For starters, amphibians do not regulate their own body temperature, like humans and other mammals can. They are what scientists call thermal conformers. The outside temperature is very important for amphibians because it plays an important role in many of the body processes of amphibians, including how much oxygen they take in, their heart rate, how they move, their digestive system, how fast (or slow) they develop and grow, and how well their immune system functions. This makes them very sensitive to temperature changes in the external environment. It also ties many species of amphibians to a particular area with certain environmental characteristics (niche). These types of species have limited dispersal abilities, which means that it's hard for them to find new homes and to spread to new areas.
The Wide World of Amphibians...
|Cartogram from Wake and Vredenburg. See references.|
The cartogram (1) above shows the distribution of amphibian species around the world. As you can see, many of the countries with the most amphibian species are in the tropics. This mirrors a general pattern of biodiversity for most kinds of plants and animals. A good rule of thumb is the closer to the equator you get, the greater the biodiversity there is. From the cartogram we can see that Indonesia has a lot of amphibian species (between 251-450 species). Now let's look at another cartogram.
|Cartogram from Wake and Vredenburg. See references.|
This second cartogram shows the distribution of amphibian species discovered between 2004 and 2007. As you can see, Indonesia is a hotspot for new discoveries. The interesting thing about this is that the number of described amphibian species has increased by about 50% over the past 20 years; in others words more than 2000 of the now-known 6300 or so species has only recently been discovered! Thus we can assume that Indonesia has many more species waiting to be discovered. But the question is, for how long?
|Diagram from Blaustein, et al. See references.|
The introduction of alien species. Alien species are those that are brought intentionally or unintentionally to a new habitat where they did not originate. In some cases the alien species is able to out-compete the native species that naturally exist in the area. Sometimes the invasive species preys upon the naturally occurring species as well (3). One example affecting amphibians on the continental US is the introduction of lake trout for recreational fishing. The lake trout feed on tadpoles, thus affecting frog populations.
Over Exploitation. This simply means that people use too many of the species in one way or another. Historically demand for frog legs as a delicacy has caused big declines in frogs in many places.
Land Use Change. This is another factor that is easy to understand; it simply refers to the way that people change the landscape. When we clear forests to make farms or to build settlements, it leaves less habitat for many types of animals, including amphibians.
Global Warming and associated changes. Over the past few decades it has become increasingly clear that people are altering the Earth's atmosphere. One change involves the amount of UV radiation that makes it to the Earth's surface. UV is a type of high-energy radiation from the Sun. This is what causes sunburns on your skin. Our atmosphere filters out most of the UV radiation, which is fortunate for us because UV is very dangerous to humans (and other animals). However, human-caused pollution has altered the atmosphere's ability to block our UV rays, and so more UV reaches the Earth's surface than before. Amphibians appear to be especially susceptible to the effects of UV radiation.
A bigger problem, though, is global warming. The changes we have unintentionally made to the atmosphere is causing the planet to warm up gradually. Over the next century or so scientists predict that the average global temperature will increase by 3-7 degrees. While this might not sound like much, it will have tremendous impacts on local environments. One really good example of this can be seen on mountains. If you've ever climbed a mountain (think of Mauna Loa or Haleakala) you've probably noticed that it gets colder as you go up. On these high mountains it gets so cold you have to put on a heavy coat! You may have also noticed that the amount of moisture changes as you go up. From a geographic perspective, what you've noticed is that you pass through several climate zones on the way to the top of the mountain (4). Each of these climate zones has different collections of plants and animals; there are different ecosystems adapted to the specific climate conditions. This variability was first described in the 19th century by uber-geographer Alexander Humboldt, who wrote about the different types of agricultural activity taking place with increasing elevation in the Andes Mountains (look for a future post on this topic; I've included a diagram on the left to give you an idea how it works). Now, think about the effects of global warming. As the whole planet warms up, these different climate zones should move up the mountain, shouldn't they? But there is only so far they can go. If it warms up too much the climate zones will basically slide off the top of the mountain, disappearing forever. If this happens we lose the species associated with those climate zones (5). Now let's think about the example of amphibians. We already learned that they are closely tied to their natural environment. This includes not only the climate but the land on which they live. When the climate zone moves up the mountain away from the land they are used to, it makes it harder for the amphibian species to survive.
|Diagram from Sodhi, et al (see references)|
More Pesticide Use. Pesticides are artificial chemicals farmers use to kill farm pests. They also use fungicides, herbicides, and a host of other chemicals to increase crop productivity. The problem is that once these chemicals get into the natural environment (often via runoff or wind) they sometimes have unexpected consequences. Chemicals may weaken amphibians, making them less successful in reproduction and development, or they may affect other parts of the food webs that amphibians depend on.
Emerging Infectious Diseases. There are numerous instances of amphibian declines being partially attributed to new or introduced diseases. One of the main culprits is a fungal pathogen with the intimidating name of Batrachonchytrium Dendrobatidis, or Bd for short. As habitats get opened up diseases are able to spread to new populations. They are also spread by humans and other invasive species. Many scientists are worried that increased temperatures will help the spread of diseases too.
So as you can see there are a number of possible causes for amphibian declines and extinctions. The reality, though, is that it is probably a combination of these factors at work. Different studies tell different stories. One recent study indicates that small range size is the most important factor, followed by body size of the amphibian. Small ranges make amphibians more susceptible to habitat destruction, but they also increase the risk posed by extreme weather events like droughts and hurricanes, which may possibly be increases in part due to increased global temperatures. Body size is an important variable because the bigger the animal is, the longer it takes it to grow and develop and hence recover from disturbances which kill off part of the population. But there are other studies as well. Part of the problem is the complex interrelationships between factors working at the local level and those operating at global scales. For example, one study argues that El Nino, a global climate event, leads to decreased snow in the Oregon Cascade mountains. Since the snow pack is not as thick, there would be less water when the snow melts in the spring, making streams and lakes shallower. The Western Toad breeds in these water bodies in the early spring, and so because there is less water protecting the eggs they are exposed to higher levels of UV radiation, resulting in increased infection and death due to water mold. Fascinating, isn't it? It's like trying to fit together the pieces of a mystery or solve a puzzle.
Amphibians and Kerinci Seblat National Park
What does all of this have to do with Kerinci Seblat National Park? Well, it's simple really. KSNP is a huge protected area that encompasses many different habitats and ecosystems. A recent inventory of herpetofauna (amphibians and reptiles) was conducted over several years to determine exactly what kind of amphibians are out there in the park. The study encompassed 14 survey sites across a variety of altitudes (50m-2000m) and general habitat types, including hilly cultivated land, highland hill forest, lowland hill forest, marshland, edificarian (near houses and gardens), flat cultivated land, secondary hill forest,
and heavily degraded forest. The study found 70 species of frogs, 4 of which had never been described before! This study was the largest of its kind ever in Kerinci, but there is still a lot of work to be done. The results show that the park, which is most known for its tigers, has other important species of note. These amphibians are important because, like we saw above, they are particularly sensitive to changes in the environment. That means that if you monitor the amphibians, you might be able to get some clues about threats to the environment before they get too severe. The amphibian census also potentially indicates small areas in the park which could be targeted for strict conservation. This would enable park managers to c
The photos of frogs are from Kurniati's inventory of amphibians for KSNP.
(1) Cartograms are neat cartographic data-visualization tools. They exaggerate the size of the country, making it bigger or smaller to illustrate the magnitude (size) of the particular variable in question. Can you find some more cartograms on the internet? What types of things to they indicate?
(2) Scientists have noted a general trend of extinctions and decreasing diversity among many types of animals, but amphibians seem to be experiencing greater than average declines and extinctions.
(3) Can you find some examples of invasive species in Hawai'i? What native species have they affected and how?
(4) Geographers classify climates on the basis of two characteristics: temperature and precipitation.
(5) Many species on mountains are unique, because mountains function as "islands" for plants and animals. What I mean by this is that they are isolated areas where populations of creatures evolve without contact with other populations, and so they rapidly diverge from other species. Think back to my post on the Wallace Line and the example of snails in Hawai'i. Do you see how the isolation of a mountain top could serve as a barrier for a species? The Hawaiian silverswords are a good example of this.
References and Further Reading
Blaustein, Andrew et al. 2011. The Complexity of Amphibian Population Declines: Understanding the Role of Cofactors in Driving Losses. Annals of the New York Academy of Sciences 1223 pp108-119.
Blaustein, Andrew, Susan Walls, Betsy Bancroft, Joshua Lawler, Catherine Searle, and Stephanie Gervasi. 2010. Direct and Indirect Effects of Climate Change on Amphibian Populations. Diversity 2, pp281-313.
Collins, James, and Andrew Storfer. 2003. Global Amphibian Declines: Sorting the Hypotheses. Diversity and Distributions 9, pp89-98.
Kurniati, Hellen. 2008. Biodiversity and Natural History of Amphibians and Reptiles in Kerinci Seblat National Park, Sumatra, Indonesia, 2005, 2006, 2007. Cibinong, Indonesia: Research Center for Biology, LIPI.
Rohr, Jason, and Thomas Raffel. 2010. Linking Global Climate and Temperature Variability to Widespread Amphibian Declines Putatively Caused by Disease. Proceedings of the National Academy of Sciences v107#18.
Sodhi, Navjot, et al. 2008. Measuring the Meltdown: Drivers of Global Amphibian Extinction and Decline. PLOS One v3#2.
Wake, David, and Vance Vredenburg. 2008. Are We in the Midst of the Sixth Mass Extinction? A View from the World of Amphibians. Proceedings of the National Academy of Sciences Vol15 Supplement 1.