Started at the Bottom, Now We’re Here…

Predicting how climate change threatens the prey base of Arctic marine predators, Florko et al., 2021 Ecology Letters. https://doi.org/10.1111/ele.13866

Image credit: Kingfisher, CC BY-SA 3.0

The Crux

We are all (unfortunately) very familiar with the effects of climate change on arctic ecosystems. Horrifying images of polar bears on small blocks of ice and the shrinking polar ice caps are but two of the many results of a warming climate, yet a great deal of the work in the realm has focused on the the charismatic, apex species (like the aforementioned polar bear). These are obviously important things to consider, but it is also necessary to look into the effects of climate change on the lower positions within food webs, as any change to these organisms and processes are likely to cascade upwards to effect the upper trophic levels (like our friend the polar bear).

Hudson Bay in North America is one such area impacted by our warming climate. Due to the changes in temperatures, the energy flowing through ecosystems has shifted away from away from species living in the ice and on the bottom. As a result pelagic (free-swimming) species are favored over benthic species (those living on the bottom of the bay), which alters the rest of the food web itself. Specifically, the fish that feed on pelagic species are increasing, while those that feed on benthic species are decreasing. Today’s authors wanted to understand how these changes in fish numbers are will affect Arctic predators, namely the ringed seal (Pusa hispida).

What They Did

The authors used a model to predict how the abundance, biomass, and distribution of fish species in Hudson Bay would change under two different climate change scenarios. Their modelling approach allowed for species-specific estimations. Usually this sort of study classifies individual species into functional groups (sets of species that share characteristics within a community), so this added precision is a major advantage of the study. Specifically, the model used empirical data on abundance and distribution of fish species from other regions as a baseline, and then quantifies relative differences in abundance, biomass, and distribution between species over time.

The two climate change scenarios were the low-emissions and high-emissions scenarios, aka best-case and worst-case scenarios. Understanding how these scenarios affected fish species would then allow the authors to make predictions for the likely consequences on the ringed seal, which depends on these fish species for survival.

Did You Know: Pinning it on the Polar Bear

Studies like these are an important reminder that Arctic ecosystems are more than just polar bears. So often we see the polar bear as the lone representative of the Arctic, but there are a multitude of other aspects of the Arctic which will suffer greatly from climate change (seal populations being a great example), and which will affect our lives severely (like permafrost thawing). Linking the entire fate of the Arctic to one charismatic species (which is already showing the ability to adapt to new conditions) is a dangerous practice.

Read More: Pinning It On The Polar Bear

What They Found

Not surprisingly, the high-emissions scenario resulted in more dramatic predicted changes for the Hudson Bay system, with more extreme increases in temperature and greater loss of sea ice. Arctic cod (Boreogadis saida), the most well-distributed and energetically valuable prey species for the seals, are predicted to decline in both abundance and biomass. However, both the abundance and biomass of seven other prey species are predicted to increase, with more dramatic changes happening under the high-emissions scenario. Specifically, for the high-emissions scenario the authors predict an increase in total biomass of the system, resulting in a more diverse system overall by the year 2100.

Arctic cod like this one are the prey species projected to be hit the hardest by the climate change scenarios modeled in today’s study (Image credit: Havforskningsinstituttet, CC BY 4.0).

Problems?

This study utilized (in my opinion) a fantastic modelling approach to understand how the upper levels of a food web are going to be affected by the predicted changes in the lower levels of a food web. However, they didn’t actually model the upper levels themselves, nor model the many other interactions within the Hudson Bay system that may change as well. While the authors made a point to discuss how they did not do this, and I understand it is likely an impossible task given the limits of computational approaches such as these, it is something that I am still curious about myself and would like to see investigated more.

So What?

Understanding changes in food webs is key to predicting how our world is likely to change in the future as our climate continues to warm. Studies like today’s are especially important, because they tackle these issues in a way that allows for a more mechanistic understanding of why food webs are going to change at the upper levels due to effects on the lower levels. I hate that our world has come to this, but I still really enjoyed seeing such an elegant analysis of the effects of climate change on an at-risk system like Hudson Bay.


Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions who doesn’t want to read about cold systems like this anymore after having hypothermia. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.

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