As a follow up on the post on an article by Beebee & Griffiths, who discussed amphibian declines and agents of decline, I would like to focus on climate change and the contribution it is making to the status of amphibian populations. For that I have chosen the article from Blaustein et al. from 2010 titled: “Direct and Indirect Effects of Climate Change on Amphibian Populations”.
Climate change as tragedy of the commons
The reason I am highlighting climate change more in this blog than for instance the chytrid fungus, which is seen now probably as a bigger killer of amphibians than climate change, doesn’t have to do with the lack of attention for climate change and the need therefore for me to draw attention to it. There are many smart and passionate people who are already dedicating themselves to the cause of battling climate change and our contribution to it. However, I don’t believe that it is clear yet to the majority of people which devastating effects climate change can have on our planet and the flora and fauna on it, especially because it adds up so vigorously to other environmental pressures. For sure we all know the image of the polar bear wandering helplessly on the continuously breaking ice floes of the Arctic Sea, but the more subtle and less visible effects of climate change throughout the world are less known and maybe harder to popularize.
So what makes climate change such an ‘interesting’ agent of decline? It shows the interconnectedness of our planet: whatever we all individually do, the results of it, our emissions, finally end up in the atmosphere where they travel indiscriminately between borders. So fighting against the consequences of climate change is in the end a global battle where everybody has to put in the effort. In this way it also is a symbol to me for the future and how we will have to deal with the ecological crisis: whether we are talking about climate change, or about conservation of biodiversity or the conservation of amphibians, these are all endeavors that require a global effort. In that way it might require a different world view than ours.
As for climate change itself already a lot has been said about that and I don’t want to regurgitate all that information. However, I did find the projections for the global rise of temperature by 2100 impressive and daunting, especially the upper limit of 6.4 °C! considering this is an average and so for particular times of the year this figure might be up even more. Such a dramatic rise in a relatively short period of time will have repercussions, no doubt about that. To give a proper comparison: as the Earth moved out of ice ages over the past million years, the global temperature rose a total of 4 to 7 degrees Celsius over about 5,000 years. We might now be looking at such a change in 100 years time.
Weather and amphibians
By first looking into responses of amphibians to extraordinary weather events that have occurred in the last couple of decades, we can begin to understand the importance of climate on their survival. The Golden Toad is one example , but Blaustein et al. mention several more. There are the mountainous frogs from the Atlantic part of Brazil described by Weygoldt in 1989 that disappeared because of extremely dry winters. In the same country an unusual frost probably was the cause of extinction of several amphibian species in the 1970s. And as a good example of the intricateness of climate and the well-being of animal species Pounds et al. (1999) reported on changes in the fauna of Monteverde cloud forest in Costa Rica, including the decline of amphibians, in response to the El Nino event. Due to El Nino changes in water availability have occurred and that has led to a decrease in the amount of mist precipitation received in the forest due to increased altitude of the cloud bank. As we will later see, humidity has a clear effect on amphibian survival.
Direct effects of climate change
Blaustein et al. mention five different direct effects of climate change on amphibians: range shifts, effects on survival, on reproduction, on development and on amphibian behavior.
Range shifts: due to climate change species may have to shift their home range; because the distribution of amphibians is determined partly by environmental factors like temperature and humidity, a shift in those factors will force some species to migrate over a certain distance to stay within this range of temperature and humidity that they need to survive. This doesn’t only mean that species might have to migrate to another latitude, but also mountainous species might have to go up the mountain to survive. In the latter case the possibilities of migration of course end once they reach the top of the mountain (article on species in Andes). To predict the consequences of these range shifts bioclimatic models are used: these models try to assess the possibilities for future potential ranges for particular species by using current distribution of this species and current climate and then translate this to future climate projections. Lawler et al. (ref) developed such a bioclimatic model for amphibians in the Americas. This model predicts that especially species in Central America are susceptible to climate change and the impact on their home range. This is because a lot of species there have a restricted range and so cannot find suitable habitat easily as well as because big climatic changes are forecasted there. (map from article)
Effects on survival: being ectothermic animals, temperature has a profound effect on many physiological characteristics of amphibians. Just as a side note, being cold-blooded doesn’t mean that the internal temperature of an animal is exactly the same as that of its environment. It just means these animals don’t have an internal heating mechanism like we have, but they can regulate their body temperature to a certain degree. A lizard basking in the sun in the morning to heat up is a good example of this regulatory behavior. Temperature can influence heart rate, locomotion, water balance, developmental rate, immune function etc. Water balance, referring to water uptake through osmosis and water loss due to evaporation, plays a big part in the lives of amphibians, because they have with almost no exception a permeable skin, which links their internal environment very closely to the external environment. This permeable skin means that external factors like humidity, precipitation and soil moisture can influence water balance heavily. Climate change resulting in a change in any of these factors might mean that the habitat becomes unsuitable for a species. An example of another effect: salamanders in the family Plethodontidae lack internal lungs and therefore can only breath through their skin. This mode of respiration requires a moist skin, so any change in humidity and precipitation could result in respiration problems and reduced survival.
Effects on reproduction: one of the first well established consequences of climate change on amphibians is the shift in phenology, the timing of reproduction. Beebee found that three anuran species in Britain are reproducing earlier with a shift in climate, and the same has been found for amphibians in Japan and newts in the genus Triturus, showing that it is a common phenomenon. However not all species respond the same; two out of six anurans in New York State didn’t shift their phenology in response to climate change. This can be explained by the different cues for breeding that are used by different species; some species respond to temperature, others to peak rainfall events. A shift in phenology leads to a shift in amphibian assemblages in breeding habitats over time; this could lead to a competitive advantage of one species over the others, for instance when larvae of the first species develop earlier and therefore have more food available.
Effects on development: Climate and especially temperature also influence the larval stage of amphibians in several ways. The development of larvae is generally speaking speeded up by a higher temperature, but for many species there exists a certain threshold beyond which survival of the larvae drops steeply. For instance, Volpe showed that for embryos of Bufo valliceps the temperature for normal development spans from 20 to 33 °C, but if temperature exceeds 36 °C the embryos will not develop properly. Climate change can push the temperature beyond this ‘tolerance band’, severely affecting population growth. The speeding up in development of larvae may also lead to a shorter development time before metamorphosis and smaller size of the adults. This acceleration of time to metamorphosis has been observed in both anurans and urodels. It is likely that there is a trade-off between rate of development and growth and that this is exacerbated by climate change.
Effect on behavior: the last direct effect of climate change is the effect on behavior of amphibians; under stressful conditions as high temperature and low humidity they seek refugia and become less active. Red-spotted newts (Notophthalmus viridescens) have been found avoiding leaving their refuges during hot and dry conditions and those that do, travel less distance. This means they will spend less time foraging which might lead to a decrease in fitness. Changes in activity may also reduce gene flow from one population to the other and make these populations more vulnerable to chance events that cause a decline in population size.
Indirect effects
Next to direct effects there are also indirect effects from climate change on amphibian populations. These can be divided into effects on habitats, food availability and effects on species communities.
Effects on habitats: the very essence of being an amphibian is living both in the water as well as on the land during your lifetime (for most species this applies). Because of this use of a ‘double’ habitat, amphibians will be more affected by changes in habitats within a certain area. For the United States it is known that although precipitation has increased in the 20th century, warming also has led to increased evapotranspiration from wetlands, ponds and streams, which could in turn lead to drying of aquatic sites and reduced productivity of populations. In coastal freshwater wetlands the rise of the sea level, storm surge and salt water intrusion can influence negatively amphibian abundance and species richness.
In terrestrial habitats a change in temperature and precipitation might give rise to increased frequency and intensity of fires, change in ground water level and increased rates of evapotranspiration. We have seen earlier that soil moisture is a very important environmental characteristic in the lives of some of the terrestrial amphibians and they face the risk of desiccation.
Climate change will also affect the composition of the vegetation in an amphibian’s habitat. Structure and composition of the vegetation influence reproduction and survival of amphibians in an aquatic habitat. On land, microclimatic factors like temperature and humidity as well as the quality and abundance of leaf litter, are determined partly by canopy structure and the species composition of the vegetation. Canopy cover over wetlands and leaf litter in breeding ponds play a strong role in reproduction of some aquatic-breeding amphibians.
Food availability: Tadpoles (larval anurans) function as ‘ecosystem engineers’ in freshwater habitats, because they are quite dominant members of the food chain. Their modes of feeding cover all the range from herbivory, detritivory to carnivory. Higher temperatures may increase the rate of primary productivity and nutrient cycling thereby providing more food to the tadpoles. On the other hand they can induce blooms of algae, cyanobacteria etc which will lead to pollution of the water. Larval salamanders are mostly carnivorous creatures that prey on aquatic invertebrates. Changes in temperature and precipitation will influence wetland hydrology and therefore impact densities of wetland invertebrates. For the UK it has been predicted that a rise in temperature of 1 °C might lead to a decline of around 20% of aquatic invertebrates.
For terrestrial amphibians an increase in the amount of precipitation can have a positive effect; for the western slimy salamander (Plethodon albaluga) it is shown that higher precipitation can lead to increased clutch size. Namely, the increased humidity on the forest floor gives more opportunities to forage. From this follows that a reduced amount of precipitation can have a detrimental effect on clutch size.
Effects on species communities: as we have earlier on noted, climate change has an impact on the distribution of amphibian populations. When populations go extinct within a certain area or their home range shifts, this affects the community dynamics in a given area. With one population moving away or going entirely extinct this leaves room for other populations to flourish. At the same time, in the adjacent area where the population might be moving to, competition will increase. Climate change has also changed community dynamics because it enables some non-indigenous species to survive in a habitat where they beforehand could not last. For example, climate change projections indicate that Cuban treefrogs, cane toads and American bullfrogs will expand their ranges throughout the world. These are just three examples of invasive species, but as you might know, they have already had a considerable impact on communities in different parts of the world.
As a last example of this indirect effect; species like the mole salamander (Ambystoma talpoideum) and the eastern newt (Notopthalmus viridescens) are keystone species within communities. They have an impact on many other members of the community and interestingly the eastern newt can also mitigate the negative impact an invasive species like the Cuban treefrog has on other native species. However, both species require intermediate to long hydroperiod wetlands for reproduction; if climate change shortens the hydroperiod (hydroperiod can be defined as the number of days per year that there is standing water at a location) of these wetlands than this could impact the whole amphibian community of these freshwater wetlands.
So we can see from the paragraphs above that many different effects are to be expected from climate change on amphibian populations. Blaustein et al. add to these direct and indirect effects the interactions with different stressors, like diseases, UV-B radiation and immunity. Climate change might for instance facilitate the spread of pathogens (like the chytrid fungus) and also in this way play a crucial role in the survival of amphibian populations. These synergistic effects however are under debate and seem to be quite hard to prove. But even without certainty on these effects it is safe to say that climate change in itself can pose a threat to viability of populations. Especially those populations in remote areas, like the tropical forests on the slopes of the Andes, that will have a hard time shifting their home range to another locality, because their habitat is so specific and dispersal opportunities are rare. For more general species this ‘rangeshifting effect’ might be less important, but still altered competition in their habitat, invasive species that can survive due to a warmer climate and changes in vegetation and soil moisture in their habitat, can have a large effect as well on these more general species. And even though nature is very adaptable, climate change is happening so relatively fast compared to evolutionary change that we have to double or quadruple our efforts to combat it and keep our climate within a safe margin.