Analysis

Loss of Predators Can Destabilize Marine Communities

 

Fishers pulling up nets to collect fish.© Marco Famaili

Fishers pulling up the nets to collect the fish enclosed in the “dead chamber” of the tuna trap in Camogli, Italy. The dead chamber is the name of the last compartment of the tuna trap where the fish would remain enclosed before being taken by the fishermen.

Scientists have long suspected that sharks and other predators were important, not because they are charismatic but because they bring balance to the overall marine community. But direct, empirical evidence for this has been lacking.

Now, a study of the Mediterranean Sea published on September 16, 2014 in the journal Ecology Letters shows that reducing the abundance of sharks, tuna, and other predators—for example, through fishing—can destabilize marine communities.

© Squalo Bianco Guigno

Scientists have long suspected that sharks and other predators were important, not because they are charismatic but because they bring balance to the overall marine community. But direct, empirical evidence for this has been lacking.

To test this hypothesis, Gregory Britten of Dalhousie University in Halifax, Nova Scotia and colleagues started with an unusual dataset from a tuna trap off the northwest coast of Italy. This stationary trap—a square net enclosure—has operated continuously for many decades. Data from 1950 through 1974 show a large decline in the abundance of predators due to fishing in the region.

Over that 25-year period, catches of all species in the trap were recorded three times a day for roughly six months each year. This kind of long-term, consistent data collection is rare, but it is useful because it allows scientists to attribute changes in the fish community to factors other than changes in the sampling method.

© Marco Faimali

Fishing continues today at the tuna trap in Camogli, Italy.

The authors divided the 51 species caught into three groups: top predators (such as large tuna and sharks), middle predators (such as smaller tunas and squid), and prey (such as pilchard and herring). Over the 25 years, the relative abundance of both predator groups declined by about 80 percent, with some large sharks almost completely absent by 1974. By contrast, the relative abundance of the prey group quadrupled. (Relative abundance is the size of each fish population relative to the others.)

Statistical analysis of the fish community showed that the predator groups had strong negative effects on their prey, which tended to stabilize fluctuations in prey abundance. As predators declined, this stabilizing effect was reduced.

By the 1970s, the dynamics of the prey species came to drive the dynamics of the overall fish community. These species have shorter generation times and higher reproductive rates, which naturally gave rise to more fluctuations and higher susceptibility to environmental perturbation.

The loss of predators led to a decline in community stability as measured by three metrics:

  • Resilience: the capacity of a fish community to recover from environmental perturbation
  • Reactivity: how much a community changes in response to environmental perturbation
  • Resistance: how variable a community is relative to environmental variation (high relative variability suggests low stability)

Further analysis showed that increasing fishing pressure over the study period was the best explanation for the decline in stability, via its effect on the abundance of predators. Other factors, like temperature and species diversity showed no trends over the period, and thus could not account for the decline in stability.

This finding is likely to apply to other fish communities. This would mean that maintaining the presence of predators is important not only for the survival of predator species, but for entire communities. It would also be important for the associated fisheries, which rely on stable, predictable yields.

 

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