Large reductions in the abundance of exploited land predators have led to significant range contractions for those species…(read more here).
This is an extremely interesting new report by Boris Worm and Derek Tittensor. They compiled global range information for 13 species of large pelagic tuna and billfish, documenting significant range contractions in 9 of those species between 1960 and 2000. This implies that these top or near-apex predators have been extirpated over much of their range. Similar extirpations and extinctions of high trophic level terrestrial predators have resulted in now well-documented top down cascading effects, such as meso-predator release (no controls on smaller or less powerful predators) and subsequent declines of herbivore prey. It’s difficult for me to infer what the top down effects might look like in the ocean partly because of the tremendous ranges of some of these species and the likelihood of refuges, but also because, unlike most terrestrial predators, these species are exploited for food; as are the lower trophic level of meso-predators. We’re so busy knocking out the entire web that we could just be dampening many would-be cascades!