Results were complex and illustrate that bat activity and species diversity are impacted by a wide variety of factors, with structural and contextual characteristics having the greatest impact. Best fitting models also mainly consisted of many variables with each one making a small contribution instead of one variable contributing the most. These models successfully show the habitat variables at multiple scales that are most associated with bat species richness and average activity. The species richness models had a higher AICc and lower R2 than the activity model, so the activity models may be better predictors of what makes high quality bat habitat, although both are worth considering when managing for bats.
Scale was one factor causing variability in results. Variables that were important at one scale were often not important at another. For instance, variables that were strongly associated with bat activity or species richness at 100 m were often not associated with it at 500/250 m (e. g. perennial ponds), and vice versa. This supports previous findings that it is important to use multiple spatial scales (Gallo et al. 2017).
There were consistent hotspots of bat activity along transects between years. These hotspots were mainly located near Metroparks, which indicates the importance of protected parks. It is unclear why so many were clustered in the northern region, but this may be related to the northern region having less cropland cover. Cold spots with no activity, while not consistent between years, were located in the southern region where there was heavy cropland cover. If cropland avoidance is influencing the location of hot and cold spots of activity, it is consistent with other findings along transects. Hotspot analysis was shown to be a reliable method for determining areas of highest activity across years.
Bats avoided areas with higher amounts of cropland along transects. Species richness and activity were lower when at least one side of the road had agricultural habitat unless the other side was natural and cropland was negatively associated with bat activity and species richness along transects. Previous studies also found that bats generally avoided areas with high agricultural cover (Blakey et al. 2017; Put et al. 2019). Most of the agriculture in the region consists of large row crop monocultures. Previous research shows that increasing farmland heterogeneity or using organic farming methods is beneficial to bat activity and diversity, so if possible, encouraging local farms to diversify their crops, plant tree rows, or use organic methods could help bats (Monck-Whipp 2018; Wickramasinghe et al. 2003).
While activity and species richness were lower when there was agricultural habitat on one side of the road, they were higher with natural habitat on one side of the road compared to points with only agricultural or residential habitat. Points with natural habitat on both sides did not have significantly higher activity or richness than those with natural habitat on only one side. Retaining natural field or forest habitat alongside the road on at least one side would be beneficial in managing for ideal bat habitat. It is worth noting that the number of bats killed by cars is also higher in these areas of high-quality habitat, so they could serve as ecological traps unless high speed traffic is limited in areas of especially high bat activity (Medinas et al. 2012; Russo and Ancillotto 2015). Also, landscape scale results showed that types of natural habitat didn’t impact bats equally.
Open habitats such as savanna and upland prairie were positively associated with activity and/or species along transects. Savanna was positively associated with both species richness and activity at transect points. Buckman-Sewald (2014) found higher activity of hoary bats, one of the most common species along transects, in savanna habitats. This may be contributing to higher activity in areas of more savanna cover along transects. The lack of clutter from tall trees in shrubs in these habitats likely make them easier for bats to forage in, especially for open adapted species such as hoary bats. While savanna habitats have increased in cover in recent years due to restoration efforts (Martin and Root 2020), additional restoration may be beneficial to bats.
Floodplain forests in larger size buffers was negatively associated with activity along transects. However, Blakey et al. (2017) found higher bat activity in floodplain forest. This could mean that the impact of floodplain forests is somewhat variable. Higher percent conifers in larger size buffers were also associated with lower activity and species richness at transect points. Previous studies have found that bats utilized conifer stands less or not at all (Yoshikura et al 2011). Many of the conifer forests in the Oak Openings region are non-native commercial pine stands that generally less diverse than surrounding forests (Abella 2010; Abella et al. 2017). Previous studies have found increased plant and wildlife diversity when removing or thinning these pine stands in the region, with additional efforts to remove and convert this land to early successional cover underway (Abella 2010; Abella et al. 2017; Martin and Root 2020). My results suggest that removing or thinning these pine stands could be beneficial for bats, as they generally avoid areas with large blocks of this habitat type. While forest cover is usually positively associated with bat activity, the presence of these specific types seems to be detrimental for bats. This could be related to low forest quality for these types or that the most common bats around road transects are open-foraging species.
Total percent forest was positively associated with activity along transects. This indicates that while specific forest types were detrimental to bats, overall forest cover still seems to have a positive impact along roads. Previous studies also found a significant positive impact of total forest cover along roads and developed areas, although there is some variability based on species (Evelyn et al. 2004; Medinas et al. 2019). There was a positive association between canopy height and species richness and between average percent canopy cover and total activity. Previous studies have also found positive associations between canopy height and cover and bat activity, especially along roads (Bader et al. 2015; Bailey et al. 2019; Jung et al. 2012; Russell et al. 2009). This combined with the forest cover results indicates that planting trees along roads could be beneficial for bats. Other structural variables, such as clutter, were also important.
For clutter, whether the impact was positive or negative depended on the height level. There were negative associations between clutter at 3-6.5 m and species richness and activity along transects. This supports previous findings that bats avoid areas of higher vegetation clutter and taller understory cover (Adams et al. 2009; Campbell et al. 1996; Lintott et al. 2015; Rainho et al. 2010). However, there were positive associations with clutter 0–3 m for the open guild at stationary points and for species richness and activity along transects. This is likely because most open habitats had higher clutter at the lower height level and less clutter at the higher height level, which is ideal for open foraging species, the most common group along transects. These results suggest that it would be advantageous for open foraging bats for land managers to reduce clutter at the 3-6.5 m level, while increasing low level vegetation along roads.
Water is another resource that should be increased along roads. Higher percent of ponds in 100 m buffers was significantly correlated with higher species richness along transects. Combined with the positive association between months water observed at transect and species richness, this indicates that water is especially important to a diverse bat community along roads. It is worth noting that number of months where water was recorded was only recorded in 2021, while previous years whether ditches were present was only noted once a season (if ditches were present once, they were noted as present all month for those years), so that may have impacted results. This is consistent with past findings (Gaisler et al. 1998).
Meters of roads were positively associated with bat activity along transects, indicating that roads and fragmentation can have a positive impact on bat activity rather than negative, albeit only for open-foraging species that forage along habitat edges. The importance of forest, water, and roads along roads was consistent with past findings (Gaisler et al. 1998; Myczko et al. 2017). The number of habitat types was positively associated with species richness and activity along roads, which indicates that fragmentation and habitat heterogeneity is positively associated with bat activity along roads. This may be because bat species foraging along roads tend to be more fragmentation tolerant and prefer to forage along habitat edges.
Northern long-eared bats were never found along transects and little-brown bats and tri-colored bats very rarely were. These species were located more frequently, although they were still rarer than other species, during surveys in protected parks in the same region during the same period (Russo-Petrick 2022). All three of these species are of conservation concern, so this indicates that roadside habitats are not preferred by these species. While some variables were consistently important, the variety in results and that no one variable had an especially large contribution to models shows how complicated the question of what habitat factors impact bats is and how much results can vary when multiple scales are considered.