Why aren't there more predators? Testing the rules that set marine food-web structure

68%, so biomass pyramids become increasingly bottom-heavy as productivity rises.

This regularity creates a paradox for marine ecology. If pyramid shape changes with productivity, size-spectrum slopes should steepen in richer systems, yet marine size spectra are famously consistent in the absence of fishing. Both observations can be reconciled only if something else varies with community biomass. Either less energy passes up each trophic step (lower transfer efficiency), or predators in richer systems eat prey closer to their own size (a contraction of the predator-prey mass ratio, PPMR), or both. These explanations have rarely been tested against each other, because transfer efficiency and PPMR are usually measured in different systems by different studies.

This PhD will test these competing explanations using one of the richest marine ecological datasets in the world. The Centre for Environment, Fisheries and Aquaculture Science (Cefas) holds four decades of standardised fish, plankton and benthic surveys across UK waters, alongside a stomach-contents archive recording individual predator and prey sizes, some dating back to the 1800s (Thompson et al. 2025). Uniquely, pyramid shape, size spectra and PPMR can all be estimated from the same samples, across gradients in space (productivity, temperature and nutrients) and time (warming, nutrient reduction and changing fishing pressure).

Research questions

The project will address three broad questions, with scope for the student to shape its direction:

• Does sub-linear predator-prey biomass scaling hold across UK shelf seas and through four decades of environmental change?
• Is the paradox resolved by changes in transfer efficiency, predator-prey mass ratios, or both?
• Can departures from this expected baseline provide a meaningful indicator of food-web condition?

CASE placement with Cefas

The project offers Cefas a route to an empirically grounded baseline for assessing Good Environmental Status under the UK Marine Strategy and OSPAR. The student will spend at least three months at Cefas, spread across the four years as a series of visits, working directly with Cefas data archives and joining the food-web indicators team to see how scientific evidence is turned into statutory assessments. Where operationally feasible, there may also be an opportunity to join a research survey aboard RV Cefas Endeavour.

Training and research environment

The student will be based in the Centre for Pollution Research and Policy at Brunel University of London, joining a group working on food-web energetics and macroecological scaling, and will also engage closely with the ecology group at Queen Mary University of London. Training will include R and reproducible workflows, Bayesian hierarchical modelling, size-spectrum estimation, analysis of large spatial and time-series datasets, and food-web and energy-flux modelling. The TREES doctoral training programme provides cohort-based training and an interdisciplinary peer network.

Candidate profile

We welcome applicants with a strong degree in ecology, marine biology, environmental science, zoology, or a quantitative discipline such as mathematics, statistics or physics, combined with a strong interest in ecology. Experience with R or another programming language and an interest in analysing large datasets would be beneficial, although full training will be provided. Above all, we are looking for a curious and motivated candidate who is keen to work at the interface of fundamental ecology and marine policy.

Supervisors:

• Dan Perkins
• Pavel Kratina
• Murray Thompson
• Beth Gillie

Informal enquiries are welcome: Dr Daniel Perkins (daniel.perkins@brunel.ac.uk)

Funding Notes:

Fully funded place including home (UK) tuition fees and a tax-free stipend at UKRI rates. Additional funding to cover full overseas fees is available for a maximum of 6 studentships.

Enquiries email name and address:

TREES.Admissions@ucl.ac.uk

Application Web Page:

https://www.trees-dla.ac.uk/apply

References

Hatton, I.A., McCann, K.S., Fryxell, J.M., Davies, T.J., Smerlak, M., Sinclair, A.R.E. & Loreau, M. (2015) The predator-prey power law: biomass scaling across terrestrial and aquatic biomes. Science, 349, aac6284.

Perkins, D.M., Hatton, I.A., Gauzens, B., Barnes, A.D., Ott, D., Rosenbaum, B., Vinagre, C. & Brose, U. (2022) Consistent predator-prey biomass scaling in complex food webs. Nature Communications, 13, 4990.

Thompson, M.S.A., Preciado, I., Maioli, F. et al. (2025) Fish functional groups of the North Atlantic and Arctic oceans. Earth System Science Data, 17, 2447–2462.