Shortlisted for the 2023 Southwood Prize

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Andrea Radici talks us through his team’s development of a comprehensive metapopulation framework, which provides a tool for assessing area-based management measures aimed at improving fisheries outcomes and describing the spatial distribution of costs and benefits. This can help guide effective spatially-distributed management.

Ecosystems and mathematics

In Chapter 151 of The Curious Incident of the Dog in the Night-Time, Christopher, a high-functioning autistic boy who is investigating over the death of his dog, discuss how simple mathematical formulas may explain apparently chaotic ecological behaviour.

In his example, Christopher illustrates that the yearly abundance of frogs in a pond may vary following very different patterns depending on the parameter defining the finite growth rate. For the population to persist, this rate needs to be larger than one. Surprisingly, dynamics result in cyclic oscillation for values between 3 and 3.57 and in completely chaotic ones for values exceeding 3.57.

“Sometimes a whole population of frogs, or worms, or people, can die for no reason whatsoever, just because that is the way the numbers work”.

I read this book a few months before attending my ecology class at Politecnico di Milano. I remember being excited by the realization, for the very first time, that my class on ordinary differential equations – complex mathematics to my eyes, whose primary application involved describing the flow of non-Newtonian fluids in pipes of different shapes – could be used to describe the dynamics of populations, communities, and food webs, just as Christopher claimed. This aspect of my studies has continued to fascinate me since then.

Managing natural resources

Bioeconomic theories suggest that the optimal yield (also called “maximum sustainable yield”) of a managed natural resource is the one that allows the long-term viability of the harvested resource itself. This leads to the apparently paradoxical outcome in which, in the case of overexploited resources, decreasing the harvesting effort produces an increase in yield – a better chance for the conservation of the targeted species, a kind of win-win solution. This is clearly visible in the case of fisheries: a temporary increase in yield leads to a sharp fall in the population, causing long-term damages to both fisheries and the targeted species.

Reducing pressure can be accomplished in several ways, including catching quotas, limitations on gears, etc., many of which have the drawback of being difficult for policymakers to control. Hence, the idea arises of closing specific areas to fishing through the creation of marine protected areas. From a theoretical standpoint, this has already been shown to work.

Connecting European hake populations

On the other side, the aim my supervisor, Paco, and I had was to investigate not the simple case of a population, but that of a metapopulation – a set of subpopulations of European hakes in the Northwestern Mediterranean Sea, comprised of habitats connected by exchange of adults and larval flow, over which the fishing pressure is heterogeneously distributed. What is the impact of closing fishery in a site over the regional scale? Are all sites equal in terms of expected impacts? Are sites located far away from the closed one affected as well?

We built a model to try to answer these questions. Luckily, we had plenty of data concerning biological traits and time series of abundance and catches by age classes (STECF European assessment), geographical distribution of habitats (MEDISEH European project), marine currents to simulate larval drift (Copernicus European project), data about fisheries (Safenet European project). Our main challenge was to assemble them to assess protection scenarios.

Winners and losers of marine protected areas

Our study suggests that some sites play a more critical role in sustaining the entire metapopulation, considering both habitat availability and larval connectivity, compared to others. Prioritizing the protection of these sites is crucial. We discovered the possibility of win-win solutions, allowing for stock restoration – even surpassing present levels – while reducing losses in terms of missed yield which, in a business-as-usual scenario, is projected to decrease by 8%.

Geographical impacts of local protection depend on certain parameters, such as the mobility of adult individuals, influencing the so-called ‘spillover’ from the protected site to the remaining areas. The positive impact of local closures on the stock appears to be locally concentrated around the protected site, while the impact on fishing yield is more evenly distributed over the domain.

About the author

Metapopulation dynamics and ecosystem connectivity has become a central focus of my research. In my PhD in agronomic science, which I defended in December, I dealt with a very different topic: plant protection from airborne pests and pathogens. The main link with my past studies was related to the (quasi-)passive transport of biological matter immerged in fluids over long distance: hake’s larvae and eggs transported by marine current before, spores and bacteria transported by wind now.

My interests deal with mathematical ecosystem modelling, with a focus on the interactions between the human activities and natural ones. I look with optimism to the urgent challenges of reconciliations of human pressures over ecosystems and I see quantitative ecology as a valid tool to investigate this issue.

Read the full article “Assessing fish–fishery dynamics from a spatially explicit metapopulation perspective reveals winners and losers in fisheries management“ in Journal of Applied Ecology.

Find the other early career researchers and their articles that have been shortlisted for the 2023 Southwood Prize here!