Ecological interactions are like a fluid dance in nature, in constant movement, sometimes smooth and synchronised, sometimes vigorous and challenging, showing that threads of change weave the harmony of life over time and space. One way of observing such ecological interactions is through the so-called “interaction networks”. Interaction networks represent the complex interactions between different species within a community, where species are nodes, and these are linked to each other if they are known to interact. Therefore, using ecological networks allows us to visualise interactions –such as those between plants and their pollinators– and to calculate a series of metrics that help researchers understand how communities are assembled and how vulnerable they are.
Traditionally, ecologists studied these interaction networks as if they were static entities: they sampled during a specific period and assumed that this selection of ecological interactions represented the past, present and future of interactions in a given site. However, we know that ecological interactions rarely behave this way; like species, they vary across time and space. This is evident to us since, depending on the time of year, we see that only some insects are active and that some plants have flowers. Still, if we visit another location or even the same site at another time of the year, we will find other species interacting. Therefore, researchers have been looking for ways to incorporate these dynamics into the study of ecological networks, considering that understanding these changes is crucial to predicting and managing biodiversity in our world, especially in times of climate change.
Keeping this in mind, Dr Sandra Hervías-Parejo and her collaborators conducted an intriguing study that employed an innovative multilayer interaction networks approach that accounts for simultaneous variations across time and space. These networks incorporate multiple layers of interactions, accounting for differences in species abundance over time and space. Nodes and edges are still present in these networks, but they are divided into separate layers, representing different types of interactions. These layers can be used to depict various forms of contacts, spatial locations, subsystems, or points in time. Connections between nodes within the same layer are termed intralayer connections, while connections between nodes in different layers are interlayer connections. This new approach holds great promise for studying ecological interactions, as it recognises that such interactions do not occur in isolation within an ecosystem but rather coexist simultaneously.
During the spring of 2021, the authors carried out five consecutive fieldworks (March to July 2021) in three different habitats on the island of Menorca in the Mediterranean Sea. They selected three sites on the island with three different habitats and recorded interactions between plants and pollinators. Additionally, they conducted flower counts on individual plants to estimate the number of open flowers, observed flowers, and flowers contacted by each visiting floral insect in randomly selected transects at each sampling site. This provided valuable data to estimate plant species abundance and flower density in the respective areas.
Based on their collected data, the researchers constructed two distinct multilayer networks: a temporal multilayer network and a spatial multilayer network. The spatial network grouped interactions into three layers (one layer for each habitat), while the temporal network grouped interactions into five layers (one layer for each sampling month). These networks allowed them to analyse the dynamic interactions between species over time and space, providing a comprehensive understanding of the ecological relationships in the studied ecosystem.
One of the most fascinating findings from this research is the significant turnover of species and interactions across space and time. While species turnover was more pronounced over time, the turnover of interactions was more pronounced in space. This means that over time, there was a greater change in the species present in different periods, while interactions showed a more significant variation when moving between different habitats. Species turnover was influenced by the presence of endemic species, species with restricted distribution and by when plants flower. On the other hand, the turnover of interactions was higher in space due to differences in species richness and abiotic factors between habitats. In other words, the interaction variation was more significant when different habitats were compared due to the unique conditions each habitat offered in terms of species composition and environmental factors. This suggests that pollinators have strategic adaptability, adjusting their interactions to different ecological scenarios.
The study also revealed that the contribution of plants and pollinators to network structure varies according to the component under consideration. While plant species played a crucial role in maintaining the cohesion of spatial networks, the importance of pollinators was correlated with both scales. This result represents, at least in part, that plant-pollinator interactions are a consequence of random encounters between plants and pollinators. And this is further reaffirmed when we look at the versatility of species. The versatility of plant species is positively related to the number of flowers in the spatial network. This happens because, in environments with limited resources, such as islands, pollinators consume available resources without specialising. In contrast, the versatility of pollinators is positively related to their abundance. This is mainly due to three species of bees (Andrena ovatula, Anthophora plumipes and Anthophora subterranea) and insects of the order Thysanoptera, which share the greatest number of partnerships with other species, both in time and space, acting as spatial and temporal connectors in networks of interaction between plants and pollinators.
The study also revealed that temporal networks displayed a higher number of modules (i.e., interconnected species groups) compared to spatial networks. This implies that species in spatial networks can maintain consistent interactions while also finding reliable partners in different habitats. As a result, they play a vital role as connectors bridging different spatial layers. Additionally, the adaptability of both plants and pollinators in temporal networks was particularly noteworthy. The exchange of species between modules in these networks suggests a higher level of flexibility and resilience among these organisms, enhancing their ability to thrive in various environmental conditions.
Furthermore, the grouping of pollinators from different functional groups indicates their strong association with one another. This highlights the importance of specific species’ presence in different habitats and during various sampling periods for sustaining these networks. These associations extend beyond mere morphological or species-related traits and contribute significantly to the ecological balance and stability of these systems.
Given all these fascinating results, this study provides valuable insights into the dynamic nature of plant-pollinator interactions and highlights the importance of considering both spatial and temporal scales. Understanding the common patterns and drivers of these interactions becomes crucial as global changes continue to affect ecosystems. These findings emphasise that short flowering periods can limit the availability of partners for plants and pollinators since time is an important issue for the encounter between two possible interaction partners, and this can lead to reduced reproduction if pollinators are unavailable during that time.
However, the remarkable flexibility observed in changing interaction partners suggests a certain level of adaptability. It remains to be seen how interaction volume and rewiring affect ecological consequences and whether species experience other adverse effects. Moving forward, this research emphasises the need to consider spatiotemporal differences in resource use to improve our understanding of mutualistic interactions. By deepening our understanding of these dynamics, we can better conserve and protect vital networks of plant pollinators and the ecosystems they support, ensuring the resilience of the natural orchestra that fuels our planet.
READ THE ARTICLE:
Hervías‐Parejo, S., Colom, P., Beltran Mas, R., E. Serra, P., Pons, S., Mesquida, V., & Traveset, A. (2023). Spatio‐temporal variation in plant–pollinator interactions: a multilayer network approach. Oikos, e09818. https://doi.org/10.1111/oik.09818
Portuguese translation by Victor H. D. Silva.
Victor H. D. Silva is a biologist passionate about the processes that shape interactions between plants and pollinators. He is currently focused on understanding how plant-pollinator interactions are influenced by urbanization and how to make urban green areas more pollinator-friendly. For more information, follow him on Twitter: @another_VDuarte