Cod Food Web
I was again amazed with the infinite ramifications of scale-free networks and the applications of this recent knowledge. Led by a few short citations in the books I’ve been reading, I started researching complex networks in the context of food webs, particularly marine food webs – binary feeding relationships between the species in a community. Until recently, biologists, government officials and even environmentalists had a very simplistic view of nature and looked at animal species as a scattered web of nodes with a short number of dependencies. However, most links among components of food webs are not so simple and may involve the interaction of hundreds of organisms.
The importance of fully understanding the dynamics of scale-free networks as been recognized by the cod fishery industry in the worst way. “The collapse of the Northwest Atlantic cod fishery has become a metaphor for ecological catastrophe and is universally cited as an example of failed management of a natural resource” (MacKenzie 1995). Peter Meisenheimer in his paper “Seals, Cod, Ecology and Mythology” collects an incisive list of six hypotheses that might have led to the demise of the once abundant cod stock:
1. Canadian elected officials and Department of Fisheries and Oceans (DFO) staff have stated that the culling of seals will benefit the recovery of Northwest Atlantic cod stocks.
2. In contrast, published reports in scientific journals, including those authored by DFO biologists, unequivocally conclude that seals are having no demonstrable impact on cod recovery.
3. “Common sense” arguments that culling seals will “obviously” benefit the fishery are premised on a mythological view of predators that is unsubstantiated by most scientific evidence.
4. Research conducted in other fisheries has indicated that the complexity of marine food webs, and the diversity of seal diets mean increased seal numbers can sometimes lead to positive effects on commercial fish stocks.
5. Consistently, recent research in terrestrial systems indicates that top predators can have a significant positive impact on numbers of herbivores by reducing numbers of smaller predators.
6. The Canadian political agenda for dealing with the collapse of the cod stocks has evolved to include a subsidized seal cull, and suppression of internal reports contradicting the “common sense” position adopted by the political leadership.
As Meisenheimer says, the use of seals as scapegoats for the failings of Canadian fisheries management is an example of a global problem in the management of fisheries and wildlife. Whether the system is aquatic or terrestrial, tropical or arctic, the predators of the world are seen as problems to be controlled, not as integral parts of a functioning ecosystem. Whenever I think of food webs I instantly recall those simple and infantile diagrams showing the carrot, rabbit and fox. Although this example is intently exaggerated I believe most of us think of a food web of any particular species as an isolated set of interactions, not having more than a few links. Of course we couldn’t be more wrong.
Prof. David Lavigne, a zoologist researcher sponsored by the Natural Sciences and Engineering Research Council and the International Marine Management Association is a leading force in combating this miscomprehension of food webs. Regarding the cod stock decrease, he also claims that seals are being used as scapegoats because government scientists are failing to look at the problem in a macro level, the way any network should to be analyzed. The image below is Lavigne’s effort to understand the complex map of interactions in a food web. This astonishing work shows the Cod food web displaying some trophic interactions for part of the Northwest Atlantic.
Copyright David Lavigne. For a larger version of this image click here.