Alien plants and flower visitors disrupt the seasonal dynamics of mutualistic networks

Abstract

Alien species can drastically disrupt ecological processes such as those involving plant-pollinator interactions, performing central roles that may affect the structure of native pollination networks. However, most studies to date have focused on a single trophic level of alien species, evaluating either the impacts of an alien pollinator or an alien plant species, and have neglected their joint effects on the seasonal dynamics of mutualistic networks. Here, we aim to fill this gap by investigating how alien plant and flower visitor (here used as a proxy for pollinator) species structure temporal networks, and how these species affect the beta-diversity of interactions across a flowering season. Our study system is located in the South Island of New Zealand, where 68% of the sampled plant species and 14% of the flower visitor species that interact with them are alien. Alien flower visitor species exhibited higher interaction degree, specialization and strength than their native counterparts, while alien plant species showed the opposite pattern. We found that invader complexes (in which alien species interact significantly more with each other than with native species) were established across the season, and interactions involving alien plant species were the main connectors of the temporal networks. Both alien plant and flower visitor species increased total interaction turnover through the flowering season by promoting interaction rewiring in the case of alien plants and by increasing species turnover in the case of alien flower visitors. Synthesis. This study provides one of the first empirical reports of alien species shaping the seasonal dynamics of plant-flower visitor networks. We demonstrate that the presence of alien species may simultaneously lead to a homogenization of plant species composition and increase the diversity of plant-flower visitor interactions through a flowering season. Additionally, we highlight the importance of considering the role of different trophic levels when analysing the impact of alien species in plant-flower visitor communities. Overall, our findings suggest that management strategies should pay particular attention to the timescale at which interactions with alien species dissolve or form, and to the consequences and drivers of such seasonal dynamics.