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Complexity and Change in a Simple Food Web: Studies in the Baltic Sea (FAO Area 27.IIId)
Stockholm University, Faculty of Science, Department of Systems Ecology.
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

An influence at one trophic level can result in dynamic impacts also on other components of a food web. These dynamics are known as trophic cascades, and can be both top-down and bottom-up. After a near-collapse of the Baltic cod Gadus morhua stock in the 1980s, its main prey sprat Sprattus sprattus increased dramatically. The main food of sprat, marine copepods, decreased during the same time period, likely a combined effect of increased predation pressure from sprat and decreasing salinities. This shortage of food for sprat resulted in decreasing quality of sprat as a food source for common guillemots Uria aalge. However, a recent increase in fishing for sprat has again resulted in better feeding conditions for guillemots.

Human impacts on this simple food web can be complex. In the early 20th century, marine mammals were abundant and nutrient levels were low in the Baltic Sea. This thesis illustrate that this situation corresponded to lower fish biomass. A reduction of seals early in the century led to reduced top-down control, which resulted in increasing fish stocks. Later, in the 1950s, the largest inflow of salt water during the century mobilized accumulated phosphorus from the deep sediments, which stimulated nitrogen fixation. Combined with increasing anthropogenic nutrient loads, this led to increased primary production and a rapid change from an oligotrophic to a eutrophicated state. This change can be termed a regime shift, which also stimulated fish production. Subsequent over-fishing of cod likely caused a second regime shift, from a cod- to a clupeid- dominated state, which led to the described effects on the common guillemots.

Several factors affect the life-history of this long-lived seabird. Bycatches in gillnet fisheries is one factor directly affecting guillemot survival, and the proportion of bycatches increased during a period of increasing fishing effort. Surprisingly, avian cholera, a previously undocumented disease in common guillemots, was found at times to cause considerable adult mortality. Common guillemot life-history information can communicate the diversity of factors influencing marine ecosystems – hopefully this can increase our understanding of how complex even "simple" food webs are.

Place, publisher, year, edition, pages
Stockholm: Institutionen för systemekologi , 2006. , 36 p.
Keyword [en]
Avian cholera, Bottom-up, Bycatch, Clupea harengus, Ecosystem, Food web, Gadus morhua, Halichoerus grypus, Regime shift, Sprattus sprattus, Top-down, Trophic cascade, Uria aalge
National Category
Ecology
Identifiers
URN: urn:nbn:se:su:diva-953ISBN: 91-7155-225-1 (print)OAI: oai:DiVA.org:su-953DiVA: diva2:200720
Public defence
2006-05-19, sal G, Arrheniuslaboratorierna, Svante Arrhenius väg 14-18, Stockholm, 10:00
Opponent
Supervisors
Available from: 2006-04-05 Created: 2006-04-05Bibliographically approved
List of papers
1. A decrease in fledging body mass in common guillemot Uria aalge chicks in the Baltic Sea
Open this publication in new window or tab >>A decrease in fledging body mass in common guillemot Uria aalge chicks in the Baltic Sea
2001 In: Marine Ecology Progres Series, ISSN 0171-8630, Vol. 224, 305-309 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-25872 (URN)
Note
Part of urn:nbn:se:su:diva-953Available from: 2006-04-05 Created: 2006-04-05Bibliographically approved
2. Fish, seabirds and trophic cascades in the Baltic Sea
Open this publication in new window or tab >>Fish, seabirds and trophic cascades in the Baltic Sea
2006 (English)In: Marine Ecology Progress Series, ISSN 0171-8630, E-ISSN 1616-1599, Vol. 323, 233-238 p.Article in journal (Refereed) Published
Abstract [en]

 

 

 

In the relatively simple Baltic Sea ecosystem, zooplankton-feeding sprat

 

 

Sprattus sprattus

is a major food source for breeding seabirds and piscivorous fish, and an important resource for

commercial fisheries. Large-scale and long-term ecosystem changes resulting mainly from over fishing

and recruitment failure of cod

 

 

Gadus morhua

, which is the main fish predator of sprat, have

affected natural-history patterns in a piscivorous seabird, the common guillemot

 

 

Uria aalge

, in a complex

way. As the sprat stock increased, leading to lower energy content of fish, common guillemot

chick body mass at fledging decreased. However, chick fledging body mass recovered in recent years

as the sprat stock diminished, which brought about corresponding increases in sprat weight-at-age

and energy content. The cod and sprat fishery affect the common guillemots in the Baltic Sea, but the

effects differ depending on the management strategy.

Keyword
Cod · Baltic Sea · Fisheries management · Gadus morhua · Common guillemot · Sprat ·
National Category
Ecology
Identifiers
urn:nbn:se:su:diva-25873 (URN)
Note
Part of urn:nbn:se:su:diva-953Available from: 2006-04-05 Created: 2006-04-05 Last updated: 2010-11-03Bibliographically approved
3. Human-induced trophic cascades and ecological regime shifts in the Baltic Sea
Open this publication in new window or tab >>Human-induced trophic cascades and ecological regime shifts in the Baltic Sea
Show others...
2007 (English)In: Ecosystems (New York. Print), ISSN 1432-9840, E-ISSN 1435-0629, Vol. 10, no 6, 877-889 p.Article in journal (Refereed) Published
Abstract [en]

The ecosystems of coastal and enclosed seas are under increasing anthropogenic pressure worldwide, with Chesapeake Bay, the Gulf of Mexico and the Black and Baltic Seas as well known examples. We use an ecosystem model (Ecopath with Ecosim, EwE) to show that reduced top-down control (seal predation) and increased bottom-up forcing (eutrophication) can largely explain the historical dynamics of the main fish stocks (cod, herring and sprat) in the Baltic Sea between 1900 and 1980. Based on these results and the historical fish stock development we identify two major ecological transitions. A shift from seal to cod domination was caused by a virtual elimination of marine mammals followed by a shift from an oligotrophic to a eutrophic state. A third shift from cod to clupeid domination in the late 1980s has previously been explained by overfishing of cod and climatic changes. We propose that the shift from an oligotrophic to a eutrophic state represents a true regime shift with a stabilizing mechanism for a hysteresis phenomenon. There are also mechanisms that could stabilize the shift from a cod to clupeid dominated ecosystem, but there are no indications that the ecosystem has been pushed that far yet. We argue that the shifts in the Baltic Sea are a consequence of human impacts, although variations in climate may have influenced their timing, magnitude and persistence.

Keyword
eutrophication, fishing, marine mammal, predation, regime shift, trophic cascade
National Category
Ecology
Identifiers
urn:nbn:se:su:diva-25874 (URN)10.1007/s10021-007-9069-0 (DOI)
Available from: 2006-04-05 Created: 2006-04-05 Last updated: 2011-03-10Bibliographically approved
4. Bycatches of common guillemot (Uria aalge) in the Baltic Sea gillnet fishery
Open this publication in new window or tab >>Bycatches of common guillemot (Uria aalge) in the Baltic Sea gillnet fishery
2002 In: Biological Conservation, ISSN 0006-8630, Vol. 105, no 3, 309-319 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-25875 (URN)
Note
Part of urn:nbn:se:su:diva-953Available from: 2006-04-05 Created: 2006-04-05Bibliographically approved
5. Adult survival and avian cholera in Common Guillemots Uria aalge in the Baltic Sea
Open this publication in new window or tab >>Adult survival and avian cholera in Common Guillemots Uria aalge in the Baltic Sea
2004 In: Ibis, ISSN 0019-1019, Vol. 146, no 3, 531-534 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-25876 (URN)
Note
Part of urn:nbn:se:su:diva-953Available from: 2006-04-05 Created: 2006-04-05Bibliographically approved

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