Introduction

The poles are among the harshest environments on our planet. In the winter, the sun never rises, air temperatures are extremely cold, and snow and ice accumulate. In summer, the sun never sets, it is warmer, and the ice melts back toward the poles. However, there is no doubt that the amount of ice is shrinking as our planet gets warmer. This is happening on the land and at sea. On the land, glaciers are melting and retreating in places like Antarctica, Greenland, and Alaska. At sea, the area of floating ice, which we call sea-ice, is becoming smaller each year [1] (Figure 1).

A National Geographic video of a starving polar bear with the caption “This is what climate change looks like” has become an internationally recognizable symbol of the effects of melting sea-ice on the Arctic ecosystem [2, 3]. The story behind the video is that polar bears depend on being able to roam over the ice in search of seals to eat. Because the ice is shrinking they are struggling to find enough food. In this article, we explain why the story may be more complex than it first appears, and how computer models of ecosystems can be used to help us understand these complexities.

Food Webs and Ecosystems

An ecosystem is a community of bacteria, plants, and animals that occupy the same habitat. All the organisms in an ecosystem are interconnected by a network of who-eats-whom relationships. We call this a food web (Figure 2). Wherever there is sunlight and sufficient nutrients, plants manufacture the materials needed for life in a process called photosynthesis. Photosynthesis provides the energy to support all the animals in the rest of the food web.

In the sea, most of the plants are microscopic and we call them phytoplankton. Phytoplankton are eaten by a variety of tiny animals up to about the size of a grain of rice, called zooplankton. Zooplankton are then eaten by larger zooplankton, jellyfish, and plankton-eating fish, such as capelin. The waste they produce (called detritus) is broken down by bacteria as it falls to the seabed, where it provides food for bottom-living animals (called benthos). Plankton-eating fish and benthos are food for predatory fish, such as cod and hake, seabirds, and mammals, such as seals and whales. In the Arctic, all these animals are hunted by the top predators, such as polar bears. Humans are also top predators in the food web, because we catch fish and hunt for seals and whales.

How is Warming of the Planet Affecting Arctic Food Webs?

Warming has a direct effect on all living organisms. Metabolism depends on temperature [4]. Metabolism includes the processes that allow organisms to produce energy from the food that they eat. All organisms have a temperature range in which their metabolism can keep them living comfortably. Tropical species are less tolerant of cold conditions than polar species. This means that, as oceans warm, we expect species found in warm regions to expand their distributions, and we expect those species that prefer colder water to retreat toward the poles. Over the past 50 years, scientists have observed some zooplankton and fish species moving toward the poles, as the oceans become warmer [5].

The effects of warming on the sea-ice are also extremely important for the Arctic food webs. The presence of sea-ice strongly reduces the amount of sunlight that enters the ocean. The shrinking area of sea-ice (Figure 1) means that some parts of the ocean are being exposed to summer sunlight for the first time in many thousands of years. The light enables phytoplankton to grow and photosynthesize in areas that have previously been barren, increasing the productivity of the Arctic.

The increase in phytoplankton could be considered a positive change, leading to more life in the sea and more food for zooplankton, benthos, and eventually fish, birds, seals, whales, and polar bears [6]. However, some of the Arctic species have evolved to depend on the presence of ice. Sea-ice provides a means for seals and walruses to get out of the water. They need to do this to rest, breed, and evade other marine predators, such as killer whales. Other mammals, such as polar bears rely on the ability to travel across the frozen sea to hunt for seals. Melting ice makes travel more difficult because thin ice cannot support the bears’ weight. On the other hand, ice is a barrier to whale species because they must be able to reach the air to breathe. Overall, it is not completely clear which of the predators in the food web will be benefited by melting sea-ice and which will be harmed by it.

Predicting What Will Happen in the Arctic

We use computer models to make predictions about the effects that changes in sea ice will have on food webs. To make these models, we first write down the key processes and components of the food web as mathematical equations. These include feeding, metabolism, photosynthesis, and who-eats-whom. Then we convert the equations into computer code. This gives us a virtual world, in which we can carry out experiments on the model ecosystem. The experiments are designed to help us understand how the ecosystem could change in the future.

We built a simple model of the Barents Sea ecosystem in the Norwegian/Russian Arctic (Figure 1). This model contains data on ice and temperature conditions, and equations to represent some of what we know about the ecosystem’s plants and animals. The main simplification is that some species are combined into groups that share similar characteristics, such as size, structure, or diet. For example, we combined all birds and mammals into a single group, because they are warm-blooded, which means that they can use energy from their food to maintain a warm body temperature independent of the environment, unlike plankton and most fish. This simplification means that, in our model, we cannot distinguish between polar bears and whales, for example. Although our simplified model is not perfect, it still reproduces the general responses of the natural Barents Sea ecosystem.

The equations in the model include parameters that determine the results. In our model, these include, for example, maximum feeding rates, or sinking rates of detritus. Selecting values for the parameters is an important step in building the model. We try to select values that allow the model to match as closely as possible to the real data from the ecosystem. This ensures that the model is as realistic as possible. Once the model is set up, we can change the ice and temperature conditions to what they might be in the future, and use the model to predict what may happen in real life. In Figure 3, we show an example of a comparison between the present day and future model results. The present-day model represents the Barents Sea as it is today. The future model is ice-free, with water temperatures 0.5°C warmer all year round. As expected, photosynthesis is higher in the future model. The animals that benefit are benthos, predatory fish, and the bird-and-mammal group. Zooplankton and plankton-eating fish are less abundant than they are today. This is because phytoplankton and detritus, upon which the benthos and zooplankton feed, settle to the seabed more quickly when there is no ice. Since we cannot distinguish between different bird and mammal predators in this model, we cannot use it to say exactly which species will be more abundant or less abundant in the future. However, the model predicts that the bird-and-mammal group as a whole will have a diet different from today’s, with more feeding on predatory fish and less on planktivorous fish.

Why Does All This Matter?

The Arctic is warming faster than the rest of the planet. Yearly averaged air temperatures in the Arctic have increased by around 2°C between 1970 and 2010 compared to 0.6°C for the planet as a whole [7]. Summer temperatures in the Canadian Arctic are now higher than at any time in the last 44,000 years [8]. Within the period of a human lifespan, the Arctic Ocean has partially emerged from beneath thousands of years of sea-ice cover.

There is a race between nations to claim the seabed and resources in the Arctic as it emerges from the ice. New routes will become possible for ships to travel between Europe, Asia, and North America. On 30 November 2017, the EU and nine major fishing nations agreed not to develop fisheries in the Arctic Ocean until at least 2033. This is supposed to allow time for scientists to develop the models needed for managing the fish stocks in a sustainable way [9]. There is much research to be completed before we have a full understanding of the effects of warming on the Arctic.

Glossary

Sea-ice: ↑ Floating ice formed by freezing of the sea surface.

Photosynthesis (foh-toh-sin-thi-sis): ↑ A chemical reaction in plants and some bacteria. It uses energy from light to convert carbon dioxide and water into organic compounds.

Phytoplankton (fie-toh-plangktuhn): ↑ Microscopic plants cells that drift in the water. There are many species of phytoplankton.

Zooplankton (zohuh-plangktuhn): ↑ A general term for a huge variety of tiny animals that drift in the water.

Detritus (di-trie-tuhs): ↑ Remains of dead organisms or waste produced by living organisms.

Benthos (ben-thos): ↑ A general term for a huge range of animals that live in and on the seabed.

Metabolism (muh-tab-uh-li-zuhm): ↑ The combination of chemical reactions required for life.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.