Article published in 'Nature' combines evolutionary and network theories to calculate how species can co-evolve in large mutualistic networks
Since Darwin's theory of natural selection in the 19th century, it has been known that interactions between species can generate responses capable of shaping the planet's biodiversity.
The classic example of coevolution by mutualism involves a parasite and its host. When the first evolves a new form of attack, the second develops another type of defense and adapts. However, when it comes to a broad network of interactions with hundreds of species – such as plants pollinated by many insects – it becomes more difficult to determine what effects drove coevolution throughout this network.
In these networks, species that do not interact with each other can still influence species evolution through indirect effects. An example of an indirect effect would be an evolutionary change in a plant caused by one pollinator that ends up leading to evolutionary changes in another pollinator.
New research has managed to quantify, for the first time, the weight of indirect interactions in coevolution. The conclusion is that the impact could be much greater than expected.
In the study, published this Wednesday (18/10) in the magazine Nature, a group of ecologists and biologists from five institutions – USP, Unicamp, University of California, Doñana Ecological Station and University of Zurich – combined evolutionary theory and network theory to calculate how species can co-evolve in large networks of mutualism.
The researchers, supported by FAPESP, developed a mathematical model to analyze interaction networks and separate the effects of direct and indirect interactions. The networks studied describe the mutualistic interactions that occur in a location, such as interactions between bees that pollinate flowers when collecting nectar or birds that consume fruits of various plant species and disperse seeds.
The study also brings important results for the adaptation and vulnerability of species in situations of abrupt environmental changes.
“The results we obtained with this approach suggest that relationships between species that do not directly interact with each other may have a greater weight than expected in the coevolution of species. Surprisingly, the indirect impact is greater on specialist species, those that interact with only one or a few species directly. To illustrate, we can imagine this process as being analogous to behavioral changes in people mediated by social networks. These changes are often caused by people they don't live with directly, but know through mutual friends,” said Paulo Roberto Guimarães Jr., professor at USP's Institute of Biosciences and lead author of the study.
75 ecological networks were analyzed, ranging from very small networks, with around 10 species, to structures with more than 300 species interacting with each other. Each network occurs in different locations on the planet, in terrestrial and marine environments. To collect the data, the team, formed, in addition to Guimarães, by Mathias Pires (Unicamp), Pedro Jordano (IEG), Jordi Bascompte (University of Zurich) and John Thompson (UC-Santa Cruz) counted on the collaboration of researchers who previously described the interactions in each network.
With the data in hand, the team divided six types of mutualism categorized into two large classes: intimate mutualisms, such as the interactions between anemones and clownfish that spend practically their entire lives in a single anemone, and mutualisms with multiple partners, such as pollination carried out by bees and seed dispersal by vertebrates, which normally establish many interactions with different species in the same location.
The results showed that species that do not interact directly can be as important as species that interact directly in shaping the evolution of a species. However, the weight of direct and indirect interactions depends on the type of mutualism.
“When the relationship is very intimate between partners in the same network – as is the case with clownfish and anemones or certain species of ants that live inside trees – what matters most are direct interactions. This is because these interaction networks are more compartmentalized. So, there are not many paths for direct effects to propagate. When the interaction is not so intimate, the indirect effects can have an even greater effect than the direct ones on the evolution of a species”, said Mathias Pires, from the Institute of Biology (IB) at Unicamp, another author of the study.
In a simulation carried out with a species-rich seed dispersal network, less than 30% of the selective effects on the specialist species were driven by their direct partners, while the effects of indirect species represented around 40%.
A matter of time
One of the clear consequences for the impact of indirect relationships is the greater vulnerability of species in situations of abrupt environmental changes. This is because the more important the indirect effects, the slower the process of adapting to changes may be.
“An environmental change that affects one species can generate a cascade effect that spreads to other species that also evolve in response, causing new selective pressures. Indirect effects can create conflicting selective pressures and species can take a long time to adapt to new situations, which can make these species more vulnerable to extinction. Ultimately, environmental changes can cause changes that are faster than the ability of species immersed in a network to adapt,” said Guimarães.
Quantifying indirect effects in complex networks is a challenge not only for Ecology. Indirect effects are a fundamental component of processes that affect the genetic structure of populations, the financial market, international relations and cultural practices.
“The interesting thing about using this method that we developed is that it can be applied in different areas. The interaction networks approach is transdisciplinary and the tools developed to answer questions about a specific topic in ecology, for example, can be used to study questions about social networks or economics, just be creative”, said Pires.
The article Indirect effects drive coevolution in mutualistic networks (doi:10.1038/nature24273), by Paulo R. Guimarães Jr, Mathias M. Pires, Pedro Jordano, Jordi Bascompte and John N. Thompson, can be read at Nature em http://nature.com/articles/doi:10.1038/nature24273.
