APR 13, 2013 LINDSEY THURMAN
What does it mean to be plastic?
Plasticity refers to the variation in phenotypes under certain environmental conditions. This means that one genotype can translate into multiple phenotypes based on environmental change. Phenotypic plasticity is widespread in nature, and can significantly alter the relationship between organisms and their [abiotic and biotic] environment.
The evolution of plasticity
The profile of phenotypes produced by a genotype across environments is known as the “norm of reaction”. It is often described by a curve that relates the contribution of environmental variation to the observed phenotype.
The level of phenotypic response to environmental change is determined by natural selection and can allow a given species to exploit novel environments. Just as in any heritable trait, genetic variation is required for the evolution of plasticity. Optimal norms of reaction can be achieved, but they are dependent upon the ratio of selection and mutation and the amount of genetic variation available.
What role can plasticity play in amphibian adaptation to climate change?
Figure 1 shows the typical life-cycle for pond-breeding amphibians from egg (embryo) to developing tadpole to emerging adult frog. Amphibians have evolved numerous unique characteristics that make their ecological role multi-dimensional. First, they have a biphasic life cycle in which the developing eggs and tadpoles are directly associated with aquatic environments. Post-development, they emerge from these aquatic systems to become terrestrial breeding adults [there are cases in which this biphasic cycle does not hold true – e.g. Pacific Giant Salamanders!]. Their tendency to dry up like a prune in the absence of moisture and their ectothermic life histories make them additionally vulnerable to climatic changes in precipitation and temperature. Their life histories require each individual to be adaptive to multiple interacting ecosystems within a single lifetime; making them both extremely vulnerable, and yet uniquely resilient, to environmental change.
Ecologists have long been interested in the way in which environmental factors influence the growth and development of amphibians. So what are some of the factors that go into making the decision of when and how quickly to metamorphose?
- Hydroperiod and water quality
- Resource levels
- Biological interactions
Amphibians have been documented plastically increasing tadpole development rate in response to these environmental stressors. Fascinating! Let’s look at some examples of phenotypic plasticity in amphibians, and discuss why this is relevant.
Examples of phenotypic plasticity in amphibians:
Couch’s Spadefoot (Scaphiopus couchii)
Newman, RA (1988) Evolution 42:774-783
In Southern Arizona’s hot desert environment, there is strong selection pressure on these tadpoles to increase rates of development and minimize the length of development since the late summer monsoon season is so short. In addition to the short wet season, tadpole densities are often very high and there is considerable competition for limited food resources. When temperatures become exceedingly high and the ponds begin to dry, these toads will plastically increase their development rate to emerge as juveniles before food, space, and water runs out.
Plains Leopard Frog (Rana blairi) and Southern Leopard Frog (R. sphenocephala)
Parris, MJ (2000) Copeia 2000:11-19
The effect of hydroperiod on tadpole growth, development, and survival was used to assess response to a drying aquatic environment on both of these species. The Plains Leopard Frog occurs throughout the Great Plains and into the Midwest. Declines have been reported throughout its range, largely due to the conversion of wetlands to agriculture. The Southern Leopard Frog is found throughout the Southeastern US and is known to be very adaptable in many freshwater habitats. Interestingly, regardless of their broad distributions and generalist tendencies, both species plastically reduced the time for tadpole development when exposed to a drying environment. Thus, we are noticing much diversity in the types of amphibians able to adjust their life histories in response to changing environmental conditions.
Mole Salamander (Ambystoma talpoideum)
Semlitsch, RD and HM Wilbur (1988) Copeia 1988: 978-983
Much less work has been done on responses of salamander larvae to pond drying, in part because most salamanders do not breed in highly ephemeral habitats. One of the only detailed experimental studies to date involved the effect of pond drying on tadpole development in Mole Salamanders, a species that breeds in Carolina bays and other shallow temporary ponds. Reproductive success varied spatially and temporally, but most tadpole mortality occurred in those ponds that dried early. Individuals able to rapidly metamorphose before pond drying were significantly smaller than their relatives who were ‘living it up’ in constant water. Thus, trade-offs may exist in a species ability to plastically respond to environmental stress and maintain the necessary growth trajectories to sustain life on land.
What about amphibians of the Pacific Northwest? Can plastic traits make some species more adaptable to rapid environmental change (i.e. Climate Change)?
In the Pacific Northwest, changes to freshwater ecosystems as a result of climate change are predicted to increase amphibian extinction rates and impose new challenges for conservationists. This region is divided into multiple ecological and climatic zones by the Cascade Mountain Range and is characterized by a maritime climate with moderate winter temperatures due to the proximity to the Pacific Ocean. Along an altitudinal acclivity, the Cascade Mountains experiences substantial temperature and precipitation gradients, with dynamic seasonal snow cover directly reflecting the fluctuating climate conditions throughout the winter.
The impacts of climate change in this mountain region will be particularly complex for amphibian species existing in snowmelt-dominated wetlands at high elevations. The relatively high amphibian species diversity makes for a unique scenario in which the effects of climate change on a single taxonomic group may be highly variable and multi-directional.
The aim of my study was to evaluate an assemblage of high elevation amphibian species for their potential to plastically respond to a warming climate scenario via rapid development. We examined temperature-induced phenotypic plasticity in individuals from three high elevation species native to the Cascade Mountain Range: Cascades frog (Rana cascadae), Western toad (Anaxyrus boreas), and Pacific chorus frog (Pseudacris regilla).
Results from my experiment showed that each of the species was able to plastically increase development rates under climate warming; however, the consequences to body size at metamorphosis were highly variable. For the Pacific Chorus Frog, often regarded as a generalist species, there were very minimal costs to body size. Unfortunately for the Cascades Frog, a species of conservation concern and candidate for listing on the Endangered Species Act, the increase in tadpole development rate in response to climate warming resulted in significant costs to body size for emerging juveniles. Thus, it appears that the most environmentally sensitive amphibian species may also be the least capable of rapidly adapting to climate warming (at least from a plasticity perspective).
Laura Blackburn, Priya Nanjappa, and Michael J. Lannoo (2001) US Amphibian Dist. Maps (http://home.bsu.edu/home/00mjlannoo/)