Bottom-up effects on top-down regulation of a floating aquatic plant by two weevil species: the context-specific nature of biological control.

Published online: 04 Jun 2014
Content type: Journal article
Journal title: Journal of Applied Ecology
DOI: 10.1111/1365-2664.12213

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Publication language: English

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Abstract

Plant nutrition (bottom-up effects) impacts a plant's ability to withstand herbivory (top-down effects) and influences phytophagous insect fecundity. These factors confound efficacy predictions for biological control purposes. We investigated the importance of these forces with regard to invasibility of Eichhornia crassipes. We also examined whether one could have predicted whether the weevil species Neochetina eichhorniae or Neochetina bruchi would have been the better biocontrol agent. Three experiments compared E. crassipes performance when subjected to herbivory by these weevils across a range of fertilizer levels. A low initial plant density experiment examined growth and flowering while allowing density to increase. A high plant density experiment evaluated reduction in biomass and surface coverage. A third experiment evaluated plant growth unconstrained by space. Herbivory effects varied between treatments; interactions varied between experiments. Herbivory reduced plant density and biomass, except in low fertilizer conditions, and consistently reduced flowering. N. bruchi alone or both weevil species together reduced growth more than N. eichhorniae alone, elucidating N. bruchi as the better agent. However, N. eichhorniae better reduced biomass and plant density under initial conditions of complete plant coverage, suggesting it as the superior choice. Synthesis and applications. Bottom-up and top-down forces acting in concert affect invasibility of plant species. Plasticity to resource availability enables invaders to persist in conditions that are unfavourable to herbivores (enemy-free space). Consequently, biological control efficacy is nuanced and context specific, so predictability requires assessment of multiple parameters across a range of conditions. Overly simplistic evaluations risk rejection of effective agents capable of mediating non-native plant invasions. These results also argue that biological control practitioners should disregard the concept that a single best agent can be identified to control an invasive plant that exploits a broad range of habitats and environmental conditions. Aquaphyte invasions often coincide with watershed nutrification so unless weed control is carried out concurrently with measures that reduce nutrient inputs, removal of infestations will merely reduce competition thereby accelerating regrowth or facilitating replacement by other undesirable species (the 'invasive treadmill'). The suppressive effects of biological control are valuable and complimentary, even when replacement by other, presumably less problematic, species may occur. These subtler effects should be considered with protocols developed for integrated control strategies. Bottom-up and top-down forces acting in concert affect invasibility of plant species. Plasticity to resource availability enables invaders to persist in conditions that are unfavourable to herbivores (enemy-free space). Consequently, biological control efficacy is nuanced and context specific, so predictability requires assessment of multiple parameters across a range of conditions. Overly simplistic evaluations risk rejection of effective agents capable of mediating non-native plant invasions. These results also argue that biological control practitioners should disregard the concept that a single best agent can be identified to control an invasive plant that exploits a broad range of habitats and environmental conditions. Aquaphyte invasions often coincide with watershed nutrification so unless weed control is carried out concurrently with measures that reduce nutrient inputs, removal of infestations will merely reduce competition thereby accelerating regrowth or facilitating replacement by other undesirable species (the 'invasive treadmill'). The suppressive effects of biological control are valuable and complimentary, even when replacement by other, presumably less problematic, species may occur. These subtler effects should be considered with protocols developed for integrated control strategies.

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