Dragonfly and Damselfly monitoring on the Writtle College estate - the first five years
Weather Data
Total monthly rainfall collected for April to September 1997 to 2001 is displayed in Table 7. The
rainfall data was collected from the Writtle College Weather Station. Over the five years, the total
rainfall for the six month periods when Odonata arc in their nymph or adult stages has tended to
increase. In particular, 2001 was the wettest year and Odonata were also in their lowest abundance
on the estate (Table 4).
Table 7. Total monthly rainfall for the five years
Rainfall (mm)
Discussion
Fourteen species of Odonata were recorded on the Writtle College estate during the monitoring
period, comprising six Damselfly (Table 2) and eight Dragonfly species (Table 3). Compared with
the total of 15 species recorded at nearby Hylands Park in the 1980s (Benton 1988), the College
estate, which is predominantly agricultural land, possesses a relatively high number of species. L.
depressa, P. nymphula and P pennipes were all observed during the monitoring period and are
species which have a localised distribution in Essex. Benton (1988) states that the presence of P.
pennipes at a site is of particular importance in a county context, as this species occurs only within
Essex river systems.
P. pennipes is particularly sensitive to pollution, especially run-off of chemicals from agricultural
land (Benton 1988), so its presence on the College estate, if only in small numbers, may indicate
that water quality is good. This species also prefers rivers and streams with an abundance of bank-
side, floating and emergent vegetation (Brooks 1997). On the College estate, the slow-flowing
Rivers Can and Wid provide the 'optimum' well vegetated habitat. Six-metre grass field margins,
which are part of the College's Countryside Stewardship Scheme Agreement, abut the aforementioned
rivers and may provide areas where immature adults can reside in tall grass.
The general trend of increasing Odonata abundance during the first four years of monitoring was
followed by very low numbers in 2001 (Table 4). This trend was particularly evident with C.
splendens which became much more common and widely distributed on the estate, but experienced
a substantial reduction in numbers in 2001 (Fig. 2; Table 6). The cause of this reduction in numbers
may be the extremely wet 2001 season (Table 7), with adults experiencing increased mortality due
to the inclement weather.
The strongholds of C. splendens on the estate were the Rivers Can and Wid, with individuals being
particularly numerous on the latter river. In Essex, C. splendens prefers slow-flowing streams and
rivers usually with an abundance of floating and emergent vegetation (Benton 1988). This habitat
Essex Naturalist (New Series) 19 (2002)
105