A decadal-scale qualitative comparison of lotic Odonata nymph surveys in northwestern Pennsylvania, USA, reveals possible assemblage restructuring

Abstract

Detecting long-term changes to Odonata assemblages requires repeated sampling at fixed locations over decadal periods. However, few such biomonitoring efforts exist for Odonata, especially at low taxonomic resolution and immature life stages. We repeated a survey of Odonata nymphs conducted nearly thirty years ago in ten streams from a northern Pennsylvania forest reserve to qualitatively explore assemblage-scale changes and identify streams supporting species of conservation concern. Our survey collected five more species among all sites than the original survey, reflecting a 15% increase in richness despite a reduced sampling effort. Rank abundances of many species changed substantially between surveys. Some, such as Calopteryx maculata, fell in rank while other species like Boyeria vinosa and Phanogomphus lividus that were rare in 1994 became the most numerically dominant. We did not detect patterns in species richness or changes in richness between surveys along gradients of unconventional natural gas extraction or timber harvest among streams. The fraction of species deemed vulnerable or imperiled within Pennsylvania grew from 22 to 38% between surveys, highlighting the regional conservation value of protected area we sampled. Our sampling methodology precludes population estimates or quantitative community comparisons due to the sampling design and inconsistencies in the literature used for identification between surveys. However, the substantial differences in rank-abundance among species between surveys suggests potential long-term changes in these lotic Odonata assemblages. The collective findings presented here emphasize the high value of data from repeated surveys with species-level taxonomic resolution.