Host specificity and species colouration mediate the regional decline of nocturnal moths in central European forests

The high diversity of insects has limited the volume of long-term
community data with a high taxonomic resolution and considerable
geographic replications, especially in forests. Therefore, trends and
causes of changes are poorly understood. Here we analyse trends in species
richness, abundance and biomass of nocturnal macro moths in three
quantitative data sets collected over four decades in forests in southern
Germany. Two local data sets, one from coppiced oak forests and one from
high oak forests included 125K and 48K specimens from 559 and 532 species,
respectively. A third regional data set, representing all forest types in
the temperate zone of Central Europe comprised 735K specimens from 848
species. Generalized additive mixed models revealed temporal declines in
species richness (-38 %), abundance (-53 %) and biomass (-57 %) at the
regional scale. These were more pronounced in plant host specialists and
in dark coloured species. In contrast, the local coppiced oak forests
showed an increase, in species richness (+62 %), while the high oak
forests showed no clear trends. Left and right censoring as well as cross
validation confirmed the robustness of the analyses, which led to four
conclusions. First, the decline in insects appears in hyper diverse insect
groups in forests and affects species richness, abundance and biomass.
Second, the pronounced decline in host specialists suggests habitat loss
as an important driver of the observed decline. Third, the more severe
decline in dark species might be an indication of global warming as a
potential driver. Fourth, the trends in coppiced oak forests indicate that
maintaining complex and diverse forest ecosystems through active
management may be a promising conservation strategy in order to counteract
negative trends in biodiversity, alongside rewilding approaches.