Extensive loss of forage diversity in social bees owing to flower constancy in simulated environments

Abstract

Many bees visit just one flower species during a foraging trip, i.e. they show flower constancy. Flower constancy is important for plant reproduction but it could lead to an unbalanced diet, especially in biodiversity-depleted landscapes. It is assumed that flower constancy does not reduce dietary diversity in social bees, such as honeybees or bumblebees, but this has not yet been tested. We used computer simulations to investigate the effects of flower constancy on colony diet in plant species-rich and species-poor landscapes. We also explored if communication about food sources, which is used by many social bees, further reduces forage diversity. Our simulations reveal an extensive loss of forage diversity owing to flower constancy in both species-rich and species-poor environments. Small flower-constant colonies often discovered only 30-50% of all available plant species, thereby increasing the risk of nutritional deficiencies. Communication often interacted with flower constancy to reduce forage diversity further. Finally, we found that food source clustering, but not habitat fragmentation impaired dietary diversity. These findings highlight the nutritional challenges flower-constant bees face in different landscapes and they can aid in the design of measures to increase forage diversity and improve bee nutrition in human-modified landscapes.

Keywords: agent-based models; bees; communication; diet; flower constancy; nutrition.

Figure 1.

The species diversity (as the percentage of all available flower species) (a–d) and Simpson’s diversity index (SDI) (e–h) in relation to colony size, food source distribution, food abundance and foraging strategy (flower constancy and communication) when food sources offered small rewards and needed time to replenish (1200 s). Colonies were either flower constant (triangle) or foraged indiscriminately (= randomly, circles); colonies either had communication (blue in (a–d); pink in (e–h)) or consisted of bees that foraged solitarily (red in (a–d); orange in (e–h)). Uniform distribution means that food sources were uniformly distributed, whereas clustered distribution means that food sources were clustered. Ten clusters per plant species were simulated (default, see §2 for more details).

Figure 2.

The species diversity (as the percentage of all available flower species) (a–d) and SDI (e–h) in relation to colony size, food source distribution, food abundance and foraging strategy (flower constancy and communication) when food sources offered large rewards and needed time to replenish (1200 s). For further explanation, see the legend of figure 1.

Figure 3.

The species diversity (as the percentage of all available flower species) (a–d) and SDI (e–h) in relation to colony size, food source distribution, food abundance and foraging strategy (flower constancy and communication) in environments with low plant diversity (four species), when food sources offered small rewards and needed time to replenish (1200 s). For further explanation, see the legend of figure 1.

Figure 4.

The species diversity (as the percentage of all available flower species) (a–d) and SDI (e–h) in relation to colony size, food source distribution, food abundance and foraging strategy (flower constancy and communication). All measurements are from environments with low plant diversity (four species), in which food sources offered large rewards and needed time to replenish (1200 s). For further explanation, see the legend of figure 1.

Figure 5.

The species diversity (as the percentage of all available flower species) (a–d) and SDI (e–h) in relation to colony size, type of fragmentation (squares versus stripes versus unfragmented; see electronic supplementary material, figure S1c,d) and foraging strategy (flower constancy and communication). All measurements are from environments with high flower species diversity (12 species), in which uniformly distributed food sources offered small rewards and needed time to replenish (1200 s). Food source abundance was intermediate in all cases, with an average of 500 food sources per plant species (approx. 6000 in total).

Figure 6.

Percentage of foragers collecting pure pollen loads in 30 bee species. Letters a and b indicate statistically significant differences (see electronic supplementary material, table S2 for details).