Many studies have recorded the effects of roads on the ecology of the surrounding areas, also known as road-effect zones. In terms of animal impacts, most of these studies focus on vertebrates (Muňoz et al. 2015), but a number of authors have also studied invertebrates (Muňoz et al. 2015), with mixed results. Many studies of road effects on insects focus on insect mortality due to vehicle collision; as well as the effects of pollutants, such as exhaust gases and de-icing salt (Muňoz et al. 2015). A third pathway of impact is via the road construction itself (Muňoz et al. 2015). We deemed these three pathways as unlikely to have played a role in our study site, as the road in Telperion incurs minimal traffic and was not constructed recently.
A fourth mechanism via which roads affect insects is by acting as barriers to dispersal (e.g., Haskell 2000; Bhattacharya et al. 2003, Fitch and Vaidya 2021) and thereby fragmenting habitats. This is the most likely mechanism via which we believed the road at Telperion could affect invertebrate abundance and formed the impetus of our study. Beetles in particular have often been documented to struggle crossing roads (Koivula and Vermeulen 2005; Noordijk et al. 2006; Yamada et al. 2010). Fragmentation can also affect beetle abundance by changing the composition of edge and interior species near road sides (Knapp et al. 2013). In contrast to these studies, we found no significant change in beetle numbers with distance from road. Similarly, although mean ant numbers seemed to increase with distance from road, this was mostly the result of two exceptionally high samples, one at ten and the other at twenty meters, and ant numbers were therefore not signaficantly affected by distance from road. The road at Telperion is not very wide, with relatively low vehicle traffic. Although road effects on invertebrate movement (Fitch and Vaidya 2021), abundance (Luce and Crowe 2001) and composition (Melis et al. 2009) have been recorded on relatively narrow or quiet roads and paths, such as ours, the severity of road impacts often depends on the characteristics of the road, with, busier roads having larger impacts (Dunn and Danoff-Burg 2007; Fitch and Vaidya 2021). This could explain the absence of an effect for ants and beetles in our study.
Although the majority of studies suggest that roads affect invertebrates negatively, there are also studies that suggest negligible (Bhardwaj et al. 2019; Spiess et al. 2020), mixed (Ste-Marie et al. 2018) or even positive (Itzhak 2008; Melis et al. 2009) road effects on invertebrates. For example, Samways et al. (1997) attributed the high ant abundance, richness and diversity at a busy South African highway to roadkills, which attract ants. Interestingly, in our study, Hemiptera numbers increased closer to the road, implying a positive road effect for this order. There are relatively few relevant studies on Hemiptera specifically, as most studies either focus on insects in general, or on the more charasmatic orders, such as Odonata, Lepidoptera and Coleoptera (Muňoz et al. 2015). However, Luce and Crowe (2001) also found increased abundance for a number of orders, including Hemiptera, closer to a quiet gravel road in Canada. A number of studies have also shown that aphid numbers, specifically, are higher next to roads (Przybylski 1979; Braun and Flückiger 1984, 1985; Martinez and Wool 2006; Cohen et al. 2021). In fact, Przybylski (1979) describes aphids and members of Heteroptera, both groups within the Hemiptera order, as “flourishing” close to roads (Przybylski 1979, page 161). This has been attributed to the redistribution of water – and as a consequence vegetation – next to roads (Cohen et al. 2021), biochemical changes within plants closer to roads (Przybylski 1979; Braun and Flückiger 1984, 1985), changes to interspecific competition next to roadsides (Samways et al. 1997), as well as reduced predator efficiency next to roads (Przybylski 1979; Braun and Flückiger 1984, 1985). In general, disturbance-tolerant species tend to dominate roadsides. We were not able to identify samples to species level in this study. Had we been able to key individuals to species, or even family level, we may have been able to shed light on the reasons behind our own Hemiptera findings. However, our results are in general alignment with these previous studies and support this notion that roadsides may be favourable habitats for species within the Hemiptera order.
In conclusion, the results presented here suggest that relatively quiet dirt roads do not affect insect abundance of the orders Hymenoptera or Coleoptera, but increase abundance within the order Hemiptera. This was contrary to our initial expectations, but in line with previous studies. Although our study is a relatively small case study, it contributes toward necessary research on the most diverse, but understudied group of animals within roadside ecology – the insects.