Effect of density and neighbours on interactions between invasive plants of similar growth form

Abstract

Invasive aquatic plants constitute a great problem in many freshwater systems. Although many studies have addressed the potential threats of invasion by exotic species in aquatic ecosystems, few studies have considered the interactions (competition, facilitation, coexistence) between exotic aquatic plants of similar growth form in invaded communities. Our aim was to investigate experimentally the effect of density in monocultures and the effect of neighbour identity and plant density on a focal species in mixed cultures. We used Elodea canadensis, Egeria densa and Lagarosiphon major as model species and conducted two experiments, one in a monoculture and another in a mixed culture following an additive design. Individuals were planted in pots with several treatments based on the identity and density of neighbour species. Our results demonstrated that the development of an invasive submerged plant could be influenced either positively (facilitation) or negatively (competition or inhibition) by the presence of more than one neighbour species of different densities. An increase in density significantly stimulated competition intensity, although the effect of density on the performance of the invader also depended on neighbour identity. A facilitative effect of E. canadensis on the apical growth of E. densa was established. However, despite these results, the competitive outcome also depended on other environmental factors (e.g. underwater radiation) and not on plant density and neighbour identity alone.

Monitoring of alien aquatic plants in the inland waters of Sicily (Italy)

“…In aquatic environments, an important factor determining invasiveness is nutrient availability. For example, experiments comparing competitiveness between Lemna minuta and L. minor found that the invasive species was dominant only under high levels of nutrient availability (Njambuya et al 2011), and therefore invasion would increase with eutrophication, as reported also for E. crassipes (Coetzee et al 2017) In mixed aquatic communities, plant density has been reported as another relevant factor affecting invasiveness, either through facilitation or competition, depending on the species (Silveira and Thiébaut 2020).…”