Precipitation effects
Social strategies can also help mitigate environmental stressors
associated with precipitation and water availability. Many of these
effects are intrinsically bound to thermal effects, working in concert
with temperature variation to determine whether conditions are favorable
or unfavorable for foraging and brood rearing. Annual variation in
precipitation can dictate activity periods, either by inhibiting
foraging in times of extended rainfall or by creating floral dearth
periods in times of drought. Indeed, for the facultatively eusocialAugochlorella aurata , drought conditions reduced brood sizes by
two to three offspring (Packer, 1990). Conversely, Schürch et al.
suggest that an increase in spring rainfall under climate change could
reduce the frequency of social nesting in Halictus rubicundus , by
delaying provisioning and reducing the time window to produce a second
brood (2016). Likewise for Halictus ligatus , high rainfall
created conditions unfavorable for worker production, with consequences
for social organization (Richards and Packer, 1995). Finally,
precipitation can pose direct survival risks that may affect the
frequency of social nesting. Heavy rainfall and flooding threaten brood
survival, which can reduce worker recruitment, decreasing colony size
and restricting opportunities for social nesting.
Precipitation can also alter features of the physical environment that
determine the costs and benefits of social nesting. For ground nesting
bees, nest excavation may be particularly costly when soils are hard,
especially during drought. Danforth suggests that the high energetic
costs of excavating dry, hard-packed soils favors communal nesting for
the desert-adapted bees Perdita portalis Timberlake, 1954 (1991)
and Macrotera texana Cresson, 1878 (1996). Drying of soils is a
predicted consequence of climate change in many regions, which may raise
the costs of nest excavation for ground nesting bees, thereby increasing
the benefits of cooperative nest excavation. Indeed, Bohart and Youssef
observed an increase in the incidence of social nesting during drought
conditions for the typically solitary sweat bee Lasioglossum
lusorium Cresson, 1872 (1976). In addition to energetic costs,
excavation of dry soils could entail increased cuticular wear,
potentially increasing risk of desiccation. However, the physiological
and behavioral consequences of dry soil excavation remain to be tested
empirically.