Pedro Jordano
Research Professor in the Integrative Ecology Department at Doñana Biological Station (CSIC)
The paper addresses a largely overlooked aspect in studies on biodiversity: the ecological interactions between species. This approach, particularly at a continental scale, is novel. While previous studies have recognized the importance of species interactions, this article reviews and builds upon an observation made years ago by Professor Daniel Janzen regarding the loss of ecological interactions: “What escapes our view is a much more insidious type of extinction [than species extinction]: the extinction of ecological interactions".
After all, no species on Earth exists in isolation; each species depends on others for survival. Therefore, the article tackles a crucial topic: current efforts in biodiversity conservation often overlook the functional aspects of biodiversity—those functions that arise directly from species interactions (e.g., pollination, mycorrhization, seed dispersal by animals, etc.).
The article provides a brilliant and compelling analysis of the diversity of interactions concerning the conservation status (IUCN) of the animal and plant species involved in these seed-dispersal mutualisms. The emphasis here is on the conservation status of the interactions themselves, rather than just the species involved.
Animal-mediated seed dispersal is essential for forest regeneration. If we lose frugivorous animals and the countless interactions they maintain with plants, forests would collapse: the seeds would simply not reach the ground in conditions suitable for regenerating plant populations. Their natural regeneration cycle would fail. In tropical rainforests, for example, more than 85% of woody species depend on frugivorous animals for seed dispersal, and in Mediterranean forests, this figure is as high as 60%.
Most conservation initiatives so far have emphasized pollination as a key mutualistic interaction that illustrates the interdependence between plants and animals (see, for example, the 2019 IPBES report). However, this article highlights the problematic status of other mutualistic interactions, such as seed dispersal, which has been relatively understudied.
The implications of the extinction of species interactions are profound. The authors show that one-third of the animal-dispersed species and their interactions are at risk of extinction in Europe, while 30% of plant species have most of their dispersers threatened or declining. This finding echoes Janzen's concept of “cryptic extinction”: interactions, and therefore their ecological functions, can be lost long before the species themselves go extinct. Even more concerning, the authors identify a persistent knowledge gap, with many species classified as "not evaluated" or "data deficient," for which we do not know their population trends.
When frugivorous animals reach low densities and become rare in a forest, their functional role as seed dispersers disappears. The loss of these dispersers leads to rapid changes in landscape dynamics, including significant changes in vegetation composition, where fleshy-fruited plant species gradually decline. This decrease in fruit availability, in turn, affects the frugivores themselves. Such landscape changes are already occurring in several ecosystems and are not slow processes; a field researcher can observe them throughout their life. It is alarming that this article shows that such trends are widespread in European biomes, from the Arctic to Mediterranean forests.
The main limitation is the lack of information about the population status of numerous plant and animal species, even in our well-studied European biomes. This prevents a complete understanding of the conservation status of these interactions in European ecosystems. At the same time, it suggests that the alarming estimates presented in the article may be underestimations, meaning that the real situation could be even worse.
The ecological processes that are likely to collapse due to a crisis in seed dispersal range from forest regeneration after disturbances such as wildfires to plant responses to rapidly changing climatic conditions on a continent already severely affected by habitat loss and forest fragmentation. Resolving this situation requires developing measures to protect habitats, fauna, and flora, ensuring the persistence of species at adequate levels to maintain their ecological functions.