[LUM#14] Dear Rare Species
How do we determine whether a species is rare? A new study highlights the need to consider the functional rarity of species in order to take their ecological role into account. It calls for a shift in conservation biology toward an ecosystem-based approach. Here’s an explanation.
It measures more than 50 centimeters and can weigh up to one kilo, making it the largest tree pigeon in the world and earning it its common name: the giant carpofage, or Ducula goliath to those in the know. This extraordinary pigeon hides its red eyes and slate-and-fawn plumage in the primary rainforest of New Caledonia, where everyone knows it by its common name, the notou. And the notou has a unique feature: its beak has a lower mandible capable of deforming, allowing it to swallow fruits more than 5 centimeters in diameter.
Its favorite food? The fruits of the pandanus, a small tree native to New Caledonia (see photo). As the only bird capable of eating them, it plays an important role in dispersing the seeds of the pandanus, as well as those of several other trees with very large fruits. But the notou is now threatened: hunted, a victim of habitat destruction, and with low reproductive rates—laying only a single egg each year—its population is declining. So what would happen to the pandanus and this entire tropical ecosystem if the giant fruit-eater were to go extinct?
Functional rarity
This question opens up a new frontier for conservation biology: that of the functional rarity of species. The notou plays a unique ecological role in its environment, making it irreplaceable. “It is an ecologically rare species because it is essential to the proper functioning of its ecosystem,” explain Nicolas Loiseau* and Nicolas Mouquet, co-authors of a study on the ecological rarity of species, funded at CESAB by the Foundation for Biodiversity Research and EDF.
Until now, a species has been considered rare when its population consists of only a small number of individuals or when it inhabits a very limited geographic area. Researchers are calling for a broader definition of these characteristics: “The role these species play in their ecosystem and the way they contribute to its functioning must also be taken into account,” emphasizes Nicolas Mouquet, scientific director of the Center for Synthesis and Analysis of Biodiversity (Cesab).
Together with their colleagues at the CNRS andthe University of Grenoble Alpes, the scientists analyzed databases containing nearly 15,000 terrestrial mammals and birds in order to map ecologically rare species around the world. This painstaking work led to the identification, alongside the notou, of New Zealand’s kakapo—the world’s only flightless parrot—which has been promoting soil decomposition in the areas it inhabits for nearly a century, thanks to its exceptionally long lifespan. Or the black macaque of Sulawesi Island in Indonesia, which feeds on 145 varieties of fruit and disperses the seeds of just as many tree species. And many other birds, rodents, lemurs, bats… “An ecological rarity concentrated in the tropics and the Southern Hemisphere, and especially on islands, ” ecologists note.
Double Punishment
How do they go about identifying these ecologically rare species? “We examine a species’ functional traits by analyzing its lifestyle: Does it live on the ground or in the canopy? Is it diurnal or nocturnal? What is its diet?” replies Nicolas Loiseau. “We consider that the more unique a species’ combination of traits is, the more important it is to the ecosystem, ” adds the researcher. And if that species goes extinct? “Then that combination of functional traits won’t be provided by another species, and the ecosystem is at greater risk of being disrupted than when a functionally redundant species goes extinct.” Redundant? “That’s how we describe a species when its functions can be fulfilled by others; it serves as a ‘safety net’ in case of extinction, ” explains Nicolas Mouquet. Losing an ecologically rare species therefore means losing not only the species itself but also its function within the ecosystem: an irreparable loss, a true “double blow,” according to the researchers.
These are all swords of Damocles hanging over these ecosystems, in a context where ecologically rare species appear to be particularly threatened, especially by global warming and human activities. “We’ve modeled the distribution of these species for the period 2050–2080 based on IPCC climate change scenarios,” explains Nicolas Loiseau, “and our results show that the negative impact will be greater for these species, with losses of more than 50% of their range for certain birds.” ” More threatened… and less well protected. “Ecologically rare species are, in general, less well protected than others, ” the researchers lament.
Paradigm Shift
This alarming finding underscores the need to take ecological rarity and the functioning of ecosystems into account in conservation programs. While some of the animals identified in this study are already considered threatened with extinction bythe International Union for Conservation of Nature (IUCN), this is not the case for all ecologically rare species. “This ‘profiling’ of ecologically rare species highlights that their preservation, even within currently protected areas, is insufficient,” laments Nicolas Mouquet . “Species conservation is too often based on their identity and population status. Yet, taking into account the uniqueness of their ecological roles is essential and should also guide conservation efforts. ” This calls for a genuine paradigm shift in the conservation policies that need to be implemented. “We hope that these new data will lead to new decision-making tools. Because a better understanding will lead to better conservation.”
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*Marbec (UM CNRS – IRD – Ifremer) Global distribution and conservation status of ecologically rare mammal and bird species. Nicolas Loiseau, Nicolas Mouquet, Nicolas Casajus, Matthias Grenié, Maya Gueguen, Brian Maitner, David Mouillot, Annette Ostling, Julien Renaud, Caroline Tucker, Laure Velez, Wilfried Thuiller, Cyrille Violle. *Nature Communications*, October 8, 2020.