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Marine food webs: community change and trophic level decline

Feedback mechanisms

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

  • Biotic mechanisms (local, contested): Once the system has shifted into the planktivore dominated regime, the high abundance of planktivores and low abundance of zooplankton may prevent the recovery of the predator population, even if fishing pressure is reduced substantially. Food webs are prone to trophic cascades when fishing pressure is high in species which have strong interactions, particularly top predators(Bascompte et al. 2005). Given that in marine food webs the average path distance between top predators and primary producers is generally short, disturbances as over-fishing may spread faster than previously thought(Dunne and Williams 2004). For example in the Central Baltic Sea the lack of cod stock recovery has been partially accounted for by the decrease in abundance of a zooplankton species (Pseudocalanus acuspes) considered as an important dietary source for cod larvae (Hinrichsen et al. 2002). The biomass of P. acuspes is on the other hand considered to be controlled by the abruptly increased planktivorous sprat (Mollmann and Koster 2002) that additionally prey on cod eggs, further suppressing cod recruitment (Koster and Mollmann 2000). Consequently, it is hard to generalize feedback mechanism based in inter species interactions, given these foodweb links can vary in strength from place to place.
  • Climate-carbon mechanism (regional, proposed): Abiotic factors like climatic forces can also have a significant role in maintaining and enforcing a regime characterized by high dominance of lower trophic level groups. Global warming, both natural and anthropogenic, is expected to increase sea surface temperature (SST) and lead to more frequent ENSO events. An increasing frequency and intensity of warm events accentuates the density contrast in the water column, inhibiting nutrient exchange through vertical mixing(Behrenfeld et al. 2006), and thereby reducing the productivity of marine food webs. Roughly half the biosphere’s net primary production is synthesized by phytoplankton in the oceans. These microscopic plants daily fix more than a hundred million tons of carbon dioxide, which in turn supports marine food webs that consume the total phytoplankton biomass every two to six days(Behrenfeld et al. 2006). Hence, inhibited mixing due to an increase in SST may substantially reduce fishery productivity by directly affecting net oceanic primary production. This phenomena has already been observed in the South American Pacific coast through satellite measurements of chlorophyll production during warm events(Behrenfeld et al. 2006)
  • Upwelling-high nutrients mechanism (regional, proposed). A parallel mechanism involving atmospheric dynamics may explain an increase in upwellings, although not necessarily an increase in fisheries productivity(Bakun et al. 2010). Increased temperatures associated with climate change promote the release of water vapor generating higher thermal low pressure cells. In other words, the difference in temperature between the air over the continental shelf and the air over the ocean increases. This physical difference increases wind stress perpendicular to the coast which in turns generates more intense upwelling(Bakun et al. 2010). However, it does not necessarily translate to higher biological productivity. In fact, nutrient-enriched food webs may become trapped in states where phytoplankton is overabundant and less mobile zooplanktivores like jellyfish (medusas) becomes their main predatory control. Under such scenario, poisonous gases like methane and sulfide are release due to dominance of microbial metabolism, being potential greenhouse-gases(Bakun et al. 2010).

Drivers

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.).

Summary of Drivers

# Driver (Name) Type (Direct, Indirect, Internal, Shock) Scale (local, regional, global) Uncertainty (speculative, proposed, well-established)
1 ENSO/warm events shock regional well-established
2 Upwellings shock regional well-established
3 nutrients input direct local well-established
4 fishing direct local well-established
5 global warming inidirect global speculative

Summary of Ecosystem Service impacts on different User Groups

References (if available)
Provisioning Services
Freshwater
Food Crops
Feed, Fuel and Fibre Crops
Livestock
Fisheries
Wild Food & Products
Timber
Woodfuel
Hydropower
Regulating Services
Air Quality Regulation
Climate Regulation
Water Purification
Soil Erosion Regulation
Pest & Disease Regulation
Pollination
Protection against Natural Hazards
Cultural Services
Recreation
Aesthetic Values
Cognitive & Educational
Spiritual & Inspirational