Energy Flow in Ecosystems Energy Flow in Ecosystems

Food Chains & Food Webs

• Food Chain: sequence of who eats or decomposes whom.

• Trophic Levels: feeding levels on the food chain

• Food Web: all the food chains (feeding levels) of all the organisms.

Ecological Pyramids

• Each Trophic level in a food chain or pyramid contains a certain amount of Biomass, the weight of all the organic matter contained in all the organisms.

• The percent of useable energy that transfers from one trophic level to the next ranges from 5-20%, on average being about 10%, called the 10% RULE.

10% Rule

http://channel.nationalgeographic.com/channel/wicked-tuna/bluefin-tuna/interactives/seafood-decision-guide/

Man Moves Down the Marine Food Chain, Creating Havoc

• By WILLIAM K. STEVENSPublished: February 10, 1998

• • AS overfishing depletes prized species like tuna, cod and swordfish, commercial fishermen are

moving farther down oceanic food webs in search of a catch, a new study has found. If this quest is pursued to its logical end, scientists warn, it will lead to a wholesale collapse of marine ecosystems.

• One symptom of this practice of ''fishing down'' the food chain is that second-level creatures normally preyed upon by the fish at the top of the chain are increasingly appearing on restaurant menus, said Dr. David Pauly, a fisheries biologist at the University of British Columbia in Vancouver who is the study's chief author. One example is squid. ''Americans wouldn't eat squid before,'' he said. ''It was used as bait, and now Americans are eating bait. It has all kinds of fancy Asian names, but it's bait.''

• The downward shift is global, according to the study, published in the current issue of the journal Science. Fisheries experts have believed for some time on the basis of anecdotal evidence that the shift was taking place, but the study is thought to be the first that has tried to document it systematically and statistically.

• Dr. Pauly and four colleagues, one in British Columbia and three at the International Center for Living Aquatic Resources Management in the Philippines, combined two sets of data to reach their conclusions. One was fisheries statistics compiled by the United Nations' Food and Agricultural Organization. The other was an analysis of the food web.

• The analysis assigned each marine species to one of four different levels in the food web; scientists call them trophic levels. On level four are top predators like sharks, tuna and swordfish. On level one, at the bottom, are grass, detritus and algae.

• The trophic levels are not sharply demarcated, said Dr. Pauly, because many sea creatures have ''ecumenical tastes'' and feed at more than one level. The model sorted out species according to their feeding habits and placed them on the 1-to-4 scale accordingly. Alaskan pollock near the top of the food web have a trophic level of 3.8, for instance, while the anchovy's level is 2.2.

• By combining the model analysis with United Nations statistics on worldwide catches of fish, the scientists calculated that the overall global trophic level of fish catches has declined to about 3.4 from 3.7 since the 1950's. The actual decline is probably greater than that, said Dr. Pauly, because statistics from the tropics are sketchy and show a weak decline, while there is an abundance of individual reports from the tropics of fishing lower on the food web.

• If the rate of decline suggested by the study continues, Dr. Pauly said, many marine food webs will be ''collapsing in on themselves'' in three or four decades.

• And because fish suitable for human consumption fall between about 2.5 and 4 on the trophic scale, he said, ''the space available for the fishery to move is quite limited.'' A simple and particularly stark example of this is the cod fishery off eastern Canada, Dr. Pauly said. With the collapse of that fishery, he said, fishermen have begun concentrating on the cod's prey, which are shrimp. Shrimp feed on bottom detritus, the lowest trophic level. ''It's mud,'' Dr. Pauly said, ''and that's when you hit the wall,'' beyond which the fishery has no more commercial value.

• The researchers offered no detailed prescription for a remedy, but they did suggest that in the next decades, fishery managers must emphasize the rebuilding of fish populations within large ''no take'' marine protected areas.

• The statistical analysis showed that once fishermen concentrate on a lower trophic level, total seafood catches go up for a while and then decline again. But at some point, Dr. Pauly said, ''you've got to hit the bottom.'' Even before the bottom is reached, the researchers wrote in Science, fisheries would effectively be depleted: zooplankton, microscopic animals found at trophic level 2, are ''not going to be reaching our dinner plates any time soon.'' Nor, said Dr. Pauly, do most people want to eat survivors like jellyfish. Some species, like the lanternfish, are suitable to eat but they are too scattered to be caught commercially.

• The study sends ''a clear message and an important one,'' said Dr. Paul K. Dayton, a marine ecologist at the Scripps Institution of Oceanography in San Diego. ''This is the very best data set in the world,'' said Dr. Dayton, who added that Dr. Pauly ''is a genius at getting huge amounts of data and synthesizing them.''

• The study has limitations, Dr. Pauly pointed out, not least because the quality of the United Nations data varies. Some countries ''clean it up,'' he said. Despite the flaws, he said, the trend seems clear.

• Along with squid and shrimp, dining tables and supermarkets contain other evidence of fishing lower on the food web, he said. One example is ersatz crab meat, which is made of the ground-up and processed flesh of second-level fish like menhaden, which is normally fare for top predators. Another example is an increased European harvest of herring, once considered a trash fish.

• Even short of an outright collapse, ''fishing down'' can have untoward effects on marine ecosystems, Dr. Pauly said. The study cited a striking example from the North Sea.

• There, cod have been so depleted that fishermen have concentrated on a second-level prey species of the cod, the Norway pout. The pout in turn preys on tiny organisms yet another level down, called copepods and krill. But krill also eat copepods, and if pout are removed the krill population expands -- at the expense of the copepods, whose numbers are drastically reduced. Because copepods are the main food of young cod, the cod population cannot recover, Dr. Pauly said.

• Aquaculture is not necessarily an answer, he said. For instance, now that Atlantic salmon are depleted, they are being reared for the table in offshore pens. But they are being fed with meal made from second-level marine fish.

• Chart: ''Tangling a Food Web'' When fishing boats net too many predators high in an ocean food web, as in this North Sea example, many species can feel the blow. The cycle starts when the cod are overfished. Fishing boats then turn to the Norway pout. 1. As pout disappear, the number of krill, small shrimp-like animals, increases. 2. The krill eat more of the small crustaceans called copepods. 3. Young cod can't find enough copepods to eat so the cod can't recover. (Source: Dr. Villy Christensen)

Primary Productivity

• The Rate at which producers produce food (capture light energy and store it as potential/chemical energy in biomass)

• Gross vs. Net Primary Productivity