[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 conservation biology to shift towards an ecosystem-based approach. Explanations.
It measures over 50 centimeters and can weigh up to one kilogram, making it the largest tree-dwelling pigeon in the world and earning it its name: the giant fruit dove, or Ducula goliath to its friends. 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: it has a beak with a lower mandible that can deform, allowing it to swallow fruits more than 5 centimeters in diameter.
Its favorite food? The fruit of the pandanus tree, a small tree endemic 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 tree, as well as several other trees with very large fruit. But the notou is now threatened: hunted, victim of habitat destruction, and with low reproduction rates—laying only one egg per year—its population is declining. What would happen to the pandanus and the entire tropical ecosystem if the giant carpophagous bird were to become extinct?
Functional scarcity
This question opens up a new horizon 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 Research on Biodiversity and EDF.
Until now, a species has been considered rare when its population is limited to a small number of individuals or when it lives in a very restricted geographical area. Researchers are calling for these characteristics to be broadened: "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 from the CNRS andthe University of Grenoble Alpes, the scientists analyzed databases containing information on nearly 15,000 land mammals and birds in order to map ecologically rare species around the world. This painstaking work identified, alongside the notou, New Zealand's kakapo, the only parrot in the world that cannot fly and thus activates the decomposition of the soil it treads on for nearly a century, its extraordinary life expectancy. Or the black macaque of Sulawesi Island in Indonesia, which feeds on 145 varieties of fruit and disperses the seeds of as many tree species. And many other birds, rodents, lemurs, bats... "An ecological rarity that is concentrated in the tropics and the southern hemisphere, and especially on islands, " note ecologists.
Double punishment
How do they identify these ecologically rare species? "We observe the functional traits of a species by deciphering 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 this species disappears? "Then this combination of functional traits will not be fulfilled by another species, and the ecosystem is more likely to be disrupted than when a functionally redundant species disappears." Redundant? "This is how we describe a species when its functions can be performed by others; it's a kind of 'insurance' in case of extinction, " explains Nicolas Mouquet. Losing an ecologically rare species therefore means losing not only the species itself, but also its function in the ecosystem: an irreparable loss, a real "double penalty" 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, notably by global warming and human activities. "We have modeled the distribution of these species between 2050 and 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 distribution range in the case of certain birds. " More threatened... and less well protected. "Ecologically rare species are generally less protected than others," the researchers lament.
Paradigm shift
This alarming finding highlights 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 endangered 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 in currently protected areas, is not sufficient," regrets Nicolas Mouquet. Species conservation is too often based on their identity and demographic status. However, taking into account the uniqueness of their ecological roles is essential and should also guide conservation actions. " This calls for a real paradigm shift in conservation policies. "We hope that these new data will lead to new decision-making tools. Because better understanding will lead to better preservation."
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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.