Note on Habitat Management for Pool Frog Reintroduction Thompson Common

Transcription

1 Note on Habitat Management for Pool Frog Reintroduction Thompson Common

2 Contents 1. INTRODUCTION AND BACKGROUND Contractual background MANAGEMENT WORK Ongoing habitat restoration at Thompson Common Recent pond restoration RESULTS OF MANAGEMENT WORK Compartment RECOMMENDATIONS Monitoring Short-term habitat management Long-term habitat management LITERATURE Introduction and background 1.1. Contractual background This work was carried out by John Baker under contract to Amphibian and Reptile Conservation as part of the second pool frog reintroduction to England. The work specified was to assist Darrell Stevens (Norfolk Wildlife Trust s Brecklands Reserves Manager) in undertaking habitat restoration at Thompson Common, the selected location for the second reintroduction of the pool frog to England. The main management tasks specified in the contract were: scrub and tree removal, pond re-profiling, aquatic vegetation management and grazing. 1

3 2. Management work 2.1. Ongoing habitat restoration at Thompson Common Norfolk Wildlife Trust (NWT) has been carrying out restoration of pingo habitat at Thompson Common Nature Reserve prior to the planned pool frog reintroduction. This has involved removal of scrub and secondary woodland that has overgrown some ponds (especially in NWT management compartments 1 and 6), reinstating grazing to maintain a more open habitat and installation of sluices to hold water on site. This restoration work favours the pool frog (which requires relatively open aquatic habitat and early succession stage ponds) and goes a long way towards achieving the management measures specified under the current contract work Recent pond restoration Darrell Stevens has also obtained a Natural England consent to cut and remove Typha from pingos, using a weed basket with a reciprocating blade mounted on a 360 excavator, and to weed wipe regrowth with Roundup Biactive. This weed basket work took place during the last week of January and the first week of February 2015 and was, in practice, a substitute for the main excavation work to be carried out at this time (as detailed in the contract to John Baker). Fig. 1: A weed basket has been used to remove Typha from ponds within NWT management compartments 1 and 6. The weed basket removed mats of Typha leaving pond profiles intact. Superficial silt was also removed, attached to the rhizomes. The southern part of management compartment 6 has been targeted for the pool frog reintroduction so this area was prioritised for vegetation clearance by weed basket. 3. Results of management work 3.1. Compartment 1 NWT management compartment 1 is the part of the site accessible by the Great Eastern Pingo Trail and includes the last three ponds where Charles Snell found pool frogs. Removal of scrub has greatly reduced shading of one of these former pool frog ponds (243) and Typha has been removed from 262 which has greatly reduced shading at this pond. The weed basket could not reach the centre of the former fishing pond (278) so there is still a small central stand of Typha in this pond which will require herbicide treatment, if it is not to spread over the pond once more. 2

4 Fig. 2: Pond 262, a former pool frog pond in October 2011 (upper) and after removal of Typha January 2015 (lower). 3

5 3.2. Compartment 6 Most ponds in the southern part of this compartment were cleared of Typha. Eleven ponds fall within an area that could be fenced off to allow greater control of grazing and this can be considered the pool frog reintroduction area. Grid references for these ponds are given in Table 1. Note that there is some difficulty in matching mapped ponds with those on the ground. The grid references provided were taken from the southern bank of ponds and should be used by GIS to produce more definitive pond locations and mapping. Table 1. Ponds within pool frog reintroduction area. Pond Code Grid reference from southern shoreline A Possibly continuous with E TL B TL A TL C TL A TL B TL C TL A* TL A* TL A TL B TL *63 and 64 have the same reference code. The ponds within the pool frog reintroduction area are a range of sizes. Two of them (57 and 68) are large ponds, with extensive shallows grading into swamp, which may make long-term control of emergent vegetation by mechanical intervention difficult (although grazing may be effective). Two ponds extend beyond the eastern site boundary fence, which may also make vegetation management difficult. The remaining seven ponds are relatively small, with discrete boundaries. Any mechanical intervention required in future should be feasible for these ponds. Ten of the eleven ponds within the pool frog reintroduction area were cleared of Typha using the weed basket. Residual emergent vegetation remains in four of the ponds and should be targeted for chemical treatment of re-growth, as permitted under the existing consent for Typha control. 4

6 Fig. 3: Two of the smaller ponds (February 2015) in the pool frog reintroduction area after vegetation clearance by weed basket. Further mechanical intervention may be feasible in these ponds. 5

7 4. Recommendations 4.1. Monitoring Monitoring will be required to determine the success of habitat management measures to date and to enable future management that is adaptive to vegetation development. During the 2015 season the growth of aquatic and terrestrial vegetation should be monitored and the observations used to determine the extent of short-term management. Monitoring will also allow seasonal changes in water level to be observed Short-term habitat management Funding has been secured (Heritage Lottery Fund s Breaking New Ground) for further habitat management work on ponds within the pool frog release area. This includes: Scraping limited sections of pond edges to remove emergent vegetation and create sections of open pond banks Mechanical excavation of emergent vegetation and superficial silt to create open water. This work is planned for the winter of 2015/16 (subject to agreement and consent from Natural England). The extent of this planned work (which ponds and how much work on each of them) will be conditional on how vegetation growth develops over the 2015 growth season. Care will be taken to avoid ponds where Crassula helmsii has been found to prevent spreading this non-native invasive plant. Any mechanical excavation will be preceded by core-sampling to establish the depth of recent silt deposition to ensure that only recently accumulated material is removed and not older deposits Long-term habitat management The pool frog release area should be grazed to maintain open terrestrial and aquatic habitat, which is consistent with the site management plan. There is some uncertainty about the intensity of grazing required to maintain pingo habitat as it was in its heyday and even whether grazing alone is now sufficient to maintain pingos at an early successional stage as learned from experience at the first release site and as noted during a recent NVC survey at Thompson Common (Yaxley, 2013). Habitat monitoring should determine whether it will be necessary to increase the grazing intensity within the pool frog reintroduction area above that of extensive grazing of the reserve as a whole. Hence, installation of stock fencing across the northern boundary of the pool frog reintroduction area (fig. 4), to fully enclose it and allow control of livestock therein, should be an option conditional on how aquatic vegetation develops in future years. If grazing management cannot prevent pond succession then Yaxley s (2013) proposal for occasional and very careful mechanical clearance of selected pingos should be implemented as a long-term programme within the pool frog release area (average rate of one pond per year). This will not only maintain some ponds at an early successional stage, but will also diversify the range of successional stages of the ponds within the whole reser 6

8 TL TL TL TL TL TL TL Fig. 4: Diagram to indicate location of proposed (conditional) stock fence (red dashed lines) relative to existing fencing (solid black lines). 5. Literature Yaxley, R. (2013). Thompson Water, Carr and Common SSSI. NVC Survey Wild Frontier Ecology, Fakenham. John Baker, March