Ground beetle (Coleoptera: Carabidae) population declines and phenological changes: Is there a connection?
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Carabid beetles are important components of terrestrial food webs, and changes in their populations may affect other species in an ecosystem (such as the prey species that carabids feed on). Significant decreases in the abundance of several carabid species have been found in this and in other studies.
The authors of this paper investigated changes in the abundance and phenology (i.e. the timing of certain life-cycle events) of carabid species. Well-known examples of phenology include the timing of tree bud opening, the arrival of migratory species, and the timing of frog spawning. Temperature is commonly a controlling factor, and recent climate change has already resulted in phenological changes in some species.
Many species are dependent on other species and their phenological patterns have evolved to be tightly synchronised. When timings shift in response to changes in the environment, mismatches may have a negative impact. For example, a prey species may become less abundant, adversely affecting predator survival. Impacts may be exacerbated by other changes, such as loss of suitable habitat.
To investigate changes in carabid abundance and phenology, the authors used ECN data for the 25 most abundant carabid beetles, collected over 18 years (1994-2011) from lines of pitfall traps at two ECN Scottish ECN sites, Sourhope and Glensaugh.
They found that eight of the 25 species decreased significantly in number between 1994 and 2011. One species, Pterostichus adstrictus, almost completely disappeared from the samples by 2011, despite being recorded in large numbers in the early years. The populations of thirteen species at the sites were regarded as being stable, whilst four species were captured more frequently over time. The general decline in ground beetles is in line with findings at other ECN sites (see Morecroft et al., 2009 and Brooks et al., 2012). There is some evidence from this study that beetle species favouring bog and dry heather moorland declined more than those that favour grassland habitats.
The authors found significant changes in some of the phenological measures they considered. For example, the first day of appearance advanced for seven species and became later for two species. The length of the activity period decreased for four species and increased for two species. Overall, 10 species advanced their activity, as measured by at least one of the phenological measures. Similar advances have been reported for plants, fish and birds. The authors suggest that, at least in the case of carabid beetles, this advancement of activity may be a result of a tendency towards milder winters.
Determining the precise cause of changes in phenology is difficult, since there are many factors to consider. For example, some species have two or more distinct periods of activity. Some species hibernate in the winter while others remain active. Temperature, day length, body size and the type and abundance of prey species all influence the timing of activity, and vary from species to species.
Declining species were found more in heathland habitats (a grouping of bog and dry heather moorland habitats) compared with grassland, emphasizing the vulnerability of heathland vegetation types, and associated insect assemblages depending on them, to environmental or climatic changes.
To answer the question in the title of this paper, is there a connection between carabid beetle population declines and phenological changes, the answer appears to be yes. Earlier initiation of activity appeared to be advantageous for some species, whilst declines in abundance showed a strong relationship with a narrowing window of activity, mainly caused by earlier cessation of activity.