|Title||Effects of different intensities and frequencies of disturbance by snail herbivory on periphyton succession|
|Year of Publication||1988|
|Degree||Doctor of Philosophy|
|Number of Pages||220 pp.|
|University||University of Louisville|
Effects of variations of snail grazing on succession, resistance and resilience of two benthic algal communities were investigated. Research was conducted in flow-through experimental systems where clay tiles were colonized for 26-28d using littoral lake water from Douglas Lake at The University of Michigan Biological Station (UMBS) and Kentucky Lake at Hancock Biological Station (HBS). Benthic algal succession was observed in UMBS soft algae, HBS soft algae and HBS diatoms, where early assemblages were replaced by late assemblages. The UMBS diatom component did not demonstrate species shifts because strong immigration pressure by the dominant Cyclotella comensis, masked succession of rare species on tiles. The architecture of communities did not always progress from low to high stature forms. Soft algal components of both systems had low-profile early forms that were succeeded by filamentous late species. However, both diatom components had high-profile early species and adnate or small motile (understory late species). Snail gut content analyses showed snail grazing selectivly removed the overstory algae. In soft algal components, grazing arrested succession by removing late filamentous species. However, snail grazing enhanced succession in diatom communities by removing high-stature early forms. Grazer-resistant species were those having low profiles. Apically attached, stalked, filamentous or flocculent species were vulnerable to removal by grazers, and occurred in significantly lower numbers where grazing was constant or frequency was high. Algal communities that developed under constant stress-level grazing (15 snails/m2) were more resistant to an additional 24h intense grazing disturbance (308 snails/m2), than communities developing without stress. Stressed communities were modified by constant removal of overstory species making them less vulnerable to an additional intense disturbance. Resilience following an intense disturbance was higher in unstressed communities. These results suggest that; (1) strong immigration pressure of a dominant taxa can obscure succession, 2) disturbance does not always arrest succession, but can enhance the process when early species are overstory forms and late species are adnate, and 3) communities developing under stress are more resistant to intense disturbance than non-conditioned communities.