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.