Bees Like Diversity: Polycultures Can Support Many Kinds of Pollinators

Introduction

Agriculture depends on pollination from insects, specifically wild bees; however, habitat alteration and climate change are impacting biodiversity, and subsequently pollination. Agriculture can either provide resources to support pollinator communities or contribute to their decline. Large-scale agriculture intensification has resulted in diminished pollinator communities, whereas agricultural diversification has been shown to support wild bees.

Wild bees offer key pollination services and can provide an alternative to managed honey bee communities, which are in decline and furthermore the rental costs of managed honey bees are also increasing. These wild bees can be categorized as specialists (bees that rely on a particular plant or plants) and generalists (bees that visit a wide variety of plants). While increasing different kinds of floral resources on a farm can help generalist pollinators, it may inhibit specialist pollinators who rely on specific pollen sources from certain crops. Specialist bees have a mutually dependent relationship with their plants, where not only do the bees rely on the plant, but the plant relies on them too. Specialist “squash bees,” for example, have an intricate relationship with squash plants and are an essential component of squash plant reproduction, as they have evolved together.

There is little known about how monocultures, as opposed to polycultures, affect specialist pollinators. This study looked at specialist and generalist bees on monoculture and polyculture farms to determine which agricultural system supports the greatest abundance and diversity of pollinators. It also examined how this abundance and diversity changes throughout the day, as well as how the proportion of agricultural land in surrounding areas affects on-farm pollinator diversity and abundance.
    
Guiding questions included:

• How does farm type affect specialist pollinators and other groups of bees visiting squash flowers?
• How do pollinator communities visiting squash flowers change throughout the day?
• How does the proportion and diversity of the area surrounding the farm affect pollinator diversity and abundance on squash flowers?

Research Process

This study took place in the late spring of 2018 in the San Joaquin Valley of California. The researchers examined small-scale farms (<40 acres) of two categories: monocultures, which grow only one crop type per season, and polycultures, which grow two or more crop types. All farms sampled had to at least grow squash, and squash was chosen as the focal crop because it is visited by both specialist and generalist bees. They sampled bees on 10 farm sites (5 polyculture farms and 5 monoculture farms). Researchers observed both wild bee communities (generalists) and specialist bees (squash bees) to compare their response to crop diversification, and bees were classified as squash, honey, or wild bees.

Researchers surveyed pollinators during two periods in the morning: just after sunrise (around 5:30am; hereafter “early morning”) and later in the morning (around 8:30am; hereafter “late morning”). The surrounding landscape was characterized using the following land-use categories: developed, water, vegetation, polyculture, or monoculture. Using the last two categories, the proportion of agriculture in the area surrounding each farm site was calculated.

Results

A total of 590 bees were observed, of which 31% were squash bees, 13% were other wild bees, and 56% were honey bees. Researchers found that on-farm diversification (i.e. polyculture farms) supported a higher abundance of squash specialist bees, other wild bees, and honey bees. Bee abundances were affected by farm type, the proportion of surrounding agricultural land, and the interaction between the two. As the proportion of surrounding agricultural land increased, squash bee and wild bee abundances both increased. Researchers found no effect of on-farm diversification or the surrounding agricultural land on the diversity, richness, or evenness of the bee communities. 

Additionally, there were differences in pollinator abundances depending on the time of morning. As the squash flowers started to close in the mid-morning, the abundance of squash bees declined on monoculture farms but remained the same on polyculture farms. Honey bee and total bee numbers remained the same from early to late morning on monoculture farms, while increasing on polyculture farms. Wild bee abundances showed no difference throughout the morning.

Implications

It was hypothesized that squash monocultures would provide ample resources for squash bees, but instead, diversified crops support higher abundances of specialists, other wild bees, and honey bees. This study shows that pollinators are vulnerable to the resource simplification caused by monoculture agriculture, and on-farm resource diversification can allow pollinator communities to flourish. It is important to have continuous floral resources for pollinators––even specialists, as they require other resources in addition to their specialty plant––which is not necessarily possible on monoculture farms as all crops bloom during the same period of time. In monocultures, however, floral resource diversification can be achieved through hedgerows or in the surrounding field margins, though this strategy may be challenging as it requires additional funds, labor, and time to implement and maintain. 

Furthermore, it is important to note that these patterns of increased pollinator abundances could also be explained by other management practices such as reduced tillage and chemical usage on polyculture farms compared to monoculture farms. Overall, diversifying crops and utilizing intercropping can increase ecosystem services such as pollination, with important benefits to specialist bee populations. This subsequently would help improve species conservation and ecosystem health while lowering costs by boosting wild bee communities and thereby eliminating the need for managed pollinator colonies.