Environment & Heritage Series Monitoring of Devil’s-bit...

Environment & Heritage Series

Monitoring of Devil’s-bit scabious as a likely means of assessing the conservation status of the Marsh Fritillary butterfly Quercus Project QU03-17

© P

eter

McE

voy

ii

Prepared for

Environment & Heritage Service

by

Jane Preston, Peter McEvoy & Neil Reid

This report should be cited as:

Preston, J.S., McEvoy, P. & Reid, N. (2008) Monitoring of Devil’s-bit scabious as a likely means of assessing the conservation status of the Marsh Fritillary butterfly. Report prepared by Quercus for Environment & Heritage Service, Northern Ireland, UK.

Quercus project QU03-17

Quercus is a partnership between

Environment & Heritage Service (NI),

and Queen’s University Belfast.

www.quercus.ac.uk

Monitoring of devil’s-bit scabious Quercus

iii

Executive Summary

1. The density and size of devil’s-bit scabious, the main larval food plant of the

marsh fritillary butterfly, was assessed at 6 sites of conservation importance in

Northern Ireland including: Aughnadarragh Lough SAC, Ballykilbeg SAC (split

into two sites for monitoring), Inishargy Nature Reserve, Montaighs Moss

SAC and Murlough National Nature Reserve and SAC.

2. There was considerable within and between site variation in the distribution

and density of devil’s-bit scabious. The number and size of each patch of

plants within each site may have important implications for the persistence of

the species at each site.

3. Devil’s-bit scabious was significantly associated with short wet swards

dominated by mosses and sedges and, to a lesser extent, tall wet swards

dominated by rushes.

4. There was no consistent decline in devil’s-bit scabious density across the

three study years; however, a dramatic decline was evident between 2005

and 2006. Patches of devil’s-bit scabious shrank in size and became less

dense. It seems likely that the cause of the observed decline was drought.

Stochastic environmental events may significantly effect species abundance

where patches are fragmented and isolated.

5. Variability in plant size, and therefore growth, within and between sites across

seasons and between years, supports the hypothesis that devil’s-bit scabious

are responsive to spatial and temporal variation in environmental conditions,

most likely soil drainage and rainfall.


iv

6. We make 5 recommendations for site management and future research:

a. Future monitoring may be more cost effective if conducted synchronously

with marsh fritillary caterpillar web surveys during autumn when devil’s-bit

scabious plants are most prevalent and at their largest. Pairing food plant

and butterfly/caterpillar abundance surveys is necessary to make

inferences about their ecological relationship.

b. If inter-annual and multi-annual fluctuations in devil’s-bit scabious

abundance are to be understood annual monitoring at specific sites of

conservation importance is required.

c. Increasing the cover and area of short wet swards on each site may

benefit devil’s-bit scabious density and distribution. Assessment of

anthropogenic manipulation of the water table, lowering of surface peat

and soil, clearing rank vegetation by burning and management of scrub

encroachment by prescriptive grazing should all be considered and

assessed as potential site management practises.

d. Increasing the number of patches of devil’s-bit scabious on each site may

help safeguard the future persistence of the plant against stochastic

environmental events.

e. Establishment of an effective ecological network throughout Northern

Ireland should increase and maintain habitat connectivity between

fragmented habitats of conservation importance increasing the likely

persistence of species such as the marsh fritillary butterfly.


v

Contents Introduction ....................................................................................................1

Methods ..........................................................................................................3

Study Sites ...................................................................................................3

Aughnadarragh Lough SAC .....................................................................3

Ballykilbeg SAC .......................................................................................3

Inishargy Bog ...........................................................................................4

Murlough SAC .........................................................................................6

Vegetation Surveys ......................................................................................6

Statistical Analysis........................................................................................8

Results ..........................................................................................................10

Within-site plant distribution........................................................................10

Spatio-temporal trends in plant density ......................................................14

Plant habitat associations...........................................................................15

Plant size....................................................................................................19

Discussion....................................................................................................21

Recommendations .......................................................................................25

Acknowledgements .....................................................................................26

References....................................................................................................27

Appendix 1....................................................................................................28


1

Introduction

Marsh fritillary butterfly (Euphydryas aurinia Rottemburg, 1775) populations have

undergone dramatic declines across Europe and the species is now extinct in

some regions e.g. northern Belgium (Schtickzelle et al., 2005). The United

Kingdom remains one of the strongholds for the species in Europe but, even here

its populations have declined substantially over the last 150 years (Schtickzelle et

al., 2005). In Great Britain, the range of the marsh fritillary has contracted by over

62%, and it has disappeared from most eastern regions (Warren, 1994). The

species remains widespread in south-western and western regions but it has

been estimated that colonies are disappearing at a rate of more than 10% per

decade (Warren, 1994). Relatively little information is available on the species in

Ireland, but its range is thought to have contracted by up to 50% (Warren, 1994).

During 1990, 58 colonies of marsh fritillary were recorded in 20 key areas of

Northern Ireland (Warren, 1994).

The marsh fritillary butterfly is listed on Annex II of the EC Habitats Directive and

Appendix II of the Bern Convention. It is also protected under Schedule 5 of the

Wildlife and Countryside Act 1981 and is fully protected under Schedule 5 and 7

of the Wildlife Order (Northern Ireland) 1985. Like other rare butterfly species the

persistence of the marsh fritillary now depends largely on successful habitat

management (Schtickzelle et al., 2005).

Like most butterfly species, the marsh fritillary depends solely on one larval food

plant, in this case devil’s-bit scabious (Succisa pratensis). Devil’s-bit scabious is

a perennial that flowers from July to October and is characteristic of damp

grassland and marsh. Typically, Devil’s-bit scabious forms large dense patches in

suitable habitat.

Joyce & Pullin (2003) suggested that two principal habitats are important for

marsh fritillary; damp neutral or acid grasslands and dry chalk and limestone


2

grasslands. Connectivity of suitable breeding habitat is essential for long-term

survival (Joyce & Pullin, 2003). Small isolated populations are at risk of

stochastic extinction events and protected habitat networks facilitate re-

colonisation.

The MOSS initiative (management of sensitive sites), implemented by the

Environment and Heritage Service, is currently under review and it is expected

that a number of existing SACs in Northern Ireland will be identified as important

areas for the marsh fritillary butterfly. Short-term fluctuations in butterfly

populations and stochastic extinction events make it unlikely that the current 6

year monitoring cycle for designated SAC features will be sufficient to detect

population change on a relevant temporal scale. Consequently, the main aim of

the current project was to assess the favourability of 6 sites for marsh fritillary by

monitoring changes in devil’s-bit scabious distribution and density annually for 3

years.

The specific aims of the project were:

1. To determine the availability of suitable habitat for the marsh fritillary

butterfly on a number of sites managed for the species

2. To map the location of suitable habitat using GPS in order to monitor

habitat over time

3. To assess the condition of the habitat for marsh fritillary butterfly by

measurement and monitoring of a selection of environmental variables

4. To determine whether current management prescriptions are effective in

maintaining and increasing the availability of suitable habitat and assess

the impact this is having on the marsh fritillary population.


3

Methods

Study Sites

Six sites in Northern Ireland where devil’s-bit scabious and marsh fritillary

butterfly are known to occur were selected for assessment and monitoring (Fig. ):

1. Aughnadarragh Lough SAC (subject to signing of MOSS agreements)

2. Ballykilbeg (a)

3. Ballykilbeg (b)

4. Inishargy Bog (Ulster Wildlife Trust Nature Reserve)

5. Montiaghs Moss SAC (EHS owned land)

6. Murlough SAC (MOSS agreement already in place)

Aughnadarragh Lough SAC

Aughnadaragh Lough is fringed by swamp and fen giving way to wet grassland,

cut-over bog and pockets of scrub and wood further from the lake shore. Drier

areas are characterised by acid grassland dominated by purple moor-grass

Molinia caerulea. The site has no current management prescriptions and remains

ungrazed.

Ballykilbeg SAC

Ballykilbeg is a relatively large site composed primarily of fen and wet grassland.

Historically, the site was extensively grazed by cattle; however, more recently,

willow and gorse encroachment has been managed by prescriptive grazing at

regulated stocking densities within fenced areas. For this study the site has been

divided into two sub-units: Ballykilbeg (a) had large, well-defined patches of

dense devil’s bit scabious growing on cut-over bog while Ballykilbeg (b) had

scattered plants at low density growing on water-logged grassland (Fig. 2).

SAC (subject to signing of MOSS agreements)


4

Fig. 1 Location of the five study sites important for devil’s-bit scabious and the marsh fritillary butterfly; 4 designated as SAC and one Nature Reserve.

Inishargy Bog

Inishargy consists primarily of cut-over bog with willow and gorse scrub

interspersed amongst wet grassland. Scrub and bracken management has been

carried out irregularly over the last 20 years by the Ulster Wildlife Trust. Some

parts of the site may have been grazed by cattle.


5

‘A’

‘B’

Fig. 2 Subdivision of Ballykilbeg SAC into ‘Ballykilbeg (a)’ and ‘Ballykilbeg (b)’ based on the distribution and density of devil’s-bit scabious.

Montiaghs Moss SAC

This Area of Special Scientific Interest (ASSI) is largely cut-over bog and is one

of the last remnants of the extensive bog system that would have once covered

most of the land south and south-east of Lough Neagh. The site is dominated by

purple moor-grass Molinia caerulea and small patches of grassland used for hay.


6

The site is interspersed with alder-willow Alnus glutinosa-Salix spp. woodland

and hedgerows and is dotted with deep pools and sectioned by a lattice of peat

trenches. Historically, the prevalence of Molinia mono-swards and a lack of

grazing to manage scrub encroachment had limited the spread of devil’s-bit

scabious. More recently, a management programme of rotational grazing and

burning to control scrub and reduce Molinia density has resulted in the species

regeneration on the site.

Murlough SAC

Unlike the other 4 wetland sites, Murlough is an acidic sand dune system. The

site is dominated by Marram Ammophila arenaria and red fescue Festuca rubra

but some areas of shorter vegetation host diverse swards rich in herbs including

common restharrow Ononis repens and wild thyme Thymus polytrichus.

Historically, the site has subject to intensive grazing. Currently, the National Trust

graze cattle, ponies and goats on the site, however, measures have been taken

to prevent access to areas that have been identified as important breeding

grounds for the marsh fritillary butterfly.

Vegetation Surveys

At each site, areas of devil’s-sit scabious were identified and their condition in

terms of density and plant size was assessed. The methodology was based on

that advocated by Butterfly Conservation NI. Sites were surveyed during autumn

2005 and spring and autumn during 2006 and 2007. Data for spring 2005 was

interpolated during analysis.

Devil’s-bit scabious occurred in well-defined, discrete, high density patches at

four of the study sites including Aughnadarragh, Ballykilbeg (a), Inishargy, and

Murlough. Each site was surveyed and the edge of each devil’s-bit scabious

patch was defined using a handheld GPS. The fate of each patch of devil’s-bit

scabious was monitored overtime using a triplet of 2m2 quadrats. One quadrat


7

was placed within each patch near its centre, a second was placed on the edge

of each patch and a third was placed beyond each patch at a similar distance

from the edge as the centre quadrat (Fig. 3).

At two of the study sites devil’s-bit scabious plants occurred at low density and

were scattered throughout each site, including Ballykilbeg (b) and Montiaghs

Moss. The extent and persistence of devil’s-bit scabious at both sites was

monitored overtime using randomly placed 2m2 quadrats (Fig. 3).

Fig. 3 Sampling protocols at sites where patches of devil’s-bit scabious were (a) large, discrete and high-density and (b) small, randomly distributed and low-density.

For each quadrat the density of devil’s-bit scabious plants was recorded as

percentage cover (Table 1). Mean plant size was taken as maximum plant

volume taken as the product of maximum plant height and width from 5 randomly

selected plants within each quadrat. The general habitat at each quadrat was

described by the percentage cover of bracken, grasses (including Molinia),

heather, mosses, rushes sedges and shrubs (Table 1).


8

Table 1 Variables collected for each sample quadrat at all sites from spring 2005 to autumn 2007.

Variable name Units Description

Devil’s-bit scabious density % Estimated percentage cover of Succisa pratensisper 2m2

Devil’s-bit scabious plant size cm3 Mean plant volume taken as the product of maximum plant height and width (cm3) from 5 randomly selected plants

Bracken % Estimated percentage cover of bracken per 2m2

Grasses (including Molinia

spp.)

% Estimated percentage cover of grasses per 2m2

Heather % Estimated percentage cover of heather per 2m2

Mosses % Estimated percentage cover of mosses per 2m2

Rushes % Estimated percentage cover of rushes per 2m2

Sedges % Estimated percentage cover of sedges per 2m2

Shrubs % Estimated percentage cover of shrubs per 2m2

Statistical Analysis

Factors affecting devil’s-bit scabious density were analysed by fitting a linear

mixed model using a restricted maximum likelihood (REML) procedure (Patterson

& Thompson, 1971) using an autoregressive error structure. Yearly data were

treated as repeated measures while site and season were treated as fixed

factors. Due to site differences in sampling protocol and habitat characteristics

four separate models were created:

Model 1 - Includes Aughnadarragh, Ballykilbeg (a) and Inishargy. All

these sites are wetland habitats and devil’s-bit scabious

patches were sampled using the triplet quadrat regime.

Model 2 - Murlough only. Whilst this site is was also sampled using

the triplet quadrat regime it was analysed separately as

sand dune floral communities are substantially different

from wetland floral communities.


9

Model 3 - Ballykilbeg (b) only. A regenerated cut-over bog habitat

sampled using the random quadrat regime.

Model 4 - Montiaghs only. A wetland bog habitat sampled using the

random quadrat regime. This site was not combined with

Ballykilbeg (b) as the character of each site was noticeably

different.

For models using data from a triplet quadrat sampling regime, each patch of

devil’s-bit scabious was given an arbitrary ID number and this was included as a

random factor while patch position (either inside, at the edge or outside the

patch) was treated as a fixed factor. For models dealing with random quadrat

sampling, each quadrat was given an arbitrary ID number and this was included

as a random factor. All two-way factorial interactions were included.

The floral character of each quadrat, taken as the percentage cover of bracken,

grasses (including Molinia), heather, mosses, rushes sedges and shrubs, was

assigned to a broad habitat category using Principal Components Analysis

(PCA). All percentage data were arcsine square-root transformed prior to

analysis (Hosmer & Lemeshow 2000). Principal components were included in

each model as covariates.

Missing values were interpolated during analysis to plug gaps in the data e.g.

spring 2005 or incidences when quadrats failed to be surveyed each season due

to GPS inaccuracy. Estimated marginal means were used plot overall trends in

devil’s-bit scabious density. Plant size was also examined using a linear mixed model adopting a REML

procedure and an autoregressive error structure. Yearly data were treated as

repeated measures while site and season were treated as fixed factors. All two-

way factorial interaction were included.

All statistics were conduced using GenStat v.6.


10

Results

Within-site plant distribution

The distribution of devil’s-bit scabious was geographically restricted within each

site:

Aughnadarragh Lough - Only two patches of devil’s-bit scabious are currently

extant; one large patch and another smaller patch in

the centre of the site (Fig. 4).

Ballykilbeg - Two large patches of devil’s-bit scabious occur in

Ballykillbeg (a) at the north of the site while plants

occur at low density and are thinly scattered across

Ballykilbeg (b) at the south of the site (Fig.5).

Inishargy Bog - A large number of small patches of devil’s-bit

scabious occur throughout the site (Fig. 6). Montiaghs Moss - Devil’s-bit scabious is thinly distributed at low density

within field parcels at the north and north-west of the

site (Fig. 7). In the north, plants occur on raised banks

along drainage ditches while in the north-west they

occur throughout Molinia dominated grassland. Murlough - A large number of well defined patches of devil’s-bit

scabious occur throughout the south and west of the

site currently grazed by ponies. Patch size varied

considerably with the largest large patches occurring

in dune slacks in the centre of the site (Fig. 8).


11

Fig. 4 Location of devil’s-bit scabious (red patches) at Aughnadarragh SAC.

Fig. 5 Location of devil’s-bit scabious in dense patches (hatched areas) and spread thinly at low density (dotted area) in Ballykilbeg SAC.


12

Fig. 6 Location of devil’s-bit scabious (red patches) at Inishargy Bog Nature Reserve.

Fig. 7 Location of devil’s-bit scabious (blue dots) at Montaighs Moss SAC. The size of the dot corresponds to the density of plants.

13

Fig. 8 Location of devil’s-bit scabious (blue patches) at Murlough National Nature Reserve SAC.


14

Spatio-temporal trends in plant density

Devil’s-bit scabious plant density varied across sites and between seasons

(Table 2). Greater percentage cover was recorded during autumn than in spring.

Highest marginal mean density was recorded at Ballykilbeg (a) during autumn

with approximately 25% cover and lowest marginal mean density at Montaighs

Moss during spring with approximately 10% cover (Fig. 9).

For those sites where discrete patches of plants were sampled using a triplet of

quadrats positioned within, on the edge and outside the patch, plant density

varied significantly with quadrat position (Table 2). This effect is largely trivial as

percentage cover will necessarily be greatest within the patch than outside the

patch. However, to assess the contribution of other factors to plant density it was

important to account for variation attributed by Quadrat_position.

There was a significant negative trend in plant density between 2005 and 2006 at

all sites expect Montiaghs Moss SAC (Fig. 10 & 11). Whilst some sites showed

further declines between 2006 and 2007 these changes were not significant.

For those sites where discrete patches of plants were sampled using a triplet of

quadrats a significant interaction between Year*Quadrat_position suggested that

temporal declines in plant density varied from within to the outside of plant

patches (Table 2). Specifically, devil’s-bit scabious plant density declined

significantly only within and at the edge but not on the outside of each patch of

plants (Figure 10). Plant density declined fastest within patches and to a lesser

degree at the edge of patches. In general, patches of devil’s-bit scabious shrank

significantly in area and become less dense between 2005 and 2007.


15

Table 2 Variables significant in REML linear mixed models of devil’s-bit scabious density across 6 sites in Northern Ireland from 2005 to 2007.

Explanatory variable(s) Fd.f. β (covariates only)

p sig.

Model 1 – Aughnadarragh, Ballykilbeg (a) and Inishargy

Year 103.822 <0.001 *** Site 5.652 0.004 ** Season 7.781 0.005 ** Quadrat_position 28.442 <0.001 *** Year*Quadrat_position 23.454 <0.001 *** Year*Site 3.844 0.004 ** Short wet vegetation (PC1) 37.221 +4.081 ± 1.514 <0.001 *** Mature scrub heath (PC2) 4.351 -0.117 ± 1.051 0.037 * Tall wet vegetation (PC3) 4.221 +0.666 ± 1.001 0.040 *

Model 2 – Murlough only

Year 69.292 <0.001 *** Quadrat_position 39.862 <0.001 *** Year*Quadrat_position 10.174 <0.001 *** Mature scrub/heath (PC1) 8.151 -4.596 ± 1.325 0.004 ** Short & wet (PC2) 3.501 +2.644 ± 1.446 0.061 ns Tall & wet (PC3) 9.381 +3.082 ± 1.249 0.002 **

Model 3 – Ballykilbeg (b) only Year 3.152 0.043 *

Model 4 – Montiaghs only

Season 10.041 <0.001 *** Wet scrub (PC1) 50.511 +9.249 ± 1.498 <0.001 ***

Plant habitat associations

The floral characteristics of each quadrat were described by PCA using five main

categories including short wet vegetation, tall wet vegetation, tall dry vegetation,

wet scrub and dry scrub heath. For brevity, PCA outputs are given in Appendix 1.

At most sites, devil’s-bit scabious density was positively associated with wet

vegetation, most prominently short wet swards dominated by mosses and sedges

but also, to a lesser extent, tall wet vegetation dominated by rushes (Table 2).

Devil’s-bit scabious density was negatively associated with dry scrub heath

characteristic of late-stage ecological succession. However, plant density was

positively associated with scrub where it remained wet e.g. in Montaighs Moss

(Table 2). Wet scrub is characteristic of early-stage succession in areas of bog or


16

waterlogged grassland. No habitat associations were found within Ballykilbeg (b),

Table 2.

Fig. 9 Mean plant density of devil’s-bit scabious during spring and autumn for six sites in Northern Ireland with all years from 2005 to 2007 combined.

SitesAug

hnad

arrag

h

Ballyk

ilbeg

(A)

Ballyk

ilbeg

(B)

Inish

argy

Montia

ghs

Murlou

gh

Mea

n m

argi

nal e

stim

ated

mea

n %

cove

rage

of S

ucci

ssa

prat

ensi

s

0

5

10

15

20

25

30SpringAutumn


17

Site

Aughnadarragh Ballykilbeg Inishargy Murlough

Mar

gina

l est

imat

ed m

ean

%

cove

rage

of S

ucis

sa p

rate

nsis

0

10

20

30

40

50

60

70200520062007

Site

Aughnadarragh Ballykilbeg Inishargy Murlough

Mar

gina

l est

imat

ed m

ean

%

cove

rage

of S

ucis

sa p

rate

nsis

0

10

20

30

40

50

60

70200520062007

(a) Spring (b) Autumn Fig. 10 Mean plant density of devil’s-bit scabious during (a) spring and (b) autumn at four sites at which the species occurred within discrete patches and was sampled using a triplet quadrat regime from 2005 to 2007.


18

Site

Ballykilbeg (b) Montiaghs

Mar

gina

l est

imat

ed m

ean

%co

vera

ge o

f Suc

issa

pra

tens

is

0

5

10

15

20

25

30

35200520062007

Site

Ballykilbeg (b) Montiaghs

Mar

gina

l est

imat

ed m

ean

%co

vera

ge o

f Suc

issa

pra

tens

is

0

5

10

15

20

25

30

35200520062007

(a) Spring (b) Autumn Fig. 11 Mean plant density of devil’s-bit scabious during (a) spring and (b) autumn at two sites at which the species occurred at low density and thinly distributed was sampled using a random quadrat regime from 2005 to 2007.


19

Fig. 12 Mean plant density of devil’s-bit scabious inside, at the edge and outside of each patch of plants for four sites at which the species occurred within discrete patches and was sampled using a triplet quadrat regime from 2005 to 2007. Namely, Aughnadarragh, Ballykilbeg (a) and Inishargy.

Plant size

Devil’s-bit scabious plant size was highly variable. Size varied significantly

between sites and between seasons (Table 3) with greater plant size recorded

during autumn than spring. Plant size also varied significantly between years but

no overall temporal trend was evident (Figure 13). Significant interactions

including Year*Site, Year*Season and Year*Site*Season suggest that plant size

varies within-sites between years, within-seasons between years and within-

seasons within-sites between years (Table 3). Therefore, plant size exhibited no

consistent spatial or temporal pattern suggesting that plant growth may vary

mainly with local environmental factors such as soil drainage, seasonal rainfall

and maximum/minimum temperatures.

Quadrat position relative to Sucissa pratensis patch

Inside Edge Outside

Mar

gina

l est

imat

e m

ean

%

cove

rage

of S

ucis

sa p

rate

nsis

0

20

40

60

80200520062007


20

Table 3 Variables significant in a REML linear mixed model of devil’s-bit scabious plant size across 6 sites in Northern Ireland from 2005 to 2007.

Explanatory variable(s) Fd.f. p sig.

Year 7.084 <0.001 *** Site 7.852 <0.001 *** Season 17.171 <0.001 *** Year*Site 4.927 <0.001 *** Year*Season 75.831 <0.001 *** Year*Site*Season 3.033 0.028 *

Fig. 13 Mean devil’s-bit scabious plant size, measured as maximum volume (cm3), for 6 sites from 2005 to 2007.

Site

Aughn

adarr

agh

Ballyk

ilbeg

(A)

Ballyk

ilbeg

(B)

Inish

argy

Montia

ghs

Murlou

gh

Mea

n Su

ciss

a pr

aten

sis

plan

t siz

e (c

m3 )

0

50

100

150

200

250

300

350

400

450

500

550200520062007


21

Discussion

Variation in the distribution and density of devil’s-bit scabious between and within

sites may be taken as one criterion to assess the conservation status of the

species and thus the likely persistence of marsh fritillary butterfly at each site.

Some sites, such as Aughnadarragh Lough SAC, have very few patches but they

are large in size containing many plants, while other sites, such as Inishargy Bog

Nature Reserve, may have many small patches. It is unclear which state is most

stable over time. However, it is likely that there is a conservation trade off

between the number of patches of devil’s-bit scabious and their size. Large

patches may contain more plants but they may be more prone to stochastic

extinction events, such as unseasonably dry weather, than a large number of well

distributed small patches. Increasing the number of patches of devil’s-bit

scabious on each site would be a sensible conservation priority.

Seasonal variation in devil’s-bit scabious density and plant size may suggest that

future monitoring would be more successful and cost effective if restricted to

autumn when plants are larger and more widespread. Autumnal surveys of

devil’s-bit scabious could also be coupled with surveys for marsh fritillary webs

further increasing the cost effectiveness of surveys and their utility in directly

coupling data on food availability and caterpillar abundance.

There was no consistent decline in devil’s-bit scabious density across the three

study years; however, a dramatic decline was evident between 2005 and 2006.

Nevertheless, patches of devil’s-bit scabious shrank consistently in size and

became less dense over the three year period. The magnitude and inconsistency

of change in temporal trends between 2005/06 and 2006/07 suggests that the

dramatic decline in density may have been the result of a stochastic

environmental event.

Habitat at the six sites selected for monitoring ranged from cut-over bog to sand-

dunes suggesting that the marsh fritillary butterfly is found in a much wider range


22

Year

Mea

n se

ason

al ra

infa

ll (m

m)

260

265

270

275

280

285

2005 2006 2007

of habitats than those in the UK. Nevertheless, throughout the six sites

examined, devil’s-bit scabious was generally associated with wet habitats. In the

sand dune system at Murlough, plants where associated with wet slacks between

the dunes. These findings are consistent with the habitat requirements of the

species known from elsewhere (Porter, 1981).

Given the species dependency on damp conditions, it seems likely that dry years,

and in particular dry summers, may negatively affect plant survival and

recruitment. The UK Met Office recorded that 2005 was the 4th driest year in the

England and Wales since 1973 (Anon, 2006). Mean seasonal rainfall for Northern

Ireland during 2005 was also lower than average and lower than both 2006 and

2007 (Anon, 2008; Fig. 13). It is, therefore, possible that devil’s-bit scabious

recruitment was lower during 2005 as plants may have suffered higher mortality

throughout the 2005 growing season and they may have been unable to set seed

during autumn. This explanation would be consistent with the dramatic decline in

plant density observed at all six study sites between 2005 and 2006.

Fig. 14 Mean seasonal rainfall between 2005 and 2007 for Northern Ireland. Data obtained from the UK Met office (Anon, 2008).


23

Trends in plant density between 2006 and 2007 are less clear across the six

study sites. A three year period is too short a time-series to determine interannual

and multiannual fluctuations in plant populations. Caution should be taken when

interpreting short time-series and few valuable conclusions

can be drawn from the current study. That said, it is clear that stochastic

environmental events, such as drought, represent a significant threat to

fragmented and isolated populations of devil’s-bit scabious and thus, presumably

the marsh fritillary butterfly (Joyce & Pullin, 2003). Establishment of an effective

ecological network throughout Northern Ireland would increase and maintain

habitat connectivity between fragmented habitats of conservation importance

(Roger Catchpole pers. comms.); thus increasing the likely persistence of

species, like the marsh fritillary butterfly, that is otherwise likely to disappear from

isolated sites.

Variability in plant size, and therefore growth, within and between sites across

seasons and between years supports the hypothesis that devil’s-bit scabious are

responsive to spatial and temporal variation in environmental conditions, most

likely soil drainage and rainfall. Close association of higher plant density with

short damp swards suggest that effective scrub encroachment management and

maintenance of local water table levels at each site would benefit the species.

Control of invasive species such as Molinia may also benefit floral diversity

(McEvoy et al. 2008) including devil’s-bit scabious.

Density-dependent processes involving grazing herbivores, including marsh

fritillary caterpillars, are also likely to affect devil’s-bit scabious density. Without a

contemporaneous assessment of caterpillar and butterfly abundance at each site

it is impossible to draw inferences about devil’s-bit scabious density and the

abundance of the marsh fritillary butterfly at each site. It remains unclear whether

devil’s-bit scabious density is a limiting factor in marsh fritillary butterfly

population growth. Further research is needed to establish the direction of the


24

relationship; it remains possible that marsh fritillary caterpillar abundance may be

a limiting factor in devil’s-bit scabious abundance.

The current study highlights the need for continued long-term monitoring of sites

of conservation importance on a fine spatio-temporal scale.


25

Recommendations We make five broad recommendations for future research and site management:

1. Future monitoring may be more cost effective if conducted synchronously with

marsh fritillary caterpillar web surveys during autumn when devil’s-bit

scabious plants are most prevalent and at their largest. Pairing food plant and

butterfly/caterpillar abundance surveys is necessary to make inferences about

their ecological relationship.

2. Future survey methodology should consider a line rather than point transect

system in order to more accurately revisit survey locations.

3. If interannual and multiannual fluctuations in devil’s-bit scabious abundance

are to be understood annual monitoring at specific sites of conservation

importance is required.

4. Increasing the cover and area of short wet swards on each site may benefit

devil’s-bit scabious density and distribution. Assessment of anthropogenic

manipulation of the water table, lowering of surface peat and soil, clearing

rank vegetation by burning and management of scrub encroachment by

prescriptive grazing should all be considered and assessed as potential site

management practises.

5. Increasing the number of patches of devil’s-bit scabious on each site may

help safeguard the future persistence of the plant against stochastic

environmental events.

6. Establishment of an effective ecological network throughout Northern Ireland

would increase and maintain habitat connectivity between fragmented

habitats of conservation importance increasing the likely persistence of

species such as the marsh fritillary butterfly.


26

Acknowledgements The current project was funded as part of the Quercus partnership between the

Environment and Heritage Service and Queen’s University Belfast. Thanks to all

those who enthusiastically participated in fieldwork including (listed in

alphabetical order of surname) John Kelly, Emma Seale, Oísín Sweeney and

Conor Wilson. Particular thanks to Emma Seale for help in gathering field data

and information on marsh fritillary butterfly ecology, advice, support throughout

the project.


27

References Anon (2006) Dry spell 2004/6. The Met Office. http://www.metoffice.gov.

uk/climate/uk/interesting/2004_2005dryspell.html Anon (2008) Northern Ireland Rainfall (mm) Areal Series; starting from 1914. The

Met Office Series Statistics. http://www.metoffice.gov.uk/climate/uk/ seriesstatistics/nirain.txt

Hosmer, D.W. & Lemeshow, S. (2000) Applied Logistic Regression. John Wiley,

New York, NY. Joyce, D.A. & Pullin, A.S. (2003). Conservation implications of the distribution of

genetic diversity at different scales : a case study using the marsh fritillary butterfly (Euphydryas aurinia). Biological Conservation.

Mc Evoy, P. (2008). Fen ecology in Northern Ireland. Quercus. A report to the

Environment & Heritage Service. Porter, K. (1981). The population dynamics of small colonies of the butterfly

Euphydryas aurinia. PhD Thesis, Oxford University. Schtickzelle, N., Choutt, J., Goffart, P., Fichefet, V. & Baguette, M. (2005).

Metapopulation dynamics and conservation of the marsh fritillary butterfly : Population viability analysis and management options for a critically endangered species in Western Europe. Biological Conservation.

Warren, M.S. (1994). The UK status and suspected metapopulation structure of a

threatened European butterfly, the marsh fritillary Eurodryas aurinia. Biological Conservation.


28

APPENDIX 1 Table 1 PCA of vegetation type at Aughnadarragh, Ballykilbeg (a) and Inishargy (PCs used in Model 1). Measure / Explanatory variable PC1 PC2 PC3 PC4 Eigenvalues 1.85 1.48 1.08 1.05 % of variance 26.43 21.14 15.47 15.10 Cumulative % of variance 26.43 47.68 63.04 78.11 Bracken -0.373 +0.252 +0.510 +0.686 Grasses (including Molinia spp.) -0.037 -0.909 -0.257 +0.131 Heather +0.290 +0.356 -0.230 +0.374 Mosses +0.833 +0.323 +0.132 -0.068 Rushes -0.160 -0.054 +0.677 -0.558 Sedges +0.828 -0.048 +0.026 -0.043 Shrubs -0.471 +0.596 -0.477 -0.330 Vegetation description Short wet

veg. Dry scrub

heath Tall wet

veg. Tall dry

veg. Table 2 PCA of vegetation type at Murlough only (PCs used in Model 2). Measure / Explanatory variable PC1 PC2 PC3 Eigenvalues 1.98 1.30 1.00 % of variance 32.92 21.58 16.70 Cumulative % of variance 32.92 54.51 71.19 Bracken +0.213 -0.596 +0.506 Grasses (including Molinia spp.) -0.771 -0.260 -0.183 Heather +0.807 +0.137 -0.164 Mosses -0.046 +0.852 -0.002 Rushes -0.050 +0.321 +0.822 Sedges n/a n/a n/a Shrubs +0.825 -0.156 -0.091 Vegetation description Dry scrub

heath Short wet

veg. Tall wet

veg.


29

Table 3 PCA of vegetation type at Ballykilbeg (b) (PCs used in Model 3). Measure / Explanatory variable PC1 PC2 Eigenvalues 1.77 1.12 % of variance 35.48 22.44 Cumulative % of variance 35.48 57.92 Bracken n/a n/a Grasses (including Molinia spp.) -0.830 +0.247 Heather n/a n/a Mosses +0.837 -0.198 Rushes -0.129 +0.087 Sedges +0.597 +0.479 Shrubs +0.108 +0.886 Vegetation description Short wet

veg. Dry scrub

heath Table 4 PCA of vegetation type at Montiaghs (PCs used in Model 4). Measure / Explanatory variable PC1 PC2 Eigenvalues 2.17 1.06 % of variance 43.39 21.27 Cumulative % of variance 43.39 64.67 Bracken n/a n/a Grasses (including Molinia spp.) -0.821 -0.262 Heather n/a n/a Mosses +0.544 -0.497 Rushes -0.491 +0.577 Sedges +0.402 +0.637 Shrubs +0.893 +0.092 Vegetation description Wet

scrub Tall wet

Veg.

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