Abstract
Pollinators can mediate facilitative or competitive relationships between plant species, but the comparative importance of these two conflicting phenomena in shaping pollinator resource use in plant communities remains unexplored. This paper provides a case example and proof of concept of the idea that the arrangement in pollinator niche space of large species samples comprising complete or nearly complete plant communities can be instrumental to evaluate the importance of facilitation and competition as drivers of pollinator resource at the plant community level. Pollinator composition data for 221 plant species from the Sierra de Cazorla mountains (southeastern Spain), comprising 85% of families and ~95% of widely distributed species of entomophilous plants, were used to address the following questions: (1) Do objectively identifiable species clusters occur in pollinator niche space ? Three different pollinator niche spaces were considered whose axes were defined by insect orders (N = 7), families (N = 94) and genera (N = 346); and (2) If all plant species form a single, indivisible cluster in pollinator niche space, Are species overdispersed or underdispersed relative to a random arrangement ? “Clusterability” tests failed to reject the null hypothesis that there was only one pollinator-defined species cluster in pollinator niche space, irrespective of the space axes (insect orders, families or genera) or pollinator importance measurement (proportions of pollinator individuals or flowers visited by each pollinator type) considered. Randomly simulated species arrangements in each pollinator niche space showed that observed means of pairwise interspecific distances in pollinator composition were smaller than values from simulations, thus revealing significantly non-random, underdispersed arrangement of plant species within the single cluster existing in pollinator niche space. Arrangement of plant species in niche space in the montane plant community studied did not support a major role for interspecific competition as a force shaping pollinator resource use by plants, but rather a situation closer to the facilitation-dominated extreme in a hypothetical competition-facilitation gradient. Results also illustrate the potential of investigations on complete or nearly complete entomophilous plant communities for addressing novel hypotheses and contributing insights to the understanding of the ecology and evolution of plant-pollinator systems.