Résumé
1. Clarifying the functional consequences of intraspecific trait
variability in response to interacting trophic levels would provide a
significant improvement in our understanding of aboveground-belowground
linkages. In particular, the effects of grazing on plant traits may
translate into altered litter quality, with potentially important
consequences for litter-feeding decomposers. Plant and litter variability
in response to grazing is expected to depend on soil fertility levels,
with tolerance and defensive strategies more commonly expressed on fertile
and poorer soils, respectively. However, how grazing and fertility
interactively alter litter quality and palatability to detritivores has
not been explored yet. 2. We conducted a cafeteria experiment with three
common millipede (Diplopoda) species feeding on leaf litter from two plant
species, the grass Bromopsis erecta and the forb Potentilla verna. Each
millipede was offered a binary choice between litter types produced by the
same plant species, but sampled in plots with distinct herbivory and
fertilization status: litter originating from grazed areas or from
one-year sheep exclosures, both in native areas and in adjacent paddocks
that received chemical N and P fertilization, as well as litter from a
25-year sheep exclusion in the native area. 3. We found that fertilization
and herbivore exclusion interactively affected Bromopsis litter quality
and palatability, whereas Potentilla was much less affected. Bromopsis
litter palatability was not affected by grazing when litter was collected
in native plots, except for the long-term exclosure which led to a very
low palatability. In contrast, and in line with our expectations,
herbivory was associated with much higher palatability in fertilized
plots. The changes in palatability were associated with important
alterations of litter quality. 4. Overall, our study demonstrates that
intraspecific variation in litter can have profound consequences for soil
functioning. It emphasizes the role of grazing as a key, but plant
species-specific factor controlling litter intraspecific variability, and
its complex interaction with soil fertility level. Moreover, our results
advocate for a better understanding of the response of the different
organisms involved in the decomposition process, in particular
litter-feeding macro-detritivores. We encourage future studies aiming at
disentangling the various and complex relationships between aboveground
processes such as herbivory and soil functioning.