New research says that the big fish are getting smaller, and the smaller fish are taking their place

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Organisms are getting smaller through a combination of species replacement and changes within species, according to new research from the University of St Andrews.

Research published in SciencesI looked at data from all over the world over the past 60 years, and from many kinds of animals and plants.

The study was conducted by an international team of scientists from 17 universities, as part of a task force led by scientists from the Center for Biodiversity and the School of Biology at the University of St Andrews; and the University of Nottingham.

Previous research has shown that the size of trophy fish in fishing competitions has decreased, and that many of the species most at risk are large.

The new study connects the dots and shows that the change in body size comes from individuals within a species getting smaller, but also that larger species are being replaced by smaller ones.

Lead author Dr Ines Martins, from the University of St Andrews, said: “At some sites, for example, smaller and smaller individuals of spiny skates are being observed, while smaller species such as mackerel are increasing in abundance.”

“Whether it’s because of what humans like to eat, or because their environment is getting too hot, it seems the big fish can’t take a break.”

The shrinkage was most common among fish, but among other groups of organisms β€” such as plants and invertebrates β€” the changes were more varied. By looking at groups of species, the study reveals that there are some complex changes taking place, with some organisms getting larger while others shrinking.

Senior author of the study, Professor Maria Dornelas from the University of St Andrews, said: “We think this indicates that when large organisms disappear, other organisms try to take their place and consume the resources that become available.”

Reflecting on the significance of these findings, Dr. Martins added: “Understanding and exploring this complexity is essential if we are to understand the mechanisms involved in how body size changes over time.”

The study also noted that a few large organisms were replaced by many small ones, with the total amount of life – known as biomass – held constant. This surprising finding supports the idea that ecosystems tend to compensate for change by maintaining the stability of the total biomass of the studied species in a given habitat. This stability has been attributed to a trade-off between reductions in body size and simultaneous increases in abundance between organisms.

These findings have far-reaching implications for our understanding of how different organisms have adapted to the challenges posed by the Anthropocene.

Professor Dornelas said: β€œIt is clear that the large-scale species replacement we are seeing around the world has measurable consequences. Organisms getting smaller have important effects because the size of animals mediates their contribution to how ecosystems function, and how humans benefit from them. Larger fish usually feed more people than smaller fish.

Dr Franziska Schrodt, chair of the working group, from the University of Nottingham, said: “Our study highlights the importance of looking at changes in species characteristics at an individual and across species level if we are to understand the effects of environmental change and climate change.” Human impacts on biodiversity globally.

“Unfortunately, we currently lack data on many organisms other than fish to draw clear conclusions – future research would benefit from greater investment in these types of measurements, especially when exploring food webs and interactions of other species.”

more information:
Ennis S. Martins et al., Large-scale shifts in body size between populations and populations, Sciences (2023). doi: 10.1126/science.adg6006

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