Possible top-down control of solitary bee populations by ambush predators

The effect of ambush predators on bee populations has so far been neglected on the assumption that predators ambushing at flowers for the arrival of pollinators are too scarce to have significant effects on population numbers. A simple mathematical model can be used to calculate the maximum predator abundance compatible with positive growth of solitary bee populations. Using published data to estimate model parameters, the model shows that medium-sized (20–80 mg dry weight) solitary bees are very sensitive to predation and their populations can hardly grow when 1–2% of flowers are occupied by ambush predators—a predator abundance well within the range reported in most studies. Smaller and larger bees, on the other hand, can easily cope with normal predator abundances: small bees do not need to visit many flowers to provision an offspring, making it unlikely that they encounter a predator, and large bees are less vulnerable to predator attacks. According to a sensibility analysis, the parameters with the greatest impact on bee population viability are the number of flowers that bees must exploit to provision an offspring and the probability that a bee, upon landing on a predator-harboring flower, is actually captured. It follows that ambush predators cannot be excluded a priori from the list of factors affecting bee diversity and abundance: small changes in the abundance of ambush predators, possibly related to changes in the availability of alternative prey, could have dramatic effects on the viability of medium-sized solitary bees.