Statistical analysis
We compared the NCE of fish on zooplankton densities across the four
experiments for the subset of zooplankton taxa that were represented in
all experiments (see Results, Table 1). We used a multivariate approach
to evaluate the overall response of the zooplankton community to the
presence of fish kairomones, and our initial analysis simultaneously
tested the effect of experiment, treatment and the interaction between
experiment and treatment on the subset of taxa using a permutational
multivariate analysis of variance (PERMANOVA, McArdle and Anderson
2001). This analysis uses dissimilarity matrices, and we chose to use a
dissimilarity matrix based on Kulczynski distances, which are
particularly robust in identifying patterns in community data (Faith et
al. 1987). Probabilities from the analysis were calculated based on
9,999 permutations. These effects were also investigated graphically by
comparing experiments and treatments across different axes generated
through non-metric multidimensional scaling (NMDS). The NMDS was based
on Kulczynski distances and was constrained to three dimensions, which
was the minimum number of dimensions that resulted in a stress measure
less than 0.20 (Clarke 1993).
The effect of treatment on the density of individual taxa within an
experiment was analyzed using quasi-Poisson regressions.