Using evolutionary history to prioritise conservation
Using a recent example from Madagascar, Tim Harris describes how the evolutionary history of biodiverse areas can be taken into account when prioritising areas for conservation.
With limited resources available for conservation, there is always a need to prioritise the protection of some areas over others.
For some time, areas have been identified as having high conservation value often based solely on their species richness — the number of species they contain.However, some have argued that not all species are equal and that the evolutionary history responsible for the current diversity found in a given area should also be taken into account in conservation planning.
It is with this idea in mind that the biodiversity indicator called phylogenetic diversity was developed. Phylogenetic diversity is a biodiversity index that measures the amount of evolutionary history in a given set of species (for example, all those found in a particular area) by adding up the relevant branch lengths (corresponding to time) in a phylogenetic tree that connects this set of species. There is growing evidence that phylogenetic diversity is being lost at a higher rate than species diversity and that extinction is not random across the tree of life. There is also mounting evidence that threatened species are often phylogenetically related.
Kew scientists investigating the evolutionary history of plants have therefore been using that information to identify priority areas for conservation. A study of phylogenetic diversity in the Cape of South Africa has shown that areas with high evolutionary diversity were not necessarily the areas with the highest species diversity (Forest, F. et al. 2007). A similar approach has been applied to species of the legume family found in Madagascar (Buerki, S. et al. 2015), where prioritisation of areas for conservation is not only urgent due to the high number of endemic species (those unique to a region) found on this tropical island, but also because less than 10% of the vegetation originally present on the island remains.
Applying phylogenetic diversity methods to Madagascar’s legumes
While other studies have compiled information on the most species rich areas of Madagascar, there is a need to study the evolutionary mechanisms responsible for the assembly of communities in this biodiversity hotspot. Previous global analyses have shown that some biodiversity hotspots identified according to species richness have significantly greater phylogenetic diversity than other hotspots. A study conducted by Sven Buerki (previously at RBG Kew, now at the Natural History Museum, London), Félix Forest and colleagues looked at whether species diversity and phylogenetic diversity followed the same or different patterns in Madagascar. The legume family was chosen as a good surrogate for plant diversity across Madagascar. Conservation decisions should not just be influenced by the current distribution of diversity but also informed by an understanding of the processes that formed these current distributions.
Understanding the evolutionary mechanisms that gave rise to the current diversity of legume species in Madagascar is not just of academic interest. It also allows the identification of locations in Madagascar that were most significant in the process of plant diversification. There is evidence that flowering plant genera in Madagascar diversified after the world’s climate became more arid two million years ago. If areas of low elevation became arid and isolated by being surrounded by higher elevation zones then these could be areas where legumes survived and diversified. Identifying these localities (referred to as refugia) would allow them to be protected and corridors established to enable the dispersal of plants under predicted changes in climate.
Reconstructing the evolutionary history of legumes in Madagascar involved obtaining DNA sequence data from 66 genera of legumes found on the island comprising endemic species. The DNA sequence data was used to generate a phylogenetic tree. The branches of that phylogenetic tree were calibrated so that the length of the branches reflected time. This study also used species distribution modelling, which shows where species would be expected to occur based on what is known about their climatic and environmental requirements.
Legume communities in Madagascar’s humid biome and in the coastal forest were found to have greater phylogenetic diversity than drier biomes. Arguably, this needs to be taken into account when prioritising protected areas. The study identified potential locations of refugia, areas that supported legume diversification after the dramatic climate change about two million years ago. Predictions have suggested that the climate in Madagascar will become more arid with a 1.1°-2.6°C temperature increase by 2100. Now that 90% of the original vegetation has disappeared in Madagascar, identification and protection of a network of refugia and corridors of dispersals are fundamental to buffer the effect of future climate change.
The importance of refugia in Madagascar has previously been highlighted by researchers examining how lemurs evolved. Some of the legume species associated with refugia are thought to have been dispersed by giant lemurs that are now extinct. After the extinction of giant lemurs, the niche they occupied has not been filled by any species that remain today. Understanding how plants diversified in the past can help us understand how to conserve plants in today’s changing world.
Placing species on the tree of life is usually done by comparing their DNA sequence with sequences from many other species. Currently, new species of plants and fungi are largely described on the basis of morphological characteristics of preserved specimens. However, some scientists have argued that, due to the speed at which DNA sequence data is being gathered, particularly sequence data produced from DNA obtained from environmental samples such as soil or water bodies and sequenced without ever having examined the morphology of the species, the phylogenetic position of a species in the tree of life is often the only information we might have about it.
The field of molecular phylogenetics allows scientists to reconstruct the evolutionary history of large scale assemblages of species and this information should be incorporated into conservation planning. It might be the only way to capture the future benefits that biodiversity may hold for humanity.
Reference
- Devey, D.S.*, Forest, F.*, Rakotonasolo, F., Ma, P., Dentinger, B.T.M.*, Buerki, S.* (2013). A snapshot of extinction in action: The decline and imminent demise of the endemic Eligmocarpus Capuron (Caesalpinioideae, Leguminosae) serves as an example of the fragility of Madagascan ecosystems. South African Journal of Botany (Special Issue: Towards a new classification system for Legumes), 89: 273-280. Access this article online.
