Chapter 1 - Núria BONADA

Regional amd local scale: convergences and divergences
Jaccard coefficient was selected as a binary method to calculate distances between variables.
This index is focused in checking similarities between variables, as double 0 are excluded. The
statistical program used was the PCORD (McCune & Mefford, 1999).
Differences and similarities between all reference sampling sites in med-regions
Physical, chemical and structural features
Once reference sites were established, a Principal Component Analysis (PCA) was performed to
check for differences in environmental variables measured. Prior to analysis, these variables
were standardized, tested for autocorrelation with r-Pearson. Skewness values for each
variable were also calculated to avoid variables that could bias the analysis (Legendre &
Legendre, 1998). PCA is a multivariate ordination analysis based on the Euclidean distance
that let to study relationship between objects (sites) and descriptors (variables) in a reduced
space. The CANOCO Program vs.4 (ter Braak, 1998) was used to compute the analysis.
Macroinvertebrate’s abundances
Common taxa in all sampled regions were used to check for differences between the relative
abundance between reference sites in the 4 sampled med-regions. Because in each med-region
a different proportion of temporary sites respect permanents were sampled, only permanent
sites were used to check similarities and differences in abundances. A non-parametric
Kruskal-Wallis test was computed with a relative abundance (in %) data matrix and using
sites in med-regions as replicates because its reference condition (Norris, 1995). Previously,
data was tested to normality with the Shapiro-Wilk’s test (that have a good power properties
compared with a wide range of alternative tests), and to homogeneity of variances with the
Barlett’s test. The STATISTICA Program was used to perform the analysis (Stat Soft, 1999).
Effect of temporality
To test if differences between permanent and temporary sites were higher than between med-
regions, a Bray-Curtis cluster with flexible clustering (β=-0.25) was performed using all
common taxa. For each med-region, macroinvertebrate relative abundance matrix in
permanent and temporary sites was compared using a MRPP analysis (Multi-response
Permutation Procedures). This method is a nonparametric method for testing multivariate
differences among pre-defined groups (permanent or temporary sites), providing the statistic A
and a p-value obtained by permutation (999 runs) as result. Because its non-parametric
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