Abstract
Resource availability in agricultural landscapes has been disturbed for many organisms, including pollinator species. Abundance and diversity in flower availability benefit bee populations; however, little is known about which of protein or carbohydrate resources may limit their growth and reproductive performance. Here, we test the hypothesis of complementary resource limitation using a supplemental feeding approach. We applied this assumption with bumble bees (Bombus terrestris), assuming that colony growth and reproductive performance should depend on the continuous supply of carbohydrates and proteins, through the foraging for nectar and pollen, respectively. We placed wild-caught bumble bee colonies along a landscape gradient of seminatural habitats, and monitored the colonies' weight, foraging activity, and reproductive performance during the whole colony cycle. We performed supplemental feeding as an indicator of landscape resource limitation, using a factorial design consisting of the addition of sugar water (carbohydrate, supplemented or not) crossed by pollen (protein, supplemented or not). Bumble bee colony dynamics showed a clear seasonal pattern with a period of growth followed by a period of stagnation. Higher abundance of seminatural habitats resulted in reducing the proportion of pollen foragers relative to all foragers in both periods, and in improving the reproductive performance of bumble bees. Interestingly, the supplemental feeding of sugar water positively affected the colony weight during the stagnation period, and the supplemental feeding of pollen mitigated the landscape effect on pollen collection investment. Single and combined supplementation of sugar water and pollen increased the positive effect of seminatural habitats on reproductive performance. This study reveals a potential colimitation in pollen and nectar resources affecting foraging behavior and reproductive performance in bumble bees, and indicates that even in mixed agricultural landscapes with higher proportions of seminatural habitats, bumble bee populations face resource limitations. We conclude that the seasonal management of floral resources must be considered in conservation to support bumble bee populations and pollination services in farmlands.
Highlights
The fitness of organisms is related to the availability and intake of multiple nutrients
Bumble bee colony weight was highly seasonal and nonlinear, with a clear-cut distinction between a period of colony growth followed by a period of stagnation (Fig. 3a)
Our study underpins the importance of seminatural habitats to conserve pollinator populations in agricultural landscapes and suggests that even landscapes with a higher proportion of seminatural habitats were characterized by limited resource availability
Summary
The fitness of organisms is related to the availability and intake of multiple nutrients. Over the past century, intensified human activity has caused widespread environmental changes and disturbances of resource availability for many organisms (Tilman et al 2001). The decline of seminatural habitats and the disturbance in flower resource availability has led to the decline of pollinator populations in farmlands (Biesmeijer et al 2006, Krauss et al 2010, Potts et al 2010). This directly concerns human well-being, as pollination is critical for the reproduction of many wild plant species and crop yields (Klein et al 2007, Ollerton et al 2011, Potts et al 2016)
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