Big fish are shrinking and small fish are multiplying, a new study shows
Three blue-cheeked butterflyfish (Chaetodon semilarvatus) in the Red Sea, Egypt. Blue-cheeked butterflyfish are a relatively large species, which can attain a maximum total length of 23 centimetres (cm), though 15 cm is most common. (Picture: Maria Dornelas)
Black-axil chromis (Chromis atripectoralis) swim in Australia’s Great Barrier Reef. The fish inhabit lagoons and reefs, and often occur in large numbers. (Picture: Maria Dornelas)
A starry goby (Asterropteryx semipunctata) in Kaneohe Bay, Hawai’i. This fish reaches a maximum size of four centimeters. (Picture: Mike McWilliam)
The body sizes of organisms around the world are becoming smaller over time, especially among fish, with important implications for food webs and humans.
Based on a media release from the University of St. Andrews
St. Andrews/Leipzig. Organisms are becoming smaller through a combination of species replacement, and changes within species. Published in Science, the research looked at time series covering the past 60 years, from many types of animals and plants around the world. The study was conducted by an international team of scientists from 17 universities, as part of a working group funded by the German Centre for Integrative Biodiversity Research (iDiv), and led by scientists of the University of St Andrews, and the University of Nottingham.
Previous research showed that the size of trophy fish in fishing competitions is going down, and that many of the most threatened species are large. This study connects the dots and shows change in body size is the result of not only individuals within species becoming smaller, but also larger species being replaced by smaller ones. "Whether it's because of what humans prefer to eat or their habitats getting warmer, big fish just can't seem to catch a break,” explains the lead author Dr Inês Martins from the University of St. Andrews.
The researchers used two sources of body size trait data — direct measurements of biomass taken in the field and average body size estimates from major trait databases — to assess the body size change across over 5,000 ecological assemblage time series spanning 1960-2020. This allowed them to designate body size changes as either a within-species change or a compositional change.
Shrinking was most common among fish, but among other groups of organisms – such as plants and invertebrates – changes were more varied. By looking across groups of species, this study reveals there are some complex changes taking place, with some organisms becoming bigger while others shrink. The authors believe this suggests that when large organisms disappear, other ones try to take up their place and use up the resources that become available.
Reflecting on the importance of these results, co-author Prof Jonathan Chase from iDiv and Martin Luther University Halle-Wittenberg (MLU), adds: "As with most things, changes through time are not always ‘one-size-fits-all’. It is important to dig into the complexity of when organisms become smaller or bigger to more thoroughly understand how body size is changing through time.”
The study also noted the replacement of a few large organisms with many small ones, while keeping the total amount of life – known as biomass – constant. This surprising result supports the idea that ecosystems tend to compensate for change by keeping overall biomass of the studied species in a particular habitat stable. This stability is attributed to a trade-off between reductions in body size and concurrent increases in abundance among the organisms.
These findings have far-reaching implications for our understanding of how various organisms are adapting to the challenges posed by humans in the Anthropocene era.
The senior author of the paper, Prof Maria Dornelas from the University of St. Andrews, comments: “It is clear the widespread species replacement we see around the world is having measurable consequences. Organisms becoming smaller has important effects as the size of animals mediates their contribution to how ecosystems function, and how humans benefit from them. Bigger fish can usually feed more people than smaller fish.”
Currently, there is not enough data to draw clear conclusions for most organisms other than fish. Collecting similar measurements – especially when exploring food webs and other species interactions – stands to significantly benefit future research.
Original publication
(Researchers with iDiv affiliation and alumni bold)
Inês S. Martins, Franziska Schrodt, Shane A. Blowes, Amanda E. Bates, Anne D. Bjorkman, Viviana Brambilla, Juan Carvajal-Quintero, Cher F. Y. Chow, Gergana N. Daskalova, Kyle Edwards, Nico Eisenhauer, Richard Field, Ada Fontrodona-Eslava, Jonathan J. Henn, Roel van Klink, Joshua S. Madin, Anne E. Magurran, Michael McWilliam, Faye Moyes, Brittany Pugh, Alban Sagouis, Isaac Trindade-Santos, Brian McGill, Jonathan M. Chase, Maria Dornelas. (2023). Widespread shifts in body size within populations and assemblages, Science. DOI: 10.1126/science.adg6006
Contact:
Prof Dr Jonathan Chase
Head of the Biodiversity Synthesis research group
German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig
Martin Luther University Halle-Wittenberg
Phone: +49 341 9733120
Email: jonathan.chase@idiv.de
Web: www.idiv.de/en/groups-and-people/core-groups/synthesis.html
Dr Inês Martins
University of St. Andrews
Email: istmartins@gmail.com
Christin Coester
Media and Communications
German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig
Email: christin.coester@idiv.de
