Students hard at work on the "Mangrove Challenge"! (Sierpe River, SW Costa Rica)
Amphibians including species of frogs, toads, and salamanders have inhabited the Earth for over 350 million years (National Wildlife Foundation, 1999). The adaptations that permitted amphibians to leave their aquatic habitats for terrestrial environments allowed these creatures to dominate for nearly seventy-five million years (Renaud, 2006). However, recent declines of global amphibian populations have left scientists mystified. In fact, approximately one-third of the world’s amphibians are threatened by inexplicable population crashes leaving some populations on the brink of extinction. Historically, amphibians have been closely monitored and observed as indicators of environmental stressors because their fragile life cycle exposes them to both aquatic and terrestrial stresses (Rohr, 2006). Researchers from around the globe gathered in 1989 for the First World Congress of Herpetology to discuss specific cases of dwindling populations that had been documented earlier in the decade (Collins, 2003). The following year the United States formed a separate workshop in the National Research Council solely dedicated to locating the cause of amphibian declines (Collins, 2003). Throughout the 1990’s and beginning of the new millennium, scientists have been vigorously working on this issue and have not yet given substantial evidence to support one specific hypothesis.
During the media blitz on the global amphibian decline, one species was utilized as the campaign’s “poster child”: the Costa Rican golden toad. Within thirty years of the species discovery, the population went extinct leaving scientists extremely dumbfounded because the species was only reported to live in the protected pristine reserve called the Montverde Cloud Forest. Unfortunately, little is known about the life history or ecological behaviors of the golden toad, so scientists have only hypothesized probable causes for the species demise.
If we never really understood the nature of the golden toad, why should we even care that it went extinct? Will we even miss this rare species? Addressing these questions exposes the complex biodiversity issues facing our current society because amphibians are not the only populations facing rapid extinctions. Species of birds, reptiles, and even mammals are experiencing increasing amounts of environmental stressors that are negatively impacting population sizes. Yes, people should care that the golden toad is now extinct! While living, the golden toad provided scientists with an amazing opportunity to study a unique and rare variety of toad in its natural habitat. Now in extinction, the population could foreshadow similar crashes in other populations and aid in isolating the factors triggering the quick declines. The Earth’s biodiversity of all living plants and animals creates a multifarious network of intricate connections. The golden toad’s extinction may have negatively affected other populations of organisms residing in the Cloud Forest.
Any extinction of a species should be questioned and investigated. Therefore, I will be researching information on the golden toad beginning with a brief description of the toad’s lifecycle. Once the species’ background has been presented, I will provide a timeline of human accounts with the species and scrutinize the plethora of extinction theories presented by the scientific community. Investigating this topic will allow me to gain strong insight into the mysterious events surrounding the toads’ ruin and explain the global implications of such a speedy extinction.
▪Life History of the Golden Toad▪
The golden toad (Bufo periglenes) was perhaps one of the most eye-catching creatures inhabiting the forests of Costa Rica. Displaying obvious sexual dimorphism, the males displayed vibrant orange coloration, while the females were jet black with scarlet spots etched by yellow rings (DeGroot, 2000). Another distinguishable characteristic between the genders was size. The adult females grew to be 42-56mm in length, which was slightly larger than the adult males who grew to be 39-48mm in length (DeGroot, 2000). Determination of a toad’s gender could only be accomplished once the toads were adults because all juvenile toads expressed similar physical appearances.
The golden toad was just one of the numerous amphibian species living in the Montverde Cloud Forest Preserve of Costa Rica. More specifically, the golden toad occupied a small range, approximately 4.2 km2, in the area known as the Cordillera de Tilaran in the cloud forest (Kasnoff, 1996). The wet, montane conditions of the forest were essential to the reproductive practices of the golden toad.
Golden toads were studied extensively during their breeding season because these amphibians lived in seclusion for the majority of the year (Kasnoff, 1996) . The toads were extremely active throughout the rainy season of the cloud forest, April through June. Due to the heavy precipitation of the rainy season, temporary deposits and pools of water accumulated large masses of golden toads eager to reproduce (DeGroot, 2000). Because the males outnumbered the females eight to one, males engaged in fierce competitive behaviors such as provoking males already in amplexus. Furthermore, the large proportions of males led to the creation of “toad balls” where 4-10 male toads would latch on to one another (Kasnoff, 1996). Two-hundred to four-hundred eggs could be produced in a single successful breeding episode. Additionally, the toads had unusually small clutch sizes for an amphibian species, but each egg was exceptionally large at three millimeters in length (DeGroot, 2000). The larval developmental stage of the golden toad required the amphibians to remain in the aquatic breeding pools for the five weeks during metamorphosis.
Beyond reproductive activities, details surrounding the general behaviors of the golden toad are relatively unknown because the toads were only observed during their active reproductive months. However, the size of the toads indicated that they feasted on small invertebrate organisms for nourishment (DeGroot, 2000). Researchers have also speculated that the golden toad relied on visual cues for recognition due to the toads’ bright coloration. In contrast, most amphibians rely on vocal recognition cues to identify other members of the population, but only two different calls were recorder during the breeding sessions (DeGroot, 2000).
Because the golden toad flourished in the same range as another presently living species of amphibian (Bufo holdridgei), physical similarities between the species has led scientists to draw parallels from the Bufo holdridgei to explain the undiscovered ecological behaviors of the golden toad. For instance, Bufo holdridgei are known to burrow underground for months following the wet season, so some researchers have speculated that the golden toads utilized a similar burrowing behavior (DeGroot, 2000). Therefore, the golden toads were not visible in the periods proceeding and following the reproductive season. Unfortunately, the extinction of the golden toad has made gaining insight into the nature of this delicate species nearly impossible.
▪Human Interactions with the Golden Toad▪
In 1967, a scientific paper entitled “An Extraordinary Toad from Costa Rica!” was published about the discovery of a unique species of amphibian, the golden toad (Kasnoff, 1996). From the time of its discovery, beholding a golden toad quickly became the quest of many curious researchers. Beginning in April, the commencement of the wet season, hundreds of golden toads could be spotted frolicking in the breeding pools. However, scientists following the annual breeding festivity noticed an alarming trend in the dwindling numbers of toads. The golden toad population declined dramatically in 1987 (Butler, 2005). Unusual weather in the cloud forest resulted in an extremely brief wet season. Consequently, the toad larva desiccated before their maturation was complete as the tiny water pools prematurely dried up. Only twenty-nine toads survived out of the possible 30,000 toad larva (DeGroot, 2000). In the following breeding season of 1988, not one single toad returned to the pools for reproduction; however, ten golden toads were found individually throughout their range. During the wet season of 1989, one male toad returned to the precious breeding pools anxious to seek a mate and was regrettably unsuccessful (Butler, 2005). A few unconfirmed studies reported sightings of isolated individuals during 1990. Since the early months of 1991, there have been no reports of golden toad sightings (DeGroot, 2000). Although the majority of the scientific community believes the golden toad to be extinct, a few scientists think the golden toads are in hiding until the prime reproduction conditions reoccur.
▪Theories Behind the Extinction▪
Deforestation is a common practice throughout the tropical forests of Central and South America. As trees and other forms of vegetation are removed for human use (usually in the form of agricultural lands), numerous species are displaced, and if a species is a habitat specialist, the species may become extinct (Collins, 2003). Certainly, deforestation has been a growing environmental concern in Costa Rica. Although the golden toad inhabited a protected national reserve, deforestation in the areas surrounding the Montverde Cloud Forest could have posed a threat to the survival of the species (Kasnoff, 1996). The majority of research does not support habitat loss as a significant factor in the extinct; however, some researchers maintain that the species proximity to deforested lands could have caused the death of adults retreating from the breeding grounds.
Climate change as a result of Global Warming is the most likely culprit in the extinction of the golden toad. Like all amphibians, the golden toad’s reproduction is water dependent; therefore, droughts, unusually strong storms, or changes in water temperature result could devastate maturing larva populations (Kirby, 1999). Breeding seasons with too much precipitation allow the larva to float away from the pool where they can be deposited and abandoned on the forest floor. In contrast, breeding seasons with too little precipitation end in larval desiccation once the shallower pools disappear (DeGroot, 2000). Studies of the golden toad revealed that it was a shortage of water that ignited the population crash of 1987 (DeGroot, 2000).
The global mean temperature has increased approximately 0.6 0C over the past one-hundred years with an accelerated rate of increase since the early 1970’s. The warmer ocean temperatures have promoted thermal uplifting in the atmosphere, which has increased the elevation necessary for cloud formation (Flannery, 2005). As the clouds are formed at increased heights, the loss of moisture in the remaining habitat (forest, grassland, or other environment) produces drier conditions, which can trigger the reproductive decline for amphibians residing in the affected habitat (Beebee, 2005). Paralleling the developing trend of increasing global temperature, the cloud forest mists of Costa Rica are declining as the condensation point has been rising in altitude, a trend that has been documented since the mid-1970s (Beebee, 2005). Even though amphibians have thrived for millions of years and survived numerous climatic changes, the rate of current warming may be too rapid for the amphibian populations to adjust and adapt (Pounds et al, 2004). Within the last twenty years, twenty of nearly fifty amphibian species have disappeared from the research preserve in the Montverde Cloud Forest (Kirby, 1999). In short, the warmer climate of the Montverde Cloud Forest most likely provoked the extinction of the golden toad as the warmer temperature disrupted the longevity of the wet breeding grounds necessary for larvae maturation.
Other indirect yet harmful side effects of global warming are decreased cloud cover and reduced levels of precipitation. Both conditions foster an environment of intense ultraviolet radiation exposure. The persistent exposure to high levels of UV-B radiation (280-315nm), the most detrimental form of terrestrial radiation, can catalyze the production of cancers and promote reproductive defects in amphibians (Blaustein et al, 2003). Extensive UV-B radiation can also inhibit the welfare of the immune system making the golden toad more susceptible to disease. Interestingly, the research sites having the highest levels of amphibian decline have also been documented as having the greatest increase in UV-B radiation (Middleton et al, 2001). High intensities of UV-B rays in conjunction with chemical pollutants or pathogens can also have synergistic effects. For example, increased UV-B radiation was experimentally shown to weaken an amphibian’s defenses to the common fungus Saprolegnia, resulting in embryo mortality (Blaustein et al, 2003). UV-B radiation does not seem to be the prime cause in the species’ extinction because the toads preferred to live in shaded areas of seclusion; however, synergistic effects of the radiation in concurrence with regional pathogens could have negatively affected the fragile golden toad population.
Any environmental stressor whether it be climatic changes or UV radiation can weaken an amphibian’s immune system allowing it to become more vulnerable to pathogens. The disease of the greatest concern in the mysterious extinction of the golden toad is a condition known as chytridiomycosis, which is caused by the fungus Batrachochytrium dendrobatidis. This disease has also been linked to the decreasing populations of ninety-three other global amphibian species (Renaud, 2006). In mature adult amphibians, the disease damages the skin by thickening the membranes and proves to be fatal when it progresses to inhibit respiration and osmoregulation (Beebee, 2005). Chytridiomycosis is easily spread amongst an amphibian population because some hosts can harbor the infectious disease for 220 days before death and the fungus can survive in stream beads (Renaud, 2006). When in water, the mature fungus protrudes a discharge filament where as many as twenty spores are released into the water eager to attach to a viable host (Blakeslee, 1997). However, studies have not concluded whether the disease is the sole factor in the golden toad decline or whether it is a secondary effect of another stressor leading the species eventual demise (Beebee, 2005). One research team believes the extinction was primarily sparked by the climate change associated with global warming (Pounds et al, 2006). Yet, the increasing temperatures provided optimum growing conditions for the fungus Batrachochytrium, while simultaneously hindering the effectiveness of the amphibian’s immune system. In this scenario, the fungal disease was the lethal secondary cause in golden toads’ quick extinction (Pounds et al, 2006).
Amphibians like the golden toad are exceptionally vulnerable to acquiring toxic chemicals from the soil, water, and air comprising their environment (Pounds et al, 2004). These pollutant materials enter the environment through the generous application of various agricultural pesticides, as well as, the discarding of industrial wastes like heavy metals. Unfortunately, amphibians are prone to toxic chemical poisoning because they can absorb the materials directly from the environment or indirectly accumulate the materials in their bodies from consuming contaminated insects (Kasnoff, 1996). In Costa Rica, profitable farmers utilize myriad forms of pesticides, including variants currently banned in the United States, to produce the unblemished produce they export. An extensive study of current pesticides practices in Costa Rica and Nicaragua revealed excessive dependence of organophosphates, carbamates, endosul fan, and paraquat (Wesseling et al, 2005). Additionally, these countries’ registration and documentation processes for pesticide distribution did not in follow the international code outlined by the Food and Agriculture Organization, a branch of the United Nations.
The most commonly used pesticide and ground water contaminant is Atrazine, an endocrine disrupter. Atrazine was first utilized for agricultural purposes in 1958 and is now used frequently in the production of maize, sugar cane, pineapple, and macadamia nuts (Thomas, 2006). This chemical compound persists and accumulates in the environment especially in the various forms of water including clouds, fog, precipitation, and aquatic ecosystems (Thomas, 2006). Atrazine levels in ponds and stream fed by agricultural runoff can have concentrations as high as 400 ppb as compared to the EPA recommendation of 3 ppb for drinking water (Rohr et al, 2006). Careful studies conducted on populations of leopard frogs (Rana pipens) and African clawed frogs (Xenopus laevis) have demonstrated that atrazine exposure levels of 0.1-25 ppb are capable of inducing hermaphroditism and sexual organ dysfunction for male toads (Rohr et al, 2006). By disrupting normal endocrine production the males have testicles containing eggs not sperm necessary for reproduction (Thomas, 2006). Furthermore, this herbicide stunts the growth of maturing toads forcing the affected animals to consume a different diet with their abnormally small jaws (Thomas, 2006). Experimentation has also linked small exposures of atrazine to extremely elevated levels of sodium ions in amphibian skin, which may disrupt general ion transport across their skins’ membranes (Cassano, 2006). Most frequently, atrazine enters the environment through highly concentrated pulses of agricultural runoff following episodes of precipitation, which exposes the amphibian populations to toxic levels of the herbicide in an enormously short span of time (Storrs, 2004). However, the herbicide lingers and accrues in the environment leaving trace amounts of the chemical during prolonged periods of lower exposure.
Another troubling issue surrounding atrazine may be the herbicide’s carryover effects on amphibians once the chemical is removed from the environment. The toxic effects of atrazine have led to increased levels of larval desiccation in salamander populations eight months after the atrazine level peaked (Rohr et al, 2006). Furthermore, salamanders exposed to sustained low-levels of atrazine were placed under severe density-dependent stress causing population decline similar to the populations exposed to a single peak episode of atrazine. Although the golden toad inhabited the pristine reserves of the Montverde Cloud Forest, pesticides including atrazine may have leached into the species breeding waters from runoff or aerially applied pesticides.
▪Other Possible Factors
Several other factors may have played a minor role in the extinction of the golden toad including overexploitation, introduced species, and acid rain. To begin, humans have overexploited amphibians for fashion, medical investigation, exotic pet trade, and most commonly food. Tens of millions of amphibians are harvested each year from the lush forests of Central and South American for food (Beebee, 2005). Yet, the golden toad was a relatively hard species to locate, so over harvesting the species for human use was probably not a significant factor in the extinction.
Introduced species can adversely affect an amphibian population by adding additional competition and even predation to the environment, causing the amphibian population to alter its habitat or general behaviors (Collins, 2003). Researchers have documented many cases where an introduced aquatic species has directly correlated to the decline of a native amphibian population. For example, the addition of trout to the California Mountain streams resulted in a reduced population of yellow legged frogs because the trout acclimated to preying on the yellow legged frog larvae as their main food source (Beebee, 2005). Since golden toad eggs and larva are extremely sensitive and vulnerable to any environmental changes, an invasive species in the breeding ponds could have reduced the number of surviving adults. Yet, researchers scrutinizing the golden toad’s limited range have not identified a possible invasive species that could have facilitated the extinction of the fragile amphibian population.
Finally, the addition of acid precipitation to the pristine forest environment may have influenced the decline of the golden toad. Acid rain from industrial pollutant can be blown hundreds of miles from the source of the pollutants depositing acidic waters to the soil, groundwater, and aquatic habitats of an area. Amphibian populations are exceedingly sensitive to changes in pH because amphibian eggs cannot mature in waters with too high of a pH (Kasnoff, 1996). However, there has not been substantial research or evidence to pinpoint acid rain as a causal factor in the demise of the golden toad.
As can be seen by the numerous theories offered by scientists, the case of the golden toad is exceptionally complex. Personally, I attribute their extinction to climate change because the warmer climate conditions weakened the immune systems of the toads leaving them vulnerable to common pathogens and synthetic chemicals. Hopefully, more research will provide the stronger evidence needed to determine the causal factor(s) in the golden toad population crash and extinction.
▪Implications of the Lost Golden Toad▪
Unfortunately, the extinction of the golden toad is not an unfamiliar occurrence. Approximately twenty other species of rare animals are missing, if not extinct, from the same region in the Montverde Cloud Forest (Blakeslee, 1997). The climate changes in the Cloud Forest have forced two species of birds to breed outside the pristine enclosure (Kirby, 1999). The highly publicized case of the golden toad ignited the search for the factors involved in the global amphibian decline. If the golden toad could disappear completely within four years, the same environmental stressors could cause other populations to decline just as rapidly!
Besides fueling efforts to examine extinctions, amphibians like the golden toad have prompted research on the effects of environmental stressors on other animals including humans. The amphibian’s skin and life cycle make it an excellent bioindicator of potentially hazardous environmental changes. For example, the data gathered on the effects of pesticides like Atrazine on amphibian physiology has given scientists the insight they need to establish theories about the effects of pesticides on humans. The reproductive dysfunctions of amphibians exposed to atrazine have similar links to the frequent occurrence of infertility amongst human pesticide workers (Thomas, 2006). Male farm workers who applied atrazine as their prime pesticide were documented as having sperm counts that were half the normal levels oh other human males (Thomas, 2006). Various other pesticides are being tested for possible human side-effects after these pesticides were shown to cause major deformities in amphibian populations. Beyond pesticides, studies have also been conducted to see the effects of invasive species, habitat destruction, pollution, and climate change on a wide spectrum of species, not limited to amphibians. Therefore, the golden toad extinction has led scientists to question whether extinctions due to environmental stressors are in the future for other organisms.
Contemplating the extinction of the golden toad has led me address the issues of conservation and preservation. As the populations of numerous species are declining, I must reiterate the need for a passionate concern about sustaining the Earth’s biodiversity. David Stokes, a prominent conservation biologist, believes humans can make the difference in preventing extinction; therefore, he adamantly urges people to reconnect with nature in order to see the creatures they are negatively impacting (Stokes, 2006). Public enlightenment of environmental issues through the means of education and hands-on experience is essential in gaining enough influence to make essential changes in our current consumer-driven lifestyle. The National Wildlife Foundation and Department of the Interior recently designed one such program dedicated to preventing further amphibian declines called “Frog Friends Forever”, which included classroom projects, children’s camps, and family-orientated educational presentations (National Wildlife Foundation, 1999). The program’s principle mission is to create active citizens in environmental issues and to promote local monitoring of amphibian populations. Although the “Frog Friends Forever” program has experienced great success, more action must be taken because the program has only launched in a few regions of the United States. A global effort is needed to conserve the present levels of biodiversity, and a global concern is needed to prevent another grave extinction like the incredibly quick decline of the golden toad!
Beebee, T.J., Griffiths, R.A. (May 31, 2005). The amphibian decline crisis: A Watershed for conservation biology?. Biological Conservation. 125: 271-285.
Blakeslee, S. (1997). New culprit in deaths of frogs. New York Times. 146(50917): C1.
Blaustein, A.R. et al. (2003). Ultraviolet radiation, toxic chemicals and amphibian population declines. Diversity and Distributions. 9: 123-140.
Butler, R. (2005). World Rainforests: The golden toad. Retrieved March 9, 2006 from http://www.mongabay.com/01golden_frog.htm.
Cassano, G. et al. (February 2006). Atrazine increase the sodium absorption in frog (Rana esculenta) skin. Environmental Toxicology & Chemistry. 25(2): 27 28.
Collins, J.P., & Storfer, A. (2003). Global amphibian declines: sorting the hypotheses. Diversity and Distributions. 9: 89-98.
DeGroot, J. (2000). Bufo periglenes: Animal Diversity Web. Accessed March 14, 2006, From http://animaldiversity.ummz.umich.edu/site/accounts/information/B fo_periglenes.html.
Flannery, T. (2005). The weather makers: How man is changing the climate and what it means for life on Earth. New York: Atlantic Monthly Press.
Kasnoff, C. (July 2, 1996). Monteverde Golden Toad. Retrieved March 10, 2006, from http://www.bagheera.com/inthewild/van_anim_gldntoad.htm.
Kirby, A. (1999). BBC News: Climate claims the golden toad. Retrieved March 10, 2006, from http://news.bbc.co.uk/1/hi/sci/tech/328776.stm.
Middleton, E.M., et al. (August 2001). Evaluating ultraviolet radiation exposure with satellite data at sites of amphibian declines in Central and South America. Conservation Biology. 15(4): 915-929.
National Wildlife Foundation. (January/February 1999). Frog friends seek to stem worldwide amphibian decline. International Wildlife. 29(2): 6-7.
Pounds, J. & Puschendorf, R. (2004). Ecology: Clouded Futures. Nature. 427(6970): 107-109.
Pounds, J. et al (2006). Widespread amphibian extinctions from epidemic disease driven by global warming. Nature. 439(7073): 161-167
Renaud, C. (May 2006). A new worry for amphibians. Environment. 48(4): 4-6.
Rohr, J. R., Sager, T., Sesterhenn, T.M., & Palmer, B. D. (January 2006) Exposure, postexposure, and density-mediated effects of atrazine on amphibians: Breaking down net effects into their parts. Environmental Health Perspectives. 114(11): 46-50.
Stokes, D.L. (January 2006). Conservators of experience. Bioscience. 56(1): 5-7.
Storns, S.I. & Kiesecker, J.M. (July 2004) Survivorship patterns of larval amphibians exposed to low concentrations of atrazine. Environmental Health Perspectives. 112(10): 1054-1057.
Thomas, P. (February 2006). Sex, lies, and herbicides. The Ecologist. 14-21.
Wesseling, C., Corriols, M., & Bravo, V. (March 22, 2005). Acute pesticide poisoning and pesticide registration in Central America. Toxicology and Applied Pharmacology. 207: 607-705.
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