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Cod v Herring – the complexity of an ecosystem

Every ecosystem is incredibly complex and even one change can have a detrimental effect on a whole range of organisms.

Here is just one simple example:

Since the 1960’s, cod populations in UK waters have seen a steady decline. Two reasons for this decline are regarded as plausible:

  1. Overfishing, which has meant a reduction in the volume of spawning cod.
  2. In more recent years sea temperatures have risen due to global warming, this in itself has not affected the population but the increased temperatures have meant a reduction in the abundance of zooplankton, a key food source for young cod. This reduced food source means decreased survival of cod larvae, so fewer fish maturing into the adult cod population.

​Now comes the twist.

Herring is a food source for cod, so inevitably fewer cod means more herring, and guess what more herring eat? More young cod and cod spawn, so even less cod.

Nothing in the natural world is as simple as 2+2 = 4. We will have to start thinking on a much broader plane if we want to understand the effects of global warming.

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