Offspring Size-Number Tradeoffs and Food Quality Feedbacks Impact Population Dynamics in a Daphnia-Algae System
Date of Award
Master of Science (MS)
Population fluctuations can be affected by a number of extrinsic and intrinsic factors, but few manipulative experiments have been conducted that can isolate these effects in consumer-resource systems. Extrinsic factors such as weather patterns or food availability can impact consumer growth and reproduction. Additionally, intrinsic factors relating to life history can have significant impacts on population growth rates. A fundamental principle of life history theory is that individuals are limited by trade-offs between survival and reproduction, and there are a variety of combinations, given environment and physiological limitations, that should maximize lifetime reproductive output. Reproductive strategies vary between species, but are also prevalent intraspecifically, with a key trade-off being whether mothers produce either many small offspring, or fewer large offspring. Theoretical modelling studies have shown that this trade-off in offspring size versus number, through maturation time, can have significant impacts on population dynamics. The objective of my research was to experimentally test the effect of an intrinsic offspring size-number trade-off on population stability and food availability in a dynamic consumer-resource system. I hypothesized that the trade-off would impact internal consumer population characteristics such as biasing stage structure towards adults, increasing adult size, and increasing population-level reproduction. I predicted that this adult-dominated stage structure would lead to instability and a low quantity-high quality food state. I experimentally manipulated the number and size of juveniles in populations of the consumer Daphnia pulex, creating a shift from many, small offspring to fewer, larger offspring. Experiments controlled for the ingestion pressure on algal prey at the time of the manipulation. Two sets of experiments were performed in order to examine the interaction of an extrinsic factor (light levels) and intrinsic population structure on dynamics: a dark experiment conducted in no light examining intrinsic structure only; and low and high light environments examining the interaction of intrinsic and extrinsic factors. Control populations, with no manipulation of offspring size/number, were established and monitored. In all experimental set-ups, the manipulated populations became dominated by large adults. Contrary to predictions, amplitudes in population biomass were lower in manipulated populations, representing higher stability in these populations. Furthermore, in high light conditions, a stable low Daphnia – high algae biomass (low quality) state was observed in manipulated populations but not in control populations. This shows a strong link between light levels as an extrinsic factor and the life history manipulation of consumer offspring size vs. number, through resource quality, that impacts population dynamics.
Wenkus, Ryan, "Offspring Size-Number Tradeoffs and Food Quality Feedbacks Impact Population Dynamics in a Daphnia-Algae System" (2016). College of Science and Health Theses and Dissertations. 166.