Abstract
Organisms must obtain elements (e.g. carbon, nitrogen, phosphorus) from the environment in specific ratios, yet elemental availability often do not match their requirements. These mismatches can make it difficult for organisms to survive, grow, and reproduce. However, organisms can adapt through genetic (i.e., evolutionary) and non-genetic mechanisms to lessen the
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negative effects of elemental limitation. The goal of this thesis was to investigate the capacity to respond to phosphorus limitation in herbivorous invertebrates (e.g. zooplankton).
Intraspecific variation is generally considered essential for rapid microevolutionary change. To determine the extent of intraspecific variation of stoichiometric traits, and thus the potential for to rapid evolution, I first performed a meta-analysis of common garden studies (Chapter 2). We documented small to moderated levels of variation in elemental content. In contrast, there was substantial variation in the acquisition, assimilation, allocation, and excretion of elements, the magnitude of which was similar to life history traits measured in the same studies. These results suggests that there is potential for stoichiometric traits to rapidly evolve.
I next investigated if the rapid evolution of stoichiometric traits could be predicted using the ecological stoichiometric framework. Specifically, I tested if the growth rate hypothesis, which posits that organismal P content is positively related to somatic growth rate, could predict the elemental composition of consumer populations (Chapter 3). The anticipated positive relationship between body P-content and growth rate was observed in populations provided high phosphorus resources during selection for fast growth rates, but not in populations provided low phosphorus resources. These results demonstrate that the success of the growth rate hypothesis as a predictive framework is dependent on the environmental context under which selection takes place.
The production of de novo heritable variation has been proposed as an alternative pathway for rapid adaptation. Therefore, I wanted to explore whether heritable adaptation in consumer populations that initially lack genetic diversity could occur on in response to stoichiometric mismatch (Chapter 4). We observed heritable adaptation to nutrient limitation in the populations with a low phosphorus exposure history in one of the two clones tested. These results suggest that de novo sources of phenotypic variation may play an important role in facilitating adaptation in populations with low genetic diversity.
As environments can change rapidly, plastic responses in addition to the previously documented heritable responses may play an important role in responding to stoichiometric mismatch. To investigate the effect of stoichiometric mismatch on a primary consumer’s functional response, I preformed a series of ingestion experiments with resources of varying elemental and biochemical content (Chapter 5). We demonstrated that at high but not low food densities, consumers with low phosphorus resources exhibit elevated ingestion rates compared to consumer with high phosphorus resources.
This thesis demonstrates that primary consumers have the capacity to mitigate the negative effects of stoichiometric mismatches using both heritable and non-heritable mechanisms. By incorporating biological realism (e.g. intraspecific variation, alternative adaptive pathways) into the ecological stoichiometric framework, we can improve our capacity to predict organismal responses to changes in elemental availability.
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