Ecological stoichiometry is the study of the balance of chemical substances in ecosystems. In freshwaters, research has focused on how the ratios of carbon, nitrogen and phosphorus in organisms and their environment affect ecosystem processes. Because autotrophs have variable stoichiometry, particulate C:N:P ratios are used to assess nutrient availability in lakes. Zooplankton have relatively fixed stoichiometry and so differences between their body stoichiometry and the stoichiometry of their food can constrain their growth. Ecological stoichiometry predicts that zooplankton with low C:P body ratios (e.g., Daphnia) will be limited by the P content of their food in lakes where seston C:P is high. The stoichiometric theory of consumer-driven nutrient recycling (CNR) predicts that the stoichiometry of a consumer will influence the stoichiometry of the nutrients they regenerate through such processes as egestion and excretion. In lakes, zooplankton with a low body N:P are expected to regenerate nutrients in a high N:P ratio, potentially shifting nutrient limitation of the food web from N to P limitation. I used data from 99 Canadian lakes to test the following: a. Are particulate C:P and N:P ratios consistent with other P deficiency indicators? b. Do seston C:P and N:P ratios affect zooplankton community composition? c. Does zooplankton community composition affect plankton P limitation as predicted by CNR? Particulate C:P and N:P ratios generally agreed with other P deficiency indicators, except dissolved phosphate turnover times (TTPO4). C:P and N:P suggested P sufficiency more often than TTPO4, possibly because these two indicators respond to P deficiency over different time scales. Most zooplankton biomass parameters were negatively related to seston C:P ratios consistent with improved food quality at lower seston C:P. There was, however, no evidence that Daphnia were more strongly affected than any other zooplankton. Turnover times of particulate P in the whole plankton assemblage were not related to zooplankton community structure parameters. However, particulate P turnover in the >200 µm size fraction increased with increasing zooplankton biomass. There was no evidence for a particular effect of Daphnia on particulate P turnover. Phosphorus deficiency indicators showed a trend of relaxing P deficiency as zooplankton biomass and the proportion of Daphnia increased. This contradicts the predictions of CNR which suggest that Daphnia should cause greater P deficiency in lakes.

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Understanding the mechanisms that control the rate and trajectory of primary succession can lead to insights for ecosystem rehabilitation. Proposed mechanisms include life history traits and nutrient limitation. To explore how these mechanisms can drive successional dynamics, I devised a stoichiometric ecosystem-level model that considered the role of nitrogen and phosphorus limitation in plant primary succession in conjunction with life history traits. This model was applied to the plant community on Mount St. Helens to check the validity of the mechanisms. The results show the competitive hierarchy of plants at the local scale can be explained by nutrient limitation and plant stoichiometry. At regional scales, life history traits interact with local processes to shape community structure and successional dynamics. At all scales, the presence of Lupinus lepidus, a nitrogen-fixer, significantly altered community dynamics and succession. This study suggests that primary succession can be examined within the framework of ecological stoichiometry. / Ecology

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Climate change is predicted to impact organismal nutritional ecology. Increased temperatures can directly accelerate physiological rate processes, which in turn, impact nutritional requirements. Climate change can also impact organisms indirectly by altering the quality and quantity of nutritional resources, creating the potential for nutritional mismatch between what nutrients are available in the environment and what organisms require. Investigation of organismal stoichiometry, particularly the balance of carbon, nitrogen, and phosphorus content of organisms, can help illuminate the extent to which changes in climate may impact organism nutritional ecology. Ants represent an excellent system to examine stoichiometry because they occur across a broad range of environmental conditions and perform important ecosystem services, such as seed dispersal, which may impact ecosystem functioning. In this thesis, I examined how climate variables influence ant stoichiometry across a broad latitudinal gradient in natural populations of three closely-related ant species in the genus Aphaenogaster. In a common garden study, I tested the extent to which such stoichiometric variation was due to plastic or evolved variation. I found significant species-specific differences in how ant stoichiometry responded to climate gradients. The northern species, A. picea contained more C, and less N and P at higher latitudes and elevation, consistent with increased winter lipid storage. In contrast, the more southern species, A. rudis, showed the opposite pattern, which may reflect N and P limitation at southern extremes. Aphaenogaster fulva, whose range is intermediate in latitude and partially overlaps with both congeners, contained more C in environments with more seasonal precipitation. Thus, these species appear to use different nutrient storage strategies in response to the variation in abiotic and trophic conditions across their range. When reared under the same feeding regime and thermal conditions, site-level differences in nitrogen storage between a northern and a southern ant population were retained over time and across years, suggesting that adaptive divergence in elemental composition is at least partially responsible for clinal patterns in the field. To connect latitudinal patterns to temporal changes projected under climate change, I evaluated how increases in temperature impact ant stoichiometry and associated functional traits at the individual and colony level using an experimental field mesocosm experiment at two sites, Harvard Forest (HF) and Duke Forest (DF). I examined how experimental increases in temperature impacted ant body size, colony demography, and nutritional status of two Aphaenogaster ant species. I found that Aphaenogaster ants at the northern site, HF, responded positively to direct increases in temperature, with increases in colony biomass, colony size, total reproductive output, and shifts toward increased nitrogen content with increases in temperature. In contrast, Aphaenogaster ants at the southern site, DF, were generally unaffected by temperature except for a decrease in maximum colony size with increases in temperature. Together, my findings provide evidence that both climate variables and evolutionary history impacts ant stoichiometry, which in turn, may impact ant colony fitness. Examination of the biochemical basis of stoichiometric trait variation is needed to ascertain the role stoichiometry may play in how ant species adapt to changing environmental conditions.

The field of ecological stoichiometry has been dominated by studies focusing on aquatic & benthic microinvertabrates with less attention given to herbivorous insects. These organisms rely on their food source(s) to supply all of the building blocks (elements) they need in order to complete their life cycle. Since insect herbivores do not have the same elemental composition as the plants they use for food the question arises; of how they go about building themselves. We investigated what happened when grasshoppers were fed diets with various macronutrient profiles, their total amounts, and when the protein quality varied. We discovered that under controlled conditions when using a high quality protein source that grasshoppers are able to maintain a strict level of elemental homeostasis, but that the elements directly related to manipulations made in the food seem to vary (carbon, which is associated with carbohydrates and nitrogen, associated with protein). We also discovered that when the quality of protein changes an immature grasshoppers elemental stoichiometry loses some of this strict homeostatic regulation.

abstract: Primary production in aquatic ecosystems is often limited by the availability of nitrogen (N) and/or phosphorus (P). Animals can substantially alter the relative availability of these nutrients by storing and recycling them in differential ratios. Variation in these stoichiometric traits, i.e., the elemental phenotype, within a species can link organismal evolution to ecosystem function. I examined the drivers of intraspecific variation in the elemental phenotype of aquatic consumers to test for the generality of these effects. Over a thermal gradient in Panamá, I found that average specific growth grate and body P content of the mayfly Thraulodes increased with environmental temperature, but that these patterns were due to site-specific differences rather than the direct effects of warmer temperature. In a meta-analysis of published studies, I found that in fishes intraspecific variation in dietary N:P ratio had a significant effect on excretion N:P ratio, but only when accounting for consumption. I tested for the effects of variation in consumption on excretion N:P ratio among populations of the fish Gambusia marshi in the Cuatro Ciénegas basin in Coahuila, Mexico. G. marshi inhabits warm groundwater-fed springs where it often co-occurs with predatory fishes and cool runoff-dominated wetlands which lack predators. Using stoichiometric models, I generated predictions for how variation in environmental temperature and predation pressure would affect the N:P ratio recycled by fishes. Adult female G. marshi excretion N:P ratio was higher in runoff-dominated sites, which was consistent with predators driving increased consumption rates by G. marshi. This result was supported by a diet ration manipulation experiment in which G. marshi raised on an ad libitum diet excreted N:P at a lower ratio than fish raised on a restricted diet ration. To further support the impacts of predation on phenotypic diversification in G. marshi, I examined how body morphology varied among habitats and among closely related species. Both among and within species, predation had stronger effects on morphology than the physical environment. Overall, these results suggest that predation, not temperature, has strong effects on these phenotypic traits of aquatic consumers which can alter their role in ecosystem nutrient cycling through variation in consumption rates. / Dissertation/Thesis / Doctoral Dissertation Biology 2017

abstract: Human-induced rapid environmental change (HIREC) influences nearly all of Earth's ecosystems through processes such as urbanization. Previous studies have found that urbanization influences biodiversity patterns, often yielding an increase in the abundance of a few urban-adapted taxa at the expense of native species diversity. The western black widow spider, Latrodectus hesperus, is a medically-important pest species that often forms dense urban subpopulations (i.e., infestations) relative to the low-density subpopulations found throughout undisturbed, desert habitat. Here, I employ field and laboratory studies to examine the population ecology and stoichiometry of this urban pest to increase our understanding of the mechanisms underlying its success. The population ecology of ten black widow subpopulations spread across metropolitan Phoenix, AZ was examined during the peak breeding season (June-August). This study revealed that arthropod prey abundance, female mass and population density of females showed significant spatial variation across the ten subpopulations. Additionally, prey abundance and foraging success, measured as the number of carcasses found in webs, were a strong determinant of female mass and population density within each subpopulation. To test the mechanisms that drive black widow infestations, I used ecological stoichiometry to examine the nutrient (nitrogen and phosphorus) composition of spiders and arthropod prey from urban habitat, desert habitat and a laboratory diet regime. These studies revealed that (1) spiders are more nutrient rich than cricket prey in the field, (2) spider subpopulations exhibit significant spatial variation in their nitrogen composition, (3) nutrient composition of urban spider subpopulations does not differ significantly from Sonoran desert subpopulations, (4) laboratory-reared spiders fed a diet of only laboratory-reared crickets are more nitrogen and phosphorus limited than field-captured spiders, and (5) cannibalism by laboratory-reared spiders alleviated phosphorus limitation, but not nitrogen limitation, when compared to field-captured spiders. This work highlights the need to examine the population ecology of species relationships, such as predator-prey dynamics, to fully understand the fecundity and population growth of urban pest species. Moreover, the integration of population ecology and stoichiometry illustrates the need to address mechanisms like nutrient limitation that may explain why urban pest populations thrive and native species diversity suffers following HIREC. / Dissertation/Thesis / M.S. Biology 2012

abstract: There has been important progress in understanding ecological dynamics through the development of the theory of ecological stoichiometry. This fast growing theory provides new constraints and mechanisms that can be formulated into mathematical models. Stoichiometric models incorporate the effects of both food quantity and food quality into a single framework that produce rich dynamics. While the effects of nutrient deficiency on consumer growth are well understood, recent discoveries in ecological stoichiometry suggest that consumer dynamics are not only affected by insufficient food nutrient content (low phosphorus (P): carbon (C) ratio) but also by excess food nutrient content (high P:C). This phenomenon, known as the stoichiometric knife edge, in which animal growth is reduced not only by food with low P content but also by food with high P content, needs to be incorporated into mathematical models. Here we present Lotka-Volterra type models to investigate the growth response of Daphnia to algae of varying P:C ratios. Using a nonsmooth system of two ordinary differential equations (ODEs), we formulate the first model to incorporate the phenomenon of the stoichiometric knife edge. We then extend this stoichiometric model by mechanistically deriving and tracking free P in the environment. This resulting full knife edge model is a nonsmooth system of three ODEs. Bifurcation analysis and numerical simulations of the full model, that explicitly tracks phosphorus, leads to quantitatively different predictions than previous models that neglect to track free nutrients. The full model shows that the grazer population is sensitive to excess nutrient concentrations as a dynamical free nutrient pool induces extreme grazer population density changes. These modeling efforts provide insight on the effects of excess nutrient content on grazer dynamics and deepen our understanding of the effects of stoichiometry on the mechanisms governing population dynamics and the interactions between trophic levels. / Dissertation/Thesis / Ph.D. Applied Mathematics 2014