Quantifying Change through Time in Spider Assemblages: Sampling Methods, Indices and Sources of Error

Norris, Kimberly

ABSTRACT: Using a sample dataset from six sites in the Southern Appalachian mountains in the United States, I evaluate the usefulness of diversity indices and similarity/difference indices for monitoring the changes in spider (Order Araneae) assemblages through time. The Shannon index, Brillouin index, Simpson’s index and Margalef’s index were correlated with each other and were able to detect successional changes in two old clear-cuts as well as disturbance to a Beech gap forest possibly due to Beech Bark Disease. Turnover, Bray-Curtis similarity, and Morisita-Horn similarity also detected successional differences between the forest types and indicated the short-term and long-term changes in a mature forest stand. Seasonal changes (early to late summer) in spider communities were consistently higher than yearly changes. I identify and discuss the implications of various sources of error that will adversely affect the accuracy and efficiency of a monitoring protocol: sensitivity of indices to sample size, sampler bias, inclusion of juveniles, taxonomic uncertainty and stochastic sampling effects. Although gross trends are still apparent in the sample data regardless of these errors, more subtle changes may be obscured. It is these subtle trends which may be more useful to managers attempting to identify disturbance before irreversible damage occurs.