Overfishing and selective fishing of large fish is a well documented phenomenon – it has been estimated the biomass of large predatory fish in oceans has declined 90% during the past 50 years. Given the much of the top-predators have been removed, one would perhaps expect to see some effects also at other trophic levels.
A recent study focussing on ecosystem structure Northwest Atlantic suggests that it is not only the biomass, but the size, of the predators that matters. In other words, in the system under study, the predator biomass has remained unchanged over the past 38 years, but average size of the predators has declined ca 60%. It is this size decline which best predicts the increase in prey biomass over the study period. Hence, it seems that by removing large sized predatory individuals, prey populations escape the most fierce predation – small predators are not as important as the large ones.
The grand conclusion becomes that apart of density/abundance-mediated interactions, trait-mediated interactions (e.g changes in body size or behavior) may be important drivers of trophic structuring in disturbed ecosystems.
Shackell NL et al. (2010) Decline in top predator body size and changing climate alter trophic structure in an oceanic ecosystem. Proc. R. Soc. B doi:10.1098/rspb.2009.1020