Climate change restructures marine food webs in unexpected ways

Most research studies that investigate the potential effects of climate change on animal and plant species do this by considering one species at a time. The ecological needs of a species, and how these will be met as the climate changes, give important information about what we might expect in terms of biodiversity in the future. Few studies, however, consider how food web interactions will affect the pace of change.

A new study, published today in the journal Proceedings of the Royal Society B, investigated how trophic relationships (the feeding of one species on another) will be affected by the poleward shift in distribution of marine species. As the ocean warms, species are expected to move in search of more suitable conditions, and this will affect not only their distributions, but also the dynamics of entire food webs. This will be particularly important for large species and commercially important fisheries on which people depend for food and a livelihood. 

Using sophisticated computer models, the researchers determined that predator-prey interactions cause many marine species, especially large predators, to shift their ranges more slowly than the climate changes would suggest. They developed a “spatially explicit food-web model” that included parameters such as metabolism, body size and optimal temperature ranges for species, and this model revealed that dynamic trophic interactions limit a species’ ability to react quickly to warming temperatures. 

For example, larger-bodied top predators stay longer than smaller prey in their tried and trusted habitats, partly because new sources of prey arrive in the predators’ historical ranges and this leads to the establishment of new food webs. One of the results of climate change in the ocean will therefore be a mixing and reshuffling of marine biodiversity. 

“The model suggests that over the next 200 years of warming, species are going to continually reshuffle and be in the process of shifting their ranges,” said lead author E. W. Tekwa, a former Rutgers postdoctoral student who is now at the University of British Columbia. “Even after 200 years, marine species will still be lagging behind temperature shifts, and this is particularly true for those at the top of the food web.”

This scenario has implications for commercial fishermen and fisheries all over the world. The researchers suggest that, while commercially harvested fish species will probably still occur in their usual ranges to some extent, their density is likely to be significantly reduced as many individuals will have moved to different regions. 

Not only will large species and commercially important fisheries shift out of their historical ranges as climate warms, but they will likely not be as abundant even in their new geographic ranges. For instance, a cod fisherman in the Atlantic might still find fish 200 years from now but in significantly lower numbers.

“What that suggests from a fisheries perspective is that while the species we fish today will be there tomorrow, they will not be there in the same abundance. In such a context, overfishing becomes easier because the population growth rates are low,” said study co-author Malin Pinsky, an associate professor in Rutgers’ Department of Ecology, Evolution, and Natural Resources. “Warming, coupled with food-web dynamics, will be like putting marine biodiversity in a blender.”

The changes in trophic relationships and food webs in marine systems will lead to different communities of animals and plants interacting with each other. The model predicts that, as temperatures warm, predator-prey interactions will prevent species from following environmental conditions that would be most suitable to them because they will be hanging back “at home,” attempting to feed on the newly arriving prey species instead of following their traditional prey as they become established in new and more suitable geographical locations. 

“These dynamics will not only be in one place but globally,” Pinsky said. “That does not bode well for marine life, and this is not an effect that has been widely recognized.”

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By Alison Bosman, Earth.com Staff Writer