Invasive species have a very negative effect on the native species in an area. They can cause a variety of problems from competition for food resources to introducing a new disease that the native species is not immune to. Larval amphibians do not have the defenses to handle introduced predators such as fish in their environment. Therefore introducing fish into these once safe and fish-free water sources will have a huge effect on amphibian population (Wells 825). Reversal of theses introductions have proven to be impossible very often as well.
In 1935 there was an introduction of a species of toad, the cane toad, into Australia. The range of this toad has expanded to reach much of the tropical habitat in Australia. They are very large reaching a size greater than 150 mm and also they can have very high species density. This make it easy for them to take over other amphibian’s native land that may not be able to defend it as well. Due to their size they also suck up most of the available nutrients. The results of this loss of nutrients can reduce invertebrate abundance as well as species richness.
These toads act as major competitors for the other native amphibians (Hero and Kriger 1992). Temperate forests are present along the west coast of both North and South America. California is a major spot for amphibian declines as it has a broad range on environments including temperate rainforest. A native species, the California red-legged frogs, has been greatly affected by the introduction of American bullfrogs. Experiments show that American bullfrogs prey on the young of this species. The California redlegged frog is extinct to 70% of its original area (Doubledee et al. 2003).
Invasive species have a large effect on both temperate and tropical forests and the ability of their native species to thrive. Pollution Chemical pollution is a large threat to aquatic environments. This is especially true to populations living in freshwater ecosystems. Over the years, humans have released a large variety of chemical substances into the water from many different processes. Some of the chemical pollutants include pesticides, heavy metals, sewage and fertilizers. In response to pollution, amphibians will either die out or simply disappear as they move to avoid polluted areas.
Pollution is a sign of human degradation on the ecosystem. Nitrogen pollution is an extremely common form of pollution because it is usually a result of runoff from fertilizers used for agriculture. In contrast, consequences of pollution on amphibian populations is hard to quantify (Wells 839). Pesticides are globally used and have both lethal and not-so-lethal effects on amphibians. Agriculture is a huge economical gain to main countries in both the West and the East. The impact of pollution is just as great in the temperate regions as it is in tropical regions.
Governments usually approve pesticides without there being any testing done to its effect on amphibian populations. Even if a pesticide is not directly being used on an area it can be transported by wind or water dispersal (Hero and Kriger 1992). For an example from Rouse et al. (1999), they studied the effect of nitrate-included fertilizer. Home to some temperate rainforests, North America was examined in in order to evaluate the potential for nitrate to affect the survival of amphibians. This was done by choosing areas that concentrations of nitrate in the water were above the threshold amphibians found here have for toxicity.
In 1991 the amount of nitrogen applied to land used globally was 72 million tons on average. The United States increased from 2. 5 million tons to almost 11. 9 million tons per year in 1985. Results shown in amphibians such as the Western chorus frog shows effects in reduced feeding and mobility. The amphibians also showed that ned developmental deformities such as bent tails and body swelling. There were also deformities in their digestive systems. The severity of the effects of the nitrate was positively correlated with areas of increased nitrate levels.
Hechnar suggested that the reason for reduced feeding was a disturbance in the symbiosis between the tadpole and gut bacteria caused by the nitrate. He also suggested that the reduced activity was due to the development of methemoglobinemia. This was also caused by the nitrate. Methemoglobinemia is the same thing as blue baby syndrome in humans. The hemoglobin is made to be unable to bind oxygen (Rouse et al. 1999). Disease The last factor that will be disused is the impact of diseases on amphibian populations. This is a concern that has been increasing rapidly.
Any of the earlier mentioned causes, especially invasive species can relate right to introduction of diseases that native amphibians cannot fight off. Although amphibians have a naturally high tolerance to infectious diseases, environmental degradation has decreased that ability. Little is known about the diseases of wild amphibians but the knowledge is beginning to increase. Just one epidemic can cause a mass mortality of many amphibians (Wells 831). The disease that is most known about in amphibians is Chytridiomycosis.
Chytridiomycosis is a fungal disease that was first found in amphibians collected from sites of mass deaths in Australia and Panama (Daszak et al. 1999). This disease attacks keratinized tissue, especially the skin of adult amphibians. This is very detrimental to the amphibian because its skin is the most important part of its physiology. It also attacks the mouths of tadpoles since they are keratinized as well (Wells 831). The only evidence of this disease is found when looking at histological sections of the epidermis.
This fungus is typically found in aquatic habitats and moist soil, prime amphibian habitat (Daszak et al. 1999). Clinical symptoms of this diseases include abnormal posture, laziness and loss of righting reflex. Lesions and ulcers or hemorrhages on the skin are also noticeable symptoms of amphibian infected with this disease. Surveys of museum specimens showed no signs of this disease which means that this disease has recently emerged on two continents. It seems that the diseases are endemic to Central America and Australia but not enough is known yet about the spread since it is so recent.
This could lead to accidental importation into other countries through international trade of infected amphibians. In both countries, population declines have been extreme and only occurring over a few months (Daszak et al. 1999). dl ampnibian populations are at a higher risk for disease in natural habitats. There is not much exploration in the relationship between habitat loss and disease when it comes to studying declines. In a study by Becker and Zamudio (2011), they showed the associations between habitat loss and factors of disease.
These factors include occurrence, prevalence, and infection intensity. The disease that was chosen was from a chytrid fungus, Batrachochytrium dendrobatidis (Bd). They studied the effects on amphibian populations in tropical regions. Habitat loss lowers species diversity by reducing natural habitats which increases population isolation and edge effects. There is also a shift in the ecosystem due to disturbances which can alter susceptibility to diseases. Bd prevalence varies with factors such as latitude, precipitation and elevation. Specifically, areas in Costa Rica and Australia were studied.
Increased habitat loss predicted lower Bd occurrence in amphibian populations in Costa Rica. However, including all effects together in all possible models, habitat loss remained a strong predictor of pathogen occurrence. In their best model they found a negative effect of habitat loss and a positive effect of species richness when it came to Bd occurrence. Increased habitat loss predicted lower Bd prevalence in Australia as well. It was also showed that infection intensity was greatest during the month of the year with the least amount of precipitation (Becker and Zamudio 2011).
