Event Category

Exit Seminar

Date

July 20, 2026

Time

12:00 pm

Location

Ramsay Wright Laboratories, RW 432 & online

Urbanization has pervasive effects on freshwater ecosystems, changing hydrology, nutrient dynamics, and habitat structure. Yet its influence on ecological dynamics across natural and constructed habitats is not fully understood. Pebesma’s thesis examines how urbanization impacts stream ecosystems and investigates stormwater management ponds (SMPs) as both novel habitats and drivers of downstream change.

Across four research chapters, Pebesma investigates diversity, function, and stability of benthic macroinvertebrate communities in natural and created habitats. The author shows that urbanization fundamentally restructures longitudinal diversity and feeding patterns in streams, causing deviations from classic predictions of the River Continuum Concept. Taxonomic diversity no longer increases with stream order, and functional feeding groups exhibit altered patterns, suggesting shifts in resource dynamics and ecosystem processes.

Pebesma further demonstrates that community stability of urban streams is structured by impervious cover and precipitation, with urban streams exhibiting increased stability in dry years but decreased stability in wet years. Less urbanized streams, on the other hand, show comparatively little variation. These results highlight how urbanization modifies the effects of climatic disturbances on aquatic systems.

Contrasting with altered stream dynamics, SMPs supported diverse macroinvertebrate communities, with community assembly processes that are consistent with natural lentic systems. The importance of dispersal limitation and environmental filtering varies across dispersal traits, and Pebesma observed evidence of biotic filtering through fish influencing both abiotic conditions and predatory invertebrate abundance. These findings suggest that SMPs can function as ecologically meaningful habitats within urban landscapes.

Despite concerns about downstream impacts, SMPs showed limited effects on diversity, functional composition, and water quality at both local and cumulative scales, although modest increases in filter feeders and shifts in community composition were observed downstream.

Collectively, these findings demonstrate how urbanization fundamentally restructures ecological dynamics in natural streams and creates novel habitats that can support complex communities. This work highlights the potential for meaningful biodiversity in urban habitats and advances conceptual frameworks for urban aquatic ecology, providing insight into how monitoring and management strategies can better understand changing ecological dynamics in an increasingly urbanized world.

Supervisor: Benjamin Gilbert

Exam chair: Russell Schachar