High abundance of a commercial fish species.
- Food web regulation mechanisms: trophic cascades / trophic triangles (local, contested): When a valuable fish population becomes substantially reduced, it may be replaced by a competitor species that performs the same ecological role. Thus, their functional role is not lost. High biodiversity increase resilience of the food web to disturbance such as fishing or climate oscillations. Fundamental mechanisms are not contested, their strength, variability and persistence is what varies and is contested.
Low abundance of the commercial fish species
- Food web regulation mechanisms: trophic cascades / trophic triangles (local, contested): When a valuable fish population becomes substantially reduced, it may be replaced by a competitor species that performs the same ecological role. However, if it is not replaced substantial changes in the food web may occur. If the commercial fish species is a fish predator, a drastic decrease in its abundance can lead to an explosion in the fish species that form its prey.
Trophic triangles occur when prey species feed on the juveniles of their predators - e.g., sprat and cod in the Baltic Sea (Moellmann et al. 2009). A dramatic increase in the prey population can therefore lead to increased predation on juvenile predators. If the system becomes locked in this regime, it may be very difficult for the predator population to recover even if fishing pressure is removed, because few juvenile reach maturity. This in turn means that the prey population remains large because the adult predator population is too small to substantially impact on the prey population. In this case, the feedback loop is due to predation in a trophic triangle.
A second mechanism related to food web dynamics emerges from competitive interactions which can lock-in a dominant species. When food web structure is modified by top-down or bottom-up forces, species can shift their dietary preferences in order to adapt to new conditions. In such a situation a new species can become a dominant species, and even once fishing effort is reduced it will continue to dominate. In this case a positive feedback loop is due to density dependent competitive interactions. Historic competitors or new competitors can be locked in at low numbers because competition has become too strong to allow population growth. For both mechanisms, nutrient input, climate and fishing are drivers that might can causes a food web to shift from one dominant competitor to another (Scheffer et al. 2008).
Allee effect (local, contested): The Allee effect refers to a situation where a decrease in the breeding population (mature individuals) leads to a reduced population growth rate. This effect is often termed depensation in fisheries. Depensation can result from the reduced likelihood of finding a mate when populations become low, or because lower population numbers reduce the per fish effectiveness of predator avoidance strategies such as schooling (Liermann and Hilborn 1997). When the population falls to a level at which population growth rates are lowered it can remain stuck in a lower population density even when fishing pressure is removed (Carpenter 2003). Low population levels due to the Allee effect may trigger or promote other trophic changes, discussed above, that can reinforce the low abundance regime.
Shift from high to low abundance of commercial fish species
Important shocks (eg droughts, floods) that contribute to the regime shift include:
- Climate anomalies such as extremes of the ENSO - PDA - NAO - HCE can push marine food webs from one regime to another (Regional, well supported): Although the underlying mechanism are not fully understood, climate dynamics often vary over decadal periods and this variation is thought to influence the structure of food webs and their inherent dynamics. Such is the case of ENSO (El Niño / La Niña Southern Oscillation), PDO (Pacific Decadal Oscillation), NAO (North Atlantic Oscillation), and HCE (Humboldt current ecosystem) (Moellmann et al. 2008, Takasuka et al. 2008, Alheit 2009, Jiao 2009, Moellmann et al. 2009)
The main external direct drivers that contribute to the shift are:
- Fishing pressure (Local to regional, well-established): Overfishing is the most common cause of fisheries collapse. Drivers of overfishing are related: unresponsive quotas, overcapitalization of boats, demand from new markets, global export of fish, subsidies for fishing, and technology improvements. Overfishing of international waters is also exacerbated by the tragedy of the commons, a dynamic where fishermen have strong incentives to fish more favoring their personal gains over the common good (Berkes et al. 2006).
- Nutrients inputs (Local to regional, well-established): Nutrients inputs can come from nutrient leakage from agriculture or urbanization. Over enriched water typically lead to the dominance of lower trophic levels and other symptoms of marine eutrophication (e.g. lower light penetration due to turbidity, algae blooms) that potentially impact fish stocks (Jackson et al. 2001, Bakun et al. 2010).
The main external indirect drivers that contribute to the shift are:
- Fishing fleet (Local to regional, well-established): – The difficulty of boats exiting a fishery, means that often fishing occurs at higher effort levels than are sustainable.
- Inflexible quotas (Local to regional, well-established): – quotas that are not lowered in response to declines in fish populations can lead to too many fish being caught to sustain the population.
- Technology (Local to regional, well-established): In the same way, technological developments substantially increase catch per unit effort, hence accelerating stocks depletion. Such is the case of storage technologies as refrigerators, geo positioning systems (GPS), sonars, telecommunications, and more efficient engines.
- Tragedy of the commons (Local to regional, well-established): many fisheries are common pool resources (CPR), making it very difficult to control who uses the resource, and very difficult to transfer the right to do so, creating a social dilemma called the Tragedy of the commons (Berkes et al. 2006). The tragedy of the commons can be exacerbated by regulatory capture: Dominant groups can shape the way a fishery works by controlling the regulation of their industry. Such regulatory capture, removes the independence of regulation for social benefits and switches it to benefit specific groups which can exacerbate the tragedy of the commons (Fisherman’s problem).
- Loss of local knowledge (Local, contested): in small scale fisheries the loss of traditional ecological knowledge can causes fisheries regulation and monitoring to become less effective because of loss of ability to interpret fisheries resilience (Berkes 2008).
- Demand for 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). Demand for food is further strengthened by the increasing preferences for sea food since the willingness to pay for it spreads over markets and increases fishing pressure.
- Trade facilities (Global, well-established): Trade accelerates the flow of fish and money spatially, masking the effect of local depletion by importing resources from somewhere (Clarke 2004, Lenzen et al. 2012).
- Urbanization (Local to regional, well-established): The development of coastal areas have lead to nutrients leaking increase, generating coastal eutrophication and sometimes dead zones (hypoxia)(Diaz and Rosenberg 2008). Such nutrient related phenomena can interact with upwellings related nutrient inputs and change the food web energy and carbon flow. [see Hypoxia]
- Global warming (Global, contested): Another set of mechanisms that may reinforce a low abundance regime of a valuable fish species are climate ocean interactions which influence marine food webs through bottom-up effects. Global warming increase sea surface temperature (SST) and may 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, 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. 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 (Bakun et al. 2010).
Global warming may play a fundamental role in destabilizing primary productivity in marine food webs (Kirby et al. 2009, Bakun et al. 2010). Such dynamics have been reported in regime shifts in kelp forests, coral reefs and estuaries. [see Coral transitions, kelp transitions]
Slow internal system changes that contribute to the regime shift include:
- Population structure (Local, well-established): At the population level, the demographic structure of the population is a slow process of change that can trigger mechanism as the Allee effect (Hutchings and Reynolds 2004). By demographic structure one refers to the distribution of class size (eggs, larvae, juveniles, adults and elders) which can trap the stock in low productive regime when altered (e.g. too many elders or too many juveniles).
- Trophic level diversity - functional groups (Regional, well-established): Functional groups are species that perform the same functions in the ecosystem (e.g. herbivores). High diversity in a functional group make its response behaviour to fishing effort slow and delayed (Jackson et al. 2001).
- Upwellings (Regional, well-established): Upwellings bring cool nutrient rich water to the surface, altering the structure of marine food webs. Upwellings are always present in the sea and follow seasonal dynamics. Climate variation impacts the strength and dynamics of upwelling. The strength and frequency of upwelling produces shaping new climate dynamics due to higher sea surface temperature, rain anomalies or ENSO-like events (Behrenfeld et al. 2006, Bakun et al. 2010).
Summary of Drivers
|Type (Direct, Indirect, Internal, Shock)
|Scale (local, regional, global)
|Uncertainty (speculative, proposed, well-established)
|demand of food
|tragedy of the commons
|local / regional
Summary of Ecosystem Service impacts on different User Groups
|References (if available)
|Feed, Fuel and Fibre Crops
|Wild Food & Products
|Air Quality Regulation
|Soil Erosion Regulation
|Pest & Disease Regulation
|Protection against Natural Hazards
|Cognitive & Educational
|Spiritual & Inspirational