Studying the observations of scuba divers as part of the Reef Life Survey programme, a recent paper has found striking latitudinal gradients in the relative abundance of fish and large mobile invertebrates, such as crabs, starfish, sea urchins and octopus (Edgar et al. 2017). Fish dominated tropical ecosystems, but in cold water latitudes they were less abundant and large invertebrates were conspicuous on the seabed. In warm waters fish need to eat more to maintain their growth, and their mobility and behaviour enable them to become the dominant predators and significant herbivores. In contrast, in colder waters while fish are present their appetite is less and they grow more slowly. This allows invertebrates to become more abundant, although the exact mechanisms of this remain to be studied.
Because fish are expanding their ranges into lower latitudes due to climate warming, this means they are likely to increase predation on, and competition with, invertebrates. Thus food webs will change not only due to the effects of warming on species distributions, but ecological effects due to predator-prey interactions.
RLS is a remarkable example of a scientlfically successful global marine citizen science programme. It includes standardised field observaitons on over 4,000 species at over 2,400 sampling sites in 50 countries. See videos of how RLS works at https://www.youtube.com/watch?v=gqgcMUkQMBY&t=21s and https://www.youtube.com/watch?v=y2IWZRT4pzw and photographs here.
Edgar GJ, Alexander TJ, Lefcheck JS, Bates AE, Kininmonth SJ, Thomson RJ, Duffy JE, Costello MJ, Stuart-Smith RD. 2017. Abundance and local-scale processes contribute to multi-phyla gradients in global marine diversity. Science Advances, e1700419.