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Test of alternative domains of attraction in the dynamics of a fishless oligotrophic lake

University of British Columbia

The theory of domains of attraction (alternative "stable" states) allows variability, thresholds and persistence as integral parts of ecological system functioning. This thesis investigates the potential for alternative domains of attraction in a zooplankton community. Two alternative states have been proposed for the zooplankton community of small Ashless, oligotrophic, mountain lakes. In the "competition state", Daphnia (Cladocera) outcompete solitary rotifers and impede predator recruitment. This state is persistent and resilient to disturbance. In the "predation state", predation by Chaoborus (Diptera) on both prey types alters community dynamics to favor rotifers over Daphnia. Persistence of the predation state requires reduced prey competition, enhanced young predator recruitment in summer and predator survival in high densities overwinter. I carried out graded field experiments using reduced Daphnia densities and predator additions in the spring to generate and test the predator state. I monitored lake and enclosure communities for three consecutive years using an adaptive sampling concept. Chaoborus recruitment was enhanced in enclosures and the new predator cohorts survived overwinter in high densities. However, developmental delays prior to the winter period prevented persistence of the predator state over several generations. Enhanced predator densities in the spring, as well as cold temperature, delayed Daphnia population onset and increase, which released solitary rotifer populations needed to feed young predator recruits in early summer. However, in the higher predator treatments, both prey types stayed depleted till late summer which resulted in delayed predator development. Laboratory experiments showed that although Chaoborus americanus can pupate at 5 ° C , they could not metamorphose into adults below 9 ° C . Cold water can delay their reproductive phase and delay timing of young predator recruitment. I conclude that Shirley Lake, under current nutrient levels, does not have two domains of attraction. Nonetheless the presence of a threshold between states enlarges the window for coexistence of weaker competitors or rare species. Thresholds lead to alternative domains of attraction in some systems, and to transient state in others. From a management perspective, extended transient states can either lead to misleading interpretation and erroneous interventions if permanent changes are expected or be used as tools to produce temporary changes.

Thesis/Dissertation

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2009-06-24

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http://hdl.handle.net/2429/9563

Zoology

University of British Columbia

UBCV

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