• biodiversity;
  • indicators;
  • Lepidoptera;
  • log-linear model;
  • statistical power;
  • UK butterfly monitoring scheme;
  • wider countryside


  • 1
    Butterflies are one of the few insect groups that can be monitored effectively and have the potential to develop national and Europe-wide trends in abundance.
  • 2
    For 20 widespread butterfly species, we assess the relative efficiency of reduced-effort schemes compared to the existing design and estimate the number of sites required to detect changes of given magnitudes over specified periods of time.
  • 3
    A scheme restricted to three counts during July and August requires twice as many monitored sites on average to achieve comparable precision to the existing 26-week scheme in the United Kingdom. Such a scheme requires 430 monitoring sites on average to achieve 80% power (5% significance level) for detecting a 25% decline in abundance over 10 years.
  • 4
    Such a reduced-effort scheme may also mean that volunteers are more willing to record in areas where they are likely to see only a few individuals of a few common species (such as on intensively farmed areas). This could potentially help to ensure that butterfly monitoring schemes achieve a more even geographical coverage and less of a bias towards areas rich in butterflies.
  • 5
    Synthesis and applications. Schemes with few sampling visits per year are cost-effective for expanding butterfly monitoring across Europe, and can be applied to national monitoring programmes and lead to effective assessment of continent-wide trends in populations.