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

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.