Regime Shifts DataBase

Large persistent changes in ecosystem services

Predator-dominated food web

• Biotic mechanisms (local, contested): Biotic mechanisms occur at the species level through biotic interactions such as predation and competition both inter and intra species. In figure 2 for example, each link between two species A (predator) and B (prey) may be thought as a balancing feedback where the higher the abundance of A the lowest the abundance of B, reducing in turn the abundance of A who depends in B as resource. Food webs are characterized by many weak interactions and few strong interactions given that species usually prey on more than one resource. The strength of the links as well as feeding strategies like omnivory determines the vulnerability of the system to undergoes trophic cascades(Bascompte et al. 2005). In general, a system dominated by predators and with many weak interactions is less likely to undergo trophic cascades. However, selective fishing , particularly on top predators, can reduce its resilience (Bascompte et al. 2005, Estes et al. 2011).

Planktivore-dominated food web

Shift from predators to planktivore dominated food webs

Important shocks that contribute to the regime shift include:

•  Warm events (local, proposed): Warm events have been reported to increase SST and hence reduce nutrient exchange(Behrenfeld et al. 2006) through the climate-nutrients feedback mechanism. Changes in climate can also influence top predator abundance or distribution. In the North Sea it has been suggested that the decrease in cod stock since 1987 was brought about by a decrease in their preferred larval food C. finmarchicus due to increase in water temperature(Beaugrand 2004).

The main external direct drivers that contribute to the shift include:

• Fishing (local, well established): Fishing of top predators is a particularly important driver of tropic cascades(Pauly et al. 1998, Pace et al. 1999, Estes et al. 2011). Fishing pressure change the link strength of food webs and activates the biotic mechanism. Balancing feedbacks from predation and competition aggregate and express favoring the abundance of different functional groups.

The main external indirect drivers that contribute to the shift?

• Global warming (global, proposed): Global warming can affect food webs in two ways. First by inducing higher frequency of warm events that in turn increase SST, water density contrast and as result less nutrients exchange (locally) in the water column. With less nutrient inputs, food webs are expected to be less productive. Second, by increasing SST, water vapor also increase air temperature contrast which in turn increase upwellings and nutrients inputs regionally. High nutrient income can also trap food webs in planktivore dominated regimes. It is not clear to what extend both effects cancel each other. However, it worth to note that the first happens in the local scale and while the second on the regional one. It seems that both extremes of high or low nutrients input driven by global warming results in reduced productivity and biodiversity of food webs.
• Demand of food (Local to regional, well established): Food demand is thought to drive fishing pressure. As market mechanism and trade facilitate the commerce of fish, more fishing effort is encouraged and stocks are depleted faster, having less time to recover(Berkes et al. 2006).

Slow internal system changes that contribute to the regime shift include:

• Biodiversity (regional, well established): Biodiversity loss, both in the sense of species richness and functional group is a slow internal variable. Models indicate that food webs are robust to loss of species(Dunne and Williams 2004). However the extinction of key species can trigger in turn extinction cascades(Allesina and Pascual n.d.).