A team of researchers led by scientists at the University of York has mapped the family tree of the ferocious ambush predators and their extinct relatives, the pseudosuchians. They then compared this with data from the fossil record to understand why there are so few living species of crocodiles, while there are 11,000 species of birds.Researchers have found that climate change and competition with other species have shaped the diversity of modern crocodiles and their extinct relatives, but the results also reveal that ecology - whether species live in the ocean, freshwater or on land - plays an unexpected key role in survival.
Researchers at the University of York have mapped the evolutionary history of crocodiles, revealing the impact of climate change, competition and ecological factors on their current limited diversity, providing important implications for conservation efforts.
The study, published today (December 4) in the journal Nature Ecology & Evolution, found that as global temperatures rise, the species numbers of crocodiles' sea- and land-dwelling relatives increase, and competition for resources with sharks, marine reptiles or dinosaurs intensifies, likely leading to their extinction. In contrast, crocodiles' freshwater cousins are unaffected by changing temperatures, but rising sea levels put them at greatest risk of extinction.
With seven crocodile species listed as critically endangered and four identified as vulnerable as the climate continues to change, the study's findings provide important implications for conservation efforts for crocodiles and other species.
Dr Katie Davis, senior author of the study from the University of York's Department of Biology, said: "The fossil record is a rich source of valuable information, allowing us to look back through time to how and why species originated and, more importantly, what caused their extinction. By studying the fossil record and mapping it against the crocodile family tree, our study reveals how important ecological thinking is when we try to predict how species will respond to today's climate change."
"With millions of plant and animal species at risk of extinction, it is more important than ever to understand the key factors behind species loss. In the case of crocodiles, many species inhabit low-lying areas, meaning rising sea levels associated with global warming could irreversibly alter the habitats they rely on." "
Crocodiles and birds share a common heritage with dinosaurs, and together with pterosaurs they form a group known as the "archosaurs" or "ruling reptiles", which date back to the Early Triassic. Pseudosuchians are a group of archosaur reptiles, defined as all species more closely related to crocodiles than to birds.
In the study, the researchers built a massive phylogenetic relationship (like a family tree) for all crocodilians and their extinct relatives, allowing them to map how many new species are forming and how many are becoming extinct. They then combined this data with data on past climate change, particularly temperature and sea levels, to assess whether species' emergence and extinction were related to climate change.
The researchers also explored whether interactions between species, such as competition, might play a role, so they calculated estimates of species numbers at any point in time and used a mathematical method called "information theory" to compare those estimates with new species and species extinctions. This allows scientists to estimate whether climate change and species interactions have a direct impact on the emergence or extinction of new species.
Dr Davies added: "Crocodiles and their extinct relatives provide a unique perspective on climate change and its impact on biodiversity in the past, present and future. Our findings advance our understanding of what factors have and continue to shape life on Earth."