Marine food webs: community change and trophic level decline
A characteristic regime shift in aquatic systems involves an abrupt increase in the dominance of lower trophic level groups within aquatic food webs. This regime shift involves a change from an ecosystem with high numbers of predatory fish to one dominated by pelagic planktivores. The shift is often initiated by high fishing pressure on top-predators followed by a trophic cascade, but can also be brought about by other environmental factors like global warming and upwelling increase. In extreme cases the food web is shortened due to disappearance of top predators and the carbon transfer pathways is dominated by microbial webs instead of the classic trophic chain. The new regime can be enforced and maintained by biological mechanisms including minimum population biomass, competition and dietary relations, or environmental conditions. Despite there being some mechanism that often dissipate the trophic cascade, food web regime shifts do have substantial impact on commercial fisheries, as well as increase the vulnerability of an ecosystem to eutrophication, hypoxia and invasion by non-native species.
Key direct drivers
- Harvest and resource consumption
- External inputs (eg fertilizers)
- Global climate change
- Marine & coastal
Key Ecosystem Processes
- Primary production
- Aesthetic values
- Food and nutrition
- Livelihoods and economic activity
- Cultural, aesthetic and recreational values
- Cultural identity
Typical spatial scale
- National (country)
Typical time scale
- Contemporary observations
Confidence: Existence of RS
- Speculative – Regime shift has been proposed, but little evidence as yet
Confidence: Mechanism underlying RS
- Contested – Multiple proposed mechanisms, reasonable evidence both for and against different mechanisms
Links to other regime shifts
This regime shift involves an abrupt reorganization of an aquatic food web due to a decrease in top predators or increase in environmental factors like temperature. Trophic cascades play a central role in these regime shifts. During these cascades groups in adjacent trophic levels often show inverse patterns in their abundance (Carpenter 2003). In other words, every other trophic level displays an increase in biomass, which leads to increased predation pressure and biomass decrease on every other trophic level. Most trophic cascades are described in ecosystems with low species diversity and/or simple food webs and/or small size (Frank et al. 2005). The magnitude of a trophic cascade varies depending on species diversity, regional oceanography, local physical disturbance, habitat complexity and fishery practices (Salomon et al. 2010). Food webs with more trophic levels and biodiversity show less trophic cascades than simpler systems (Salomon et al. 2010); due in part to omnivory and the variation on trophic interaction strength (Bascompte et al. 2005). However some documented examples includes subtidal reefs in New Zealand (Shears and Babcock 2002), Caribbean coral reefs (Bellwood et al. 2004), the Gulf of Maine (Steneck et al. 2004), and the North Atlantic (Kirby et al. 2009).
Predator-dominated food web.
Ecosystems in this regime are characterized by high predatory fish, low planktivorous fish, high zooplankton and low phytoplankton abundance. Such ecosystems can be subject to human influence, such as intensive fishery, but exhibit so called “natural compensation” e.g. via species richness and omnivory (Pace et al. 1999) that maintain the regime. Despite high productivity, such ecosystems are less likely to suffer from eutrophication due to high grazing pressure on phytoplankton. Ecosystem services associated with predator-dominated food webs includes food provision, higher biodiversity, better disease and pest control, as well as cultural services like recreation and aesthetic values (e.g. diving, sport fishing).
Planktivore-dominated food web.
Ecosystems in this regime are characterized by low predatory fish, high planktivorous fish, low zooplankton and high phytoplankton abundance. In extreme cases the actual number of trophic levels (TLs) can decrease. Ecosystems with lower trophic level dominance are more vulnerable to eutrophication due to lowered grazing pressure on phytoplankton, as well as invasion of planktivore organisms like jellyfish. A planktivore-dominated food web can affect the fluxes of carbon, transforming aquatic systems -coastal and lakes- from sinks to sources of green house gases (Bakun et al. 2010; Estes et al. 2011). Other ecosystem services like fisheries and recreation are expected to be significantly reduced.
Drivers and causes of the regime shift
Trophic cascades are usually initiated by fishing pressure that have direct or indirect impacts on the abundance of top predators. The response time to these drivers varies across ecosystems. In the case of coastal areas it has been found that the ecosystem response time to overfishing can vary from decades to centuries where multiple predators exist (Jackson et al. 2001).
Other factors that affect the regime shift are global warming and demand of food and fiber. Global warming have effects on upwelling, water circulatory systems that bring nutrients from the bottom of the sea towards the surface. By altering upwellings, food webs receive either too much or too little nutrients. In both cases the energy transferred towards higher trophic levels diminish as nutrients input variability increase. Demand of food strongly drives fishing, being the main cause of lost of top predators over the world.
How the regime shift works
Predator dominated food webs occur in environments with low disturbance both from fishing and nutrients input. They usually have four or more trophic levels. The regime is maintained by biotic mechanism that comprise predation and competition both intra and inter species. Abiotic factors also play a role maintaining the regime, by allowing an intermediate level of nutrient inputs from the deep ocean necessary for sustaining the food web.
In general, a combination of low top predator productivity due to poor recruitment or somatic growth combined with high exploitation pressure are conditions under which regime shifts in aquatic food webs are likely to occur (Daskalov et al. 2007). Overfishing can unbalance trophic relations, changing the configuration of competition and predation. Weak competitors may become stronger given less fishing pressure on such stocks, or by favoring the reproduction of it’s preys by fishing its competitor species. In addition, changes in nutrients inputs can represent an advantage for lower trophic levels, allowing them to escape the control of its predators.
As result, the ecological function of certain species groups is changed, allowing over abundance of immediate lower level. For example, the decrease of top-predators allows over abundance of meso predators that in turn reduces planktivorous fish and increase zooplankton. Further increase in nutrients from upwellings can lock the food web in a state dominated by planktivore fish as jellyfish. Low nutrients input, on the other hand, would reduce the amount of energy available for the food web and as a result an abrupt reduction of higher trophic levels is expected.
Impacts on ecosystem services and human well-being
Shift from predators to planktivore dominated food webs
Commercial fish stocks that are locked into a regime where they are maintained at low levels of abundance has obvious economic consequences on fisheries industry. However, these fisheries related shifts are not always negative, as for example a decrease in top predators can increase the catches of mid-level prey (e.g. shrimps and clupeids) (Frank et al. 2005). When an ecosystem-wide regime shift occurs it affects not only targeted groups, such as commercial fish (Daskalov et al. 2007) but also impacts other ecosystem functions. Biodiversity as well as ecosystem resilience may be reduced by a shift from an ecosystem with high top predator diversity and dominance to a system with lower trophic level dominance. This means that ecosystem may become more vulnerable to both climatic and anthropogenic change. Such ecosystems may more easily suffer from eutrophication, hypoxia and invasion by non-native species. Eutrophication and hypoxia in turn lead to increased algal blooms and decrease the recreational values of water bodies.
Predators and commercial fish stocks have remained under 10% of their previous sizes even after decades of fishing closure (Ainley and Blight 2009). Food production, primary productivity, pest regulation, and cultural services are the ecosystem services most affected by the reduction of food webs, which in turn increases the risk of fisheries collapse. Depletion of fish stocks has been estimated to have affected the employment of roughly 14 million fishermen, 12 million of which correspond to artisanal fisheries (Hassan et al. 2005). Fish catches are projected to further decrease in the 21st century affecting protein sources for people, especially in poor regions (Hassan et al. 2005). The estimated current contribution of fisheries to human protein consumption is 29 million tons produced industrially and 24 million tons in small-scale fisheries (Hassan et al. 2005).
Options for enhancing resilience
In food webs that exhibit strong top-down control, decreasing the fishing pressure on top predators is an obvious strategy (Scheffer et al. 2001; Frank et al. 2005). However, due to the hysteresis effect, return to a predator-dominated food web may often require substantially lower fishing levels than that which induced the regime shift, together with additional measures. Hence, restoration is usually costly, especially to fisherman (Moellmann et al. 2009). As mentioned above, environmental factors can play an important role in maintaining alternative food web regimes. Nutrient loading is one of the key parameters controlling processes such as eutrophication and hypoxia. However, decreasing nutrient loads in marine environments requires large scale actions and often collaboration at international level (e.g. Helsinki Commission Baltic Sea Action Plan in the Baltic Sea). Hence, the time-frame of these actions in addition to time-lag in ecosystem response is often long. Climatic variation cannot be directly controlled by human actions. However, climate impacts can be mediated by employing other management measures that contribute to maintaining healthy marine ecosystems with high biodiversity and resilience, which then can compensate for changes in climate.
Options for reducing resilience to encourage restoration or transformation
In addition, in small closed food webs systems (3 to 4 trophic levels), biomanipulation has been suggested to manage one of the symptoms of degraded marine environments, namely eutrophication. This involves increasing the population of predatory fish such as bass, pike and walleye through stocking or reduced angling quotas. Increased populations of these predators leads to a decrease in the level of zooplanktivores. This in turn allows an increase in the population of planktivores that graze on the algae and zooplanckton, helping to reduce the algal density and phytoplankton respectively(Smith and Schindler 2009). This option, however, has limited reach when it comes to open marine foodwebs, where seasonal migrations and metapopulation dynamics are common.
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