Pollution and Climate Change
Lankan Croakers Leaping To Extinction
An ongoing research indicates that Sri Lankan tree frogs are in a critical state. Taruga eques are an endemic, endangered frog species confined only to the central hill regions of the country. Amphibians with cysts (membranous sacs or cavities of abnormal character in the body, containing fluid) have less survival rates compared to normal amphibians.
A tropical country, Sri Lanka, has a high species diversity and along with the Western Ghats, in India, earned a name as a main biodiversity hotspot in the world.
Within the Asian region, Sri Lanka has the highest species density for amphibians, reptiles and mammals. However, recent studies indicate that several species, particularly amphibians, are under threat, especially due to man-made activities and climate change.
All amphibians are directly impacted by the natural, chemical and physical characteristics that influence their habitats and affect their physical development, maturation and amphibian survival.
Any changes in these factors lead to the collapse of their entire ecosystem and endanger their lives. Anthropogenic activities are the main cause of the contamination of pristine amphibian habitats and that makes amphibians prone to parasitic infections by reducing their immunity.
This is the main reason for the 100% abnormality index—all the captured amphibians in a definite habitat had cysts. Therefore, the protection of these pristine ecosystems is essential for the conservation of amphibians.
The aforesaid study, conducted by Praneeth Silva under the supervision of Prof. W. A. D. Mahaulapatha, reveals a number of threats to the tree frogs in the country
Silva a BSc (Special) graduate from the University of Sri Jayewardenepura is currently working as the Department Research Assistant for the Wildlife Management and Conservation, Department of Zoology, University of Sri Jayewardenepura.
Silva’s study concentrates on the parasitic infections of the amphibians found within the central hill region of the country including the Horton Plains National Park. At the end of the sampling period a total of nearly 1,000 of amphibians (including larvae, meta morphs, juveniles and adult life stages) belonging to four families and 11 species were recorded and all the infected cyst bearing amphibians were classified based on the severity of the infections.
According to the data analysis, 84.24% amphibians had cysts and of them, 3.91% were recorded within the park. All of these infections were observed only in the Taruga eques species, and not in others. Taruga eques species is arboreal.
Field studies conducted in various rain forests, have revealed that tree frogs act as predominant intermediate hosts of the helminthes larvae that include trematodes, cestodes and nematodes. Herpetologists have observed that tree frogs infected at larval stages, remain viable as metamorphs and later emerge from water and then continue their existence as infected tree frogs.
Sixty per cent of the infections recorded in the amphibians inside the park were mild (1-3 cysts bearing amphibians) and 40% were moderate (4-6 cysts bearing amphibians). In contrast, 77.55% of the infections recorded in the amphibians outside the park were moderate and only 22.45% of the infections were mild.
The main reason for this situation is that most of the ecosystems have been exposed to anthropogenic related activities such as the use of water bodies as garbage dumping sites and release of chemicals through agricultural processes.
Most of the surficial abnormalities (cysts) were detected in the premature juvenile stages of the amphibians, 58.87% and 78.70% respectively within and outside the park. This may either be that the adult populations of amphibians are less susceptible to abnormalities against different environmental stressors or they have reduced survivorship.
Most cysts have appeared within the musculature of the hind limbs, especially highly muscular upper leg (thigh) region and fewer of them were recorded in the forelimb and abdominal regions of amphibians.
Cysts are caused by both pathogens and parasites. Chytridiomycosis is a fungal infectious disease of amphibians and it has a devastating effect on amphibians and leads to severe population declines. Batrachochytrium dendrobatidis is the causative agent of Chytridiomycosis; that generates an invasive growth in host skin, affecting deeper layers and maturing into thick-walled cysts on the host epidermis.
However, most of the cysts examined in this study were observed as emerging within musculature around limb structures of the amphibians. These may have been caused by parasitic infections since Batrachochytrium dendrobatidis cysts form in the deeper layers of the epidermis. Therefore, these cysts could be due to attack by Trematode metacercariae as they are generally found above the skeletal muscles around the limb structures.
Amphibian abnormalities due to trematode infections depend on the stage of limb development, at which infections occur. Therefore, the timing of the infection is a critical determinant in forming abnormalities. Previous parasitic induction experiments in laboratories have revealed no effect on limb development or survival of tadpoles infected at paddle stage, but encysted parasite is able to remain viable even at adult stages of amphibians. Hence, these findings prove that infected amphibians, found in this study, have been exposed to the Trematode parasites at their paddle stage.
The study has revealed that the critical nature of the problems, faced by Taruga eques, points to the need for mandating urgent measures to carry out extensive research based conservation work.
Philautus is another genus of tree frogs. According to the literature, 19 species of endemic Philautus amphibians have become extinct between 1850 and 1940. Of the surviving Sri Lankan tree frogs, 51 species are in threatened category and 72 are threatened state amphibians. These data indicates the urgent need for comprehensive research-based conservation work to protect the Sri Lankan tree frogs community including Taruga eques.
Several amphibian species are subjected to fluctuations through taxonomic history. Sri Lanka’s amphibians are believed to have numbered more than 250 in the late 19th century. They are mostly confined to the rain forests of the country’s wet zone. However, at present it has been estimated that only around 119 amphibian species exist in Sri Lanka.
Many biotic and abiotic factors affect amphibian survival. As for the biotic factors, predators, parasites and pathogens are the major elements while chemical contaminants resulting from anthropogenic activities have been recognized as a main component for modification of aquatic habitats.
Today, worldwide mass die-offs of amphibians have been due to parasitic infections, especially attack of helminthes. However, the occurrence and the severity of prevalence of parasitic diseases are considered an interaction of all biotic and abiotic factors in a particular ecosystem.