In the early 1990s
there was relatively little in the literature on habitat management
for marsh fritillary, and, following the 1983 survey, David Wheeler
the site manager was very aware of the potential fragility of the
isolated population at Rhos Llawrcwrt. He had initially been
concerned that increasing stocking rates might have an adverse
effect on the butterfly population, particularly as, after the
first winter of pony grazing, the 1992 adult index fell to 711 from
the 1991 value of 1204. However, the larval index increased from
154 in 1991 to 236 in 1992. Clearly, the index data needed careful
interpretation, but the population did not appear to be suffering
from the change in grazing management. In 1993 and 1994, both
indexes remained high. Superficial observation and some simple
surveillance of vegetation structure showed that that the average
sward eight was decreasing, the litter layer was breaking up and
the larger patches of Molinia were no longer increasing in area.
All this data gave the manager the confidence to continue with the
winter grazing.
By 1994, the
concepts of 'favourable condition', 'attributes' and 'limits' had
been introduced to management planning. The next revision of the
plan needed to incorporate these into the objectives. Favourable
condition for marsh fritillary ¦needed to take account of the
cycle of the population in response to the parasitic wasp and other
natural factors.
Population size was
the obvious attribute to utilise, and David was fortunate in
already having data on this attribute in the form of adult and web
indexes. The reliability of adult index data was more sensitive to
conditions at the time of collection: poor weather for a couple of
weeks during the flight period might prevent adults from flying and
suppress the annual index to a point where it was an unreliable
indicator of population size. The accuracy of any adult count also
relied heavily on the competence and skills of the observer.
Conversely, the data collection window - larval webs was relatively
long and so optimum conditions could be selected. Webs were
relatively easy to locate and they did not fly away before they
could be identified. David Wheeler, therefore, opted to use
the larval web index as an attribute of population size. The
collection of adult index data would continue as a surveillance
project. Based on the precautionary principle, the lower limit for
the web attribute of webs was set at 50. If in any year the index
fell below 50, the managaer would critically examine the management
and quality of habitat to identify potential problems. The
larval-web surveillance project therefore became a monitoring
project. Planning guidance introduced the concept of limits that
were used to express a range for any of the factors that may affect
a feature. For the first time, the plan could include some
quantified requirements for the habitat.
David Wheeler
selected abundance of larval food-plant and average sward height as
factors for which I could set operational limits. There already
surveillance projects for these two factors, and the manager could
use historical data to support the identification of limits. There
was still no reference to the habitat requirements of the butterfly
in the management objectives for vegetation features.
