Thesis Defense: Colin Martin

Title: Seasonal Variability Emerges as a Driver of Stream Biodiversity

Program: Biology MS

Committee Chair: Jodi Brandt

Committee: Jodi Brandt, Trevor Caughlin, Jen Cruz, Charles Hawkins

Abstract: Humans have dramatically changed seasonal variability across many ecosystems. Seasonal variability
may increase biodiversity by generating heterogeneous habitats, yet also reduce aquatic diversity by
intensifying water limitation and habitat contraction. Using nine years of seasonal remote-sensing data
paired with 1,718 stream surveys, we tested whether macroinvertebrate biodiversity in drylands
responds more strongly to water availability or seasonal variability. Seasonal variability had a larger
main effect on macroinvertebrate communities than water availability, and was comparable to
established environmental drivers, including pH and elevation. Water availability and seasonal variability
interacted: seasonal variability increased richness most where water was scarce and had little effect
where water was abundant. Taxa responded uniquely to water availability and seasonal variability,
indicating that distinct hydrologic regimes support different components of biodiversity. Recognizing
seasonal variability as a fundamental dimension of ecological state can improve how science, policy, and
management identify and protect the hydrologic regimes that sustain biodiversity.