Date of Thesis



Emerging aquatic insects provide important cross-ecosystem subsidies to riparian ecosystems and elicit responses in riparian predators. Abundances and types of emerging insects change as streams increase in size from headwater streams to large rivers, changing subsidy dynamics along a river continuum. Flux of emerging insects into riparian zones is determined by a combination of secondary production per unit area of stream surface and stream width, while which taxa emerge at a particular location depends on position along the river continuum as predicted by the river continuum concept. Predators in riparian zones along streams take advantage of this flux of insects to subsidize their diets. Riparian spider communities tend to be particularly influenced by emergence events due to their various feeding strategies and ability to track preferred prey and are generally observed in high densities where aquatic subsidies are high. Similarly, riparian beetle densities tend to increase with the flux of aquatic insects. However, little work has been conducted on the relationship between invertebrate predator densities and aquatic subsidies along a large stream-size gradient. We hypothesized that 1) predator abundance and biomass would correlate with aquatic insect emergence and 2) spiders using different hunting strategies will be distributed in a predictable manner along the river continuum based on availability of certain types of prey, i.e. ground hunting spiders would be more abundant where ground dwelling prey is abundant. We collected riparian spiders, beetles and emerging insects from 1st-7th order streams in three different sub-basins of the Susquehanna River. We measured emergence rates (emerging individuals/m2 of stream surface) and calculated flux rates (individuals entering 1m length of riparian zone), as well as predator densities. Average emergence rates were not significantly different among stream orders due to high variation among sites of the same order, but emergence flux increased with stream order and was highest at 7th order sites. Arboreal spider and ground beetle densities were positively related to emergence flux and reached highest densities at larger stream orders while ground spiders showed no significant pattern with emergence flux or stream size. Horizontal orb weavers were the only spider group to show significant differences in density and biomass among stream orders, reaching highest densities and biomass at higher order sites. Densities of horizontal orb weavers were related to emergence rates, while other predator groups were related to emergence flux, potentially highlighting prey preferences of riparian predators. Our results indicate that riparian predator abundances are highly influenced by the flux of emerging insects, both of which increased with river order. However, certain predators might respond to differences in prey type and abundance, creating a continuum of riparian predators reflecting changes in aquatic emergence.


Cross ecosystem subsidies, Spider, Beetle, Predator, River continuum, Stream size gradient, Aquatic insect, Riparian

Access Type

Masters Thesis (Bucknell Access Only)

Degree Type

Master of Science



First Advisor

Matthew E. McTammany