Biological control alters competition between Pontederia crassipes Mart. (Pontederiaceae) and Salvinia minima Baker (Salviniales: Salviniaceae) in a eutrophic waterbody

Pontederia crassipes remains a highly invasive weed globally, posing significant threats to aquatic ecosystems. Megamelus scutellaris, a biological control agent released in South Africa effectively manages this weed, even at eutrophic, and high-elevation cool sites. Biological control of P. crassipes at Hartbeespoort Dam, a hypertrophic system in South Africa plagued by nuisance macrophyte growth, was followed by the invasion of an aquatic fern, Salvinia minima. This study explains the population dynamics of P. crassipes and S. minima at Hartbeespoort Dam, as a function of M. scutellaris herbivory to P. crassipes. Competition and herbivory effects on both species were evaluated using an additive series analysis in mesocosms. Multiple regression analyses indicated that in the absence of herbivory on P. crassipes, its interspecific competition coefficient was 4 times greater than that of S. minima. The competitive vigour of S. minima more than doubled in the presence of herbivory on P. crassipes. Consequently, in the presence of the biological control agent, no statistically significant difference in interspecific competition for both species could be found. These results highlight how biological control of one species can alter the population dynamics between two competing species. While biological control of S. minima is under consideration in South Africa, in the absence of nutrient remediation, the system will remain vulnerable to further invasion.