October 2022 I gave a short flash talk on my poster at the BSBI conference.
Gallery
Sea Mayweed Tripleurospermum on Great Minnis's
An Eyebright species Euphrasia nemorosa on Roe, note also a white form of Odontites vernus (Red Bartsia). This form was common on several of the islands.
MG1e grassland with abundant Knapweed Centaurea on Roe.
H7b heath with Bell Heather Erica cinerea on Darragh.
A Six-Spot Burnet moth Zygaena filipendulae nectaring on Sneezewort Achillea ptarmica on Darragh.
A Hummingbird Hawkmoth Macroglossum larva on Lady's Bedstraw Galium verum on Darragh.
Introduction
Strangford Lough is a large sea lough in Vice-county H38 (Down),
in the North-East of Ireland. Formed by glaciation, it has many
iconic drumlin islands.
These are of conservation importance, especially for their grassland
and saltmarsh, and for birds and seals. However, the flora is poorly
studied, if at all.
Agricultural improvement on the mainland has led to these islands
becoming refugia for many grassland species (such as Linum catharticum,
Euphrasia nemorosa).
I recorded the plants and NVC plant communities of eleven islands
around the townland of Ballymorran, which can be grouped by what
grazes them:
None
Craigaveagh, Parton, Shamrock
Geese
Inisharoan
Sheep
Green
Geese, Sheep
Inishanier, Roe
Geese, Cattle
Drummond, the Minnis's
Sheep, Cattle
Darragh
Methods
Surveying
Plants were recorded in March, July, August, and September of 2022.
Each island was visited at least once between July and September.
A quadrat on Roe.
Mapping
Between July and September, the islands were surveyed with 2×2m
quadrats following the NVC.
These data and satellite imagery were used to classify and map
the plant communities on each island.
Species richness and area
The number of plant taxa (species-level and above) present on
each island was calculated. Uncertain IDs were excluded.
The area of each island was measured using Google Earth.
Analysis was done in R and plots produced with the ggplot2 package.
Maps were created in GIMP.
Kayaking
Accessibility is likely one of the reasons these islands have
been under-studied.
I kayaked out to the islands, which is much cheaper, easier,
and lower-impact than using any other kind of boat.
I brought my notebooks, field guides, camera, food, and other
equipment out in a drybag which fits behind the seat.
This is a great way to do fieldwork - I'd recommend it! Many
other islands could be surveyed this way without the need for a
larger boat.
Me kayaking in front of Darragh.
Area and species richness
Species richness correlates with the area of the island, with a linear
relationship when the values are log-transformed (fig. 1;
,
,
). Residual
variance is undoubtedly at least partially
caused by differences in grazing, although
the sample size is too small to test these
effects. It would be particularly interesting to
see whether Darragh is an outlier for its size,
and whether sheep-grazed islands are
significantly less diverse.
Fig. 1: log-log graph of island species richness and area.
Plant communities on the islands
The islands are dominated by grassland, with MG1 and MG5 being
prominent. These are interspersed with bramble and blackthorn scrub,
and the ground flora in some of this is quite rich, especially on
Darragh.
On the islands where geese graze and other birds nest, the plant
communities can be quite odd, possibly due to high soil nitrogen and
selective grazing by geese. The sward here is open and dominated by
Heracleum sphondylium, Oenanthe crocata, and Centaurea nigra, with
Arrhenatherum elatius and Holcus lanatus also abundant.
This grassland community appears to mostly correspond to the
Centaurea nigra sub-community of Arrhenatherum elatius grassland (MG5e).
Hogweed Heracleum dominated sward on Drummond.
Saltmarsh is also present on some of the islands; on Shamrock
there is a very clear gradation from Festuca arundinacea-dominated
MG12 through the upper and lower saltmarsh.
The ponds have a wide range of plants growing in water and mud,
with Persicaria maculosa often dominant.
There are also iris beds and rush pasture, with Juncus acutiflorus
commonly present.
Redshank Persicaria maculosa and Marsh Yellow-Cress Rorippa palustris in the large pond on Drummond.
Darragh island management
Darragh Island (fig. 2) is owned by the National Trust and has
a management plan which was published in 1999, following biological
surveys of Trust-owned land on the Lough (Lister & Alexander, 1999).
This plan recommended the continuation of winter cattle grazing “with
a few goats present all year” in order to maintain species-rich
grassland and prevent succession to scrub, which was seen as a
threat at the time.
Since the management plan was published, the goats have been
removed and replaced with sheep, which are present all year round.
Winter cattle grazing has continued.
With current management, brambles and bracken have continued to
invade the species-rich grassland. Many grassland species are selectively
grazed by sheep, impacting flowering and seed production.
Orchis mascula was last seen in the 1980s (Brown, pers. comm. August
2022). There has been a similar decline in Dactylorhiza fuchsii between
2016 and 2022, from tens of flowering individuals to none (personal
observation). Selective grazing by sheep seems a likely culprit.
With changes in management, scrub could be reduced without harming grassland plants.
Fig. 2: plant communities of Darragh Island. Scale bar = 100m.
Conservation on the islands
The majority of the islands are part of the Strangford Lough
(Part 3) ASSI and are managed for species-rich grassland and birds
to varying degrees of success. However, most of Roe Island (fig. 3)
is not part of any ASSI and is ignored as a species-rich grassland
in Down.
Livestock farming on the islands is entirely dependent upon the
National Trust to ferry animals. This service aims to “control the
growth of scrub and providing excellent habitat for local wildfowl
and migrating birds to thrive” (National Trust, 2020). However, current
management on many of the islands is suboptimal for species-rich grassland
and other habitats.
Several of the islands, particularly Roe and Green, are dominated by
species-poor Holcus and Arrhenatherum swards to the exclusion of other
species.
Changing grazing regimes to encourage grassland species would benefit
the wildlife of all of the islands greatly. Undertaking regular monitoring
and making evidence-based conservation decisions would be a fantastic
change to their management.
Fig. 3: plant communities of Roe Island. Scale bar = 100m.
Acknowledgements
Thanks to all the landowners: Kathryn McBride, John Dynes, Leanna McBride,
and Alison Oliver. Thanks to the county recorder Graham Day for help with species
ID. Thanks to Wayne Liang for help with statistics. Thanks to my parents, Judith
and Keith Dalzell, for help with transport and kayaking!
Huge thanks to the BSBI who generously provided a plant study grant to make
this research possible.
References
Lister, J.A. & Alexander, K.N.A. 1999. Strangford Lough, Co Down (excluding
wildfowl, waders & marine)1998 Survey Incorporating 1985 Survey. Internal report
(biological survey). National Trust.
