The big pike eats the small perch. The perch eats the minnow. The minnow eats insect larvae. This structure of eating and being eaten keeps bodies of water in balance. However, this is currently changing worldwide. Not necessarily because species are disappearing, but because the composition of fish communities, their body sizes and their roles in the food web are shifting.
Researchers from the German Centre for Integrative Biodiversity Research (iDiv) in Leipzig, together with colleagues, have analyzed data from almost 15,000 fish communities from around the world - over a period of up to 70 years. Their findings have been published in the scientific journal Science Advances: Even where the overall number of fish species shows no clear trend, a lot has changed.
When disturbances spread faster
Prof. Ulrich Brose, research group leader at iDiv and at the Friedrich Schiller University Jena, explains the ambivalence of the findings. "Increased connectivity can cause disturbances to spread more quickly between species. At the same time, it can also increase the ability to absorb stresses such as warming, nutrient enrichment or fishing pressure." How future food webs will react to global change therefore remains highly uncertain.
Similar patterns have been observed in oceans and freshwaters around the world. It is not a coincidence, but a global trend. Prof. Jonathan Chase, research group leader at iDiv and at Martin Luther University Halle-Wittenberg, emphasizes that the full picture only emerges when thousands of studies are combined. This is the only way to recognize how consistent and widespread this restructuring actually is.
For research, this means that the mere number of species is not enough to assess the state of an ecosystem. Changes in body size, feeding habits and interactions provide decisive indications of the extent to which a system has already changed. Anyone who wants to protect water bodies must also keep an eye on food webs in the future.
Original publication:
Juan D. Carvajal-Quintero, Maria Dornelas, Lise Comte, Juliana Herrera-Pérez, Pablo A. Tedesco, Xingli Giam, Ulrich Brose, Jonathan M. Chase (2026). Degradation of fish food webs in the Anthropocene. Science Advances
