Response of summer squash pollinator density to habitat loss in Southeast Yunnan, China

The response diversity theory claims that functional pollinators differently react to environmental change in a range of spatial and/or temporal scales. Diverse responses of pollinators could contribute to the stabilization and resilience of ecosystem pollination functions by the reorganization of pollinator community. To date, however, little remains known about the responses of Chinese pollinators to environmental changes, especially considering the rich pollinator diversity represented in China. We selected 18 summer squash fields in Guangnan County, Yunnan Province, to study the reaction of bee communities to habitat loss. In our study, habitat loss was represented with two change gradients-one gradient of natural and semi-natural habitats surrounding the squash fields in landscapes (used to investigate landscape effect on pollinators) and one gradient of distance away from natural and semi-natural habitats (used to investigate distance effect on pollinators). While 8 fields were selected to explore the landscape effect, 12 fields were used to explore the distance effect, and of which two fields were simultaneously used for both experiments. Flower visitors were sampled once in the 18 fields on sunny days from June to July 2015. Land use and land cover were derived from Landsat satellite image and were identified as secondary forest, grassland, water, dryland, wetland and building. Secondary forest and grassland were treated as natural and semi-natural habitats, which were beneficial for management and wild bees. Then dryland and wetland were treated as farmland, which were believed to have a diminishing effect on wild bee population. The Pearson correlation coefficient was calculated between pollinator density and percentages of semi-natural habitats and farmlands at a range of nested spatial scales (250 m, 500 m, 750 m, 1 000 m, 1 500 m, 2 000 m and 2 500 m). Responses of honey bee density and bumble bee density along the percentages of natural and semi-natural habitats and farmlands were explored at spatial scales with the highest correlation coefficient. Honey bee and bumble bee density changes along the distance away from natural and semi-natural habitats were also explored. We found that bumble bees and honey bees were the dominate pollinators for squash flowers in the study area. Ants and wasps also visited the squash flowers, but they were not analyzed due to their extremely low densities. Bumble bee density and honey bee density were associated with natural and semi-natural habitats and farmlands at different spatial scales. The percentage of natural and semi-natural habitats was significantly negatively correlated to that of farmlands at spatial scales ranging from 500 m to 2 500 m. Bumble bee density increased along the gradient of natural and semi-natural habitats in landscapes, but honey bee density was not influenced by surrounding natural and semi-natural habitats. While bumble bee density declined as surrounding farmlands increased in landscapes, honey bee density was not influenced by farmlands. Moreover, bumble bee density declined along increasing distance away from natural and semi-natural habitats, but honey bee density was not influenced by the distance to natural and semi-natural habitats. The different responses of honey bees and bumble bees to habitat loss could be related to pollination services of squash. Our findings indicated the need for a special attention by farmers and researchers to understanding and managing crops that are reliant on wild pollinators.