Diversity is affected by an interaction between disturbance and invader addition
By factorially invading a community of five bacterial species disturbed at four different frequencies, we tested for interactive effects of pulse frequency and invasion on the diversity of a stably coexisting community. We found that the best fit model is the full model (Eq. 1) (see Supplemental Information).
Resident diversity (effective number of species) of the uninvaded community increased with pulse frequency (effect size: 14.86, p<10-4) but decreased with pulse2 (effect size: -23.97, p<10-3), resulting in a hump-shaped relationship with frequency, as found in similar systems (Bucklinget al. 2000). There was a small positive main effect of invader addition on resident diversity, as seen in 16-day samples (Fig. 2; effect size: 1.54, p<0.002). However, we found the interaction between pulse frequency and invasion to have a very large negative effect on resident diversity (effect size for pulse*invasion: −20.28, p<10-4; effect size for pulse2*invasion: 22.62, p<0.006; Fig. 2). This caused some resident species to become non-detectable (<1 CFU on the agar plate) and presumed extinct at high pulse frequencies in the invaded treatments (Fig. 2). We therefore found that the invasion byP. aeruginosa reduced diversity at high pulse frequencies but not at low.
Importantly, we note that the invasion qualitatively changes the pulse disturbance-diversity relationship of the resident community in our model (Fig. 2). Without invasion, the resident diversity shows the expected unimodal pattern (concave down) with increasing pulse frequency. With invasion, however, the resident diversity shows a slight but statistically significant U-shaped pattern (concave up) with increasing frequency.