Do managed bees have negative effects on wild bees?: A systematic review of the literature

Abstract

Managed bees are critical for crop pollination worldwide. As the demand for pollinator-dependent crops increases, so does the use of managed bees. Concern has arisen that managed bees may have unintended negative impacts on native wild bees, which are important pollinators in both agricultural and natural ecosystems. The goal of this study was to synthesize the literature documenting the effects of managed honey bees and bumble bees on wild bees in three areas: (1) competition for floral and nesting resources, (2) indirect effects via changes in plant communities, including the spread of exotic plants and decline of native plants, and (3) transmission of pathogens. The majority of reviewed studies reported negative effects of managed bees, but trends differed across topical areas. Of studies examining competition, results were highly variable with 53% reporting negative effects on wild bees, while 28% reported no effects and 19% reported mixed effects (varying with the bee species or variables examined). Equal numbers of studies examining plant communities reported positive (36%) and negative (36%) effects, with the remainder reporting no or mixed effects. Finally, the majority of studies on pathogen transmission (70%) reported potential negative effects of managed bees on wild bees. However, most studies across all topical areas documented the potential for impact (e.g. reporting the occurrence of competition or pathogens), but did not measure direct effects on wild bee fitness, abundance, or diversity. Furthermore, we found that results varied depending on whether managed bees were in their native or non-native range; managed bees within their native range had lesser competitive effects, but potentially greater effects on wild bees via pathogen transmission. We conclude that while this field has expanded considerably in recent decades, additional research measuring direct, long-term, and population-level effects of managed bees is needed to understand their potential impact on wild bees.

Fig 1. Hypothesized interactions between managed bees and wild bees.

Wild and managed bees may interact indirectly (dashed lines) through either bottom-up effects on shared resources (including pollen, nectar, and nesting sites), or by altering top-down interactions through shared antagonists such as pathogenic organisms.

Fig 2. PRISMA flow diagram.

A flow diagram showing the process for a systematic review including the number of studies processed, reviewed, and analyzed at each step in the review process.

Fig 3. Publication trends.

The total number of published studies over time from 1900–2016 that examined the effects of managed bees on wild bees via three reviewed mechanisms: competition for resources, changes in plant communities (specifically native and exotic plant populations), and transmission of pathogens. While the literature search began in 1900, the first publication within these topical areas did not occur until 1964.

Fig 4. Reviewed effects of managed bees on wild bees through competition for shared resources.

Variables reported by studies examining the competitive effects of managed bees on wild bees including (A) managed bee metrics (independent variables), (B) potential mechanisms (explanatory variables), and (C) wild bee responses (dependent variables).

Fig 5. Reported results from reviewed studies on the effects of managed bees on wild bees.

The total number of reviewed studies that found positive (+), neutral (0), negative (-), or mixed effects of managed bees on wild bees through (A) competition for shared resources, (B) changes in plant communities, and (C) transmission of pathogens. Studies within each category are divided into those that examined managed bees within their native range, and those that studied managed bees within their introduced range.

Fig 6. Reviewed effects of managed bees on wild bees through changes in plant communities.

Variables reported by studies examining the effects of managed bees on plant communities including (A) managed bee metric (independent variable), (B) potential mechanism (explanatory variable), and (C) plant responses (dependent variable).

Fig 7. Reviewed effects of managed bees on wild bees through transmission of pathogens.

Variables reported by studies examining the effects of managed bees on wild bees through pathogens including (A) managed bee metric (independent variable), (B) potential mechanisms (explanatory variable), and (C) wild bee responses (dependent variable).