The potential for elevated soil salinity to enhance the ecological trap effect of roadside pollinator habitat

The management of roadside habitat may be an important component of the conservation of declining pollinators, but insect mortality along roadsides can be high, creating a potential ecological trap. Runoff from winter applications of deicing salt can increase soil salinity along roadsides, and if this results in higher sodium levels in floral nectar, it could increase the attractiveness of roadside plants to pollinators, possibly accentuating the ecological trap effect. We compared the soil salinity in a highway median to soil collected from nearby fields and compared the Na+ concentrations in nectar from common milkweed (Asclepias syriaca) from these two habitats. We also manipulated soil salinity in field populations of A. syriaca by adding brine solution to document changes in nectar Na+ and pollinator visitation. We found that soil salinity in the median of a highway in Clarke County, VA was 2.3 times higher than the salinity of field soil, however nectar from A. syriaca in the two habitats did not differ significantly in Na+ concentration. Addition of brine to field soil increased soil salinity 17-fold and significantly increased the level of Na+ in A. syriaca nectar. Pollinators visited A. syriaca umbels that received the brine treatment twice as frequently as controls. Implications for insect conservation: We conclude that elevated soil salinity is capable of increasing nectar Na+ and the attractiveness of A. syriaca flowers to insect pollinators, but roadside soil salinity in our study was not high enough to enhance an ecological trap effect of roadside vegetation.

Automated summary

Management of roadside habitat is crucial for the conservation of declining pollinators, but insect mortality along roadsides can create an ecological trap. Runoff from deicing salt can increase soil salinity and nectar attractiveness, potentially exacerbating the ecological trap effect. However, the roadside soil salinity in this study did not reach the level to enhance the ecological trap effect.