Extinctions drive far greater declines in species with extreme physical attributes and unique ecological strategies, and this has far-reaching impacts as important ecosystem services are irretrievably lost
Thanks to runaway climate change, Earth is experiencing a global mass extinction event that is unprecedented in human history. But this global extinction crisis is more than just the widespread loss of species, it is an exceptional loss of species with unique physical forms and structures — a process referred to as ‘taxonomic homogenization’ or ‘morphological homogenization’.
Homogenization is occurring amongst birds. Morphologically diverse bird species are decreasing much faster than one would predict than if species were being randomly wiped out. In other words, those birds that serve unique — and possibly irreplaceable — functions in their habitats are most likely to disappear fastest. These are the birds with the most extreme and recognizable anatomic features, like the ‘upside down’ beaks of filter-feeding flamingos, the powerful downwardly curved beaks of hornbills, and the spoon-shaped beak of the spoon-billed sandpiper. Birds with distinct physical traits are critically important for earning a living by relying on rare ecological strategies such as pollinating flowers, spreading seeds, controlling pests or creating burrows or tree hollows that other animals can use that ensure the proper functioning of ecosystems.
But are species extinctions driving far greater declines than predicted of birds that rely upon these rare ecological strategies? If so, what impacts does this have on habitats — and on people? Are valuable ecosystem services being rapidly lost forever?
Emma Hughes, lead author of a newly published study has already spent several years of her career examining broad morphological traits in birds. Notably, she has studied the global distribution of different beak shapes, and was an integral part of the ongoing Mark My Bird citizen science project, where she produced 3D scans of bills from more than 8000 bird species and curated the associated morphology dataset. (I contributed heavily to this online project during its early days.)
“My interest in birds drove me to go to Uni to study Zoology — I’m the first person in my family to get a degree”, Dr Hughes told me in email. “My degree solidified my enthusiasm for research, and I ended up working on a project [where I was] 3D scanning birds’ beaks in order to quantify the diversity and evolution of bill shape across the whole family.”
Now a newly-minted macroecologist who just completed her PhD at the University of Sheffield in May, Dr Hughes is continuing her avian morphology studies as a Research Assistant at the Natural History Museum at Tring.
“I was interested in using this data to measure trait diversity at a global scale – where is it more diverse than expected? Seeing local bird species decline and become increasingly threatened with extinction from certain areas, alongside the wider biodiversity extinction crisis inspired me to ask what would happen to additional aspects of diversity such as morphological and phylogenetic diversity if these species were lost.”
The central question of Dr Hughes’ current research is whether species extinctions can drive exceptional declines in morphological and phylogenetic diversity in birds.
To determine the effects of body traits on a species’ likelihood to go extinct, Dr Hughes spent several years measuring beak size and shape, leg and wing lengths, and body size of 8,455 bird species from the collections of study skins held by several large natural history museums. (This is a large sampling of the class aves; there are just a few more than 10,000 formally described species of birds alive today.)
In addition to measuring birds, Dr Hughes and her team also examined the avian tree of life (phylogeny) and measured evolutionary differences there, as well as identifying differences in behavioral traits such as migration, foraging and song.
Dr Hughes and her collaborators then conducted statistical analyses to quantify similarities and differences amongst the measured traits from different species, and graphed these data into a ‘morphospace’. The data mostly clustered around a dense core group of species with similar appearances and body sizes, with fewer, more diverse forms at the edges of the morphospace.
With these data in-hand, Dr Hughes and her collaborators sequentially removed species’ data from their analysis from the most endangered to the least threatened, based on the assessments of the International Union for Conservation of Nature’s (IUCN) Red List of Threatened Species, which ranks species according to their probability of going extinct. Repeating the analysis after each cohort was removed, they found that avian morphological diversity is likely to decrease at a greater rate than expected through species extinctions alone due to global climate change. At the same time, the phylogenetic diversity did not decrease overall amongst those species that were least likely to go extinct.
Dr Hughes and her collaborators then investigated which geographical regions could be most affected, by analyzing data from birds found in these regions. They found that species are trending towards greater morphological homogenization in 12 of the 14 habitat types they studied.
“The Himalayan mountains and foothills are at particular risk, and it’s likely that the loss of trait diversity will be considerable”, Dr Hughes stated.
Additionally, tropical islands, such as Hawaii, which have already lost all of its distinctive honeycreepers, are also vulnerable to homogenization. In some of these habitats, there are no organisms that can replace these lost species’ unique ecological roles.
Why are the Himalayan uplands and foothills, which cover such large expanses, particularly at risk from morphological homogenisation?
“It is likely that the considerable loss of morphological diversity in the Himalayan ecoregions is partly driven by the loss of vultures — the most imperiled group of birds”, Dr Hughes replied in email. “Vultures, as large-bodied, obligate scavengers, fill distinct areas of morphospace. Vultures provide vital ecosystem services by removing decaying carcasses, which could otherwise increase the direct transmission of infectious diseases and increase populations of opportunistic scavengers (i.e., dogs and rats) that spread rabies and bubonic plague.”
Are any other ecoregions at particular risk of homogenisation?
“Another region containing assemblages at risk of morphological homogenization are the dry and moist forest ecoregions of South Vietnam and Cambodia, where there is also exceptionally high expected loss of phylogenetic diversity”, Dr Hughes explained in email. “CR [critically endangered] and EN [Endangered] species are therefore likely to be phylogenetically unique and exhibit sets of traits that the surviving species pool does not contain. Indeed, highly threatened species [in these ecoregions] are amongst the highest evolutionarily distinct and globally endangered classified species including giant ibis (Thaumatibis gigantea, ranked second by EDGE), Bengal florican (Houbaropsis bengalensis, ranked seventh by EDGE), and white-shouldered ibis (Pseudibis davisoni, ranked sixteenth by EDGE).”
EDGE — Evolutionarily Distinct and Globally Endangered — is a metric system that combines Endangered conservation status with the phylogenetic distinctiveness of a particular taxon.
The study also found that body size is strongly impacted, with giant ostriches and tiny hummingbirds both being at increased risk of extinction.
“We do find strong evidence to support the hypothesis that the largest and smallest species are likely to be most at risk of extinction.”
Overall, Dr Hughes and her collaborators found that losing endangered species leads to species being more similar to each other in terms of their morphology, leaving the more unique species at a disadvantage.
“This suggests that species threatened with extinction are found in higher densities at more extreme trait combinations than those not at risk of extinction.”
What questions is Dr Hughes researching now?
“It would be great to include species gains and look at how this changes morphospace”, Dr Hughes replied in email. “We only examine extinction and current species distributions in this paper, but there is also future range expansion invasion to consider.”
Invasive species are more likely to be generalists, and capable of exploiting the urban human environment.
“[W]e might expect on average to see more seed-eating/generalist species that have a more ordinary beak shape, of medium-small size. Something like mynas or house sparrow perhaps”, Dr Hughes explained in email. “Because of this, we could even be underestimating the extent of morphological homogenisation!”
“As species go extinct, you expect the traits that they represent to also be lost”, Dr Hughes pointed out. “But what we found was that with morphological diversity, the traits were lost at a much, much, much greater rate than just species loss could predict. This is really important because that can lead to a major loss of ecological strategies and functions.”
Emma C. Hughes, David P. Edwards, and Gavin H. Thomas (2022). The homogenization of avian morphological and phylogenetic diversity under the global extinction crisis, Current Biology 32 | doi:10.1016/j.cub.2022.06.018
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