We already know that climate change will force species to shift their distributions as climate zones move towards poles and to higher altitudes with increasing global temperatures. Several studies have looked at how biodiversity patterns across countries and continents are likely to change as a result, but fewer have taken the extra step to explore what these changes mean for the tree of life, that is, to the evolutionary lineages of biodiversity.
We tend to measure biodiversity using species and species richness as our guiding unit: the more species the higher diversity. But biodiversity spans across multiple scales, and to date we recognize at least three levels of biodiversity: 1) the diversity of different ecosystems (such as coral reefs, rain forests and the tundra); 2) the diversity of species across those different ecosystems, and; 3) the genetic diversity within and across species.
Genetic diversity is important as genes and genomes form the building blocks of evolution. Changes in the environment may change the genetic composition of a population and, under certain conditions, may lead to the emergence of new inheritable features and even new species. The more genetic diversity, the better changes a population or a species, or life in general, has for adapting to new environmental conditions, such as global climate change. One way of understanding the genetic diversity behind species is through a phylogenetic tree that shows which species are more closely related, and hence genetically more similar, and which ones are more distinct. Each branch on a phylogenetic tree represents an evolutionary lineage, joint branches ascending from a common ancestor.
From conservation perspective phylogenetic trees are useful for several reasons. First of all, they help us to identify old and unique lineages of evolution. These tend to be species that have been around for a very long time (tens or hundreds of millions of years) and have few or no living relatives left. Species like the reptile Tuatara (Sphenodon, two known species) and the lobe-finned fish Coelacanth (Latimeria, two known species). These species represent the last living examples of their genetic lineage, and their extinction would mark the permanent loss of some of the genetic diversity currently present on Earth.
But phylogenetic trees are useful in other ways as well. Combined with information on species distributions, they can help us to identify locations of not just high species diversity but also of high genetic diversity – locations where several branches across the whole tree of life co-occur. These are of particular interest when deciding where to place our next protected areas. Phylogenetic trees can also help us to identify locations with high levels of recent diversification, that is, places where many recent branches co-occur. These may indicate locations of ongoing evolutionary processes, sort of the cooking pots of evolution, which could potentially produce more genetic biodiversity in the relatively near future.
I recently had the honour to be involved in an interesting research project that looked at how climate change is likely to impact the phylogenetic diversity of the iconic eucalypt species in Australia. This great project, led by Carlos Conzalez-Orozco, my friend and former colleague Laura Pollock, Andrew Thornhill and many others, was just published in Natura Climate Change today. Eucalypts are a particularly interesting group of species, not only because they are hyper-diverse and host some of the tallest flowering plants in the whole world, but because they are quite unique to Australia and are thought have separated from rainforest ancestors around 70-65 million years ago – hence the group harbours a significant amount of the global genetic diversity of plants. In our study we looked at how future climate change is likely to alter the distributions of these iconic species by 2085, and what does that mean to phylogenetic diversity of eucalypts.
Our findings paint a stark picture of the future for eucalypt species under climate change, as a 3 °C rise in temperature over the next 60 years would see a decline of suitable habitat for 88% of the 657 species of eucalypts we studied. For 16 species the increase in average temperatures would result in total disappearance of their suitable climate, quite likely leading to significant range reductions and potential extinctions. Only 9%, or 61 species out of the 657, are likely to come out as winners in the future, expanding their ranges together with changing climatic conditions.
These changes in species distributions will also reshape the phylogenetic diversity patterns of eucalypt species in Australia. Our research shows that areas along Australia’s southern, southeast, and southwest coasts will become highly important for the conservation of old and also recently evolved eucalypt lineages, as species distributions move towards coastal regions and towards south along the coasts. As climate warms, some of the current concentrations of narrowly distributed old eucalypt lineages are expected to disappear from central Australia, while the remote Kimberley region will increasingly be a refuge for rare, ancient lineages. An alarming finding was that the loss of suitable climate space is not likely to be equally distributed over the eucalypt tree of life but we expect rare and old lineages to be most severely affected, anticipating loss of genetic information that is not necessarily present in other species. Overall, our results predict both loss of phylogenetic diversity within regions, and an increased homogenization over the landscape, which means less difference between regions. Including considerations of phylogenetic diversity has therefore provided us a much more comprehensive picture of the likely impacts of future climate change on eucalypts and how the biodiversity within this iconic group of Australian species is likely to be distributed in the future.
It is worth noting that as the distributions of species shift towards coastal areas, also the conservation efforts will increasingly focus on coastal regions where human population density is highest. This may lead to increased conflicts between conservation and different land use needs in the future. Due to the high complexity of the analyses, our results were produced considering only one climate scenario that predicts the increase of 3 °C. Naturally, if we manage to significantly reduce greenhouse gas emissions in the near future and halt the warming under 3 degrees, some of the predicted impacts could be reduced. In contrast, under more severe climate change these changes will be even more drastic.
Phylogenetic approaches reveal biodiversity threats under climate change. Nature Climate Change.
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