DOI: 10.5061/dryad.4j0zpc8pc
title: Data from: Densities of the endangered Large blue butterfly Phengaris arion vary by 100-fold in restored conservation grasslands, providing a tool to prioritise future introductions

description: A long-term study was made of the carrying capacities (K) of UK grasslands
 restored to support the endangered butterfly Phengaris arion. This iconic
 species is the focus of major restoration programmes and provides a
 blueprint for the conservation of other threatened insects. P.
 arion larvae are phytophagous during their early instars before
 switching to become obligate social parasites of Myrmica ant colonies for
 their main growth. A mechanistic model, incorporating life-table
 measurements of natality and mortalities, was used to predict the value of
 K on 19 restorations. Predictions were compared with actual butterfly
 densities measured for up to 33 consecutive generations per site over 43
 years. The model suggested that individual sites vary by up to a
 hundred-fold in the density of butterflies each site would support.
 Observed densities closely correlated with model predictions, with the
 most productive site supporting 129-times more butterflies per square
 metre than the least productive one. Of seven life-table
 parameters modelled, the three that explained observed butterfly densities
 best describe the distribution and fitness of its primary host-ant
 species, Myrmica sabuleti. Population densities correlated most
 closely with the density of host-ant nests. Annual fluctuations
 in P. arion numbers attributable to the weather were one to two
 orders of magnitude lower than the differences between site carrying
 capacity densities. Even extreme events, such as drought, caused
 perturbations one order of magnitude smaller than inter-site carrying
 capacity variations. Synthesis and application. Observed
 population dynamics of P. arion conform with theories of a definable
 ceiling for insect numbers, controlled by density-dependence and limited
 by a larval resource that is typically more specialised, and rarer, within
 sites than was once perceived. At a practical level, the model provides a
 useful tool for determining which future sites should be prioritised for
 the persistence of this endangered species and for new restorations. More
 generally, the results support two concepts: (i) once suitable management
 has been implemented, effects of intrinsic site characteristics on numbers
 greatly exceed inter-annual fluctuations; and (ii) understanding how the
 quality of larval habitat varies between sites is paramount when restoring
 the size and resilience of populations of declining
 invertebrates.  
description: # Densities of the endangered Large blue butterfly Phengaris arion vary by
 100-fold in restored conservation grasslands, providing a tool to
 prioritise future introductions
 [https://doi.org/10.5061/dryad.4j0zpc8pc](https://doi.org/10.5061/dryad.4j0zpc8pc) ## Description of the data and file structure The Excel file tabulates data collected in the field on restoration sites for Phengaris arion, a social parasite as a larva of the ant Myrmica sabuleti. The rows refer to each of 19 sites. The columns data consist of 3 columns describing the fitness of the host ant Myrmica sabuleti on each site; site area and ant equivalents calculated as described in methods section of the article; and estimated and observed carrying capacity densities (K) for each site together with the standard errors and ln values of these.   ### Files and variables #### File: Thomas\_et\_al\_japplecol\_data.xlsx **Description:** field data used to calculate estimated and mean observed carrying capacities of 19 UK restoration sites for the butterfly* Phengaris arion*  ##### Variables * **SITE**: Refers to the study site. The sites are represented by: * **D**: Dartmoor * **C**: Cotswolds * **S**: Somerset * % host ant presence * ant 'nests’ m-2: The density of ant nests per square meter observed at each site. * mean host ant weight in milligrams * density of host ant nests per square metre of habitat * site area (square metres) * ant equivalent of each site  * estimated carrying capacity density (K) for Phengaris arion * observed carrying capacity density (K) for Phengaris arion * ln values and Standard Errors of K ## Access information Other publicly accessible locations of the data: * none Data was derived from the following sources: * the authors' fieldwork