Introduction
Climate represents a major selective force in the evolution of sociality. Climatic conditions can shape developmental rates, nesting strategies, biotic interactions, and other processes that determine the relative costs and benefits of group formation (Blumstein et al., 2022; Fisher et al., 2021; Menzel and Feldmeyer, 2021; Moss and While, 2021; Wilson, 1971). Social living can give rise to emergent strategies for coping with climatic stressors (Arnold, 1988; Fahrenholz et al., 1989; Klok and Chown, 1999). Observed patterns in global biogeography support hypotheses linking climate to social evolution, with distributions of social organisms falling along gradients of temperature, precipitation, and climatic stochasticity (Jetz and Rubenstein, 2011; Lukas and Clutton-Brock, 2017; Purcell, 2011). These interactions between climate and sociality raise important questions about the fate of social organisms and social phenotypes under changing climate.
Climate change is predicted to have major consequences for the survival, health, and distributions of organisms globally, including ecologically and economically important animals like pollinators (da Silva et al., 2023; Forrest, 2016; Potts et al., 2010). Bees (Hymenoptera: Apoidea: Anthophila) are the primary animal pollinators of terrestrial ecosystems (Ollerton et al., 2011). Bees possess some of the richest diversity of social behaviors of any animal taxon, with multiple independent origins of eusociality and a broad range of non-eusocial forms (Michener, 1974; Wcislo and Fewell, 2017). Within the past decade, researchers have increasingly investigated species-level social traits as possible predictors of bee responses to environmental change (Forrest et al., 2015; Hall et al., 2019; Harrison et al., 2018). Social organization in bees correlates with other behavioral and life history traits, like foraging strategy and phenology, that can influence susceptibility to climate change stressors. However, a unifying pattern linking sociality to climate change vulnerability has remained elusive.
Climate change is also likely to shift the expression and geographic distribution of social behaviors across animal populations. Expected consequences of climate change include not only increasing average temperatures, but also increasing climate variability and increasing frequency and severity of extreme events like drought, flooding, and wildfire (IPCC, 2022). For bees, many of which can flexibly express sociality in response to environmental conditions (Wcislo and Fewell, 2017), the social impacts of these shifts could be particularly profound. Nevertheless, very few studies have directly assessed the impacts of climate change on bee social organization. Still, these effects may be extrapolated from known relationships between bee sociality and local environmental conditions. Drawing from this literature, we make predictions about the consequences of climate change for bee sociality and identify promising directions for future research.
Box 1: Notes on bee social biology and terminology
The ancestor of bees was solitary, as are the vast majority of extant bee species (Debevec et al., 2012). Nevertheless, diverse forms of social living are known to be found in all currently recognized bee families except the Stenotritidae (Michener, 1974; Wcislo and Fewell, 2017). The extent of and capacity for social living among bees is likely to be vastly underestimated, given that social strategies may be present at low levels in otherwise solitary populations (Wcislo, 1997, 1993; Yagi and Hasegawa, 2012). Social organization in bees takes a diversity of forms, and includes several independent origins of (and reversals from) eusociality (Danforth et al., 2003; Kocher and Paxton, 2014). Among and within taxa, bee societies vary considerably in group size, in the degree of reproductive skew and morphological specialization, and in the genetic relatedness of group members (Michener, 1974; Ostwald et al., 2022b; Wcislo and Fewell, 2017). Particularly instructive for social evolutionary studies are the many lineages in which social living is a facultative state, which may be determined by environmental conditions.
The studies we review here vary considerably in their categorizations of social states. For the purposes of this review, we will define sociality as multi-female group living within a nesting context, with or without reproductive division of labor (sensu Wcislo & Fewell, 2017). We consider communal living as a social category because it entails behaviors and characteristics distinct from solitary living that are relevant to environmental tolerance, for example, by enhancing nest defense from predators and parasites, by generating advantages in nest excavation and initiation, and by enabling social thermoregulatory behaviors. In general, we will broadly compare social groups to solitary individuals, but emphasize that sociality represents a broad spectrum of behaviors with corresponding diversity in their responses to climate conditions.