“Which fruit should I eat?” – a decision both migratory and resident birds have to make over and over each autumn. This decision – and the consequences of it is studied in the Early View paper “Consistency and reciprocity of indirect interactions between tree species mediated by frugivorous birds” by Daniel Martinez and co-workers. Below is the authors’ summary of the paper:
Do fruiting plants compete or facilitate each other for frugivores providing seed dispersal? This question has been previously answered through single-species, short-term studies, that have evidenced indirect interactions between plants. Nevertheless this leaves unanswered another important question. How variable through time and across species within a community are these interactions? Resolving this question will help us to reveal the actual relevance of these interactions in natural systems.
Our study was developed in a straightforward plant-frugivore system. Three species of fleshy-fruited trees (hawthorn Crataegus monogyna, holly Ilex aquifolium and yew Taxus baccata) coexist in the secondary forest of the Cantabrian mountain range (NW Spain). Their seeds are mainly dispersed by a common assemblage of frugivorous blackbirds and thrushes (Turdus spp.). During autumn and winter both resident and wintering birds have to choose where to feed, among the fruiting trees belonging to the three plant species. Decisions are taken not only considering a given individual tree, but also the trees standing in its neighborhood, with the aim of optimizing in which to perch.
Far from general patterns of competition and/or facilitation between these tree species, we find that, like often in nature, variability seems to be the rule. The arising of indirect interactions and their sign shifted between species and across years. Plant-frugivore systems, even those simple like this, are functionally complex. The abundance and spatial distribution of fruits changed from year to year. While some tree species increased their crops other became scarce. Birds faced very different fruiting scenarios every autumn and, thus, the costs and profits of feeding on different trees changed from year to year.
We do not attempt to explain the rules driving indirect interactions within a community, but to show their complexity, consistency between years and reciprocity between species. Taking this variability into account is crucial to understand the role of indirect interactions in the structuring of natural communities.
Artwork and photo credits: Daniel Martínez.