The functional response of aquatic plant invaders

The functional response of aquatic plant invaders
Philip S.R. Weyl1, Mhairi E. Alexander2, Julie A. Coetzee3, Jaclyn M. HILL1
1Department of Zoology & Entomology, Rhodes University, Grahamstown, SA
2School of Science & Sport, University of West Scotland, Scotland
3Department of Botany, Rhodes University, Grahamstown, SA

Refining the protocols for risk assessment is one of the major challenges in invasive species management. Invasive species impacts are often highly speculative and as a consequence, developing robust predictions about the potential impacts of invasive species can be challenging. Invaders are regularly associated with rapid and efficient resource allocation; accordingly, comparing the functional response (defined as resource uptake as a function of resource density) of invasive and native species may help predict impacts of emerging invaders. The functional responses of three South African native (Lagarosiphon muscoides, Myriophyllum spicatum and Lagarosiphon major) and one invasive (Egeria densa) submerged macrophyte species were compared under two temperature regimes; 20⁰C and 30⁰C. Functional response was defined as nitrogen uptake rates (as a function of nitrogen concentration) of ammonia (NH3) over 4 hours under full light. All plants demonstrated a clear type II functional response characterized by a decelerating intake rate, where plants were likely limited by their capacity to take up nitrogen. Functional response curves and maximum uptake rates (Vmax) were similar between species at 20⁰C, but L. major showed a significantly reduced Vmax at 30⁰C. Nitrogen affinity (1/Ks) was slightly higher in both temperature regimes for all native species, with L. major showing the highest affinity for nitrogen at 30⁰C. Overall, despite the clear type II curves, functional response was a poor predictor of invasion potential. However, nitrogen affinity appears to be marginally higher in the native species, suggesting some potential for native plants to outperform their invasive counterparts.

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