_Effects of fishing on the food web structure
- The Food Web Structure
- Food Chain Structure
- Trophic Levels
- Trophic Cascades
- Top-Down Cascades
- Bottom-Up Cascades
- How fishing is effecting the food web structure?
- Case Study: Eastimating the impacts of fishing on dependent predators a case study in California Current
- Case Study: Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean
- Citations
The Food Web Structure
_
The food web structure is the feeding connections between organisms within an ecological community, these connections are also known as the food chain. Every living creature is interconnected with other creatures, each rely on a food source that is of another organism; without that other organism as a food source survival would be difficult. In each ecosystem every species has a position; for example to have a functioning community every individual has a job, every position is as important as the other. In human society there is a working class that makes up the largest portion of the human population, than to the middle class that is smaller than the working class than finally the rich which is the smallest class. Each class is important to a functioning human society.
Food Chain Structure
Figure 2: Trophic Level interactions
__For an
ecosystem, the largest group is the producers which can be interpreted to be
the base of the food web structure they are also known as the autotrophs (Figure
1). Autotrophs produce their own energy
through photosynthesis the use of sunlight to convert into energy. The next group is the consumers also known as
the heterotrophs, within the consumers are three groups the primary, secondary,
tertiary and so on. Comsumers, or
heterotrophs, requires the consumption of another living organism. Primary consumers eat producers, can also be
called herbivores; zooplankton is a primary consumer to phytoplankton,
producers, that uses light and nutrients to create energy (Figure 2). Secondary consumers rely on primary producers
as a food source; to continue the chain fish, secondary comsumers, will consume
zooplankton, primary consumers (Figure 2).
Further along the food chain seals will feed on fish making seals the
tertiary consumer feeding the secondary, consumers the fish. Finally on this specific food chain there is
a quaternary consumer, also known as the apex predator, the shark, quaternary
consumer, preys on seals the tertiary consumer.
Apex predators can be found at the top of the food web, or in figure 1,
the top of the food pyramid.
Each organism has a variety of other organisms that it will consume and will consume it. There are multiple links to a single organism, an ecosystem is variable according to situation.
Each organism has a variety of other organisms that it will consume and will consume it. There are multiple links to a single organism, an ecosystem is variable according to situation.
(A great site to learn more about the food web plus children worksheets and quizzes are available at http://www.enchantedlearning.com/subjects/foodchain/)
Trophic Levels
_ The trophic level is the position of an organism relative to that start of the food chain. In the sections above, the food chain are the feeding connections between organisms of the prey they consume and the predators that will consume them. There are multiple steps from the beginning of the chain to the end, each of those steps are trophic levels from the producers through all the consumers till the apex predator
_Trophic Cascades
_ Trophic cascades are predator and prey interactions; as a predator population increases, it will in turn affect prey population number 1 by decreasing the population, than prey number 1’s food source will increase. For example as orca populations increase their prey, sea otters, population will decrease; than leaving room for the sea otter’s prey population to increase which are sea urchins. Sea urchins in this case will be the primary consumers to the producers,kelp; kelp abundance will decrease (10) (Figure 3). This is a trophic cascade as well as trophic interactions. The trophic interactions are the ways organisms within a food web interact, their connections to each other by predator and prey relationships. Studying trophic cascades are important; understanding the anthropogenic affects of removing a trophic level by hunting and/or fishing a species that is a part of the food web can have dramatic affects on trophic interactions
Figure 3: For figures 3-6, arrow thickness is relative to population size. For example in figure 3 orca population has increased so the arrow pointing to sea otter is thicker than the arrow pointing from sea otter to sea urchin because orcas have decrease sea otter population. With a decrease in sea otter population there is an increase in sea urchin population so the arrow from sea urchin to kelp is thick (This is the same for figure 4, 5 and 6).
Top-Down Cascade
_ A top-down cascade is the trophic cascade of the removal of top predators that can be either the quaternary and/or tertiary consumer, can cause shifts and changes in abundance in primary consumers and producers (4). This disrupts the natural order of the trophic cascade; in the orca example if orca population decline greatly than sea otter population increase declining sea urchin populations (Figure 4). But if sea otter population is decrease orca population will also decrease because their food source is dwindle, although sea urchin populations will increase (Figure 5).
Bottom-Up Cascade
_ A bottom-up cascade is the removal of either primary consumer and/or producers. Using the same example, when kelp abundance decreases that limits the food source to sea urchin which will have a decline in their population. Sea otters depend greatly on sea urchins as a food source, when sea urchin population decrease there will be a decline in sea otter population (Figure 6). This type of trophic cascade of bottom-up affects every member of the food chain community.
__ Trophic cascades are important
they are a natural population control between species. Anthropogenic additions deviates the natural
abundance of species population that many species are seen today as endangered
to very rare to finally extinct.
How is fishing effecting the food web structure?
__ Since the industrial age, commercial fishing has
exponentially increased its productivity due because of the growing human
population demand for seafood. Fisheries
management has failed in controlling the rate of target species capture, “the
global marine fish catch is approaching its upper limit” [1]. With a continual demand increasing year to
year overharvesting fish stocks had lead to collapses in many fisheries and
close to collapse in other fisheries [2].
The vase removal of many individual fish from various target species has
lead to many impacts of the abundance and a shift the food web structure. Fishing has also altered the physical
properties of target species; commercial fishing not only targets a specific
species but also the larger individuals within that species [3]. Economically the larger individuals will have
more financial compensation rather than the small individuals. This technique of fishing out the larger
individuals has lead to a decrease in the mean size of target fish
species. This largely affects the species
survival within the marine ecosystem, decrease their ability to protect and
defend themselves. Management policy
makers need to take into account various aspects of the food web to create the
least impacting management plan by consulting researchers that study ways
fishing is impacting the food web structure.
Studies look at ways fishing is impacting the food web structure by studying the trophic interactions; there are many species that feed on multiple trophic levels that makes assigning specific trophic levels difficult. Also at different development stages of fish, trophic levels will change. Juvenile fish are limited to feeding on smaller organisms so juvenile fish will compete with lower trophic levels but as an adults their once competitor become their prey [3].
Impacts of fishing on the food web structure are the changes in abundance of predators can affect prey abundance [3]. The strength of trophic interactions is important, because dramatically decreasing a predator or prey population can have cascading affects. (Figure 4, 5 and 6 are examples of changes in abundance in predator and prey species) Removal of either prey or predator species can shift the food web; fishing a predator species decreases the predation on a prey species. Let’s call that prey species one; an increase of prey species one’s population will increase pressures on their prey species, which will be called prey species two. Prey species one will be a primary consumer species and prey species two will be a producer species. Increase pressure on prey species two, a plant species that create habitat for the ecosystem, decrease abundance will dramatically alter the structure of the environment that multiple species rely on for shelter (8). Another shift is the fishing of a prey species, predator species that rely on that prey species now have a reduce population to feed on. The decline of prey species cannot sustain the predator species so to compensate the predator species must shift to another prey species. This shift increase pressures on another prey species because there may be another predator species that prey on them, there are now multiple predator species preying on one prey species. This will result in a drop in the prey species that will be unable to sustain both predator species, so there will be a decrease in all prey and predator species (9).
Studies look at ways fishing is impacting the food web structure by studying the trophic interactions; there are many species that feed on multiple trophic levels that makes assigning specific trophic levels difficult. Also at different development stages of fish, trophic levels will change. Juvenile fish are limited to feeding on smaller organisms so juvenile fish will compete with lower trophic levels but as an adults their once competitor become their prey [3].
Impacts of fishing on the food web structure are the changes in abundance of predators can affect prey abundance [3]. The strength of trophic interactions is important, because dramatically decreasing a predator or prey population can have cascading affects. (Figure 4, 5 and 6 are examples of changes in abundance in predator and prey species) Removal of either prey or predator species can shift the food web; fishing a predator species decreases the predation on a prey species. Let’s call that prey species one; an increase of prey species one’s population will increase pressures on their prey species, which will be called prey species two. Prey species one will be a primary consumer species and prey species two will be a producer species. Increase pressure on prey species two, a plant species that create habitat for the ecosystem, decrease abundance will dramatically alter the structure of the environment that multiple species rely on for shelter (8). Another shift is the fishing of a prey species, predator species that rely on that prey species now have a reduce population to feed on. The decline of prey species cannot sustain the predator species so to compensate the predator species must shift to another prey species. This shift increase pressures on another prey species because there may be another predator species that prey on them, there are now multiple predator species preying on one prey species. This will result in a drop in the prey species that will be unable to sustain both predator species, so there will be a decrease in all prey and predator species (9).
Case Study: Eastimating the impacts of fishing on dependent predators a case study in California Current
_
In the California Current seabird rely on juvenile rockfish (Sebates spp.) as a food source especially during the breeding season. Seabird breeding success is related to the abundance of juvenile rockfish, fishing adult rockfish is affecting seabird productivity. Seabird productivity is determined by the number of chicks that are produced from a mating pair of adult seabirds. This study was conducted in Southeast Farallon Island near San Francisco, California (Figure 8). Rockfish are long-lived with late sexual mature age, commercially fishing has removed adult rockfish before they reach sexual maturity resulting in a lower abundance of juvenile rockfish. Seabird productivity, abundance and distribution are sensitive to changes in prey resources. Most seabird species are long lived with a low rate of reproduction, fishing takes away advantages of ideal conditions for seabird reproduction that can have a long term population effects. Low abundance of juvenile rockfish increase the amount of time eggs and chicks are unprotected increasing vulnerability to predation because parents need to spend more time searching for food.
Overall results show that increase fishing has lead to decrease seabird productivity leading to a long term decline in seabird populations (Figure 9). This is an example of bottom-up cascade, a prey species abundance is low affecting a predator species productivity.
Overall results show that increase fishing has lead to decrease seabird productivity leading to a long term decline in seabird populations (Figure 9). This is an example of bottom-up cascade, a prey species abundance is low affecting a predator species productivity.
Case Study: Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean
_ A study by Meyer et al. 2007 study the abundance of 11 great sharks that prey on elasmobranchs (rays, skates and small sharks), there has been a steady decrease in the great shark species for decades and an increase in the elasmobranchs species in coastal northwest Atlantic (Figure 7). Fishing is the source of the decline in the great shark species, the removal of these apex predators has created a top-down cascade effect. The increase abundance of the cownose ray that preys on bay scallops (Argopectan irradians), soft-shell clams (Mya arenaria), hard clams (Mercenaria mercanaria) and oysters (Crassostrea virginica) has lead to the collapse of scallop fisheries and created competition with humans for food. Also inhibiting the recovery of hard clams, soft-shell clams and oysters; increase predation from elasmobranchs plus humans increase pressure on primary consumer species that are unable to recover.
This decrease of apex predator has cascading effects on lower trophic levels. Also the eastern seaboard’s longest continuous shark-target survey (UNC) showed mean length of a few great shark species has decline by 17-47%; overexploitation has left few mature individuals in the populations. There is an increase in juvenile hammerhead populations because the lost of the apex predators that prey on juvenile hammerheads.
This study has showed the lost of apex predators can shift an ecosystem to an increase abundance of elasmobranchs. It is predicted that the decrease of scallops and bivalves will alter the structure in seagrass habitat, which is already highly degraded from anthropogenic affects. This alternation of the food web structure can be expected in other ecosystems that have their apex predators removed, a top-down cascade.
This decrease of apex predator has cascading effects on lower trophic levels. Also the eastern seaboard’s longest continuous shark-target survey (UNC) showed mean length of a few great shark species has decline by 17-47%; overexploitation has left few mature individuals in the populations. There is an increase in juvenile hammerhead populations because the lost of the apex predators that prey on juvenile hammerheads.
This study has showed the lost of apex predators can shift an ecosystem to an increase abundance of elasmobranchs. It is predicted that the decrease of scallops and bivalves will alter the structure in seagrass habitat, which is already highly degraded from anthropogenic affects. This alternation of the food web structure can be expected in other ecosystems that have their apex predators removed, a top-down cascade.
Citations
_ 1. Botsford
WB, Castilla JC and Peterson CH (1997) The Management of Fisheries and Marine
Ecosystems. Science 277: 508-515 DOI: 10.1126/science.277.5325.509
2. Roughgarden J and Smith F (1996) Why fisheries collapse and what to do about it. Ecology 93: 5078-5083
3. 2006. Evidence for Ecosystem Effects of Fishing. Dynamic Changes in Marine Ecosystems: Fishing, Food Webs, and Future Options. The National Academies Press. Washington, D.C. Pp 23-57
4. Sieben K, Rippen AD and Eriksson BK (2011) Cascading effects from predator removal depend on resource availability in a benthic food web. Marine Biology 158:391-400. DOI 10.1007/s00227-010-1567-5
5. Field JC, MacCall AD, Bradley RW and Sydeman WJ (2010) Estimating the impacts of fishing on dependent predators: a case study in the California Current. Ecological Applications 20:2223-2236.
6. Myers RA, Baum JK, Shepherd TD, Powers SP and Peterson CH (2007) Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean. Science 315:1846-1850. DOI: 10.1126/science.1138657
7. Jackson JBC and Sala E (2001) Unnatural Oceans. Marine Science 65:273-281.
8. Jackson JBC, Kirby MX, Berger WH, Bjorndal KA, Botsford LW, Bourque BJ, Bradbury RH, Cooke R, Erlandson J, Estes JA, Hughes TP, Kidwell S, Lange CB, Lenihan HS, Pandolfi JM, Peterson CH, Steneck RS, Tegner MJ and Warner RR (2001) Historical Overfishing and the Recent Collapse of Coastal Ecosystems. Science. 293:629-638. DOI: 10.1126/science.1059199
9. Kaider MJ and Jennings S 2004. Ecosystem Effects of Fishing. IN: Hart PJB and Reynolds JD (ed) Handbook of Fish Biology and Fisheries: volume 2, Fisheries. Blackwell Publishing. Malden, MA. Pp 342-366.
10. Estes JA and Duggins DO (1995) Sea Otters and Kelp Forests in Alaska: Generality and Variation in a Community Ecological Paradigm. Ecological Monographs 65: 75-100
11. Polunini NVC and Pinnegar JK 2004. Trophic Ecology and the Structure of Marine Food Webs. IN: Hart PJB and Reynolds JD (ed) Handbook of Fish Biology and Fisheries: Volume 1, Fish Biology. Blackwell Publishing. Malden, MA. Pp 301-320.
Figures
1. http://www.enchantedlearning.com/subjects/foodchain/trophiclevels.GIF
2. http://www.enchantedlearning.com/subjects/foodchain/
3-6. Orca: http://danielliug7.edublogs.org/files/2011/11/orca-24capsf.jpg
Sea otter: http://www.eaglewingtours.com/userimages/Image/sea-otter.jpg
Sea urchin: http://lhsvirtualzoo.wikispaces.com/file/view/2009_04_17_1_sea_urchin.jpg
Kelp: http://www.foreverlookingood.com/wp-content/uploads/2008/12/macrocystis-pyrifera- kelp.jpg
7. Myers RA, Baum JK, Shepherd TD, Powers SP and Peterson CH (2007) Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean. Science 315:1846-1850. DOI: 10.1126/science.1138657
8. Field JC, MacCall AD, Bradley RW and Sydeman WJ (2010) Estimating the impacts of fishing on dependent predators: a case study in the California Current. Ecological Applications 20:2223-2236.
9. Field JC, MacCall AD, Bradley RW and Sydeman WJ (2010) Estimating the impacts of fishing on dependent predators: a case study in the California Current. Ecological Applications 20:2223-2236.
Videos:
http://www.youtube.com/watch?v=v_aSl3iL7rM PBS “Jean-Michel Cousteau: Ocean Adventures, Kelp Forest, PBS.” Youtube. 20 May 2008. Web. 4 December 2011.
2. Roughgarden J and Smith F (1996) Why fisheries collapse and what to do about it. Ecology 93: 5078-5083
3. 2006. Evidence for Ecosystem Effects of Fishing. Dynamic Changes in Marine Ecosystems: Fishing, Food Webs, and Future Options. The National Academies Press. Washington, D.C. Pp 23-57
4. Sieben K, Rippen AD and Eriksson BK (2011) Cascading effects from predator removal depend on resource availability in a benthic food web. Marine Biology 158:391-400. DOI 10.1007/s00227-010-1567-5
5. Field JC, MacCall AD, Bradley RW and Sydeman WJ (2010) Estimating the impacts of fishing on dependent predators: a case study in the California Current. Ecological Applications 20:2223-2236.
6. Myers RA, Baum JK, Shepherd TD, Powers SP and Peterson CH (2007) Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean. Science 315:1846-1850. DOI: 10.1126/science.1138657
7. Jackson JBC and Sala E (2001) Unnatural Oceans. Marine Science 65:273-281.
8. Jackson JBC, Kirby MX, Berger WH, Bjorndal KA, Botsford LW, Bourque BJ, Bradbury RH, Cooke R, Erlandson J, Estes JA, Hughes TP, Kidwell S, Lange CB, Lenihan HS, Pandolfi JM, Peterson CH, Steneck RS, Tegner MJ and Warner RR (2001) Historical Overfishing and the Recent Collapse of Coastal Ecosystems. Science. 293:629-638. DOI: 10.1126/science.1059199
9. Kaider MJ and Jennings S 2004. Ecosystem Effects of Fishing. IN: Hart PJB and Reynolds JD (ed) Handbook of Fish Biology and Fisheries: volume 2, Fisheries. Blackwell Publishing. Malden, MA. Pp 342-366.
10. Estes JA and Duggins DO (1995) Sea Otters and Kelp Forests in Alaska: Generality and Variation in a Community Ecological Paradigm. Ecological Monographs 65: 75-100
11. Polunini NVC and Pinnegar JK 2004. Trophic Ecology and the Structure of Marine Food Webs. IN: Hart PJB and Reynolds JD (ed) Handbook of Fish Biology and Fisheries: Volume 1, Fish Biology. Blackwell Publishing. Malden, MA. Pp 301-320.
Figures
1. http://www.enchantedlearning.com/subjects/foodchain/trophiclevels.GIF
2. http://www.enchantedlearning.com/subjects/foodchain/
3-6. Orca: http://danielliug7.edublogs.org/files/2011/11/orca-24capsf.jpg
Sea otter: http://www.eaglewingtours.com/userimages/Image/sea-otter.jpg
Sea urchin: http://lhsvirtualzoo.wikispaces.com/file/view/2009_04_17_1_sea_urchin.jpg
Kelp: http://www.foreverlookingood.com/wp-content/uploads/2008/12/macrocystis-pyrifera- kelp.jpg
7. Myers RA, Baum JK, Shepherd TD, Powers SP and Peterson CH (2007) Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean. Science 315:1846-1850. DOI: 10.1126/science.1138657
8. Field JC, MacCall AD, Bradley RW and Sydeman WJ (2010) Estimating the impacts of fishing on dependent predators: a case study in the California Current. Ecological Applications 20:2223-2236.
9. Field JC, MacCall AD, Bradley RW and Sydeman WJ (2010) Estimating the impacts of fishing on dependent predators: a case study in the California Current. Ecological Applications 20:2223-2236.
Videos:
http://www.youtube.com/watch?v=v_aSl3iL7rM PBS “Jean-Michel Cousteau: Ocean Adventures, Kelp Forest, PBS.” Youtube. 20 May 2008. Web. 4 December 2011.