Abstract
Submersed macrophytes have important ecological functions in many streams, but fostering growth of beneficial native species while suppressing weedy invasives may be challenging. Two approaches commonly used in management of terrestrial plant communities may be useful in this context: (1) altering resource availability and (2) establishing desirable species before weeds can invade (priority effects). However, these approaches are rarely used in aquatic systems, despite widespread need for sustainable solutions to aquatic weed problems. In artificial stream channels in California, USA, I conducted experiments with asexual propagules of non-native invasive Myriophyllum spicatum (Eurasian watermilfoil) and native Elodea nuttallii (western waterweed) to address the questions: (1) How does light availability affect relative performance of the two species?; (2) Does planting the native earlier than the invasive decrease survival or growth rate of the invasive?; and (3) Do light level and priority effects interact? The relative performance between E. nuttallii and M. spicatum had an interesting and unexpected pattern: M. spicatum had higher growth rates than E. nuttallii in the zero and medium shade levels, but had similar performance in the low and high shade levels. This pattern is most likely the result of E. nutallii’s sensitivity to both very low and very high light, and M. spicatum’s sensitivity to very low light only. Native priority did not significantly affect growth rate or survival of M. spicatum, possibly because of unexpectedly poor growth of the E. nuttallii planted early. This study suggests that altering light levels could be effective in reducing growth of an invasive macrophyte, and for changing the competitive balance between a native and a non-native species in the establishment phase. Further investigations into the use of priority effects and resource alteration for submersed macrophyte management are warranted, given their mixed results in other (limited) studies.
Highlights
The role of resource availability in determining relative performance of plant species is often examined in the context of competition between native and non-native invasive species
The mean relative growth rate (RGR) of E. nuttallii in the concurrent treatment was highest in the low shade level, intermediate in the zero and medium shade levels, and lowest in the high shade level—41.1% lower than the low shade level
My experiments tested the effect of light availability on relative performance of a native and an invasive submersed macrophyte when planted in a competitive environment, which is arguably more relevant to natural systems
Summary
The role of resource availability in determining relative performance of plant species is often examined in the context of competition between native and non-native invasive species (hereafter, ‘invasives’). Higher resource levels are commonly thought to favor invasive over native species, as plant traits that confer invasiveness (e.g., high growth rates and fecundity) are often associated with higher resource use [1,2]. Many studies do show a competitive advantage of invasive species over natives in high resource conditions and vice-versa [3]. Research into the competitive outcomes between natives and invasives across resource levels has focused largely on terrestrial plant communities, with relatively little attention to aquatic plant (submersed macrophyte) communities, in flowing systems. Few, if any, studies address how reductions in light may affect relative performance of invasive over native macrophytes when grown in a competitive environment
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