Abstract
Predicted increases in atmospheric carbon dioxide (CO2) concentrations often reduce nutritional quality for herbivores by increasing the C∶N ratio of plant tissue. This frequently triggers compensatory feeding by aboveground herbivores, whereby they consume more shoot material in an attempt to meet their nutritional needs. Little, however, is known about how root herbivores respond to such changes. Grasslands are particularly vulnerable to root herbivores, which can collectively exceed the mass of mammals grazing aboveground. Here we provide novel evidence for compensatory feeding by a grass root herbivore, Sericesthis nigrolineata, under elevated atmospheric CO2 (600 µmol mol−1) on a C3 (Microlaena stipoides) but not a C4 (Cymbopogon refractus) grass species. At ambient CO2 (400 µmol mol−1) M. stipoides roots were 44% higher in nitrogen (N) and 7% lower in carbon (C) concentrations than C. refractus, with insects performing better on M. stipoides. Elevated CO2 decreased N and increased C∶N in M. stipoides roots, but had no impact on C. refractus roots. Root-feeders displayed compensatory feeding on M. stipoides at elevated CO2, consuming 118% more tissue than at ambient atmospheric CO2. Despite this, root feeder biomass remained depressed by 24%. These results suggest that compensatory feeding under elevated atmospheric CO2 may make some grass species particularly vulnerable to attack, potentially leading to future shifts in the community composition of grasslands.
Highlights
The largest annual increase in global atmospheric CO2 emissions in the last 50 years occurred during 2010 [1]
Plant responses Plant biomass was unaffected by elevated atmospheric CO2 concentrations (eCO2), for either grass species, C. refractus plants were significantly bigger than M. stipoides largely due to higher shoot mass (Table 1)
This study demonstrates that eCO2 negatively affects a grass root herbivore when feeding on M. stipoides, a C3 grass, but not on C. refractus, a C4 grass
Summary
The largest annual increase in global atmospheric CO2 emissions in the last 50 years occurred during 2010 [1]. Such increases will impact on ecological communities and the species interactions within them. It is widely observed that elevated atmospheric CO2 concentrations (eCO2) reduces the nutritional quality of plants for herbivores [2]. A meta-analysis of over 100 published studies demonstrated that while both carbon (C) and nitrogen (N) increased in both roots and shoots, C increased at an accelerated rate relative to N and led to an average increase in C:N ratios of 11%, effectively reducing nitrogen concentrations in both roots and shoots [3], either by dilution or reallocation [4]. Only four studies have investigated the effects of eCO2 on root feeding insects [8,9,10,11] and none have investigated this for grasses
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