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Are species communities a super-organism or simply a random assortment?

Published: 26 January 2024
A pile of sand surrounded by a ring of stones in a meadow.

Yuval Zelnik and his colleagues propose a parameter that quantifies how collectively integrated a species community is. If the parameter´s value is close to zero, it means that the individual species do not interact much. If the value is above one, the collective integration is high. "This tool will help us to understand the nature of different ecosystems", says Zelnik, researcher at SLU and one of the authors of a just recently published paper.

What is an ecosystem, and how should we understand the community of species within it? Should we think of it is a collection of species uniquely tied to one another with a common purpose, like the Casteller human-tower (left picture below)? Or perhaps it is an aggregation of random individual species, with little connection between them, like people in a train station (right picture below)?

This question, and a wide range of answers as suggested above, has been haunting ecology for over a century.

Two photos showing the Casteller human-tower to the left and Liverpool Street Station to the right.

Left: The Casteller human-tower. Photo: Roger Carvell, Wikimedia Commons. Right: Liverpool Street Station. Photo: Eric Sala and Tània García, Wikimedia Commons

Two old ways of looking at ecosystems

We can think of Frederic Clements, who (1917) wrote that “concept of the formation as a complex organism [...] represents the only complete and adequate view of vegetation”, and contrast that with Henry Gleason who wrote (1926) “an association is not an organism, scarcely even a vegetational unit, but merely a coincidence”. Surprisingly, despite much work and thought in this direction, no consensus has been reached about how to best think and understand a community of species.

A new parameter to understand species interactions

In a paper published in the journal Ecology Letters this week, Zelnik and colleagues attempt to answer the question in a simple and concrete manner. They propose a collectivity parameter, that quantifies how collectively integrated a species community is. If the collectivity parameter’s value is close to zero, the community is largely made up of individual species that do not interact in a significant way. If the parameter’s value is above one, then collective integration is high, and our intuitions on how individuals species behave within a community no longer hold true.

The significance of the collectivity parameter is demonstrated by three empirical signatures:

1) Perturbation depth: If a species goes extinct, will other species that do not directly interact with it be affected?

2) Temporal unpredictability: will we fail to predict the long-term dynamics of a species to stress from its initial response?

3) Biotic contribution to the realized niche: will the community be more important than abiotic conditions in determining how well a species does?

In a community with high collectivity, the answer to all these questions is: Yes! This underscores the non-intuitive nature of high-collectivity communities, and also suggests that we need different methods to understand such communities. Essentially, we need tools that go beyond a reductionist approach of taking things apart in order to understand them.

“This is an important step in our attempt to understand how and when species interactions determine the type of community we see. In particular, our study proposes a method to quantify how collective a community is, and demonstrates some ramifications of this collectivity. For instance, we can ask questions like, do some communities behave like super-organisms? Will some species extinctions not affect the ecosystem because of the type of community the species is embedded in?” says Yuval Zelnik, researcher at SLU and the lead author of the study.