The effect of the mixture is globally synergistic across
exposure scenarios
Synergistic effects dominated the process of decomposition across
exposure scenarios, with the number of nonadditive effects largely
surpassing the probability of retrieving significant differences by
chance. Of the 206 differences between data pairs of expected and
observed values (seven litter processing variables across the three
exposure scenarios), 92% differed from zero, of which 55% were
positive and 37% were negative. The mixture promoted
higher-than-expected colonisation by decomposers (60% of data pairs)
and detritivores (57% of data pairs), while mass loss was globally
additive (41% of data pairs positive and 59% negative). The effect of
the mixture on decomposers resulted mostly from the increased
sporulation rates, but, to a lower extent, also from the increased
number of species. Concerns on the consequences of biodiversity loss for
ecosystem structure and function have driven much of the research on the
effect of mixing litter, but interestingly only a few studies include
measures of how riparian biodiversity loss may affect diversity of the
detritus-based food webs (but see Abelho, 2009; two studies reviewed by
Gartner & Cardon, 2004; Chapman et al., 2013). The synergistic effect
of the mixture on fungal richness may indicate that the simplification
of riparian diversity will be followed by microbial diversity loss, as
suggested by Chapman et al. (2013).
The global synergistic effect across exposure scenarios clearly shows
that mixing litter stimulated colonisation by decomposers and
detritivores, but not mass loss, once again highlighting that mass loss
or breakdown rates do not fully capture the effect of mixing litter on
the process of decomposition.