Performing phylogenetic triage

How to advise those in the unenviable position of making these decisions was the focus of a meeting on ‘Phylogeny, extinction risks and conservation’ last week at the Royal Society, where the central issue was how to exercise this kind of “phylogenetic triage” in the face of inevitable biodiversity loss.

We need some way to choose which species will be the focus of conservation efforts, and knowing a species’ evolutionary relationships with others gives us a measure of its distinctiveness: how much unique evolutionary history would we lose along with it?

Of course, nothing in biology is ever quite this straightforward, and there are different ways in which we might measure evolutionary distinctiveness. A second factor to take into account is risk: different species have different risks of going extinct, as captured by the International Union for Conservation of Nature (IUCN)’s ‘Red List’ that lists species on a spectrum from “Least Concern” through “Endangered” to “Extinct”.

Ben Collen and Samuel Turvey both discussed the EDGE metric, which they have recently developed with colleagues from the Zoological Society of London and which combines evolutionary distinctiveness with extinction risk in order to generate a ranking of species that are both at risk and biologically unique.

Evolution versus function

But being more evolutionarily distinct is not the only thing – or even arguably the most important thing – that makes a species more worth saving. “Functional” traits must also be taken into consideration – unique biological traits and behaviours, or unique roles in an ecosystem. Corals, for example, notoriously create intricate 3D reef environments in which other species can flourish.

Measuring and saving functional diversity directly was a hot topic of discussion, with a key problem being that it’s particularly difficult to measure: knowing everything about a species’ biology and interactions with other species is much more difficult than knowing only its evolutionary history. Fortunately, evolutionary distinctiveness is actually a pretty good proxy for these other important properties of species.

Corals provide a good example of functional triage, as described by Danwei Huang of the University of Iowa. Some corals produce much more complex structures than others, and a healthy reef usually has a mix of types – but we might make the choice to save only the more complex, resulting in a low diversity of corals themselves, but a maintenance of the diversity of other marine species the resulting coral reefs support.

Species in space

Finally, a few speakers went beyond the species to discuss spatial considerations in biodiversity, which may have more relevance to political processes. Laura Pollock discussed how the Australian state of Victoria is currently opening parts of its national parks for tourism development, and the focus then naturally becomes which areas to save to achieve the minimum threat to biodiversity.

Her particular focus is eucalypts, a diverse native clade of plants, for which there are detailed survey records of their presence and absences. Because of this, it’s possible to investigate at high resolution which areas contain the most evolutionarily diverse set of species, and are thus the highest priority for conserving – and also to model the effects that different policies might have on the levels of biodiversity compared with this optimal scheme.

The current policy is straightforward: ring-fence for protection areas containing specific particularly rare species. But from the point of view of the total biodiversity that would be protected across the state, the modelling shows that this is far from optimal. It’s a striking example of how the practical demands of conservation – conserving areas, not species – can sometimes be at odds with more traditional species-based metrics.

Who chooses?

The Victoria study highlights a point made again and again during the meeting: what you measure depends on what you’re trying to save, and that’s a decision made by politicians – albeit with scientific input. In this way conservation science is notable for the close connection between scientists and policy makers: scientists advise policy-makers, but the different arguments for saving biodiversity – a few were outlined by Robert May in his Q&A for BMC Biology – also inform the scientific metrics used.

For example, Jon Paul Rodriguez of the IUCN argued that while species extinction is easy to measure, if we want to preserve “ecosystem services” then perhaps we should instead measure ecosystem collapse – even though it is particularly tricky to define.

The question “What should we save?” does not have a clean, correct answer, and will usually be decided by policy makers, not scientists. What scientists can do is to quantify the different forms of diversity that exist, and make sure that those decisions are informed.