The red-eyed tree frog (Agalychnis callidryas) is found in the tropical areas of the Americas. Tropical areas are rich in biodiversity – but the organisms found in these areas are also the most at risk from climate change.
Credit: Wikimedia
Every little niche in the variable landscape of Earth is packed with a range of plants and animals, insects and microbes. Evolution is responsible for this amazingly rich biodiversity, and yet it also responsible for the constraints on individual species’ distributions and abundance.
We have surprisingly little understanding how these constraints and limits are set, even though they are responsible for setting extinction risks.
One of the main threats to our biodiversity comes from the direct and indirect threats associated with climate change. Direct threats include droughts, heat waves and fires that destroy vegetation, kill animals and change the composition of our ecosystems.
Indirect threats come about for many reasons; plants needed by herbivores for food might have disappeared, or the empty space created by drought, fire and other disturbances may have been invaded by weedy species.
Some species are more susceptible to climate change than others. Species that cannot cover much ground are particularly affected by these threats because they are less able to find refuges or re-colonise areas that have become favourable again. Those that require moist and cool conditions are likely to succumb under increased climatic variability.
Plants with low rates of seed dispersal and without large seed banks in the soil are likely to be particularly prone because they cannot evade stressful periods. Animals that cover large areas in search of food are prone to extinction because they cannot survive in small refuge areas. Yet these ecological explanations tell only part of the story – ultimately, limits are set by an absence of adaptive evolution.
Through evolution, species could overcome their low tolerance levels to heat or cold, increase their mobility and change their timing of reproduction to match the arrival of favourable conditions.
Evolution can be very rapid within species, particularly when species have short generation times, as is the case for microorganisms, many insects, annual plants and some small mammals. There are large genetic differences between individuals within a single species, and this variability can be readily accessed to mount evolutionary responses to climate change.
A species possesses several different forms of the same gene, called alleles, and these alleles are selected to produce local adaptation. Consider a plant species that occupies an altitudinal gradient spanning several hundred meters. Within that species, a population from low-altitude site would almost always perform much better at low altitude than a population from a high-altitude site that had been moved (and vice versa).
This is because they have the right genotype – the combination of all alleles that determines the traits of an individual. Populations within a species have genetic adaptations to deal with local conditions.
Local adaptation is a feature of many plant and animal populations that live across ecological gradients because they possess genotypes that have a high fitness under particular conditions.
Populations also contain a store of different alleles that can be accessed through natural selection when environmental conditions change. This store of genetic variation was accessed by insect pests to evolve insecticide resistance, weeds to evolve herbicide resistance and snails to evolve thick shells to keep out predators.
It can also be accessed to produce adaptive responses in populations exposed to increasingly hot or dry conditions. Numerous selection experiments on species such as the fly Drosophila melanogaster, the bacterium Eschericia coli and mice show that genetic variation within a population is sufficient to make populations much more tolerant of hot, cold, and dry conditions.
This means that evolutionary responses can provide resilience in organisms faced with climate change. This resilience is already evident in natural populations that are showing adaptive changes in response to climatic stresses.
Examples include changes in patterns of diapause timing in mosquitoes, shifts in body size in birds and mammals and changes in flowering time in plants.
If so much genetic variation seems to exist in populations, perhaps all species could evolve to counter the coming threats due to changes in climate.
It seems reasonable to suggest that plants and animals could develop the ability to survive in hotter and drier conditions, or move to a more appropriate climate or even enter inactive stages to avoid disturbances.
Unfortunately, however, most of our knowledge on adaptive potential comes from model organisms. But the model species we use in genetics research are the unusually hardy and widespread species.
In reality, it is a relatively small number of species that have very wide distributions, from the tropics to cool temperature areas. The vast majority of species are restricted to a single climatic region.
This makes it difficult to generalise from what we know about adaption in a relatively small number of model organisms to other organisms. And to date, few attempts have been made to compare the evolutionary potential of groups of related species that occur in different environments.
In 2009, my team from the University of Melbourne and collaborators from Monash University, also in Melbourne, set out to test the evolutionary potential of species from different climates in a situation where the climate became drier. We compared the amount of genetic variation that 12 species of Australian Drosophila flies possessed for desiccation resistance – some widespread species and some from a tropical area.
It turned out that flies from tropical rainforests have almost no genetic variation, whereas the widespread species had abundant variation.
This means that the ability of tropical species to evolve and cope with drought conditions is surprisingly limited. Widespread species, on the other hand, can evolve quickly to cope with very low relative humidities, becoming as tough as some arid zone species after only a few generations. The same patterns were found for cold resistance.
Climate change is expected to lead to drier conditions, so these findings point to an increased risk of extinction in rainforest species from the wet tropics whose distribution is limited by low humidity and cold conditions. Though, these results do need to be extended to other groups of animals, and under different climate stresses.
Tropical species start off being highly susceptible to dry conditions. And because of the absence of evolutionary resilience, these species cannot easily increase their resistance levels to counter any reduction in rainfall.
There is a lot at stake. Much of the world’s plant, bird and insect biodiversity is restricted to moist tropical regions and a narrow range of altitudes within these regions.
These results also point to an evolutionary explanation for an ecological observation: species restricted to a particular climate range lack the genetic variation that would have allowed them to expand their range in the past.
What explanation might account for the lack of genetic variation? One possibility is that the genes required to mount an evolutionary response have degraded. This could have occurred through a continued accumulation of mutations of the genes.
If genes are no longer under selection for thousands of years, mutations will cause the genes to lose functionality as they decay. Patterns of gene degradation are easily found in comparisons of genomes between similar species.
For instance, humans have lost many of the functional genes involved in tasting bitter compounds, while these genes are still present in other primates.
Presumably we lost these genes because we no longer need to discriminate many of these compounds in our food.
Another possibility is that the genes needed to mount an adaptive response have been lost because they are at a disadvantage under other conditions. This can occur if the same genes have multiple effects on traits. For instance, genes that code for proteins that help protect other proteins during heat stress can lower survival and interfere with reproduction in the absence of stressful conditions.
Distinguishing between these explanations will depend on the identification of genes involved in thermal and aridity responses, genes controlling the induction of inactive phases and other traits across a range of species.
Research is needed to understand the genetic basis of evolutionary limits. Unfortunately, this area of investigation has received little attention over the last few decades.
If evolutionary limits are generally due to a lack of genetic variation in species, then the specialist species that make up most of the world’s biodiversity may have trouble persisting under climate change through adaptation and evolution and will become extinct.
This makes regions rich in specialist species relatively more prone to species loss due to climate change. In particular, organisms that are limited to high altitudes in the wet tropics may be particularly susceptible to climate change because they have nowhere to go and will need to evolve in order to persist, yet they are most likely species to lack such evolutionary resilience.
Ecologists have become very good at mapping predicted changes in species distributions. By using a combination of current species distributions and how these are matched with climate, the climatic variables that are associated with species distributions can be identified.
They can then be used in future forecasts of regional climate change to gain an understanding of how species ranges might be altered.
It is now the turn of evolutionary biologists to look deeply into the genomes of species and identify signatures of limits across groups of organisms. Hopefully, we will gain insights into the reasons why species fail to adapt and remain constrained to a narrow set of environmental conditions, and thereby become threatened.
Ary Hoffmann is a professor in genetics and zoology at the Bio21 Institute of the University of Melbourne.
