The ecology and conservation of threatened damselflies

to this watercourse and the variety of movement directions may reflect this. The circular
variance of the directional mean was 0.92. This suggests the mean was not a good
representation of the set of input vectors (as the variance is close to one, its maximum)
and that there was little directional bias to these movements (Gaile and Burt 1980).
However, if orientation is considered (the angle of movement without direction), the
mean axis was 87° - 267° and the circular variance 0.32 (maximum 0.5 for a 180º
distribution). This indicates that the orientation vectors were somewhat better
represented by their mean and that movements along the angle of the main water body
were more common.
To examine the effect of wind direction on direction of movement, the difference in
angle between each first mature movement and the mean wind direction during that
movement was calculated. As wind direction can vary considerably (Fig. 2.12; circular
variance of wind directions between 10am and 7 pm during study = 0.72), only those
movements with an inter-catch interval of 1 or 2 days were used (n = 580). There was
a small tendency towards movements approximately 50° different from the mean wind
direction during that movement (Fig. 2.13). However, the circular mean of the
difference between wind and movement direction was 85.7°, which is close to the
median of possible differences (90º) and there was no strong trend in the distribution of
differences (circular variance = 3.5, maximum 0.5 for 180° distribution). This suggests
that adult I. pumilio are able to compensate for wind. This was true even at higher wind
speeds as there was no correlation between wind speed and the difference between
movement and wind direction (Pearson correlation coefficient = 0.003, P = 0.94).
32 Science Report – The ecology and conservation of threatened damselflies