Abstract
Root- and shoot-feeding herbivores have the capacity to influence one another by modifying the chemistry of the shared host plant. This can alter rates of nutrient mineralization and uptake by neighboring plants and influence plant–plant competition, particularly in mixtures combining grasses and legumes. Root herbivory-induced exudation of nitrogen (N) from legume roots, for example, may increase N acquisition by co-occurring grasses, with knock-on effects on grassland community composition. Little is known about how climate change may affect these interactions, but an important and timely question is how will grass–legume mixtures respond in a future with an increasing reliance on legume N mineralization in terrestrial ecosystems. Using a model grass–legume mixture, this study investigated how simultaneous attack on lucerne (Medicago sativa) by belowground weevils (Sitona discoideus) and aboveground aphids (Acyrthosiphon pisum) affected a neighboring grass (Phalaris aquatica) when subjected to drought, ambient, and elevated precipitation. Feeding on rhizobial nodules by weevil larvae enhanced soil water retention under ambient and elevated precipitation, but only when aphids were absent. While drought decreased nodulation and root N content in lucerne, grass root and shoot chemistry were unaffected by changes in precipitation. However, plant communities containing weevils but not aphids showed increased grass height and N concentrations, most likely associated with the transfer of N from weevil-attacked lucerne plants containing more nodules and higher root N concentrations compared with insect-free plants. Drought decreased aphid abundance by 54% but increased total and some specific amino acid concentrations (glycine, lysine, methionine, tyrosine, cysteine, histidine, arginine, aspartate, and glutamate), suggesting that aphid declines were being driven by other facets of drought (e.g., reduced phloem hydraulics). The presence of weevil larvae belowground decreased aphid numbers by 30%, likely associated with a significant reduction in proline in weevil-treated lucerne plants. This study demonstrates how predicted changes to precipitation patterns and indirect interactions between herbivores can alter the outcome of competition between N-fixing legumes and non-N-fixing grasses, with important implications for plant community structure and productivity.
Highlights
Species Interactions and Climate ChangeEcological communities form a network of directly and indirectly interacting species (Wootton, 1994; Polis, 1998)
Using a model grass–legume mixture of Harding grass (Phalaris aquatica L.) and lucerne, this study addressed the effects of water stress and root nodule herbivory by S. discoideus on both the plant–plant interactions between lucerne and Harding grass and on foliar-feeding pea aphids (A. pisum), one of the most damaging pests of lucerne [see Ryalls et al (2013a) for review]
Grasses and lucerne plants subjected to drought at week 8 were shorter than those grown under ambient and elevated precipitation, height did not vary significantly between plants grown under ambient and those grown under elevated precipitation
Summary
Species Interactions and Climate ChangeEcological communities form a network of directly and indirectly interacting species (Wootton, 1994; Polis, 1998). Changes in host plant nutrients (e.g., amino acid concentrations) often underpin interactions between aboveground and belowground herbivores, with knock-on effects on plant community structure and productivity (Johnson et al, 2013). All trophic interactions, both direct and indirect, are specific to the species combinations involved (Kos et al, 2015) and may be altered by climate change (McKenzie et al, 2013). Such changes may propagate through communities and sway competitive advantages between species (Johnson et al, 2011; Barton and Ives, 2014; Jing et al, 2015). Effects of drought are of longer duration and are considered to have far more disruptive effects on plant–insect interactions (Pritchard et al, 2007)
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