Unclusterable, underdispersed arrangement of insect-pollinated plants in pollinator niche space

Pollinators can mediate facilitative or competitive relationships between plant species, but the comparative importance of these two conflicting phenomena in shaping community-wide pollinator resource use remains unexplored. This paper examines the idea that the arrangement in pollinator niche space of plant species samples comprising complete or nearly complete regional or local plant communities can help to evaluate the relative importance of facilitation and competition as drivers of community-wide pollinator resource use. Pollinator composition data for insect-pollinated plants from the Sierra de Cazorla mountains (southeastern Spain), comprising 85% of families and ~95% of widely distributed insect-pollinated species, were used to address the following questions at regional (45 sites, 221 plant species) and local (one site, 73 plant species) spatial scales: (1) Do objectively identifiable plant species clusters occur in pollinator niche space ? Four different pollinator niche spaces were considered whose axes were defined by insect orders, families, genera and species; and (2) If all plant species form a single, indivisible cluster in pollinator niche space, Are they overdispersed or underdispersed relative to a random arrangement ? "Clusterability" tests failed to reject the null hypothesis that there was only one pollinator-defined plant species cluster in pollinator niche space, irrespective of spatial scale, pollinator niche space or pollinator importance measurement (proportions of pollinator individuals or flowers visited by each pollinator type). Observed means of pairwise interspecific dissimilarity in pollinator composition were smaller than randomly simulated values in the order-, family- and genus-defined pollinator niche spaces at both spatial scales, thus revealing significantly non-random, underdispersed arrangement of plant species within the single cluster existing in each of these pollinator niche spaces. In the undisturbed montane habitats studied, arrangement of insect-pollinated plant species in pollinator niche space did not support a major role for interspecific competition as a force shaping community-wide pollinator resource use by plants, but rather suggested a situation closer to the facilitation-dominated extreme in a hypothetical competition-facilitation gradient. Results also highlight the importance of investigations on complete or nearly complete insect-pollinated plant communities for addressing novel hypotheses on the ecology and evolution of plant-pollinator systems.