By 2005, further
research on marsh fritillary had been completed. One of the most
significant outcomes was an understanding of the extent of the
habitat that a population requires to ensure long-term viability.
The management planning guidance was also revised, and the
objective structure was changed to make it more accessible through
the inclusion of a 'vision'.
The 2005 revision of
the management plan in accordance with new guidance notes (Fowles
2004), took account of the entire NNR/SAC. Previously, the
objective had focused on the western block because it contained the
majority of the butterfly population. The revision continued to
build on past objectives and experience.
2005 marsh
fritillary objective:
Vision for the marsh
fritillary population
There will be a very
large butterfly population at Rhos Llawrcwrt which will be viable
in the long term. Because the marsh fritillary is parasitised by a
wasp, the number of butterflies in the population will vary over a
cycle of several years, but, during the peak years, a visitor
taking a walk through the site on a sunny day in June will see
several hundred adult butterflies. In these years, the
caterpillars, feeding communally in silken webs on their
food-plant, devil's bit scabious, will be found in their thousands
throughout large areas of Llawrcwrt and Cors y
Clettwr.
Rosettes of the
food-plant will be both very numerous and widespread throughout the
cattle-grazed rhos pasture, growing amongst a short turf of
grasses, sedges and flowering herbs, with scattered tussocks of
purple moor grass and rushes providing shelter for the caterpillars
in wet weather. This colourful wet grassland mosaic will extend
throughout Llawrcwrt, Cors y Clettwr and the fields which were
drained and reseeded for agriculture in the 1980s but have reverted
back to rhos. Dense mixed hedges of hawthorn, hazel, mountain ash
and other locally native species grow around the boundaries and
between fields and offer vital shelter to the breeding adult
butterflies during poor weather in what is otherwise a very exposed
landscape with little shelter.
There are a number
of smaller breeding populations of marsh fritillary on rhos pasture
sites within 5 km of the National Nature Reserve. Butterflies from
Rhos Llawrcwrt will occasionally visit and breed on these sites,
and butterflies from the smaller populations will visit Rhos
Llawrcwrt. This exchange of butterflies will help to keep all
populations in a healthy condition.
Factor: Extent and
distribution of marsh fritillary habitat
Upper limit: not
required
Lower limit: Within
the SAC boundary:
26 ha of available
habitat (the likely distribution is c 9 ha in the core
compartments, c 12 ha in the low-density compartments and c 5 ha in
the compartments for reversion)', including 10 ha of good condition
habitat.
Factor: Quality of
marsh fritillary habitat Upper limit: not required
Lower limit: 50% of
the marsh fritillary habitat within compartments 5 and 6
is
described as 'good
condition habitat with dense Succisa'
And
50% of the marsh
fritillary habitat within compartment 14 is described as good
condition
And
Scrub covers less
than 10% of compartment 15
Attributes and
limits
Population size:
This is the only attribute that may be used as a performance
indicator for this species. Other attributes of quality, such as
productivity and sex ratio, are difficult and time consuming to
measure. In relation to determination of population size, it is
most appropriate to consider the larval stage. Eggs are difficult
to find and for this reason alone, abundance cannot be
systematically evaluated. Data collected on adult butterflies
cannot always be relied on to indicate population size because of
observer difficulties related to the mobility of the butterflies.
The abundance of Iarval webs will therefore be considered as the
sole attribute for this species.
Abundance of larval
webs: Larvae feed communally in webs, which are easily observed and
reasonably static. Historically, a large amount of sample data on
webs had been collected from a series of 53 transects in the
current 'core' area for the butterfly on site - compartments 3, 5,
6, 7 and 10. Data has been analysed annually and an 'index'
calculated which represents the total number of webs on all
transects observed in any one year. In order to allow comparison
with historical data, the performance indicators will include
reference to the annual index collected on the 53
transects.
In addition, the
performance indicators must take account of web abundance elsewhere
on site, namely compartments 2, 3,4, 12 and 14, which have
historically been occupied at low density, and compartments 16 and
18, which are currently semi-improved grassland but have potential
for reversion.
Because of the
host-parasite relationship with the wasp, the marsh fritillary
population fluctuates significantly over time. Historical data from
the site indicate that there is approximately a 10-year cycle
between population peaks. If the butterfly population were not
being affected adversely by other factors, it should be relatively
high for half of that period.
The following
site-specific performance indicator has been devised:
Attribute:
Larval webs
Upper
limit: not required
Lower
limit: over any 10-year period:
The web index count
will be greater than 50 for at least 5 of the years.
And
Annually:
• Larval webs
continue to be present in all management compartments
• A minimum
total of 50 webs are present in compartments 12 and 14
• A minimum
total of 50 webs are present in compartments 16, 18, 22 and
24
Annual data on adult
abundance will be collected as a surveillance project. Although
adult data is not as reliable as larval data, it may still be used
to confirm that any annual cohort of larvae are successfully
producing adult butterflies and that there is a direct relationship
between trends in the abundance of larvae and adults.
Status of marsh
fritillary: unfavourable recovering (Wilkinson 2005)
The 2005 revision of
Favourable Conservation Status meant that, for the first time since
1992, the status of the feature was deemed to be unfavourable. This
is because there is insufficient 'good' and 'suitable' habitat on
the NNR at present. However, there is potential to increase the
area of both categories of habitat through reversion of the
improved pastures on the NNR and achieve Favourable Conservation
Status in the future.
