Bees may drive the sexual reproduction of four sympatrically distributed cacti in a vanishing coastal Mediterranean ecosystem

Background. Sympatric plant species that share pollinators may have similar mating systems because their floral traits are subject to comparable canalization imposed by pollinators. However, if each sympatric species bears specialized floral morphology, each species may attract different pollinators. Our study aims to describe the pollinator diversity and pollination systems of four taxa of Eriosyce that co-occur in an endangered coastal Mediterranean ecosystem in Central Chile. We took two approaches in our study: we assessed the composition and similarity of flower visitors among taxa, and we characterized the breeding systems to determine dependence on pollinators and self-compatibility.

Methods. We performed field observations to characterized pollinators during two consecutive years (2016-2017). Additionally, we performed pollination experiments to elucidate reproductive modes using three treatments: manual cross-pollination, automatic self-pollination, and control (unmanipulated individuals).

Results. We observed one bird species (Giant hummingbird Patagona gigas only visiting E. subgibbosa) and 14 bee species (13 natives plus Apis mellifera) visiting cacti of the genus Eriosyce. We observed variation in the similarity of intra-specific pollinator composition between years and among Eriosyce species within the same year. Individuals of E. subgibbosa were visited by less number of species (2016 = 4; 2017 = 2), while E. chilensis (2016 = 4; 2017 = 8), E. chilensis var. albidiflora (2016 = 7; 2017 = 4) and E. curvispina var. mutabilis (2016 = 7; 2017 = 6) were visited by a richest guild of visitors (up to 10 bee species each).Autonomous pollination was unfeasible inE. chilensis, which depend on bees to achieve their reproductive success. Eriosyce subgibbosa, visited mainly by the Giant hummingbird, depends on pollinators to achieve reproductive success. Both E. chilensis var. albidiflora and E. curvispina var. mutabilis were visited by a diverse assemblage of non-social native bees, showing some degree of autonomous pollination and self-compatibility.

Discussion. Pollinator diversity analyses showed considerable pollinator differences between the species with ornithophilous flowers (E. subgibbosa) and remain taxa which solely dependent on Apoidea species for pollen transfer. The high diversity of native bees among sympatric Eriosyce may be a caused by their microclimatic differences at spatial (differences among cacti microhabitats) and temporal levels (differences of climatic conditions between August to December when different Eriosyce species bloom). Our study contributes to unveiling the evolutionary mechanisms for pollinator partitioning of sympatric close-related plant species. Furthermore, it improves understanding of threatened species reproductive system and ecological interactions, especially to E. chilensis and E. chilensis var. albidiflora, whose studied populations are the only known for these taxa.