Climate change: underwater forest decline

Ling S. D., Johnson C. R., Frusher S. D. & Ridgway K. R. (2009) Overfishing reduces resilience of kelp beds to climate-driven catastrophic phase shift. Proc Natl Acad Sci USA 106, 22341-5.

Aim: 
To examine whether fishing has reduced kelp bed resilience and thus increased the risk of catastrophic overgrazing by the range-extending urchin C. rodgersii
Type of Study: 
Combination of natural experiment (historical and contemporary contrasts inside/outside sanctuaries) and manipulative experiment (urchin predation) in the field and aquaria
Key Results: 
Lobsters accounted for 92%-100% of observed predation events on urchins. In predation experiments only very large lobsters were successful predators of Centrostephanus rodgersii. Overlaying observed predation events with a physical model based on the size of the arc of the first pair of legs on lobsters, revealed that the upper theoretical limit predicted by the model was in close agreement with the ceiling of observed successful predation events. In situ only C. rodgersii >60 mm test diameter (TD) were observed to exist exposed on the reef surface and are vulnerable to lobster predation. . Given the size distribution of emergent C. rodgersii in eastern Tasmania, we estimated the minimum sized lobster capable of preying on C. rodgersii under natural conditions to be approximately 140 mm carapace length. Modelling of annual population projections using the apparent survival rates revealed significantly reduced survival for C. rodgersii inside MPAs (0.094/pop.annum) relative to the fished reefs (0.613/pop.annum). This demonstrates that fishing, by removing large predatory lobsters, has reduced the resilience of kelp beds against the climate-driven threat of the sea urchin and thus increased risk of catastrophic shift to widespread sea urchin barrens
Treatments: 
Protected (MPA) and fished reefs, predation rates with lobers of different sizes
Response: 
Management must aim to prevent further phase shifts to sea urchin barrens because the strong hysteresis effect makes rehabilitation of existing barrens to kelp beds exceedingly difficult. Rebuilding the size and abundance of reef predators will increase the resilience of kelp beds and thus reduce the likelihood of widespread sea urchin overgrazing in the first instance
Models: 
ANOVA and regression
Reviewer: 
Thomas Wernberg
Locations: 
Eastern Tasmania, Australia
Response variable : 
Urchin size and survival
Replication: 
2 regions kilometers apart; one fished and one MPA site in each
Ecosystem: 
Temperate reef
Full Reference: 
Ling S. D., Johnson C. R., Frusher S. D. & Ridgway K. R. (2009) Overfishing reduces resilience of kelp beds to climate-driven catastrophic phase shift. Proc Natl Acad Sci USA 106, 22341-5.