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Originally identified in 1997.
Petalwort was originally identified at Dawlish Warren in 1997 by Dr David Holyoak in two general areas of Dawlish Warren, one population close to the Visitor Centre and the second broadly spread across the Greenland Lake grassland. Subsequent surveys took place in 1999, 2001, 2003, 2012 and 2019.
Outlook uncertain for survival of the plant at Dawlish Warren.
Following the Beach Management Scheme, initiated by the Environment Agency in 2017, it is likely that a dynamic dune system will redevelop in parts of the site. If the dunes erode as predicted by the Environment Agency, theses areas may become inundated with seawater, destroying the existing population and decreasing the chance of natural regeneration through spore dispersal.
Attempts to relocate the plant.
Teignbridge District Council Green Spaces Rangers at the Warren have already undertaken some experimental translocations of Petalwort and habitat creation to areas behind the newly constructed wall near the Visitor Centre (most likely to be protected from any inundation by the sea), under licence from Natural England.
2023 survey change of methodology.
The survey was undertaken in March 2023 and initially attempted to use an identical method to that employed by Lansdown in 2019. However, this soon became impractical because the hollows were not visible as discrete entities within the dune slack. This problem was compounded by a lack of geographical co-ordinates for individual hollows that could have helped to accurately relocate them.
Therefore, a slightly different approach was taken. A careful search of habitat that appeared to have potential to support Petalwort in Greenland Lake Slack was undertaken and, where discrete colonies were found, they were georeferenced using a Garmin hand-held navigational receiver.
Visitor Centre population not found.
A search of the slack just west of the Visitor Centre where Petalwort had previously been translocated was also made but proved unsuccessful.
619 thalli were found in approximately 300m2 of Greenland Lake Slack.
619 thalli were found in approximately 300m2 of Greenland Lake Slack, in more or less similar places to where it has been seen previously. Most colonies supported relatively low numbers of individuals (fewer than 100).
Counts of Petalwort at the Warren varying greatly over time.
Monitoring of the metapopulation of Petalwort at Dawlish Warren has now been undertaken for 20 years. In that time, the counts of Petalwort have varied greatly.
It is unclear how many thalli were present before 2003, although the counts from previous surveys indicate that at least 2000 were counted from the site at that time. The most reliable count is from 2003, with a total of ca. 3100 thalli in the two slacks. Lansdown (2019) suggested that counts fell after that and stabilised in 2012, although by then the population was confined to Greenland Lake Slack.
The current findings indicate that numbers of thalli have halved since 2019. It is not clear if the prolonged flooding of parts of Greenland Lake Slack may have played a significant role in the numbers of visible thalli at the time of the most recent assessment. The assessment places emphasis on the findings that east of the main track, the only thalli seen were in areas where the slight changes in ground height would have elevated the plants above the main flood zone.
Change of condition assessment for the plant.
Reliance on a monitoring approach that requires clear delimitation of damp hollows is no longer practical. The 2019 monitoring provided an opportunity to test the condition assessment attributes and targets previously recommended (Lansdown 2012). He proposed an alternative target encompassing extent and abundance expressed in a simple form:
If five or more hollows support Petalwort and the total number of thalli recorded exceeds 1,000, including male and female individuals, then the metapopulation may be considered to be in favourable condition.
Because of the difficulty of locating and identifying the wet hollows, it is unclear how many were occupied in March 2023. Survey indicates that a minimum of five hollows were occupied, possibly more. Both sexes were represented but the total thallus count falls well short of the stated target. On this basis, the population would be considered to be in unfavourable condition.
For future monitoring, the survey proposes that it would be pertinent to employ an alternative target for metapopulation extent and condition assessment:
If Petalwort occupies 20 or more 10m OS grid squares and the total number of thalli recorded exceeds 1,000, including male and female individuals, then the metapopulation may be considered to be in favourable condition.
According to either target condition, the total thallus count results from 2023 indicate the population is in unfavourable condition.
Likely causes of population decline are complicated.
The report suggests that the likely reasons for the decline in number of thalli seen in the current assessment may be complicated and, without further investigation of e.g., ground hydrology, entirely speculative.
However, it may have been driven by the interplay of three main factors:
• natural succession and gradual drying of Greenland Lake;
• changed patterns of recreational trampling by visitors and vehicular movements by staff across Greenland Lake Slack; and
• climate change-linked increased summer desiccation and winter flooding of slack habitat reducing the availability of suitable Petalwort habitat.
It is also possible that climate change could be affecting the water table. 2022 was the hottest year on record in England and the driest since 1935.
Conclusion.
The report returns to the idea of translocating turves to areas thought to be safe from ingress by seawater, encouraging a population to establish away from Greenland Lake Slack.
With the first attempt to translocate Petalwort to the slack just west of the Visitor Centre seemingly now unsuccessful, it remains to be seen how successful plans to relocate turves to another translocation (to a different place) will be.
In the slack immediately west of the Visitor Centre, promotion of greater public access across the edge of the slack may naturally restore suitable trampled habitat within a few years. Hence, even if Petalwort does not find its own way there, future translocation of turves containing Petalwort from Greenland Lake Slack may ‘take’ better than into other receptor sites.