Factors contributing to the decline of an endangered flightless longhorn beetle: A 20‐year study

World‐wide decreases of insect abundance and diversity are of major concern because of their importance for ecosystem functioning and the stability of ecosystems. Various studies reported dramatic declines of butterflies, wild bees and beetles in agricultural areas. Yet, evidence for decreasing abundance in cryptic insect species is scarce. Using a transect‐count technique, we monitored the relative population size of the endangered flightless grassland longhorn beetle Iberodorcardion fuliginator in 13 dry, semi‐natural grassland sites in the border region of Switzerland, France, and Germany at yearly intervals over 20 years (1999–2018). To disentangle potential causes for changes in I. fuliginator abundance over time, we recorded quantitatively the plant communities in all sites in 2004 and 2017 and changes in other habitat characteristics. We found that the overall abundance of I. fuliginator individuals decreased by 90% over 20 years: at one site the population went extinct, at five sites the populations were critically decreasing, at four sites the populations were decreasing and at only three sites population size remained stable. Linear models revealed that the factor ‘change in plant species composition’ is the main driver for the decrease in beetle abundance. Alternative models indicated that – in addition to vegetation changes – area of suitable habitat and low heat load affected the probability of decreasing population size. Our study shows that gradual habitat deterioration measured as reduction in grass cover and change in plant species composition negatively affect the abundance of the highly specialised beetle I. fuliginator.