Unexpected coexistence of a native and an invasive macrophyte: a functional versus environmental niche perspective

In this study, we aimed at investigating the mechanisms underlying the coexistence of a native and an invasive macrophyte (Egeria najas and Hydrilla verticillata), by analyzing the overlap and breadth of their functional and environmental niches in the Upper Paraná River basin (Brazil). We adopted a probabilistic hypervolume approach using relative niche size and uniqueness as metrics of niche breadth and divergence. We compared the two species niches and the variance within each species’ populations when occurring in monospecific stands or in mixed growth conditions. We found that the two species have partially different functional niches and they both expand their functional niche when co-occurring, revealing increased traits variability. This suggests that both species have a different and higher ability to exploit available resources when co-occurring with each other, which might explain their coexistence. However, H. verticillata was not greatly affected by the presence of E. najas since it does not show any environmental niche shift in its presence; nonetheless the latter species is still able to thrive given its more generalist behavior in terms of environmental conditions. Our results suggest that coexistence between the investigated native and invasive species is facilitated more profoundly by functional rather than environmental adaptations.