It is clear that roads have definite effects on wildlife populations for a variety of reasons including habitat fragmentation, runoff, pollution, visual disturbance and increased mortality. Owing to the consistent and pervasive nature of noise and its apparent or at least potential widespread effects, it is clearly an area that needs to be addressed (see Forman and Alexander(43) Forman et al.(46) for reviews on this subject). Indeed in many cases it appears that noise may have a significant effect on both numbers of individuals, species diversity and breeding.
Invertebrates are too poorly studied at present for any definitive conclusions. Some significant use of roadside areas by some species (e.g. butterflies, bees) is indicated, but there are also many other species that should be investigated (particularly the aquatic species that may decline as road density changes). Although sparse, the studies that have looked at the response of fish would suggest that normal traffic noise would not be sufficiently great to disturb those species that have been looked at so far. Roads do provide a barrier to the movement of reptiles and amphibians; however the effect of noise is less clear. Recent work suggesting that vehicle noise can arouse toads from their burrows is of concern since this could affect survival and is one area that could be looked at in a series of controlled studies where sound levels and the associated behavioral response are more systematically studied.
The most comprehensive experimental studies on the subject (41, 96-100) demonstrate that many (although not all) species of small breeding birds in both grassland and forest habitats appear to avoid areas in proportion to the traffic noise and volume at distances up to three thousand meters. It is also important to note that the other studies that review an extensive number of species found some to be negatively affected by the presence of roads, but most species were neutral and a few species to increase in numbers presumably due to food or habitat provided by rights-of–way (2) (see also Appendix A). Further, several studies have found that roadside verges can provide breeding habitat for birds – however, without more information on the populations at greater distances from the road it is difficult to determine if the same effect reported in the Netherlands was also present. What these studies do suggest is that the situation may be more complex than roads simply providing a barrier to all breeding. As an illustration, the review by Way(126) records that (in Britain) roadsides have been recorded as breeding habitat for 20 of 50 mammal species, all 6 reptiles, 40 of 200 bird species, 25 of 60 butterfly species, 8 of 17 bumblebee species and 5 of 6 amphibian species. Road noise would appear an unlikely impediment to species that are able to successfully breed so close to the source (it should be noted that the numbers relative to adjacent areas would be important in indicating their relative importance and this information is not provided in this study). A summary of some of the major findings with respect to birds shows little, if any contradiction in results, rather some species are negatively effected and others occur more frequently nearer roads due factors such as prey availability or vegetation type (see Appendix A).
A further example of the complexity involved is shown by the study of Gutzwiller and Barrow(59) where a number of bird species densities were influenced by the presence and/or number of roads; however, a number of landscape factors including the amount of development and vegetation type were also found to be significant predictors in many of the models.