Climate Change is Causing A Decline in Bombus occidentalis by Reducing its Floral Resources

Abstract

Concerns of the decline of pollinators and their services have recently been raised. While several hypothesis have been studied, such has the shift of flowering phenology so it no longer overlaps with pollinators and habitat fragmentation, this study investigated the decline of the nectar robbing bumblebee species Bombus occidentalis and the flower that it robs, Ipomopsis aggregata. B. occidentalis depends on I. aggregata because it is out-competed by other bumblebee species, of which none visit I. aggregata. This study tested the hypothesis that B. occidentalis has declined using data that Graham Pyke collected in 1974. I also tested the hypothesis that using the proportion of robbed I. aggregata is a strong predictor of the abundance of B. occidentalis. I then investigated whether B. occidentalis is dependent on both I. aggregata and a patch of mixed flowers. Lastly, I investigated whether there is more nectar robbing occurring later in the bloom of I. aggregata than early in the bloom. I found that B. occidentalis has declined since 1974. Proportion robbed is a strong estimator of bee abundance. In the early part of I. aggregata’s bloom the number of flower species in the nearest patch of mixed flowers significantly explained the proportion robbed. Later in the season the distance to the nearest patch of mixed flowers explained the most variation of proportion robbed. B. occidentalis is dependent on patches of I. aggregata that are close to or mixed with other species of flowers. There was more robbing later in the season and this is most likely because of increased competition for nectar by other species of bees as more workers are produced.

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