1. Although ecologists commonly categorize species in terms of their functional roles, function diversity occurring at the level of the individual is often dismissed.
2. Multi-female colonies of the spider Anelosimus studiosus serve as habitat for a myriad of arthropods, and colony members display notably polymorphic behavioural tendencies: females exhibit either an ‘aggressive’ or ‘docile’ behavioural phenotype.
3. We manipulated the phenotypic composition of colonies (100% aggressive, 50% aggressive and 50% docile, 100% docile) and tested its effects on species interactions between A. studiosus and its web associates, and among the web associates themselves.
4. We found that the phenotypic composition of A. studiosus colonies significantly impacted interactions within their web. In colonies of all aggressive females, the relationship between A. studiosus (−) and its web associates (+) was exploitative and web associates negative impacted each other’s performance. In colonies of all docile females, the relationship between A. studiosus (+) and its web associates (+) was facilitative and web associates positively influenced each other’s performance. Colonies of mixed phenotype had intermediate interactions.
5. Our data suggest that (i) the mixture of behavioural trait variants within groups can mediate the nature of both direct and indirect species interactions, and (ii) community structure can affect which social group compositions enjoy highest fitness.