Balancing species numbers and phylogenetic diversity

Which ‘books’ do you save as the library of life burns?

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 prioritization; they try to use their limited resources to maximize achievable goals. A large part of what EDG does is developing tools to make this process more efficient, to preserve as much biodiversity as possible within limited budgets (for example, consider Decision Point #76)

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 in life’s genetic library, representing the millions of years of evolution that have led to unique species (Cadotte and Davies 2010). Losing a species is like losing a book from this genetic library, and the unique information (ie, genes) associated with it.

Biodiversity is more than number of species

Figure 1: A phylogenetic tree showing evolutionary relationships for four hypothetical species, and the costs associated with conserving these species. Species D is more isolated on the phylogenetic tree than the other species, and contains more unique genetic information. However, its conservation costs are also comparatively high. If the conservation budget for these species is limited to $600,000, should only one very distinct species be sponsored?

Figure 1: A phylogenetic tree showing evolutionary relationships for four hypothetical species, and the costs associated with conserving these species. Species D is more isolated on the phylogenetic tree than the other species, and contains more unique genetic information.
However, its conservation costs are also comparatively high. If the conservation budget for these species is limited to $600,000, should only one very distinct species be sponsored?

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 (Fig. 1). 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 (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 on which species to prioritize, and the Zoological Society of London now has a dedicated program to conserve threatened, phylogenetically distinct species (www. edgeofexistence.org).

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 (Schluter 2001), and the reluctance of managers to abandon the traditional conservation benchmark of species diversity, has led to debate on the relative importance of conserving species numbers versus phylogenetic diversity (Winter et al. 2012, Rosauer and Mooers 2013).

Minimizing the trade-off

The good news is that, with careful planning, sets of species can be conserved that minimize the trade-offs between the goals of species numbers and phylogenetic diversity. We used a Project Prioritization Protocol (PPP) that has been used for threatened species in New Zealand (see Decision Point #29) to demonstrate how this could be done (Bennett et al., 2014).

Naturalist Mark Carwardine (with a kākāpō on his head) and Stephen Fry in the BBC TV series, Last Chance to See. The series explored the efforts to save some of the world’s most rare and critically endangered animals. How much ‘weight’ should be given to saving a species based on its uniqueness or phylogenetic difference?

Naturalist Mark Carwardine (with a kākāpō on his head) and Stephen Fry in the BBC TV series, Last Chance to See. The series explored the efforts to save some of the world’s most rare and critically endangered animals. How much ‘weight’ should be given to saving a species based on its uniqueness or phylogenetic difference?

By iteratively varying the importance of species’ phylogenetic distinctiveness in the ranking protocol, we were able to find the suite of species projects that minimized 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 over 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.

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 over 95% of both the maximum possible species numbers and phylogenetic diversity.

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 (ie, species) as possible, but we also want to save as much total information (ie, unique genes) as possible. By carefully applying appropriate conservation decision frameworks, an effective balance between both goals can be achieved.


Two rare ‘books’

Two phylogenetically-distinct (but very expensive) species.

The short-tailed bat – the only living member of its family. (Photo by Jane Sedgeley)

The short-tailed bat – the only living member of its family. (Photo by Jane Sedgeley)

The short-tailed bat (Mystacina tuberculata) is the only living member of its family, its last remaining relative having probably gone extinct in the last century. It is threatened by loss of its old-growth forest habitat, and also by invasive cats and stoats, since it spends a lot of its time foraging on the ground (which is unique for a bat). Unfortunately, costs to ensure this species’ continued survival are estimated to be over $60 million NZD over 50 years. The good news is that some colonies survive on offshore islands that are relatively safe from cats and stoats, and so this species is not necessarily doomed to extinction.

The critically-endangered kākāpō (Strigops habroptilus), has likely been genetically isolated from other parrot species for tens of millions of years. As a flightless bird, it is particularly threatened by feral cats, and its population is now so low that inbreeding may be reducing hatchling success. Controlling these threats and ensuring the survival of this species would cost more than $40 million NZD over 50 years. Fortunately, the kākāpō’s recovery has received a helping hand from public-private partnerships. This reflects the fact that this large flightless parrot has become a bit of a media star and cause celebre of the conservation world meaning it’s likely there will be strong public support for funding its conservation into the future.

The critically-endangered kākāpō, has likely been genetically isolated from other parrot species for tens of millions of years. (Photo by Dianne Mason)

The critically-endangered kākāpō, has likely been
genetically isolated from other parrot species for
tens of millions of years. (Photo by Dianne Mason)


More info: Joseph Bennett j.bennett5@uq.edu.au 

References 

Bennett JR, G Elliott, B Mellish, LN Joseph, AIT Tulloch, W Probert, MMI Di Fonzo, JM Monks, HP Possingham & RF Maloney (2014). Balancing phylogenetic diversity and species numbers in conservation prioritization, using a case study of threatened species in New Zealand. Biological Conservation 174: 47–54.

Cadotte MW & JT Davies (2010). Rarest of the rare: advances in combining evolutionary distinctiveness and scarcity to inform conservation at biogeographical scales. Diversity and Distributions 16: 376–385.

Schluter D (2001). Ecology and the origin of species. Trends in Ecology and Evolution 16: 372–380.

Rosauer DF & Mooers AO (2013). Nurturing the use of evolutionary diversity in nature conservation. Trends in Ecology and Evolution 28: 322-323.

Winter M, V Devictor & O Schweiger (2012). Phylogenetic diversity and nature conservation: where are we? Trends in Ecology and Evolution 28: 199–204.

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