As we have seen in an earlier blogpost, predation by salamanders might have a positive effect on the amount of carbon that is stored in the soil. This is because they are the apex predators in temperate forests and eat the invertebrates responsible for shredding the leaf litter. Less invertebrates means that more leaf litter ends up locked away in the soil through humification. In this way salamanders provide a service to us humans, as they help to mitigate some of the human-induced CO2 in the atmosphere and hopefully evade too great a climatic change. However, a recent research has shown how salamanders themselves can be the victims of climate change. Scientists compared measurements on present day woodland salamanders in the Appalachian habitats to measurements on the same species that were done from the 1950s onwards. The results showed that size in these salamanders had decreased notably in only a few generations in most species. The researchers suggest this is a response to climate change. A warmer environment leads amphibians like these salamanders to expend more energy on metabolism, leaving less energy left for growth. This may impact their survival rates: a smaller salamander for instance has more potential predators and therefore is more prone to predation.
The Appalachian Mountains in the United States are a hotspot for salamander diversity with 10% of the worldwide known species occurring here. Climate models predict for these Appalachian habitats shifts in seasonal temperatures and moisture conditions in the near future. As we have seen before such climatic change can have serious effects on amphibians: ranging from a change in breeding period, migration, hopping higher up the mountain, to changes in colour pattern. While we as mammals and endothermic animals might be quite adaptable to a changing environment (although we all know the devastating effects a heat wave can have), amphibians are ectothermic animals ánd require moist conditions for their reproduction and survival. A warmer environment can therefore lead to less activity and a higher energy expenditure, because metabolism speeds up in these animals with higher temperatures. The drying up of ponds, and the environment in general, can limit reproduction and lead to desiccation of individuals.
A major response to climate change in animals is a reduction in body size. Scientists assume this arises, because a higher temperature limits activity, for instance animals stay in the shade longer and have less time to go out and feed, and because of the before mentioned speeding up of the metabolism. A smaller size in its turn may lead to a change in activity patterns, as smaller animals may opt for searching for food in the cover of the dark to avoid predators. A smaller size will probably also affect survival, as such an animal will be on the menu of more predators and competition with larger conspecifics or individuals from other species can be impeded. Other interactions between and within species than competition will be influenced too, as for example mate preference in females might be directed towards the larger males.
Plethodontid (woodland) salamanders are particularly sensitive to a warmer and drier climate; these are lungless salamanders that rely on both respiration and water uptake through the skin. Both warmer and drier conditions will impede these processes and make daily life harder for them. It is no coincidence therefore that the species richness in these salamanders is correlated with cool and moist conditions. And with a warmer and drier climate ahead, climate models predict range contractions for all species of these salamanders and especially for high elevation populations in the Appalachian mountains. The researchers then asked themselves what the impact of climate change on the physiology of these salamanders could be?
The study was designed as a resurvey of 78 historic sites that had been visited from the 1950s onwards. These sites had been included in surveys by the American herpetologist Highton and the researchers followed his methodology as much a possible. They sampled a total of 212 populations of 15 different Plethodontid species, that were living at a medium altitude between 400-1700 meters. They determined for each individual, 9450 in total, the species, sex, age class, snout to vent length and the temperature of the body. The researchers then compared these new measurements with the old ones, stemming back from 1950-2007.
The results remarkably showed an average 8% change in body size that occurred over about 55 years. For seven species a significant change was measured, and for 6 out of 7 species this meant that they had shrunk significantly. One species even showed an 18% reduction in body size, or almost one fifth. The results were most dramatic in regions that have become warmer and drier; in the United States these regions are found at lower latitudes. Because ‘shrinking’ of organisms has been proposed as an important response to climate change, several hypotheses to explain this shrinking have been coined. One, which stated that relatively more juveniles will be present in a population, can be discarded for this research, as no juveniles were included (and still and effect was shown). Two other explanations focus on a smaller individual body size or alternatively a decrease in minimum adult body size. The results from this study support the first one the most, as the reduction only occurred in the largest size classes and not in the smallest ones (which is what you would expect if minimal body size decreased)
The change in individual body size can be caused by a genotypic change or an environmentally induced response: in the latter case the phenotypic plasticity of the animal enables it to respond to climate change. It is the built-in range of characteristics that an individual carries with it. An excellent example of this in humans is the ability to tan, i.e. to change skin colour in order to cope with more sunlight. This phenotypic plasticity does not require genetic change, but reflects the capacity of the animal to withstand environmental change with is current genetic baggage.
Although we do not know whether the reduction in body size is a genotypic response or an example of phenotypic plasticity, it makes a big difference which mechanism is at work: if these salamanders are able to respond genetically that fast to a changing environment then evolutionary adaptation to climate change might be within their reach. The alternative explanation of phenotypic plasticity doesn’t rule out that evolutionary adaptation might occur as well, however if it’s the only mechanism at work then the range of climate change to which these creatures can respond adequately might be too limited for them to survive a changing climate.