Sharks and rays have populated the world's oceans for around 450 million years, but more than a third of the species living today are severely threatened by overfishing and the loss of their habitat. An international research team led by palaeobiologist Manuel A. Staggl from the University of Vienna has now investigated whether and how global warming influences the diversity of sharks based on climate fluctuations between 200 and 66 million years ago. According to the study, higher temperatures and more shallow water areas have a positive effect, while higher CO₂ levels have a clearly negative effect. The study was recently published in the scientific journal "Biology".

There have been five mass extinctions in Earth’s history - and one group of animals has survived them all: cartilaginous fish, which include sharks and rays, have inhabited the oceans for more than 450 million years; more than 1,200 shark and ray species are known today. However, more than a third of these species are now seriously threatened due to their habits and increasing over-exploitation and habitat destruction; extinction would also have a direct impact on many ecosystems.

"The current rapid climate warming could also have a negative impact on this group of animals - in an international study based on earlier climate changes, we have now looked at how exactly this might impact rays and sharks," explains palaeobiologist Manuel A. Staggl from the University of Vienna. The international team investigated the driving forces behind the biodiversity of sharks and rays during the so-called Jurassic (200-143 million years ago) and Cretaceous (143-66 million years ago), a heyday of shark and ray evolution with a wide range of different environmental conditions. Fossilised shark and ray teeth were used to determine the species diversity for each age and compared with the climate data of the respective age. "We wanted to understand which environmental factors influence the diversity of sharks and rays in order to be able to develop possible future scenarios with regard to current global warming," says Jürgen Kriwet, professor of palaeobiology at the University of Vienna.

Higher CO₂ level as a decisive environmental factor

The results show that three environmental factors are decisive: Higher temperatures and more shallow water areas have a positive effect; however, a higher carbon dioxide (CO₂) concentration clearly has a negative effect. This is the first time that the negative impact of CO₂ concentrations in the ocean has been so clearly described: "We cannot yet fully explain the exact mechanisms behind the negative effect of CO₂ on the biodiversity of sharks and rays," says Staggl. However, laboratory studies on sharks and rays living today have revealed direct physiological effects of higher CO₂ concentrations on the animals - from impacts on the animals' senses to changes in the skeleton during embryonic development. In any case, the fossil record shows that a higher CO₂ content contributed to the extinction of individual shark and ray species.

Climate change as an opportunity?

On the other hand, the current global warming may also harbour opportunities for sharks and rays: Rising sea levels and higher temperatures have already been beneficial for the biodiversity of these predators in the past - firstly due to the increase in shallow coastal waters and secondly due to the global expansion of warm waters that offer stable conditions all year round.

The rise in sea level in particular appears to have been of great importance. "The resulting habitats in shallow seas that cover large continental areas are real biodiversity hotspots; sharks and rays were able to colonise them very quickly and efficiently thanks to their adaptability," explains Staggl. Due to the sometimes significantly higher temperatures during the Jurassic and Cretaceous, the tropics and subtropics were able to expand further north and south, and without distinct seasons, stable and more complex ecosystems with a greater diversity of species were able to develop.

No bright future

However, assuming that sharks and rays are facing a bright future would be too short-sighted, explains Staggl: "The environment is currently changing particularly quickly - unfortunately probably too quickly for the animals and their ecosystems." Together with overfishing, habitat loss and the rise in CO₂ levels in the oceans, it is unlikely that these predators will benefit greatly from global warming.

In order to reduce the environmental pressure on sharks, urgent measures are therefore needed to protect them. This is not just about protecting the sharks and rays themselves, but also about preserving entire ecosystems. "Because without the top predators, the ecosystems would collapse," emphasises Kriwet: "By protecting sharks and rays, we are investing directly in the health of our oceans and therefore also in the people and industries that benefit from these ecosystems," says the professor of palaeobiology.