Reduced-effort schemes for monitoring butterfly populations.
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. 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. 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. 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. 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.