The response of ground-dwelling spiders (Araneae) and hoverflies (Diptera: Syrphidae) to afforestation assessed using within site tracking

Incentive schemes which provide grants and annual premiums for establishing forests on private land mean the majority of recent afforestation in Ireland has taken place in agricultural grasslands. This study is the first site level assessment of the change in diversity of two important arthropod groups as a result of afforestation. Five agricultural grassland sites were sampled one year before afforestation and seven years after afforestation using pitfall traps for active ground-dwelling spiders and Malaise traps for hoverflies. At each study site open grassland habitats were sampled for both species groups and additional sampling for ground-dwelling spiders only was conducted in hedgerow habitats. A positive effect of afforestation on ground-dwelling spider diversity was observed over time. Total species richness increased in open and hedgerow habitats and forest specialist species richness also increased in open habitats. This was attributed to increased canopy cover and changes in structural layers of the habitat, including increased cover of field layer vegetation and litter. There was no effect of afforestation on hoverfly species richness over time and a mixture of open associated, woody vegetation associated and water-associated species were present both pre- and post-planting. This lack of an obvious effect may be due to the confounding effects of hoverfly movements across landscapes. These results indicate that afforestation in a predominantly agricultural landscape is likely to increase arthropod biodiversity across the landscape by increasing the number and type of available habitats. However it is important to also consider habitat quality. Protecting and promoting aquatic zones, broadleaf trees and shrubs, glades and rides, and flowering plants will enhance spider and hoverfly diversity in afforested areas. The current trend of planting on private lands provides an important opportunity to plant native tree species and restore forest types lost through anthropogenic activity.