Functional diversity does not explain the co‐occurrence of non‐native species within a flow‐modified African river system

Globally, there is growing concern on the occurrence of multiple non‐native species within invaded habitats. Proliferation of multiple non‐native species together with anthropogenic‐driven habitat modifications raise questions on the mechanisms facilitating the co‐occurrence of these species and their potential impact within the recipient systems. Using the Great Fish River system (South Africa) which is anthropogenically‐modified by inter‐basin water transfer (IBWT), as a case study, this research employed trait‐based approaches to explore patterns associated with the co‐occurrence of multiple non‐native fish species. This was achieved by investigating the role of functional diversity of non‐native and native fishes in relation to their composition, distribution and environmental relationships. Nineteen functional traits that defined two broad ecological attributes (habitat use and feeding) were determined for 13 fish species that comprised eight native and five non‐native fishes. We used these data to, firstly, evaluate functional diversity patterns and to compare functional traits of native and non‐native fishes in the Great Fish River system. Secondly, we employed multivariate ordination analyses (factor analysis, RLQ and fourth‐corner analyses) to investigate interspecific trait variations and potential species‐trait‐environmental relationships. From a functional diversity perspective, there were no significant differences in most functional diversity indices between native and non‐native species. Despite interspecific variation in body morphology‐related traits, we also found no clear separation between native and non‐native species based on the ordination analysis of the functional traits. Furthermore, while RLQ ordination showed broad spatial patterns, the fourth‐corner analyses revealed no significant relationships among species distribution, functional traits and environmental variables. The weak species‐trait‐environment relationship observed in this study suggests that environmental filtering was likely a poor determinant of functional trait structure within the Great Fish River. Modification of the natural flow regime may have weakened the relationship between species traits and the environment as has been shown in other systems.