Mixed company: a framework for understanding the composition and organization of mixed‐species animal groups

Abstract

Mixed‐species animal groups (MSGs) are widely acknowledged to increase predator avoidance and foraging efficiency, among other benefits, and thereby increase participants' fitness. Diversity in MSG composition ranges from two to 70 species of very similar or completely different phenotypes. Yet consistency in organization is also observable in that one or a few species usually have disproportionate importance for MSG formation and/or maintenance. We propose a two‐dimensional framework for understanding this diversity and consistency, concentrating on the types of interactions possible between two individuals, usually of different species. One axis represents the similarity of benefit types traded between the individuals, while the second axis expresses asymmetry in the relative amount of benefits/costs accrued. Considering benefit types, one extreme represents the case of single‐species groups wherein all individuals obtain the same supplementary, group‐size‐related benefits, and the other extreme comprises associations of very different, but complementary species (e.g. one partner creates access to food while the other provides vigilance). The relevance of social information and the matching of activities (e.g. speed of movement) are highest for relationships on the supplementary side of this axis, but so is competition; relationships between species will occur at points along this gradient where the benefits outweigh the costs. Considering benefit amounts given or received, extreme asymmetry occurs when one species is exclusively a benefit provider and the other a benefit user. Within this parameter space, some MSG systems are constrained to one kind of interaction, such as shoals of fish of similar species or leader–follower interactions in fish and other taxa. Other MSGs, such as terrestrial bird flocks, can simultaneously include a variety of supplementary and complementary interactions. We review the benefits that species obtain across the diversity of MSG types, and argue that the degree and nature of asymmetry between benefit providers and users should be measured and not just assumed. We then discuss evolutionary shifts in MSG types, focusing on drivers towards similarity in group composition, and selection on benefit providers to enhance the benefits they can receive from other species. Finally, we conclude by considering how individual and collective behaviour in MSGs may influence both the structure and processes of communities.

The relationship between morphology and behavior in mixed‐species flocks of island birds

“…The flocks characterized here appear to be structured largely by complementary rather than supplemental benefits, sensu Goodale et al. (2020). We do not note any examples of convergence in plumage or call that might suggest the flocks were shaped by supplementary benefits.…”

“…In New Guinea, mixed‐species flocks are found that appear to have both elements of the supplementary and complementary models of Goodale et al. (2020). Flocks centered around the Papuan babbler ( Garritornis isidorei ), for example, include constituent species that mimic the babbler's plumage and call (Bell, 1983; Diamond, 1987).…”

“…The benefits of mixed‐species flocking can result from the association with individuals of the same or similar species (i.e., supplementary benefits), or complementary interactions among species in which interspecific differences provide benefits to group members (Goodale et al., 2020). Due to the intensity and complexity of interspecific interactions in mixed‐species flocks, constituent species may be exposed to novel biotic selective pressures (Harrison & Whitehouse, 2011).…”

Diffusion of Social Information in Non-grouping Animals

“…As the above examples suggest, social information may originate not only from conspecifics, heterospecific individuals are also likely to be an important, and for a long time overlooked, source of social information (Seppänen et al, 2007;Webster and Laland, 2017;Sridhar and Guttal, 2018). Using social information produced by heterospecifics might be preferable when territoriality or competition among conspecific individuals limit the frequency of direct interactions (Goodale et al, 2020); in line with that, foraging in mixed-species groups has been predicted to be more beneficial under several ecological scenarios than foraging in single species groups (Gil et al, 2017). Nongrouping animals may encounter heterospecific social cues more frequently than cues originating from conspecifics; on the other hand, such cues can be less reliable or relevant as well.…”

“…similar physiological characteristics and motivation assemble in aggregations and display some temporary synchrony in their patterns of activity (e.g., some fish shoals or locust swarms), while in other organisms, like eusocial hymenopterans, they form highly structured, and stable societies (Ward and Webster, 2016). Animals often aggregate with heterospecifics and create mixedspecies groups as well, especially when they share resources or are subject to the same threats (Goodale et al, 2017(Goodale et al, , 2020. Because of the apparent diversity in animal grouping behavior, however, the dichotomous categorization of animals into group-living and solitary species can be misleading.…”