Ground beetle (Coleoptera: Carabidae) population declines and phenological changes: Is there a connection?

Long-term monitoring data were analyzed to reveal correlations between declining ground beetle (Coleoptera: Carabidae) populations and phenological changes at two Environmental Change Network sites in Scotland. The potential role of advancing phenology as an adaptation function in population stability was investigated. Analysis focussed on the 25 most abundant species over an 18 year sampling period. Pitfall trap catches were used to calculate mean activity-densities both for the whole sampling period and for dates limited to those within the activity period. Several phenological measurements were calculated (i.e. first day of appearance, peak activity date, median activity, length of activity and winter inactivity periods, and the last day of presence) for each species. Robust non-parametric estimation was used to model changes in both activity density and phenology. Eight species declined in activity density over time, three increased and fourteen showed no change. The mean rate of decline was greater than that of increase. Most of the species included in the analysis changed their phenology. Advancing onset of activity and earlier cessation were the most pronounced changes. However, a slow advancing trend in the peak activity was also shown. Only Nebria brevicollis, an autumn species with recorded winter activity, extended its activity period to later dates, suggesting that cessation of activity for the remaining species may be more closely linked to photoperiod. The earlier termination of activity shortened substantially the activity window for several species. Declines in activity density showed a strong relationship with a narrowing window of activity, mainly caused by earlier cessation of activity. Declining species were found more in bog or dry heather moorland habitats compared to grassland, emphasizing the vulnerability of these vegetation types, and associated insect assemblages depending on them, to environmental or climatic changes. The reciprocal relationship found between the trend of timing of initiation of activity and changes in activity-densities suggests that populations with a higher capacity to advance their phenology are less prone to decline. Since phenological changes may drive changes in populations, investigating phenological variables is encouraged in both research and conservation planning. (C) 2014 Elsevier Ltd. All rights reserved.