Abstract
In the absence of long‐term monitoring records, paleoecology can be used to extend knowledge of species and community ecology into the past. The rare and declining aquatic plant Najas flexilis is a priority species for conservation across Europe, and is an ideal candidate for paleoecological study; not only are historical records of the plant sparse, but its seeds are commonly found and well preserved in lake sediment cores. In this study, we investigate the timing and causes of decline at two UK sites at which N. flexilis has recently become extinct: Esthwaite Water (England) and Loch of Craiglush (Scotland). For both sites, multiple paleoecological indicators and available historical biological records and monitoring data are compared to numbers of N. flexilis seeds enumerated in dated sediment cores representing the last 150 years. At Esthwaite Water, N. flexilis seeds were found in abundance in association with indicators of a clear, oligo‐mesotrophic, mildly alkaline lake. Eutrophication led to the disappearance of N. flexilis in the 1980s. By contrast, far fewer N. flexilis seeds were found in a core from Loch of Craiglush, and the current period of N. flexilis absence was found to be one of several over the last 100 years. Species represented in cores taken from Loch of Craiglush were indicative of slightly more acidic conditions than Esthwaite Water. Given that N. flexilis favours circumneutral to alkaline conditions, it is possible that Loch of Craiglush has not always been favourable for the plant. These findings have important implications for future conservation efforts, particularly at Esthwaite Water where they suggest that recent failed attempts to reintroduce the species may have been premature. More generally, this study demonstrates the value of paleoecological techniques as a means to provide the long‐term context that is often missing from conservation planning and management.
Highlights
Consistent, reliable documentary records and long‐term monitoring both play an important role in conservation
The exceptions are (1) a few seeds found in sediments dated from the 2000s in ESTH9, which could represent recent failed attempts to reintroduce N. flexilis to Esthwaite Water from seed; and (2) seeds found in sediments dating from around 2007 in CRAIB, which, when compared with the results of intensive SCUBA surveys, create an unexplained “false positive” effect similar to that found at Loch of Butterstone by Bishop et al (2018)
Before around 1915, N. flexilis was relatively abundant at Esthwaite Water, and, according to both the macrofossil record and an early vegetation survey undertaken in 1914 (Pearsall, 1920), it co‐occurred with a plant community typical of a mildly alkaline lake
Summary
Consistent, reliable documentary records and long‐term monitoring both play an important role in conservation. Salgado et al (2010) used macrofossils found in a sediment core to establish that the plant community of Loch Leven, Scotland, had undergone a shift from Isoetes spp. dominance towards a Potamogeton spp. and Chara spp. community long before regular systematic monitoring of plants began These findings led to the reassessment of macrophyte conservation targets at the lake. Many of the perceived threats to N. flexilis are readily explored through paleoecological proxies This applied paleoecological study investigates the history of occurrence and causes of decline of N. flexilis at two sites in the UK from which N. flexilis has been lost in the past 50 years or so: Esthwaite Water (English Lake District) and Loch of Craiglush (Perthshire, Scotland). To evaluate the causes of N. flexilis decline by inferring parallel lake environmental changes using macrofossil, diatom, and cladoceran remains
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