Baines CB, McCauley SJ, Rowe L. Electronic Supplement.
The interactive effects of competition and predation risk on dispersal in an insect.
Appendix

Supplementary methods and results
Notonectid collection and holding tanks
In July 2012, ten holding tanks (378 L) were prepared in the same manner as the
experimental tanks (filled with water, and stocked with leaves, artificial vegetation, and
an inoculation of zooplankton). We also provided the holding tanks with dead crickets to
supplement zooplankton as food for the notonectids.
Notonectids were collected from a pond at KSR over a nine-day period (July 24 –
August 1). The pond is small (estimated to be approximately 13,000 m2 using Google
Earth) and fishless. On each collection day, notonectids were divided roughly equally
among the ten holding tanks. By the last day of collection, each tank held ~100
notonectids.
Experimental notonectid densities
The densities used in this experiment were 19, 38, and 60 notonectids per tank.
These densities correspond to approximately 50, 101, and 159 notonectids per cubic
metre of tank volume, and 18, 36, and 56 notonectids per square metre of tank surface
area (tank surface area = 1.35m × 0.79m = 1.07m2). We did not estimate the density of
notonectids in the source pond from which the experimental animals were collected.
However, previous studies have yielded density estimates for this species (Notonecta
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undulata). Density per unit volume estimates from ponds in Connecticut ranged from 5214 individuals per cubic metre [1], and estimates of density per unit surface area from
pipe sampling in ponds in Michigan ranged from 0.25-63 individuals per square metre
with a single observation of 345 per square metre (E. E. Werner et al. unpublished data).
These estimates support our own observations that the densities in our experiment fall
within a natural range, and that under some conditions, these animals experience densities
higher than the maximum used in our experiment. In order to explore the role density
may play in shaping dispersal dynamics, our densities were chosen to span a wide, but
biologically realistic, range of N. undulata densities.
Emigration rates in first round
We counted the notonectids remaining in each tank on two separate occasions. In
the first round, the notonectids from the tanks in blocks 1, 2, and 3 were counted on three
consecutive days, starting one day after the notonectids had been placed in the
experimental tanks. In the last round, the notonectids from the tanks in blocks 1, 2, and 3
were counted on the three last days of the experiment. The experiment lasted 12 days in
total. Here, we present the results from the first round.
Statistical analysis
We used a GLM with a quasibinomial error distribution and a logit link (i.e.
logistic regression with overdispersion) to model the effects of fish presence, notonectid
density, and their interaction on dispersal status (emigrated or philopatric) as of the first
round.
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Results
578 notonectids had emigrated by the end of the experiment. A majority (330 or
57%) of these dispersers emigrated within the first three days. This rapid dispersal has
been observed in N. undulata in similar experiments (C. Baines et al., unpublished data).
The effects of density and predation risk were similar in the first and last rounds
(Figs. 1 and S1). The proportion of individuals that emigrated by the first round depended
on density (22=6.197, p=0.0451). There was no main effect of predation risk (21=0.397,
p=0.529), but there was a trend toward an interaction between density and predation risk
(22=5.288, p=0.071). Since much of the emigration had occurred by the end of the first
round, these results suggest that the densities experienced at the beginning of the
experiment were important in shaping the observed patterns of emigration.
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Baines CB, McCauley SJ, Rowe L. Electronic Supplement.
Figure S1. Mean proportion of individuals that emigrated from the tanks within the first
three days of the experiment +/- s.e. Low, medium, and high density tanks contained 19,
38, and 60 notonectids, respectively.
References
[1] Bennett DV, Streams FA. 1986 Effects of vegetation on Notonecta (Hemiptera)
distribution in ponds with and without fish. OIKOS 46, 62-69.
(doi:10.2307/3565381).
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