Predation, parasitism, and competition
For many bee lineages, sociality may have arisen as a strategy for mitigating the effects of inter- and intraspecific interactions, especially parasitism, predation, and intraspecific competition (Lin and Michener, 1972; Wcislo and Fewell, 2017). Climate change will impact the distribution and abundances of diverse communities of bees’ competitors and natural enemies, with variable consequences for bee social behavior. In populations under strong selective pressure from parasitism or predation, sociality can enable bees to forage without leaving their nests unattended. Active defensive behavior by guard bees, or even simply the presence of a bee in the nest, can successfully deter natural enemies (Kukuk et al., 1998; Mikát et al., 2016; Zammit et al., 2008). Abrams and Eickwort observed cleptoparasitic bees (Nomada , Apidae) entering solitary nests of the sweat bee Agapostemon virescens Fabricius, 1775 (Halictidae), but never communal nests, which were continuously guarded (1981). Similarly, solitary Ceratina australensis nests were more severely parasitized by chalcid wasps (Eurytoma sp.) than social nests of the same species (Rehan et al., 2011). Importantly, social nesting can also provide insurance against nest failure in the event of foundress mortality (Gadagkar, 1990; Queller, 1994). In one study of the facultatively eusocial sweat bee Megalopta genalis Meade-Waldo, 1916, adults in the nest successfully defended brood against raiding ants, but orphaned brood all succumbed to ant predation (Smith et al., 2003).
Similarly, in environments characterized by strong intraspecific competition, sociality can provide strategies for securing and safeguarding limiting resources, especially food and nesting substrate. Social nests of the facultatively social carpenter bee, Xylocopa pubescens Spinola, 1838, contain a non-reproductive guarding female and a reproductive forager (Gerling et al., 1981). In one study, the presence of a guard in the nest prevented pollen robbing by conspecifics and also allowed the dominant reproductive to complete longer foraging trips (Hogendoorn and Velthuis, 1993). Importantly, the relative costs and benefits of tolerating a guard (i.e., a reproductive rival) in the nest depended on local resource availability and therefore the intensity of pollen robbing (Hogendoorn and Velthuis, 1993). Competition over nests is also a driver of social evolution in some bees, especially when nest substrate is limited or costly to exploit. Shifts in nest substrate availability may even drive social evolutionary transitions, as for one stem-nesting allodapine bee, Braunsapis puangensis Cockerell, 1929. The recent introduction of B. puangensis to Fiji accompanied by a shift to communal nesting from the ancestral strategy of reproductive queueing (da Silva et al., 2016). Because Fiji has no native stem-nesting bees, low competition for nesting substrate in their introduced environment may have expanded opportunities for egalitarian sociality (2016). Finally, nests may be limiting not due to a shortage of substrate, but due to costs of exploiting that substrate. For large carpenter bees (Xylocopa ), the high metabolic costs of wood nest excavation may favor sociality via nest inheritance strategies (Ostwald et al., 2021). Indeed, for one population of Xylocopa virginicaLinnaeus, 1771, high-density conditions led to an increase in social nesting due to saturation of available nests (Vickruck and Richards, 2021). Under climate change, as species are redistributed in time and space (phenological and geographic shifts), we expect social evolutionary consequences of these changes in selective pressures related to parasitism, predation, and intraspecific competition.