Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes.

Mixed-species groups of birds, fishes and mammals have traditionally been described in taxa-specific journals. However, mixed-species systems are actually more widely found when one includes aggregative (non-moving) systems, such as those common in amphibians and invertebrates. The objective of this special issue is to dispel the idea that mixed-species phenomena are a 'niche topic' to ecology and instead explore how taking a mixed-species perspective can change our conception of important ecological patterns and processes. A mixed-species perspective starts by understanding the relative abundance and positioning of individuals of different species and their behavioural synchrony; it is enriched by understanding differences between species in their vulnerability/attractiveness to predators, their potential for competing with other group participants and their use as a source of public information. Contributions to the special issue show how the mixed-species perspective can change our ideas about invasion ecology, island biogeography, keystone species, mimicry, predator eavesdropping and more. Rather than seeking synthesis, the special issue celebrates the taxonomic and conceptual breadth of the field of mixed-species groups, with detailed descriptions of many distinctive systems. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Mixed-species groups of herbivorous reef fishes show variable responses to ecosystem perturbations in the Lakshadweep Islands, India.

Herbivorous reef fishes provide a vital function in reef ecosystems by removing algae and making space available to coral recruits. The high abundance of herbivores in the reefs of the Lakshadweep islands has potentially aided in reef recovery and helped avoid a phase shift to an algal-dominated system, despite most areas having suffered massive coral losses. Mixed-species grouping in herbivores could potentially benefit both the participant species and the reef ecosystem by improving foraging efficiency. We examined the grouping propensity and species richness for three types of herbivore groups after a mass-bleaching event in 2010 and a mass recruitment event in 2015. The species richness and number of parrotfish groups, as well as the grouping propensity of common species, declined starkly across years, indicating that these groups may have formed in response to the mass-bleaching event, slowly diminishing as the reefs recovered. Conversely, large surgeonfish, which varied in richness and propensity across islands and aspect, are probably influenced by local processes. Small surgeonfish only increased in species richness and number in 2015, which may have been in response to the recruitment event. Thus, herbivorous fishes may respond differently to local ecosystem perturbations and play different roles in reef recovery. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Wave exposure reduces herbivory in post-disturbed reefs by filtering species composition, abundance and behaviour of key fish herbivores.

Harsh environmental conditions limit how species use the landscape, strongly influencing the way assemblages are distributed. In the wake of repeated coral bleaching mortalities in Lakshadweep, we examined how wave exposure influences herbivory  in exposed and sheltered reefs. We used a combination of i. field observations of fish herbivore composition, abundance and activity across 6 exposed and 6 sheltered reefs; ii. experimental manipulations in a subset of these reefs (herbivore exclosures); and iii. opportunistic observations of fish recruitment, to determine how exposure influences herbivore biomass and herbivory. Species richness, biomass, abundance, total bite rates and species-specific per capita bite rates were lower in exposed compared to sheltered reefs, linked to strong environmental filtering of species composition, abundance and behaviour. For some critical species, this environmental filtering begins with differential recruitment and post-recruitment processes between exposures. Bite rates at sheltered sites were dominated by just a few species, most being laterally compressed surgeonfish that may find it difficult accessing or surviving in wave-battered shallow reefs. Exclosure experiments confirmed  that exposed reefs  were less controlled by herbivores than sheltered reefs. In post-disturbed reefs like Lakshadweep, environmental gradients appear to be key mediators of critical functions like herbivory by determining species composition, abundance and behaviour.

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

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.