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Balancing Species Numbers and Phylogenetic Diversity

By Joseph Bennett

The current extinction crisis can be thought of as a fire in the genetic library of life. In the scramble to save as much as we can, we want to save as many books (i.e. species) as possible but we also want to save as much total information (i.e. unique genes) as possible.

The global extinction crisis shows no signs of abating, and conservation funding falls far short of what is necessary to stop declines in biodiversity. Thus, either implicitly or explicitly, conservation agencies engage in prioritisation: they try to use their limited resources to maximise achievable goals.

Traditionally, biodiversity has often been viewed as species diversity. However, other measures of diversity are gaining acceptance. One of the most prominent of these is phylogenetic diversity – the diversity of evolutionary relationships among species. Phylogenetic diversity can also be thought of as the information contained in life’s genetic library, representing the millions of years of evolution that have led to unique species. Losing a species is like losing a book from this genetic library, and the unique information (i.e. genes) associated with it.

The amount of unique genetic information contained in a species is associated with that species’ phylogenetic distinctiveness – the relative isolation of its branch on the tree of life. A species with no close relatives (whose lineage has been isolated on the tree of life for many millions of years) contains more unique information than one with recently evolved close relatives.

The loss of the more distinct species means the loss of these millions of years of evolution, along with unique genetic information that may have been useful for science or for adaptation to future environments. It’s like losing a rare old manuscript from life’s genetic library. Thus there is great incentive to conserve unique species in particular and phylogenetic diversity in general.

Conservation agencies are increasingly considering phylogenetic diversity in their decisions about which species to prioritise, and the Zoological Society of London now has a dedicated program to conserve phylogenetically distinct species that are threatened.

Unfortunately, highly distinct species can sometimes be expensive to conserve because they can have special requirements (and the actions required to conserve these “very different” species may have less complementary value than for other species). This has the potential to set up a dilemma between conserving highly distinct species and conserving the maximum number of species possible.

Too much focus on expensive, highly distinct species could even have the perverse result of conserving low phylogenetic diversity, if resources used on a single distinct species could have been used to conserve several others with higher aggregate phylogenetic diversity.

This problem, as well as a possible disconnect between evolutionary uniqueness and evolutionary potential, and the reluctance of managers to abandon the traditional conservation benchmark of species diversity, has led to a debate about the relative importance of conserving species numbers versus phylogenetic diversity.

The good news is that, with careful planning, sets of species can be conserved that minimise the trade-offs between the goals of species numbers and phylogenetic diversity. We have used a Project Prioritisation Protocol that has been employed for threatened species in New Zealand to demonstrate how this could be done.

By iteratively varying the importance of species’ phylogenetic distinctiveness in the ranking protocol, we were able to find the suite of species projects that minimised the sacrifice in either species numbers or total phylogenetic diversity that could be conserved within a given budget. We needed to use an iterative approach because there was no mathematical solution to the problem, when the realistic constraints of cost, probability of success and benefits for species projects are considered.

We showed that the best solutions meant giving up on a few very unique species that were so expensive that they would have seriously reduced the total number of species and even the total phylogenetic diversity that could be conserved. But the best solutions were still able to reach more than 95% of both the maximum possible species numbers and phylogenetic diversity.

What this means for conservation agencies is that it may be possible to satisfy both sides of the species numbers versus phylogenetic diversity debate. In a realistic situation where aspects such as probabilities of success, benefits and project costs across many years are considered, this may mean a careful process of choosing among candidate groups of species. But the rewards are worth the effort.

By carefully applying appropriate conservation decision frameworks, an effective balance between both goals can be achieved.

Joseph Bennett is part of the Environmental Decisions Group at the University of Queensland.