Monitoring global biodiversity is essential for understanding and countering its current loss. However, monitoring of many species is hindered by their difficult detection due to crepuscular activity, hidden phases of the life cycle, short activity period and low population density. Few statistical power analyses of declining trends have been published for terrestrial invertebrates. Consequently, no knowledge exists of the success rate of monitoring elusive invertebrates. Here data from monitoring transects of the European stag beetle, Lucanus cervus, is used to investigate whether the population trend of this elusive species can be adequately monitored. Data from studies in UK, Switzerland and Germany were compiled to parameterize a simulation model explaining the stag beetle abundance as a function of temperature and seasonality. A Monte-Carlo simulation was used to evaluate the effort needed to detect a population abundance decline of 1%/year over a period of 12 years. To reveal such a decline, at least 240 1-hour transect walks on 40 to 100 transects need to be implemented in weekly intervals during warm evenings. It is concluded that monitoring of stag beetles is feasible and the effort is not greater than that which has been found for other invertebrates. Based on this example, it is assumed that many other elusive species with similar life history traits can be monitored with moderate efforts. As saproxylic invertebrates account for a large share of the forest biodiversity, although many are elusive, it is proposed that at least some flagship species are included in monitoring programmes.

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Conservation of the threatened and protected European stag beetle (Lucanus cervus) mainly focuses on the availability of dead wood as larval habitat. However, as the larval ecology of this species remains poorly studied, less attention has been given to other habitat requirements such as ambient temperatures for the development of the larvae. To design proper guidelines for the preservation of this iconic species, the growth of stag beetle larvae is compared between outdoor containers under a warm sunny and those in a cold shady treatment. Populations originated from the Veluwe (Netherlands) and Colchester (United Kingdom). The shady microclimate led to lower temperatures, which resulted in higher larval weights before molting and an additional year to complete larval development for a part of the population. However, weights were lower than in the warm microclimate when comparing larvae from the same age. This is explained by the longer development time for the different stages. Finally, we found higher larval weights for larvae originating from the Veluwe (Netherlands) compared with larvae from Colchester (UK). We conclude that larvae of L. cervus can cope with shady, colder microclimates. Larval development time in general takes two to three years and depends on temperature. Larvae grow mainly in the warm season while weight remains constant, or even decreases, during the cold season with an approximate threshold between 10 to 15 C.

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