Macrophyte diversity–abundance relationship with respect to invasive and native dominants

• Invasive and native dominants altered the diversity–abundance relationship.
• Patterns were consistent over time and with environmental heterogeneity.
• Plant diversity can increase abundance.
• But dominant species can have a strong effect on diversity–abundance relationships.
• Dominant species play an important ecological role, whether exotic or native.

Many experiments have demonstrated positive relationships between plant diversity and abundance, but few have examined the diversity–abundance relationship in non-experimental systems, aquatic systems, or in the presence of invasive (non-native dominant) species. We sampled the aquatic macrophyte community of a large lake in northern Wisconsin to examine: (1) the diversity–abundance relationship and (2) the influence of invasive and native dominant species on that relationship. Over repeated sampling efforts we observed an overall positive correlation between species richness and macrophyte abundance, but trends were less positive where the dominant species, Potamogeton robbinsii (native), was present, whereas, Myriophyllum spicatum (invasive) showed no consistent significant trend. These trends held in the context of other environmental variables known to affect macrophyte diversity and abundance as well as across multiple years. Our observational data are in agreement with current ecological theory on diversity–abundance relationships, mainly that: (1) plant diversity can increase abundance and (2) dominant species can have a strong effect on diversity–abundance relationships. Such effects might be more typical of invasive species (e.g., M. spicatum), but in our system a less-widespread native (P. robbinsii) had more consistent and significant effects, illustrating the difficulty of determining management actions based on simple classifications or observations. Our results from an aquatic macrophyte community highlight the ecological importance of dominant species, whether exotic or native.