Spiders on lowland heath: Functional diversity and abundance in relation to heathland structure

Kelly, Caroline (2023) Spiders on lowland heath: Functional diversity and abundance in relation to heathland structure.

A dissertation submitted in partial fulfilment of the requirements of the degree Master by Research (MRes)Habitat loss and fragmentation has had negative effects on biodiversity worldwide. The Dorset Heathlands, despite historical changes and drastic losses, are home to some of the rarest species in the UK. The dominant plants, ericaceous shrubs, vary in structure according to age, creating a heterogeneous landscape of different habitats. Conservation and restoration management can influence patterns of growth stages which can alter species composition and diversity. The diversity of different functional traits of species can be defined by their activity, which is related to their use of resources. The overall aim of this study examines the influence of heathland vegetation structure and habitat type on the abundance and functional diversity of spiders. Sampling was conducted within the Purbeck Heaths National Nature Reserve (PHNNR) over 30 sites (10 dry heath, 10 restored dry heath and 10 wet heath); each sample site measured 20m x 20m. A combination of pitfall traps, sweep netting and vacuum methods were undertaken fortnightly over two months (May and June 2023). Vegetation structure data were also gathered by cover estimates and the use of drop disc to measure the vegetation height within sites. Including juveniles, a total of 1,661 individual spiders were collected across all three methods from 66 species across 18 different families, with 835 mature individuals across 16 families used in the analyses. The abundance of individual spider species with different functional traits, and diversity, were compared across the different habitat types and related to vegetation structure, using contingency tables and chi-square tests, ANOVA with post hoc tukey, generalised linear models, and Multimodel inference analysis in R. Results indicated that the wet heaths were significantly different from the dry and restored heaths. Wet heaths had higher percentage cover of pioneer heather, whilst restored dry heaths contained a higher percentage cover of building heather as well as all shrub species and the dry heath had higher percentage cover of bare ground, mature heather, litter layer and moss lichen layer. The wet heaths had higher abundance and species richness of spiders and were more functionally diverse. The restored heaths had significantly higher frequency and abundance of the traits ground dwellers, nocturnal species and those with preference of living on the ground than the dry habitat. In addition, the wet heaths e had significantly more of the orb web weavers and those that build an orb web trait than the dry heaths. The most significant variables influencing the frequency of spider functional traits and species were vegetation height and the habitat type of wet heath. The functional traits ground hunter, capture of prey through pursuit, no use of a web, nocturnal and ground dwellers had highest abundance in the study and the most significant inverse relationship with vegetation height. The wet heath had significantly more frequent traits of the tangled cribellate web builders, cathemeral species and those that preferred to live in vegetation than were in the restored dry heath. The most abundant spiders were Pardosa pullata, predominantly recorded on the wet heath, and P. nigriceps on the dry. The wet habitat type was significantly different from the dry habitat for species Pardosa pullata, Mangora acalypha and Drassyllus praeficus. The wet habitat type was significantly different from the restored dry habitat for the species Dysdera erythrina and Dictyna arundinacea. Overall, the results suggest that the current heathland structure, particularly the vegetation height, influences the diversity and abundance of spider functional traits, generating additional evidence to support the importance of heterogeneity and the mosaic theory of heathland management. Further research is required across the wider PHNNR, over a longer sampling period to include additional sampling methods further to the understanding of how conservation and restoration efforts affect biodiversity as a whole.