Ecological and functional niches comparison reveals differentiated resource-use strategies and ecological thresholds in four key floating-leaved macrophytes

Niche theory has been widely used in ecology; however, few studies have attempted to combine information on functional and ecological niches (i.e., variation in traits and environmental requirements), especially for freshwater macrophytes. In this study, we aim to describe the functional and ecological niches of four key nymphaeid species (Nuphar lutea, Nymphaea alba, Nelumbo nucifera, and Nymphoides peltata) to investigate their environmental tolerance and functional adaptability. Twelve Italian populations per species were sampled. Functional and ecological niches were determined using hypervolumes based on eight functional traits and environmental variables, related to leaf structure and economics spectrum, and to water and sediment quality. Among the three Italian native species, N. lutea and N. alba showed intermediate niche size and position in the functional and ecological space, although N. alba appears to be less competitive due to a small functional niche and lower trait performance, which could explain its general tendency to decline. N. peltata appeared more specialized in its environmental requirements and characterized by highly acquisitive leaves, while the invasive alien N. nucifera exerted its competitive success by distinguishing its functional niche and expanding its ecological niche, through high investment of resources in leaves. Overall, all four target species share similar ecological niches, colonizing eutrophic ecosystems typical of intensive agricultural landscapes, but show different patterns in their functional niche. We demonstrate the applicability of an approach based on both functional and ecological niches to unravel species' adaptation and strategies.