Insights from Plant Reproductive Trade-offs and Plant-pollinator Interactions at Contrasting Ecological Scales

Abstract

<p>Global change is threatening both plant and pollinator species, leading to a rapid loss of ecological interactions. Surprisingly, our knowledge of plant reproductive ecology and plant-pollinator interactions remains largely disconnected and there is a need to better understand how plant reproductive strategies affect, and are affected by, the structure of plant-pollinator interactions. This thesis tries to progress knowledge by investigating (i) plant reproductive trade-offs; (ii) how these tradeoffs determine plant-pollinator interactions; (iii) the effects of pollinator sharing for plant reproduction; and (iv) methods to assess the state of vegetation communities, as this can influence the condition of pollinator communities.</p> <p>First, I compiled information on 17 reproductive traits for more than 1506 flowering plants from 64 plant-pollinator networks distributed across the globe to explore plant reproductive trade-offs and how they influence interactions with floral visitors. Second, using 60 of these plant-pollinator networks, I examined the structural properties of network motifs (subsets of interactions) that describe both direct and indirect interactions among plants and pollinators. I then assessed over- and under-representation of the different plant and floral visitor functional groups on the distinct motif positions and position combinations. Third, I explored one of the consequences of indirect interactions, heterospecific pollen deposition on floral stigmas, by creating an experimental co-flowering community with heterogeneous traits and contrasting phylogenetic relatedness. This allowed me to investigate the impact, mechanisms and competitive structure of heterospecific pollen arrival. Finally, because plant and pollinator species interactions depend on the state of natural systems, I investigated the performance of three remotely sensed metrics (two cover metrics and one novel indicator of ecological condition) to assess the state of vegetation across environmental gradients.</p> <p>Of the flowering plants recorded, two main reproductive trade-offs were evident (‘flower number - flower size’ and ‘pollinator dependence’) that explained over half of the reproductive trait variation. These plant reproductive trade-offs determined their interactions with floral visitors worldwide. The analysis of network motifs revealed that the structure of motifs in real plant-pollinator networks is non-random. Further, the different floral visitor functional groups associated with distinct motif positions describe contrasting ecological roles. Interestingly, the combination of functional groups within motifs cannot be retrieved by their observed probabilities indicating the presence of underlying ecological processes that constrain motif composition. In the experimental plant communities, heterospecific pollen deposition showed a prevailing negative effect on seed production. These impacts were mainly driven by pollen recipient traits and specific pollen recipient - pollen donor combinations of traits. In addition, the structure of pollen competition was hierarchical with clear ‘winner’ and ‘loser’ species. Lastly, the different remote sensing metrics tested to assess vegetation cover and condition indicated good performance on tree-dominated communities but they were not reliable for specific ecosystem types from arid regions.</p> <p>Greater understanding of plant reproductive ecology is clearly essential to progress knowledge on the global spectrum of plant trait variation, but also to understand the structure of plant-pollinator interactions (e.g., pollination syndromes). Further, pollinator-mediated interactions among plants (e.g., competitive or facilitative processes) and the consequences for plant fitness need to be considered to disentangle the main ecological drivers of species coexistence. Finally, there is an urgent need to develop precise indices that can capture the state of vegetation for species conservation and enable better understanding of the different ecological processes that mediate plant-pollinator interactions.</p>