Appendix S2: Disturbance frequency in the system: Variation and relative impacts on
species richness, volume of occupied functional space and species packing.
There is a large body of literature addressing the effect of disturbances on species
diversity and community assembly. Highly influential early studies on the subject
suggested that species diversity should be highest under intermediate levels of
disturbances (Intermediate Disturbance Hypothesis (IDH); Connell 1978) and this
assumption is also central to the dynamic equilibrium hypothesis (DEH) tested in our
study (Huston 1979). However this hypothesis has been challenged theoretically (Fox
2013), experimentally (Violle, Pu & Jiang 2010), and a review of literature showed that
the disturbance-diversity relationship was not more likely to be U-shaped than linearly
positive, negative or even non-significant (Mackey & Currie 2001). These results do not
necessarily contradicts the DEH (Huston 1979, 1994, 2014) where the effect of
disturbances on diversity is explicitly modulated by the growth rate of the vegetation.
However, an apparent issue that could be problematic is that the IDH and DEH explicitly
described different aspects of disturbances (i.e. frequency and intensity) as having the
same relationship with species diversity. Indeed, Miller, Roxburgh & Shea (2011)
showed that these different aspects of disturbances can not only produce different shapes
of the disturbance-diversity relationship – which would be in itself a potential explanation
for the results of Mackey and Currie (2001) – but that frequency and intensity interact
together to drive diversity. This has important implications for the study of community
assembly along disturbance gradients, highlighting the importance of explicitly
differentiating between different disturbance facets.
1
The time interval since the last disturbance, as a measure of disturbance
frequency, was recorded for each plot (see Fig. S2.1 for an illustration of the variation in
disturbance frequency within sites). This was expressed in years, so that a plot disturbed
(by mowing or grazing) twice a year has a value of 0.5 and a plot abandoned for 5 years
has a value of 5. Unfortunately, and unlike disturbance intensity, not all sites did
manipulate this variable, so that it varies mainly in 4 out of 8 sites (Fig. S2.1).
We tested the ideas expressed in Mackey and Currie (2001) with mixed effect
models in the same way as in the main manuscript but replacing disturbance intensity by
disturbance frequency to see if different patterns in species richness, occupied volume of
functional space and species packing variation would emerge. We also tested if any
interaction between disturbance intensity and frequency in driving the taxonomic or
functional metrics does exist, but none were detected (results not shown).
The models for the variation in species richness showed that the patterns observed
when using disturbance frequency (Fig. S2.2) were very similar to the ones found with
disturbance intensity (Fig. 2 of the main manuscript). Only a few trends were not similar
(e.g. Fig. S2.2, panel a) and these were related to extrapolation of the models, i.e. parts of
the gradient for which no real data were available. On the contrary, the results of the
models for the variation in occupied functional space (corrected for species richness)
showed different results between models including disturbance intensity (Fig. 3) and
frequency (Fig. S2.3). In the case of the models with disturbance frequency, the expected
unimodal relationship between disturbance and functional diversity (Navas & Violle
2009) was clearly detected except under the most favorable climatic conditions (Fig.
S2.3, panels d-f). Note that these unimodal trends depend again on extrapolations from
2
the model where no real data are available. For the species packing models, there was no
effect of the nutrient index, regardless of the disturbance aspect used. Furthermore, using
disturbance frequency instead of disturbance intensity led to a similarly general decrease
in species packing with increasing disturbances (Fig. S2.4), except under unfavorable
climatic conditions where there was a slight increase in species packing along the
gradient of disturbance frequency.
These results support (at least partially) the conclusions of Mackey & Currie
(2001) by showing that functional diversity – and to a lesser extent taxonomic diversity –
responds differently to different aspects of disturbance regimes. A thorough interpretation
of these differences is difficult in our context (and out of the scope of this study) because
disturbance frequency varied in 4 sites only. However, these results highlight the
importance of specifically identifying which aspect of the disturbance regime is being
considered and to take it into account for the interpretation.
3
Fig. S2.1. Boxplot of the frequency of disturbances measured at the plot level across the
eight sites. The y-axis is log-scaled.
4
Fig. S2.2. Predicted species richness (SR) from a significant linear mixed model (see
main manuscript for the construction of the models) along a nutrient index (NI; a, b, c)
and disturbance frequency (DF; d, e, f) gradients. Grey sections of the curves indicate the
range of values on the x-axis for which actual observed species richness values were not
available, for each combination of the other variables. These sections are extrapolations
from the models. Formula: SR ~ NI*DF*GSL + NI2*DF2 + NI2*GSL + DF2*GSL
5
Fig. S2.3. Predicted standardized values of occupied volume of functional space
(occFSstand) from a significant linear mixed model (see main manuscript for the
construction of the models) along nutrient index (NI; a, b, c) and disturbance frequency
(DF; d, e, f) gradients. Grey sections of the curves indicate the range of values on the xaxis for which actual observed species richness values were not available, for each
combination of the other variables. These sections are extrapolations from the models.
Formula: occFSstand ~ NI*DF + NI*GSL + DF*GSL + DF2*GSL
6
Fig. S2.4. Predicted SES of species packing (SESPack) from a significant linear mixed
model (see main manuscript for the construction of the models) along a disturbance
frequency gradient. Grey sections of the curves indicate the range of values on the x-axis
for which actual observed species richness values were not available, for each
combination of the other variables. These sections are extrapolations from the models.
Formula: SESPack ~ DF*GSL
7
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