"Beyond the Numbers"
The hidden transition from ecosystems ruled by apex predators to those crowded by smaller, mid-level feeders.
Image Credit: Scientific Frontline
Scientific Frontline: Extended "At a Glance" Summary: Reorganization of Global Fish Food Webs
The Core Concept: Long-term global data indicates a widespread restructuring of marine and freshwater fish food webs, characterized by a shift toward smaller-bodied species and altered feeding relationships, even in ecosystems where overall species richness remains stable.
Key Distinction/Mechanism: Unlike traditional biodiversity metrics that rely primarily on species counts (richness), this ecological shift highlights underlying structural changes. Because the size of predators and prey governs feeding rules, the decline of large top predators and the rise of mid-level, generalist feeders create denser, more highly connected food webs. Ecosystem degradation is occurring via shifting biological traits and interactions rather than direct species loss.
Origin/History: The phenomenon was detailed in a massive global synthesis led by researchers from the German Centre for Integrative Biodiversity Research (iDiv), Martin Luther University Halle-Wittenberg (MLU), and Friedrich Schiller University Jena. By analyzing time-series data spanning up to 70 years across nearly 15,000 fish communities, the research team formally published their findings in Science Advances on February 24, 2026.
Major Frameworks/Components:
- Trait-Based Ecology: The methodological shift from counting species to tracking specific biological traits, particularly body size and diet specialization.
- Trophic Network Dynamics: The redistribution of biomass across trophic levels, noting a specific decline in apex predators (e.g., sharks, goliath groupers) and a compensatory increase in mid-level predators and primary consumers.
- Food-Web Connectance: The theoretical pillar addressing the density of predator-prey interactions; structural reorganization has led to broader, overlapping diets among generalist feeders.
- Perturbation Propagation: The framework used to understand how human-driven stressors (eutrophication, warming, overfishing) ripple through an ecosystem based on its network structure and buffering capacity.
Branch of Science: Marine Biology, Freshwater Ecology, Biodiversity Synthesis, and Conservation Science.
Future Application: Integrating food-web theory and trait tracking into next-generation global biodiversity monitoring systems. This approach will improve the accuracy of predictive ecological models and inform targeted conservation strategies designed to maintain structural ecosystem functionality, rather than simply preserving species counts.
Why It Matters: Ecosystem functionality dictates resilience. Monitoring species richness alone is insufficient and masks severe ecological degradation. Because altered food webs determine how human-driven pressures propagate through an environment, understanding these structural changes is critical to anticipating how marine and freshwater ecosystems will respond to ongoing global changes.
Species numbers alone do not fully capture how ecosystems are changing. In a global study led by researchers from the German Centre for Integrative Biodiversity Research (iDiv), the Martin Luther University Halle-Wittenberg (MLU), and the Friedrich Schiller University Jena, scientists analyzed long-term data from nearly 15,000 marine and freshwater fish communities. They found that fish food webs have changed substantially over recent decades, even in places where the number of species (species richness) has remained stable. Published in Science Advances, the study shows consistent shifts in species composition, body size, and feeding relationships, highlighting that changes in species traits such as body size and interactions can alter ecosystem structure without obvious changes in species richness.
Smaller fish, different food webs
The researchers combined time series data spanning up to 70 years with information on fish body size, diet, and trophic position. While overall species richness showed no consistent trend, species composition changed strongly over time. Across many ecosystems, communities increasingly consisted of smaller-bodied fish species.
“We often say, “big fish eat small fish,” and in nature it’s true—it’s an ecological rule. Fish predators are usually larger than their prey, and this size difference determines who can eat whom. When the size of predators or prey changes, feeding relationships shift, reshaping food webs and how ecosystems function,” says first author and iDiv alumnus Dr Juan Carvajal-Quintero. He worked on the newly published study while working as a postdoctoral researcher at iDiv’s synthesis center sDiv, and is now an Assistant Professor at Dalhousie University (Canada).
The study found that fish food webs have become more densely connected, with species interacting with a broader range of prey. This reflects an increase in generalist feeders – species that are less specialized in their diets. While the proportion of large top predators like sharks, goliath groupers, muskellunge, and marble trout declined, mid-level predators as well as primary consumers increased, reshaping the distribution of species across trophic levels.
“Together, these results indicate a widespread reorganization of fish food webs, affecting both their structure and function. Increased connectance may accelerate the spread of perturbations among species, yet it may also enhance overall buffering capacity against disturbances such as warming, eutrophication, or fishing pressure. As a result, the responses of future food webs to global change remain highly uncertain,” said Prof Ulrich Brose, research group head at iDiv and the University of Jena.
Food-web structure determines how effects propagate through an ecosystem. When large top predators are lost and generalist feeders with overlapping diets dominate, the consequences of human-driven pressures — warming, overfishing, nutrient loading — can ripple more widely across species.
Similar patterns across ecosystems worldwide
The researchers found similar trends in both marine and freshwater systems, across many regions of the world, suggesting these changes reflect broad, long-term reorganization rather than local responses. “No single study could reveal this. It’s only by synthesizing nearly 15,000 fish communities spanning decades and linking compositional changes to food-web theory that we can see how consistent and widespread this restructuring really is,” says senior author Prof Jonathan Chase, research group head at iDiv and the MLU.
The findings show that monitoring species richness alone may miss important aspects of biodiversity change. Tracking species traits and interactions provides additional insight into how ecosystems are reorganizing. Integrating food-web perspectives into biodiversity research and monitoring helps improve our understanding of ecosystem change and may support future conservation efforts.
Reference material: What Is: Ecosystem
Funding: The study was funded by the German Research Foundation (DFG; FZT-118), among others.
Published in journal: Science Advances
Title: Degradation of fish food webs in the Anthropocene
Authors: Juan D. Carvajal-Quintero, Maria Dornelas, Lise Comte, Juliana Herrera-Pérez, Pablo A. Tedesco, Xingli Giam, Ulrich Brose, and Jonathan M. Chase
Source/Credit: Martin Luther University Halle-Wittenberg | Zum Seitenanfang
Reference Number: mb022426_02
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