For tens of millions of years, sharks have played a crucial role in marine ecosystems. But what happened for their functional diversity to be at its lowest historical level today?
Sharks, survivors of numerous environmental crises over the past 250 million years, saw their functional diversity peak around 20 million years ago during the Miocene epoch. This diversity, defined by traits such as body size and diet, has gradually declined over the past 10 million years.
To understand this evolution, researchers from Swansea University and the University of Zurich analyzed over 9,000 fossilized and current shark teeth, representing around 500 species. These well-preserved teeth in the fossil record allow deducing the functional traits of ancient sharks.
Jack Cooper, a Ph.D. student at Swansea University, explains that the size, shape, and types of tooth edges are reliable indicators of sharks' functional traits, such as their size and diet. Through these measurements, the team quantified the functional diversity of sharks throughout the Cenozoic era, from 66 million years ago to the present day.
The decline in functional diversity of sharks was mainly caused by the disappearance of ecologically unique and specialized species. The extinction of the megalodon, for example, eliminated a superpredator with no equivalent among current sharks. This loss reduced the range of ecological roles fulfilled by modern sharks compared to their ancestors.
In light of this finding, researchers warn against current human threats, such as overfishing, which accelerate shark extinctions and further reduce their ecological contributions. Dr. Catalina Pimiento emphasizes that identifying modern species occupying part of the Cenozoic functional space could help guide conservation priorities to preserve this vital functional diversity.
Functional diversity
Functional diversity is an ecological concept that describes the variety of ecological roles played by different species within an ecosystem. Unlike taxonomic diversity, which counts the number of species, functional diversity focuses on the specific traits of species, such as body size, diet, reproductive methods, and other biological and behavioral characteristics.
Functional traits allow determining how species interact with their environment and each other, thereby influencing ecological processes like primary production, organic matter decomposition, and the regulation of prey and predator populations. By measuring functional diversity, scientists can assess the resilience and stability of ecosystems in the face of disturbances, as well as their ability to provide essential ecosystem services.
For example, in shark studies, functional diversity is quantified by examining traits such as the size and shape of teeth, which reflect the diet and trophic roles of different species. High functional diversity indicates a wide range of ecological roles filled by sharks, while low functional diversity suggests a reduction in this range, often due to the extinction of unique and specialized species.