibian chytrid fungus (Batrachochytrium dendrobatidis) has massively compounded this problem, and was a major impetus for the development of the ACAP.

The fact that many amphibian species are being driven to extinction is irrefutable. What actions will be most effective in preventing these extinctions remains very much an open question. The ACAP suggested a very important role for captive breeding, but is captive breeding a realistic solution? Hundreds of amphibian species are being driven to extinction by a myriad of factors including disease, habitat destruction, introduced species, and overharvest. Do we really have the resources to harbor even a fraction of these declining amphibian species in captivity? The ACAP provides some insights into the costs of captive breeding programs, and these figures are an eye-opener. The estimated 5-year cost to house 100 amphibian species in zoos is $41,000,000! Where is this money going to come from? Will it be siphoned away from already-scarce funds used for conservation efforts currently being implemented in the wild?

The idea of housing hundreds of species in zoos brings with it difficult issues that go well beyond financial considerations. One of the most important is what the fate of those captive populations will be. For species threatened primarily by habitat destruction, efforts at preserving pieces of habitat could be followed by release of animals back into the wild. However, the fact that disease is playing such an important role in causing global amphibian declines greatly complicates the role of captive breeding. For example, the amphibian chytrid fungus is spreading around the world, and once it is present at a site it is apparently there forever. Under this scenario, releasing disease-free animals back into the wild will not be an effective conservation strategy. The released populations will simply become reinfected and succumb. Numerous examples from the field of disease ecology indicate that the only solution to the effects of disease is evolution between host and pathogen. Putting amphibians into zoos and keeping them free of disease will halt the process of host-pathogen evolution, and will ensure that the captive populations will not have a chance of survival in the wild.

Another serious concern related to captive breeding is the unavoidable genetic effects on the captive population. These effects arise from the novel environment in which the animals are held, which will always be markedly different from that in the wild. These changes to the environment induce new selective pressures that can result in rapid changes to the animals themselves. A recent article published in the journal, Science, showed that steelhead trout raised in hatcheries had greatly reduced reproductive capabilities when released back into the wild. The reduction in reproductive capabilities accrued at the astounding rate of 40% per generation, so every generation had approximately half the reproductive capability of the previous generation. Based on these striking results, the authors concluded that "even a few generations of domestication may have negative effects on natural reproduction in the wild" and that "the repeated use of captive-reared parents to supplement wild populations should be carefully reconsidered".

There are no easy answers here, but there sure are enough difficult questions to give me pause when contemplating the role of captive breeding programs for hundreds of the world's amphibian species.

Back to The Mountain Yellow-legged Frog Site.

Hope for Frogs in Southern California?

The southern mountain yellow-legged frog (Rana muscosa) has declined precipitously during the past century (check out www.mylfrog.info for details), and nowhere has this decline been more severe than in the Transverse Ranges in southern California. Mountain yellow-legged frogs are now extinct from 98% of the sites they formerly inhabited and were listed as Endangered in 2002.

For the past decade, Robert Fisher and colleagues (U.S. Geological Survey) have been studying the eight populations of mountain yellow-legged frogs remaining in southern California in a last-ditch effort to prevent their extinction. Although the causes of the frogs' decline remain relatively poorly understood, trout introduced into the streams inhabited by the frog have been suspected as playing a role for many years. As such, the California Department of Fish and Game (CDFG) recently stopped stocking trout in those locations where mountain yellow-legged frogs were present. In addition, the CDFG began actively removing nonnative rainbow trout from one location (Little Rock Creek) to determine the effect of this removal on a remnant mountain yellow-legged frog population.

Surveys conducted in 2007 provide the first indication that these fish removals are beginning to have their desired effect (summary report available here). In 2000, a small population of mountain yellow-legged frogs was present above a natural barrier at the headwaters of Little Rock Creek, but were absent below the barrier (where trout were present). In 2001, the U.S. Forest Service built a fish barrier 1.5 km downstream of the natural barrier, and since 2002 the CDFG has been removing trout from the reach between the two barriers. Trout removals are conducted once per year via electrofishing.

In 2006, USGS scientists found a juvenile mountain yellow-legged frog in the trout-removal reach. In 2007, as many as seven mountain yellow-legged frogs were detected in the same area. The appearance of frogs coincided with the apparent elimination of trout from the upper portion of the trout-removal section.

Given these hopeful (although preliminary) results, the CDFG needs to immediately step up their fish removal efforts in Little Rock Creek. In recent years, fish removals have been conducted by CDFG volunteers and removal efforts have been limited to one two-day session per year. The electrofishing effort should be increased to include at least a few weeks of fish removals per year, and electrofishing could potentially be supplemented by efforts using gill nets placed into stream pools.

The CDFG deserves a lot of credit for the current Little Rock Creek frog recovery project, but has to redouble their efforts to carry the project to a successful conclusion.

Back to The Mountain Yellow-legged Frog Site.

Garter Snakes - The Anaconda of the Frog World

In last week's post, I wrote about the relative lack of food for mountain yellow-legged frogs at high elevation sites, and some of the tactics they use to fill their bellies despite this scarcity. Despite this disadvantage of high elevation sites compared to lower elevation sites where food is more plentiful, the high elevation habitats also have a distinct advantage - a virtual lack of predators, especially garter snakes.

Three species of garter snakes inhabit the range of the mountain yellow-legged frog, each in a very specific habitat niche and all with a strong affinity for water and for amphibian prey. The Valley garter snake (Thamnophis sirtalis fitchi) is found at elevations up to about 5,000'. The Sierra garter snake (Thamnophis couchi) is found at elevations of 5,000' to 8,000', and the mountain garter snake (Thamnophis elegans elegans - photo above) is found at elevations of 8,000' to 11,000'. These snakes congregate at water bodies containing amphibians, and mountain yellow-legged frogs are a major component of their diet.

Habitats above 11,000' are generally free of snakes, and it is in these highest elevation lakes that the densities of mountain yellow-legged frogs reach their peak. Densities of three frogs per foot of lake shoreline are not uncommon, and at such sites tadpoles by the thousands often aggregate in shallow, warm waters. These densities are a remarkable sight to behold when all other life forms are so scarce, and are in large part a consequence of a lack of predators. Not only are garter snakes absent, but so too are invertebrate predators such as large beetles that prey on tadpoles. Bears and coyotes might come by on occasion and eat a few frogs but their visits are relatively rare. And so, frogs have these highest elevation alpine lakes largely to themselves.

Mountain yellow-legged frogs have evolved some interesting behaviors to reduce the risk of predation. For example, tadpoles reduce their swimming activity when confronted by the smell of a garter snake. In other amphibian species, the presence of predator scents causes tadpoles to develop a more streamlined body shape and a more powerful tail. It remains to be seen whether mountain yellow-legged frogs show similar developmental responses to predators. Given that mountain yellow-legged frogs don't move much during their lives, I've often wondered whether frogs from low-elevation habitats have evolved anti-predator behaviors that are absent in frogs from the highest elevations.

Given that mountain yellow-legged frogs have coexisted with predators for millenia, you can bet that they have a host of tricks that we have not yet discovered to reduce their chances of become someone else's meal.

Back to The Mountain Yellow-legged Frog Site.

Frogs at 12,000 feet?

In the back of our minds, I think most of us have a picture of what frog habitat looks like, perhaps a warm pond filled with lily pads and logs. Given this preconception, it comes as quite a surprise to see mountain yellow-legged frogs basking by the thousands at lakes in the Sierra Nevada that lie well above timberline, some at elevations as high as 12,000' (3,660 m). How do they survive in these cold, stark conditions?

The fact that frogs are ectothermic, as described in my 4/25/08 post, plays an important role in allowing mountain yellow-legged frogs to flourish at high elevations. Given the cold water and air temperatures even in mid-summer, food requirements are relatively modest, allowing frogs to add the necessary fat reserves to get them through the long winter despite low densities of prey.

Despite the general paucity of food in this harsh environment, food is actually plentiful during short periods and mountain yellow-legged frogs take full advantage of this ephemeral abundance. In early summer (a few weeks after ice-out), mayflies and chironomids ("midges") transform from aquatic larvae into terrestrial winged adults and provide a brief dietary bounty for the frogs. The mayfly, Ameletus edmundsi, thrives in cold, high elevation lakes, and its emergence style makes it particularly vulnerable to frogs. Unlike many mayfly species that emerge well offshore, A. edmundsi crawls up onto a shoreline rock and emerges there. In lakes harboring mountain yellow-legged frog populations, I've watched frogs eyeing A. edmundsi larvae from the second they crawl out of the water and then engulfing the winged adult just as it completes metamorphosis. Midges provide a similar feeding opportunity for frogs. On windy days, chironomid adults aggregate in mating swarms on the lee side of boulders. Within minutes of these aggregations forming, mountain yellow-legged frogs will arrive and spend hours picking off chironomid adults when they land on the rock to rest.

During late summer, most aquatic insect emergences are over and mountain yellow-legged frogs depend more heavily on terrestrial prey. Anything crawling along the lake shore that fits in their mouths is fair game. I've watched grasshoppers get blown into the water, swim back to shore, and be eaten by a frog as soon as they crawl up onto a shoreline rock. Although mountain yellow-legged frogs generally spend their time right at the lake shore, some take advantage of another food source away from water: ants. Frogs will position themselves along ant trails and pick off the hapless ants as they pass by. The metamorphosis of Pacific treefrogs (Pseudacris regilla) from tadpole to froglet in late summer provides another food source. The froglets don't have the well-developed hopping skills of adults and make easy prey for the much larger mountain yellow-legged frog.

Despite the various tricks that mountain yellow-legged frogs employ to fill their bellies, food is scarce in this high elevation environment, especially when there are thousands of frogs at a lake all competing for the same limited food resource. And yet, these high elevation lakes offer the frogs an important advantage over lakes at low elevations: a lack of snake predators. More about that next week....

Back to The Mountain Yellow-legged Frog Site.