Biodiversity loss in fish ecosystems

Fish are a main protein source for over a billion people worldwide and is one of the most traded food commodities. However, many commercial fisheries have become unsustainable due to practices such as overfishing and habitat destruction. The pressure of commercial fisheries created a new category of species depletion: commercial extinction. Fish populations are depleted to the point that it is no longer economically feasible to fish for them. While not extinct, these species are certainly no longer playing their traditional roles in their ecosystems. Because there is little evidence to the contrary, there has been the common impression that marine species and ecosystems are generally in good shape. However, as more is learned, that impression is turning out to be wildly misconceived. Here is another good example for that:

A new study conducted by researchers from Singapore, Ireland and the UK shows that as the fish diversity of complex marine food webs declines, fish production resists the change, thereby masking ultimate rapid loss. The researchers used a unique high performance mathematical model known as the Population-Dynamical Matching Model (PDMM) to represent thousands of coexisting species dynamically interacting as a food web in a complex ecosystem. The model allows the representation of features such as prey-switching, where predators preferentially target prey species in abundant supply in order to improve their chances of survival. At the end of a gradual assembly stands a model food web very similar to the natural process. 

The team sequentially deleted fish species from such complex model marine ecosystems and after each deletion, observed the dynamics of the remaining fish species and measured the change in total fish production.

The study results showed that the prey of the deleted fish species tended to resist the biodiversity loss by increasing their production, which helped to mitigate the decrease in total fish production. However, once the biodiversity losses became too great and only about a third of the species in a food web remained, a collapse ensued whereby production sharply declined with further species loss, as the relatively small percentage of remaining species were unable to provide much compensation.

This resistance to change in wild fish ecosystems may be masking a more serious decline in production potential in situations where biodiversity loss is already reported, posing the danger of providing a false sense of assurance until the biodiversity losses become too great.