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.
