Sarah Weisberg is holding the bottom of a plankton net—a large net for catching the small critters that drift in surface oceans and are key links in the marine food webs—as part of an effort to gather data aboard the R/V Seawolf. Credit: Ellie Heywood

— By Chris Gonzales, Freelance Science Writer, New York Sea Grant

Stony Brook, NY, November 28, 2024 - Scientists are working on a new theoretical approach to explain how ecosystems might respond to climate change.

At root, this new theory is based on merging two existing ones.

One is trait-based ecology. In this view, we tend to focus on the functional characteristics of species in an ecosystem. For example, let’s think of all of the fish in the Mediterranean Sea. We might describe those fish as a collection of different species, or we might think of them in terms of their traits – like how many big fish there are compared to small ones. The latter approach describes more about the actual functioning of the sea than the former. 

A weakness of trait-based analysis is that it focuses on the static properties of the ecosystem, rather than the dynamics of change. But in the real world, abrupt changes are commonplace – and very important to anticipate. In a new paper, scientists point out that a second theory, called regime shift theory, can merge with trait-based ecology to help us think about and be ready for sudden, dramatic shifts.¹ 

Sarah Weisberg, a fisheries ecologist and the lead author of the paper, has been thinking about these topics as part of her National Marine Fisheries Service–Sea Grant Joint Fellowship.

“We often study fish species in isolation, while in reality fish live in dynamic communities of interacting species,” said Weisberg. “The direct climate impacts on one species can indirectly affect another, but this is often rather difficult to study. One way forward to understanding these complex climate change impacts at the community level is to merge together two streams of ecological theory, specifically ‘trait-based ecology’ and ‘ecological regime shift theory.’ We believe that these two theories are highly complementary, and that by blending them we can better understand, and therefore manage, marine fish communities under a changing climate.”

Studying change and resilience

The scientists begin with the understanding that changing environmental conditions, such as rising temperatures, lead to observable changes in traits at the individual level. These changes at the individual level in turn influence the fitness and function of populations. 

Scientists use the term “community” here to refer to many species, living together and interacting. Community ecology also takes into account features such as the size of those organisms.

At the community level, scientists aim to quantify trait distributions such as size, generalist or specialist designation, residents versus migrators, and so on.

However, changes in these distributions are neither immediate nor linear, and scientists anticipate abrupt and perhaps irreversible regime shifts.

This also raises the important question of how resilient an ecosystem is.

Most empirical studies today track communities taxonomically, rather than in terms of their traits. The scientists hope to see more work done to track and analyze the dynamics of trait distribution responses. They expect such an approach would lead to a more holistic understanding of warming impacts on communities.

Applying the theory to North Atlantic zooplankton and fish communities

Weisberg and her team applied the principles of this theoretical approach to understand warming impacts on zooplankton and fish communities in the North Atlantic.

Empirical studies of marine systems have a long history of records related to size, in part because marine food webs are strongly size-structured. 

A type of copepod, Calanus finmarchicus, once dominated North Atlantic zooplankton communities. These zooplankton were large in size, and very fatty—making them attractive prey for fish and whales. In recent times, their numbers have declined, giving way to smaller zooplankton, who provide less energy to their predators.

Meanwhile, within fish communities, Atlantic cod (Gadus morhua) were highly successful prior to warming. Cod are relatively long-lived, slow-growing top predators. In more recent times, cod have become smaller and now exert less dominance in the food chain.

As waters warm, scientists are both predicting and seeing such changes to community traits.

Fish and plankton getting smaller

With these changes, scientists have been predicting that fish would begin to have smaller body size. This has been observed in the North Atlantic. They also predicted seeing smaller body size in the zooplankton community. This has been observed as well.

Yet trait changes do not appear instantaneously, or even quickly. Instead, they can shift suddenly. Scientists expect lags as well as abrupt shifts in ecological systems, depending on their resilience.

The scientists are calling for a model that uses both regime shift theory and a trait-based approach to understand the data.

Such an approach could help them uncover functional shifts like the one to smaller fish size that began around the year 2000 in the North Atlantic Ocean.

Applying these approaches early

Above all, these ecological changes are happening in living communities, which are resilient. The scientists note that we cannot expect environmental change to be smooth. By applying these two approaches early, they hope to start to observe and recognize early warning signs of functional regime shifts.

References

¹ Merging trait-based ecology and regime shift theory to anticipate community responses to warming. Sarah J. Weisberg, et. al. Global Change Biology. October 2023. https://doi.org/10.1111/gcb.17065 


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