Nurse species and indirect facilitation through grazing drive plant community functional traits in tropical alpine peatlands
A Danet, S Kéfi, RI Meneses, F Anthelme
Facilitation among plants mediated by grazers occurs when an unpalatable plant extends its protection against grazing to another plant. This
type of indirect facilitation impacts species coexistence and ecosystem
functioning in a large array of ecosystems worldwide. It has nonetheless
generally been understudied so far in comparison with the role played by direct
facilitation among plants. We aimed at providing original data on indirect
facilitation at the community scale to determine the extent to which indirect
facilitation mediated by grazers can shape plant communities. Such experimental
data are expected to contribute to refining the conceptual framework on
plant-plant-herbivore interactions in stressful environments.
We set up a two-year grazing exclusion experiment in tropical alpine
peatlands in Bolivia. Those ecosystems depend entirely on a few, structuring
cushion-forming plants (hereafter referred to as nurse species) in which
associated plant communities develop. Fences have been set over two nurse
species with different strategies to cope with grazing (direct vs indirect
defenses), which are expected to lead to different intensities of indirect
facilitation for the associated communities. We collected functional traits
which are known to vary according with grazing pressure (LDMC, leaf thickness
and maximum height), on both the nurse and their associated plant communities in
grazed (and therefore indirect facilitation as well) and ungrazed conditions.
We found that the effect of indirect facilitation on the associated plant
communities depended on the functional trait considered. Indirect facilitation
decreased the effects of grazing on species relative abundance, mean LDMC and
the convergence of the maximum height distribution of the associated
communities, but did not affect mean height or cover.
Synthesis Nurse species and grazing jointly affected the structure of
the associated plant community through indirect facilitation. Our results
together with the existing literature suggest that the
grazer-nurse-beneficiary interaction module can be more complex than expected
when evaluated in the field.