for example once fisheries have depleted the large typically higher value predatory fish at the top of the food web they have been seen to then progressively redirect effort towards smaller often less valuable species at lower trophic levels pauly et al this situation referred to as fishing down the food web can also be associated with increased fishing intensity to offset the lower value of the smaller species which can further negatively affect diversity and productivity see folke et al once these effects take place an ecosystem might lose its resilience and become more vulnerable folke et al once an ecosystem has shifted the process may be difficult to reverse scheffer and carpenter scheffer carpenter and de young