Species interactions have predictable impacts on diversification (2021) Zeng and Wiens, Ecology Letters. https://doi.org/10.1111/ele.13635

Image Credit: MacNeil Lyons/NPS, CC BY 2.0

The Crux

No organism on the planet lives in complete isolation from other organisms. Many organisms serve as a food source for others, and even apex predators have to compete for their food. Species interactions like predation, competition, and parasitism directly impact organisms in their daily lives, but there is also a possibility that these same species interactions have had an impact on much longer timescales. That is, species interactions may have had a direct effect on the diversity of life on our planet.

Species interactions have been previously shown to affect diversification rates (see Did You Know?), so the question that today’s authors asked was whether there is a general trend to the effects of species interactions on diversification rates? Specifically, do species interactions with negative fitness (such as being killed by a predator) impacts decrease diversification rates, and do species interactions with positive fitness (such as successfully parasitizing a host) impacts increase diversification rates?

Did You Know: Diversification Rates

In an evolutionary sense, diversification is the process by which one ancestor evolves and changes into multiple descendant species. For example, the common ancestor of lions, tigers, and house cats diversified into those descendant felines. The diversification rate is simply how quickly that process occurs. The greater the diversification rate, the faster that descendent species split from the original ancestor.

What They Did

The authors first conducted a literature search for all studies that investigated the impact of species interactions on diversification rates. Then, they classified each species interaction as “positive” or “negative” for the focal individual in the interaction. For example, if the focal organism in a predation study was the prey item, then predation would be detrimental for the focal organism. Conversely, if the focal organism in a predation study was the predator, then predation would be a positive interaction. After classifying all of the collected examples of species interactions, they then ran a series of statistical tests to understand if there were any general trends.

What They Found

In general, positive species interactions tended to significantly increase diversification rates, while negative interactions tended to decrease them, though this was not a significant relationship. Additionally, there were no differences between the different types of species interactions. That is, mutualism (two organisms mutually benefitting one another) did not increase diversification rates more than commensalism (one organism benefitting from another, while the other doesn’t benefit or suffer from the relationship).

Problems?

Though the authors took the time to conduct a thorough literature search, the studies included in this paper were not representative of a systematic sampling of all organisms. As a result, these interesting results come from a relatively small group of organisms. This is not to discount what the authors have done, instead it is merely an important caveat to consider that these patterns may or may not hold up when further analyses are included.

So What?

The results from today’s paper not only provide an interesting and exciting insight into the potential mechanisms behind the diversity we see in nature today, but they also run contrary to the idea that species interactions only matter on smaller spatial and temporal scales. Further research into the exact mechanisms behind how these patterns came about is crucial to gain an understanding of how species diversity came about in the distant past.

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Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions. 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.