Thursday, April 15, 2010

Prey populations explode as predators get smaller.

When top predators are removed from ecosystems their prey and/or competitors increase due to decreased predation and competitive release. However, can changes in behaviour, or body size, of the predators also cause this effect? If true, this would be most evident in heavily exploited marine ecosystems where size selective fishing has lead to rapid reductions in the size of top predators. The authors in this study used a 38 year time series to examine the relationship between predator size and prey biomass within such an ecosystem, the Western Scotian Shelf.

Their analysis showed that since the mid 1990's predator biomass has remained relatively constant. If one species of predatory fish was overfished it tended to be replaced by another species of predatory fish. Yet, despite no changes in predator biomass, prey biomass has increased by a huge 300%. Statistically, what matched this increase most closely was a decrease in the size and body mass of fish at higher trophic levels. The mean lengths of benthivores decreased by 21%, piscivores by 8%, and planktivores by 16%. When translated into body mass large benthivores decreased by 59%, medium benthivores by 48%, piscivores by 45%, and planktivores by 34%. For example, a haddock in the 1970's weighed, on average, 2 kg, but now weighs approximately 0.8 kg.

The empirical results from this study support the hypothesis that reduction of predatory fish size is the dominant factor in the underlying explosion of prey biomass. Why would this occur? Larger predators have been shown to be more successful at capturing prey due to their faster swimming speeds, and greater visual acuity. Thus, larger predators can consume more prey per unit time than smaller predators, and as a result larger predators can regulate their prey populations more effectively. As predators get smaller, a reduction in predation pressure results, leading to large increases in prey populations such as the pattern observed in this study.

Shackell, N., Frank, K., Fisher, J., Petrie, B., & Leggett, W. (2009). Decline in top predator body size and changing climate alter trophic structure in an oceanic ecosystem Proceedings of the Royal Society B: Biological Sciences, 277 (1686), 1353-1360 DOI: 10.1098/rspb.2009.1020

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