Motorways change grasshopper and cricket assemblages in a Croatian grassland
Written By Fran Rebrina, Klaus Reinhold, Nikola Tvrtković and Andreja Brigić
29th April 2022
Terrestrial invertebrates, including insects, are still largely neglected in sustainable road planning, despite growing evidence of the negative effects of roads on this crucial component of terrestrial ecosystems. In particular, assemblage-level responses of insects to road proximity could provide a good indication of the extent to which major roads affect ecosystem processes (i.e the road-effect zone). As one of the best-studied sound-communicating insect groups, grasshoppers and crickets (orthopterans) have been considered in road-ecology research primarily as model organisms for studying acoustic interference by traffic noise. Species-specific responses to noise exposure and/or road mortality are likely to have implications at the assemblage level, but no research has investigated this so far. Accordingly, we were interested whether and how orthopteran assemblages change with distance from a motorway in a grassland habitat, and which road-influenced environmental factors are mainly responsible for the observed changes.
Sampling orthopterans along the largest Croatian motorway
We conducted our study along the A1 motorway (part of the European route E71) in Ličko Polje karst field, Croatia, in eight locations within unmanaged grasslands on the northern side of the road. Each location included five sampling sites at different distances from the motorway: 10, 25, 50, 100 and 500 m. Orthopterans were sampled at each site once a month from June to October, using pitfall trapping and sweep-netting. During these visits, we also measured six environmental factors at each site: air temperature and humidity, soil temperature and moisture, vegetation height, and ambient noise level. We calculated orthopteran abundance, species richness, taxonomic diversity, and conservation value at each site. We also calculated abundance and species richness for two groups of species that communicate acoustically using low-frequency (< 10 kHz) or high-frequency signals (> 10 kHz), respectively. Since low-frequency acoustic signals are more prone to being masked by traffic noise, these two groups likely have a different sensitivity to traffic noise.
Figure 1. (A) Map of the study area (rectangle) with sampling locations indicated by red dots; (B-F) Sampling sites at (B) 10 m, (C) 25 m, (D) 50 m, (E) 100 m, and (F) 500 m distance from the A1 motorway within a grassland habitat in Ličko polje karst field, Croatia. Photos taken at sampling location 2.
Orthopteran assemblages show distance-dependent responses to a motorway
Our results demonstrate, for the first time, that orthopteran assemblages change with distance from a major road. We found that abundance, species richness, taxonomic diversity and, consequently, the conservation value of orthopterans show a significant decrease at the sites closest to the motorway. This response, however, was observed only in the assemblages sampled by pitfall traps and was primarily associated with the group that communicates using low-frequency signals. Thus, it seems that detectable negative impacts of the investigated motorway are confined to a narrow zone between 10 m and 25 m from the road and mainly affect ground-dwelling species producing low-frequency signals. In contrast, orthopteran species richness, diversity, and conservation value peaked at 25 m distance from the motorway, again only in the assemblages sampled by pitfall traps. These results show that, in the investigated grassland ecosystem, the orthopteran responses to road proximity (1) depend on distance to the road and (2) are more efficiently detected by pitfall trapping than by sweep-netting. This could be the result of either the difference in sampling effort (one-shot vs. continuous) or lower efficiency of sweep-netting in sampling larger, primarily ground-dwelling but potentially road-affected species.
Figure 2. Differences in orthopteran assemblages sampled by pitfall traps at different distances from the motorway: (A) abundance; (B) species richness; (C) true diversity (Shannon index); (D) true diversity (Simpson index); (E) conservation value (Grasshopper Conservation Index). Different letters indicate statistically significant differences among estimated means (p < 0.05).
What are the environmental drivers of orthopteran assemblage responses?
We found that the motorway influenced orthopteran assemblages primarily through changes in vegetation height, akin to the effects of vegetation succession. Increased presence of shrubs and ruderal vegetation in the roadsides could account for more diverse assemblages at 25 m from the motorway, allowing for the occurrence of both typical grassland and (open) shrubland species. On the other hand, negative influences of traffic noise and/or road mortality may at least partly explain the decrease in most assemblage parameters within 10-25 m from the road. Larger ground-dwelling orthopterans using low-frequency signals are potentially highly vulnerable to acoustic interference at this distance, but may also avoid roadside sites due to low soil moisture associated with skeletal soil.
Implications for roadside management
The results of the present study show that appropriate vegetation management, maintaining native shrub vegetation of sufficient height and density in addition to naturally occurring grassland vegetation, could increase the conservation value of roadsides for orthopterans in grassland ecosystems. However, negative impacts of traffic noise and/or road mortality on orthopterans, while occurring at a relatively small spatial scale, still emphasize the need for adopting noise and mortality reduction measures to make road verges suitable habitats for species producing low-frequency signals.
Author information:
Fran Rebrina (e-mail: rebrinafran@gmail.com)
Klaus Reinhold (e-mail: klaus.reinhold@uni-bielefeld.de)
Nikola Tvrtković (e-mail: nikolatvrtkovic71@gmail.com)
Andreja Brigić (e-mail: andreja.brigic@biol.pmf.hr)
Source:
Rebrina F., Reinhold K., Tvrtković N., Brigić A., 2021. Motorway proximity affects spatial dynamics of orthopteran assemblages in a grassland ecosystem. Insect Conservation and Diversity, Early Access. DOI: https://doi.org/10.1111/icad.12546
Source of sound files: DORSA (Digitized Orthoptera Specimens Access; (https://www.dorsa.de/) and Orthoptera Species File (http://orthoptera.speciesfile.org/HomePage/Orthoptera/HomePage.aspx)
Editor:
Jochen Jaeger and Rodney van der Ree
Cite this summary:
Rebrina F., Reinhold K., Tvrtković N., Brigić A. (2022). Motorways change grasshopper and cricket assemblages in a Croatian grassland. Edited by Jaeger, J. and van der Ree, R. TransportEcology.info, Accessed at: https://transportecology.info/research/grasshopper-cricket-assemblages [Date accessed].
