Ecological Archives E000-000-S5
Henriksson, J. Yu, D. A. Wardle, J. Trygg, G. Englund. Year. Weighted species richness
outperforms species richness as predictor of biotic resistance. Ecology VOL:pp-pp.
Appendix S5. Relationships between species richness and weighted species richness
Fig. E1. Relationships between species richness and weighted species richness for
introductions of Arctic char, tench, zander and whitefish. The weighted species richness is the
sum of component scores from the PLS-GLR analyses. The weights describe the contribution
of resident species to invader success. Thus the weighted species richness depends on which
species is introduced. To make the two measures comparable, we lumped rare species as
described in “Methods; Model 4 – species identity hypothesis” and we multiplied weighted
species richness with -1. For three of the introduced species, Arctic char, tench and whitefish,
the PLS-GLR analyses produced two orthogonal components. However since the second
components explained a minor fraction of the variance in the matrix of presences/absences of
residents, we only show the first component (explained variance in first and second
component were, Arctic char: 36.6% and 10.2 %; tench 6.7% and 3.3%; zander: 27.3% and
0%; whitefish: 16.0% and 4.3%). We also indicate the presence of the species that we found
to best predict invader success in the identity models. A) The relationship between weighted
and unweighted species richness is positive for Arctic char introductions, because most
resident species had a negative influence of the success of this species. This pattern is
reinforced by the fact that pike, which made the strongest contribution to resistance, mostly
present in species rich lakes. B) The negative relationship between weighted and unweighted
species richness occurs because positive interactions had a strong influence on invasion
success of tench. Rudd has a negative effect on resistance and is present most often in speciesrich lakes, meaning that the highest resistance is found in species poor lakes that do not have
rudd. C) The weighted species richness is negatively correlated to species richness for zander
introductions. The relationship is negative because all resident species had a positive effect on
invasion success of this species. Bleak has a strong negative effect on resistance, which is
shown in the figure; where bleak is present the weighted species richness is low. D) The
negative and positive contributions to resistance for whitefish are of approximately equal
importance. Here there is a positive relationship between weighted and unweighted species
richness because the species that make strong negative contributions to resistance tend to
occur in species-rich lakes. Ruffe, which is the species that had the strongest individual
contribution to the resistance, is one of these species.