Does flower richness attract bees?

Are bees attracted by flower richness? What are the Implications for ecosystem service-based policy

Ecosystem services in rural landscapes ensure quality of life in human societies. Agri-environmental policies aim to implement biodiversity-based land-use solutions. Pollinator declines and the loss of their forage basis have received particular attention. A novel agri-environmental measure or eco-scheme option is bee-forage fields with melliferous flower crops in Estonia. This field should be combined either as three monocultures or a mixture. We challenge the largely biodiversity-oriented agricultural policy designs, i.e., wild pollinators benefit mostly from species-rich areas. Pollinator-oriented, species-rich flower seed mixtures are costly and areas complicated to maintain, but their service provision efficiency is poorly known.

The quality of ecosystem service provision can be described with three main properties (Kütt et al. 2018):

• functional diversity,
• functional stability and
• functional intensity.

Species diversity is associated well with the stability of ecosystem service provision (service duration and resilience to disturbance) and with functional diversity of the ecosystem (the diversity of distinct service providers). Functional diversity and intensity tend to saturate along the species diversity gradient (e.g. well shown for biomass production), because most of the service is provided by a small subset of (co-) dominant species – this is called functional redundancy among species.

A case study: Are bees attracted by flower richness?

An original multi-site experiment was developed to estimate the effect of plant diversity on the quality of flower-based services. Field test sites were distributed over an area of approximately 75 x 75 km in south-eastern Estonia. Sites were positioned in agricultural landscapes near groups of honey bee hives. To provide equal choice options for bees and to reduce the effect of confounding contextual factors (e.g., landscape structure), each test site consisted of eight randomly ordered strip segments of different flower species or diversity levels. Different seed mixtures were sown in excess into consecutive 10 cm wide x 10 m long strip sections in random order – the long and narrow strip design makes the detection and counting of flower visitors very time efficient. The treatments consisted of three different monocultures, one low-diversity mixture of the same three species, and four different high-diversity mixtures. Among the high-diversity treatments, one treatment consisted of a high-diversity mixture with balanced species proportions – this was the treatment with highest diversity (high species number and high evenness). The other three high-diversity treatments had one forage species as a (co-)dominant (the same species used in monoculture), meaning a high species number but with a lower evenness of species. Plant flowering and the number of pollinators were surveyed weekly over the season.

Service quality was estimated by service properties such as functional stability (as flowering duration, i.e., number of weeks with abundant flowering) and functional intensity (foraging activity of insect pollinators).

We expected that a diversity of simultaneously flowering plant species at a site can create a distraction for flower-visiting foragers, and therefore, a low-diversity mixture or monoculture can be more attractive for pollinators to forage. We separately addressed three groups of flower-visiting insects (potential pollinators): honey bees, bumblebees, and other conspicuous insect pollinators (i.e., solitary bees, syrphids, and butterflies combined) because the shape of the activity-diversity relationship may differ between these groups as a result of their social behavior.

Flowering duration (stability) between Species Mixtures and Monocultures
The results showed that the abundant flowering period lasted longer in all kinds of species mixture than in monocultures. The low-diversity mixture and all types of high-diversity mixture had equivalently long-duration flowering. The crop species-level analysis confirmed the systematically shorter flowering period of monocultures. To conclude, the three-species mixture was sufficient to ensure long continuity of abundant flowering, i.e. flower service provision stability (functional stability).

How Plant Diversity Influences Foraging Behavior of Bees

In contrast, the foraging activity of honey bees and bumblebees was greatest in monocultures and in a low-diversity mixture, while a balanced high-diversity mixture was least attractive. The foraging activity was already lower when an abundant melliferous plant species was flowering within a high-diversity mixture (i.e., the mixture with high species richness but with low evenness). A similar pattern was observed for honey bees and bumblebees. Other conspicuous flower visitors were overall characterized by low numbers of specimens, but there were hints of analogous differences in activity counts in relation to the diversity gradient.

To note, honey bees and bumblebees had different phenological patterns of foraging activity – the number of honey bees was relatively uniform during each summer, while bumblebees had an unimodal activity pattern, with the maximum number of flower-visiting specimens in July (the mid of vegetation season). The visiting activity of other pollinators was too low to detect general trends within and between years.

Biodiversity’s Complex Impact on Ecosystem Services

The role of biodiversity in shaping the provisional quality of ecosystem services is still debated. This flower field test provides evidence that plant species richness is not a comprehensive indicator of the ecosystem service provision quality. We found that human perception of service provision quality cannot be linearly transferred to other flower service intermediating groups. We showed that flower mixtures could be highly effective when composed of only a few true bee-forage plant species that are morphologically, functionally, and phenologically complementary. Flower fields or bee mixtures should also focus on the optimization of blooming phenology (i.e., sequential flowering) to reduce competition for pollinator attention, as flower diversity causes the suppression of forage activity even on a dominant species within other flowers.
We showed that bumblebees would profit from measures optimal for domestic honey bees. At the same time, many wild solitary pollinators could not react efficiently to the short-term appearance of man-made forage habitats in the same way, or they had low abundance in general.  For the efficient planning of bee fields and to support other pollinators, a mosaic of various monocultures and low-diversity mixtures is the ecologically most efficient field design, but a multi-functional landscape mosaic should also contain permanent (semi-)natural habitats. A specific smaller flower-field type might be an autumn flowering south-facing quick-snack patch near a beehive group or fragments of (semi-)natural habitat.

References:

Liira, I. Jürjendal, Are bees attracted by flower richness? Implications for ecosystem service-based policy, Ecological Indicators, Volume 154, 2023, 110927, ISSN 1470-160X, https://doi.org/10.1016/j.ecolind.2023.110927.