Rachel Jones, Senior Ecologist, explores local-scale management impact on the Lulworth Skipper.
Many butterfly and moth species persist in networks of interlinked populations known as metapopulations1. In a stable metapopulation local extinctions are balanced by colonisations but habitat loss and fragmentation due to land abandonment, urbanisation and conversion to agriculture disrupts this balance. In fragmented habitats, smaller patches support smaller, more extinction prone populations and more isolated populations have a reduced likelihood of colonisation following a local extinction. This can increase the vulnerability of metapopulations to extinction2, especially in years where habitat or climate conditions are suboptimal3.
Working across metapopulations to increase patch size (and therefore population size) and improve connectivity is a key strategy in species conservation4. This involves conservation work at a landscape-scale5 to focus on size, quality and connectivity of sites (bigger, better, and more joined up6). However, in heavily fragmented landscapes creating new patches or forming physical linkages between them can be difficult to achieve following habitat loss or where land is limited and subject to competing demands7. Furthermore, many threatened species are habitat specialists and creating or restoring semi-natural habitats in highly modified landscapes can be complex due to restricted access, time consuming and costly particularly where land is degraded8,9.
The Lulworth Skipper Thymelicus acteon is an example of a specialist species persisting in a metapopulation in a fragmented landscape. In the UK this butterfly is restricted to a 40km stretch of the south Dorset coastline and is associated with south-facing chalk and limestone grassland where the larval food plant Tor-grass Brachypodium rupestre can grow in tall tussocks10,11. New research by the University of Exeter and Butterfly Conservation uses four decades (197810, 199711, 201012 and 2017) of habitat and population data across the entire UK range of the Lulworth Skipper, in combination with metapopulation models, to test how barriers to landscape-scale conservation can be overcome by management at the local (site-based) level.
Results showed that there had been no change in the overall range of Lulworth Skipper over 40 years. The number of populations was similar in 1997 (77) and 2017 (79), but 34% of populations showed turnover (colonisation or extinction) in this time. Population density (number of butterflies per hectare) of Lulworth Skipper was closely linked to vegetation characteristics indicative of habitat management (vegetation height and food plant frequency), these characteristics changed through time alongside changes in density. The modelled effect of habitat on population density showed high densities were found in habitats with taller vegetation (20-35cm) or high food plant cover (>90%) with 27cm vegetation height and 100% food plant cover supporting the highest population densities.
Population density was reduced at shorter and taller vegetation heights and where the frequency of food plant was lower. Simulating effects of habitat quality on metapopulation dynamics 40 years into the future suggests that coordinated changes to two key components of quality (vegetation height and food plant frequency) towards optimum would increase patch occupancy above the range observed in the past 40 years and outside the existing distribution. In contrast, the deterioration of habitat suitability below threshold levels leads to metapopulation retraction to core sub-networks of patches, or eventual metapopulation extirpation.
The results have three important applications. Firstly, for the ongoing habitat management for Lulworth Skipper and other mid-successional species which are sensitive to management but require some management to maintain suitable conditions. Lulworth Skipper requires an intermediate management intensity (e.g. light grazing) or frequency (across space or time) which aims to increase food plant cover and manage vegetation height towards optimum. Secondly, results outline applications for metapopulation modelling incorporating habitat quality to help identify core sites important for regional persistence, habitat barriers to range expansion, habitat thresholds for persistence and guide spatial targeting of management (e.g. in landscapes supporting species with very different habitat requirements).
Lastly, the results highlight how changes to habitat quality can help overcome constraints imposed by habitat patch area and spatial location on rates of colonisation and local extinction as a result of habitat fragmentation. Improving habitat quality can increase population density which increases the carrying capacity of a patch without physically enlarging it. At higher population densities dispersal can be increased without directly creating linkages between populations. Local habitat management therefore plays a key role in landscape-scale conservation particularly in crowded landscapes where scope to create new habitat patches or linkages is limited and for species persisting in metapopulations where density is influenced by habitat quality. Habitat management should be supported by appropriate advice and funding (such as agri-environment schemes) and co-ordinated across landscapes to maximise results. The monitoring of population density, and the monitoring and management of local (site-level) habitat quality, represent effective and important components of conservation strategies in fragmented landscapes working alongside other landscape-scale initiatives.
We thank F. Bell, I. Clark, S. Woodley, K. Pradel, B. Goodyear and G. Pearman for their fieldwork and analysis, J. A. Thomas for supplying the 1978 data. Particular thanks also to the National Trust, the Lulworth Estate and all private landowners that supported the study and permitted access for surveys. The work was funded through a UK Natural Environment Research Council studentship (grant reference: NE/N00857X/1
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