Seed dispersal mechanisms modulate Janzen‐Connell effects in Mediterranean forests through antagonists and mutualists

Abstract

Antagonist and mutualist organisms mediate the long‐standing, classical Janzen‐Connell effects and yet they are rarely investigated, especially when they share plant species hosts. Here, we propose that plant species create an “antagonists‐ and mutualist‐scape” which, coupled with seed dispersal mechanisms, will govern the relative survival rates of juveniles within the landscape and influence their later life stages. We combined data on fully stem mapped plots of woody plant species in two Mediterranean forests (Mixed Forest of Segura [MFS] and Mixed Forest of Jaen [MFJ]) with data on the associated communities of leaf epiphytic fungi, leaf pathogens, sap‐sucking insects and chewer insects hosted by each woody plant species. This allowed us to indirectly test the Janzen‐Connell hypothesis by investigating the placement of plant species with contrasting seed dispersal mechanisms (fleshy fruited vs. dry‐fruited plant) within the antagonists‐ and mutualist‐scapes. More specifically, we determined the degree of host‐specificity of plant‐associated organisms and defined the antagonist‐ or mutualist‐scapes for focal plants as the mean number of antagonist‐ or mutualist species the focal plant shares with its (sapling or adult) plant neighbours, respectively. We then applied multivariate spatial point pattern analysis to test whether saplings and adults of the focal species tended to be located in favourable sites of the antagonist‐scapes (i.e. were neighboured by fewer antagonist species than expected) or were distributed independently of the antagonist‐scapes. A substantial proportion of organisms was associated with more than one plant species, with organisms at MFJ being more “host‐specific” than organisms at MFS. Our spatial analyses revealed spatial patterns that differed strongly between fleshy‐fruited and dry‐fruited species and were, in most cases, consistent with our expectations. For example, saplings of dry‐fruited species were placed in favourable sites in the saplings antagonist‐scapes, whereas saplings of fleshy‐fruited species were independently placed with respect to the saplings antagonist‐scapes. Our results show how the placement of plant species within the antagonist‐scapes changes during the ontogeny, and how this is influenced by Janzen‐Connell effects and the seed dispersal mechanisms. We provide a new approach and indications about the importance of generalist antagonists and mutualists as drivers of the spatial assembly of plant communities. Read the free Plain Language Summary for this article on the Journal blog.