Multi-scale effects of farmland management on dragonfly and damselfly assemblages of farmland ponds
Highlights
► Species richness was influenced at smaller scales for damselflies than dragonflies. ► A greater number of landscape variables had an effect on exuviae than on adults. ► Increased uptake of England's basic AES related to decreased species richness. ► Exuvial species richness was higher at ponds with buffers. ► Exuvial richness increased with water amount, not pond number, in the landscape.
Introduction
The escalating worldwide demand for agricultural products is having a detrimental effect on biodiversity (Scherr and McNeely, 2008). In an attempt to reverse environmental degradation, most European countries have introduced agri-environment schemes (AES), which are considered the most realistic policy-tool to integrate biodiversity with economically sustainable production systems (Vickery et al., 2004, Scherr and McNeely, 2008), although their effectiveness remains uncertain (e.g. Kleijn et al., 2011).
In Britain, farmland covers 76% of the land area (FAOSTAT, 2009). Post-war agricultural intensification has resulted in habitat loss and fragmentation, reducing habitat connectivity. This, together with more intensive farm management practices and increased pollution and eutrophication, contributed to the extinction of three species of Odonata in the 1950s (British Dragonfly Society, 2010). Out of 40 native British odonate species, four are currently classified as ‘Endangered’ and two as ‘Vulnerable’ (Daguet et al., 2008). Odonates are also declining worldwide (Clausnitzer et al., 2009). Of 137 native European species, 15% are threatened and 24% are declining. Deterioration of the farmland biotope has been implicated in their declines (IUCN, 2010, Kalkman et al., 2010).
Ponds are the obligate habitat for at least 35% of British odonate species and secondary habitat for a further 38%. However, 50% of UK farmland ponds have been lost over the past century and only 8% of the remaining ca. 482,000 ponds are of good quality (Carey et al., 2008). Although ponds are still widespread, and regionally sustain the highest levels of aquatic biodiversity compared to other waterbodies (Williams et al., 2003), there is a paucity of financial incentives and policy frameworks to create, maintain or improve them. For example, ponds are not considered in the Water Framework Directive (Davies et al., 2008a).
Odonates are effective bio-indicators of both aquatic and terrestrial habitats and, therefore, of agricultural land, including farmland ponds (Clark and Samways, 1996, Briers and Biggs, 2003). Odonates have a bipartite life-cycle: at a local scale they depend on healthy waterbodies for growth and emergence during their larval stages, and egg deposition during their adult stages, and at a larger scale adults depend on the quality of the terrestrial landscape for dispersal, feeding and roosting (Corbet, 1999). As a result, the presence of odonates at ponds not only reflects pond quality but also reflects the quality of the surrounding farmed landscape, and a failure to consider aquatic habitats at the landscape-scale has exacerbated odonate declines (Declerk et al., 2006, Thompson and Watts, 2006). Ponds need to be considered within the landscape matrix (the ‘pondscape’: Boothby, 1997) as odonates benefit from shorter distances between ponds (Sherratt et al., 1999).
Dispersal is a fundamental aspect of the odonate life-cycle: after emergence, tenerals (immature adults) will move away from the waterbody to mature, forage and roost (Corbet, 1999). Movements between roosts and ponds will occur daily, while dispersal to other waterbodies will typically occur less often. Species size affects dispersal: the larger dragonflies will typically disperse longer distances in search of new resource patches, while the more slender damselflies tend to remain near the natal pond (Banks and Thompson, 1985, Conrad et al., 1999). Consequently, the pond's surrounding land-use may affect damselfly and dragonfly dispersal in different ways (Samways and Steytler, 1996).
Agri-environment schemes (AES) were launched in the UK in 1987 (Table S1 in Supplementary Information). They all include some pond-specific options that could potentially benefit odonates, such as buffering in-field ponds in improved grassland or arable land, maintenance of high quality ponds, and pond creation and restoration. Other non-pond options can also be beneficial; for example, areas with lower fertiliser input will have a positive effect on pond quality. Some species may benefit from matrix restoration through AES, particularly species whose ranges or wetland habitats are vulnerable to climate change, species of low mobility and high habitat specificity, and species with fluctuating populations (Donald and Evans, 2006). These features are common to many odonates (Hassall et al., 2007, Allen and Thompson, 2010).
This study presents the results of extensive exuvial (cast-off larval skin at emergence) and adult odonate surveys of farmland ponds in lowland England, in relation to characteristics of the surrounding landscape. The aims of the study were to: (i) identify the scale at which the surrounding landscape influences species richness of odonates most strongly, with the hypothesis that landscape affects dragonflies at a larger scale than damselflies, due to the latter being less mobile; (ii) test if, and at what scale, the presence of other waterbodies increases odonate species richness; (iii) test if some types of land-use and AES surrounding ponds affect odonate species richness; and (iv) suggest recommendations for odonate conservation in intensively farmed lowland landscapes.
Section snippets
Study sites
Twenty-nine farmland ponds (mean pond area ± SE: 353 ± 63.4 m2) were randomly selected (with regard to location and size) in the River Ray catchment (Buckinghamshire/Oxfordshire, UK) and were treated separately purely for logistical purposes. The Ray is a 283 km2 catchment of alkaline waters within lowland agricultural land, with clay as the main top substrate. Pond selection was unbiased with regard to odonate species richness levels, as these were unknown. The only selection requirement was that
Overall species richness: adults and exuviae
A total of 17 species was recorded as adults in each of the three study years (Table S4), comprising eight damselfly and nine dragonfly species. Overall, 11,025 exuviae were collected (2006: 3316; 2007: 2325; 2008: 5384), from seven damselfly and nine dragonfly species (Table S4). The maximum species richness at a pond was: (i) Total exuviae: 12; (ii) Total adults: 15; (iii) Dragonflies-exuviae: 7; (iv) Dragonflies-adults: 9; (v) Damselflies-exuviae: 7; (vi) Damselflies-adults: 6. Minimum
Landscape responses depend on sub-order and life-cycle stage
Species richness of adult and exuvial dragonflies was more influenced by variables at a larger scale (1600 m) compared to damselflies (100/400 m); similarly, Kadoya et al. (2008) found the occurrence of dragonflies to depend more on landscape composition than damselfly occurrence. Current strategies for odonate conservation are most likely to benefit the less mobile damselflies, by focusing on increasing pond quality and their immediate surroundings (such as establishing pond buffer strips). This
Conclusions
Odonate species richness is affected by land-use at different spatial scales, and conservation needs to focus on creating more heterogeneous landscapes (Foster and Soluk, 2006). Odonate responses to land-use, while varying with the mobility of sub-orders and life-stages, support the argument for the development of policies that encourage contiguous farms to apply for AES (Merckx et al., 2009b) in order to form continuous managed landscapes. AES can play an important part in achieving this, if
Acknowledgements
This work was funded by the Esmée Fairbairn Foundation. We express our gratitude to Alison Poole for early comments, landowners who allowed us access to their land, and volunteers who assisted with fieldwork. We are grateful to two anonymous referees whose comments enabled us to improve the manuscript.
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