Linking agricultural systems, secondary and conserved forests through complex networks: Pollination as an ecosystem function and an ecosystem service

Background/Question/Methods

Anthropogenic disturbance affects pollinators that are crucial for the reproduction of most tropical plant species and for the production of many economically important crops.  Tropical dry forests are among the most heavily used and least conserved ecosystems in Mesoamerica and more than two thirds have been  fragmented and transformed in secondary forest.  The study of the quality of these habitats for pollinator use, and of the regeneration of forests and its dependence on animal pollination are ecological priorities for study.  This study aims to understand the links among plant-pollinator systems of different habitat types of a tropical dry forest of western Mexico, through visitation networks.  The objectives were 1)to compare pollinator diversity and network structure of visitation networks of early, intermediate and late successional sites of tropical dry forest, and of agricultural crops around these forests; and 2) to identify the pollinators that link these systems and their foraging resources.  The study was conducted in the tropical dry forest Chamela-Cuixmala Biosphere Reserve and surrounding agricultural and secondary forests.  Three sites of each of three successional stages were used, as well as sites of several crops dependent on animal pollination (cucumber, pepper, mango, melon, tomatillo and watermelon) to construct plant-pollinator networks. 

Results/Conclusions

Pollinator abundance was greater at the early successional sites, where floral resources were also greater.  The main difference in floral resources among successional sites was due to greater abundance of lianas' flowers during the summer at early successional sites.  Rarefaction analyses indicate that pollinator species richness is not significantly different among successional stages.  Nevertheless, specialization of late successional sites is greater than that of early successional sites, and network asymmetry was greater in early successional sites, indicating differences in network structure along forest succession.  Twenty one native bee species (seven of which were common), as well as the honeybee visited the studied crops.  These bee species foraged on resources of species common to all three successional sites, as well as resources unique to late successional sites.  We show that protected continuous forests provide crucial foraging resources to pollinators of economically important crops year round and the main native visitors are social species. This is the first study to link natural and agricultural systems through pollination networks, and to evaluate pollination network structure along succession.