Over the past 150 years, numerous populations of native shrubs worldwide have increased in density and cover, effectively invading neighboring grassland communities. The consequences of shrub encroachment vary depending on the ecosystem and group of organisms evaluated. In the American Southwest, desert grasslands have been extensively replaced by Larrea tridentata, causing decreased plant cover and diversity. This reduction in floral reward diversity is likely to affect insect pollinators, and consequently ecosystem function, due to pollinator’s major role in plant reproduction. However, the floral resources provided by the encroacher species itself might be considerable. The consequences of shrub encroachment in this habitat are therefore likely to be complex, and will depend on the extent that the invading species can be utilized by the pollinator community as well as on whether the decreased abundance of forb species can successfully attract and sustain those pollinators which cannot rely on the shrub. This study evaluates the effect of L. tridentataencroachment on the diversity and abundance of bee communities. Thirty-minute bee surveys were conducted at side-by-side plots (grassland vs. shrubland) once a week from July to September 2015 and from May to September 2016 to evaluate differences in overall bee community composition.
Results/Conclusions
A total of 1442 bees belonging to 31 different genera were collected visiting a total of 29 plant species. Overall bee abundance was higher in the grassland sites, compared to the neighboring shrubland. NMDS ordination plots show that bee community composition was markedly different in shrubland sites compared to immediately adjacent grassland sites, with bee community composition clustering significantly by habitat. Plant-pollinator network analysis showed that the grassland network is slightly more nested, diverse and specialized, suggesting that it is slightly more robust than the shrubland network. Shrubland sites were clearly dominated by L. tridentata and its associated bees. In contrast, grassland sites had several plant species with relative high number of bee visitors, contributing to an overall increased diversity in this habitat.
Bee diversity worldwide is highest in arid-temperate areas, such as the American Southwest. Shrub encroachment and the consequent displacement of the unique grassland bee community in this region, effectively reducing overall diversity, is therefore cause for concern.