No description available.

The maternal rearing environment (MRE) of an organism can be a key determinant of an organism's host choice decisions, its own fitness, or the fitness of its offspring. Here, it is investigated if the MRE of an organism can influence lower or higher trophic levels. A series of reciprocal cross-over experiments was conducted using pea aphids (Acyrthosiphon pisum), bean (Vicia faba) or pea (Pisum sativum) plants, and an aphid natural enemy, the parasitoid wasp Aphidius ervi, as model organisms. In each experiment, pea aphid offspring experienced either the same or an alternative plant host to that experienced by their mothers. This PhD showed that the MRE of pea aphids and parasitoid wasps was not a main contributory factor of host choice decisions or offspring fitness but influenced mother parasitoid wasp fecundity. Additionally, the MRE of pea aphids influenced the foliar nutrient concentration of pea plants when infested with the aphid's offspring. First, over shorter infestation periods, variation in foliar nitrogen and essential amino acid concentrations of pea leaves could be explained by pea aphid MRE. Over longer infestation periods, variation in foliar nitrogen and essential amino acid concentrations of pea leaves was explained by a combination of pea aphid MRE and aphid genotype. Second, the 13C concentration of pea leaf tissue, an indicator of stomatal aperture and leaf water stress, varied with pea aphid MREs over longer infestation periods. However, stomatal conductance and the expression of abscisic acid-responsive genes did not vary in a manner that was consistent with leaf water stress. Additional components of an organism's maternal rearing conditions are considered, including symbioses, as a more realistic MRE compared with that observed in nature. Taking account of MREs could provide a better understanding of the factors influencing the fitness of many organisms interacting in natural and managed ecosystems.

No description available.

No description available.

Thesis (Ph. D.)--University of Chicago, Dept. of Ecology and Evolution, August 2003. / Includes bibliographical references. Also available on the Internet.

No description available.

This thesis examines broad-scale patterns in the food web structure of lake ecosystems. The unifying framework for this analysis is the concept of trophic position, which represents the energy-weighted trophic path length leading to a consumer or population. Trophic position was estimated for over 550 populations of fish (16 species) using quantitative dietary data. A method to estimate trophic position of aquatic consumers using stable isotope ratios was developed; this involved correcting for within and among-lake variation in delta15N of primary consumers (organisms used to represent the base of the food web). We report an overall correspondence between dietary and isotopic estimates of trophic position; for each species, trophic position generally ranged the equivalent of one trophic level unit among-populations. The concept of discrete trophic levels provided only a qualitative description of energy flow pathways in aquatic food webs. Among-population variation was much greater than within-population variation in trophic position of lake trout. Long-standing hypotheses about the determinants of food chain length were examined; food chain length was most closely correlated with species richness and lake area. / The trophic position approach was used to examine two separate environmental problems relevant to lakes: the bioaccumulation of persistent contaminants in food chains and the impacts of invasive species. Trophic position was the major determinant of PCB levels in lake trout; these relationships were used to characterize biomagnification factors (BMFs) for this and a number of other contaminants. Furthermore, the introduction of smelt into lakes was linked to increased levels of PCB and Hg contamination in lake trout. Stable isotopes were used to quantify the impacts of smallmouth bass and rock bass invasions on food webs leading to lake trout. Lake trout from invaded lakes exhibited reduced consumption on littoral prey fish; a food web shift that is likely to have detrimental impacts on native lake trout populations.

I examined the distribution of stable isotope ratios of nitrogen $( delta sp{15}$N) and carbon $( delta sp{13}$C) in fish and aquatic invertebrates. Animals are enriched in $ delta sp{15}$N compared to their diet and I hypothesized that among-lake variation in the $ delta sp{15}$N of a top predator should reflect variation in the length of the food chain leading to it. A comparative study of a biomagnifying contaminant, mercury, confirmed that the presence or absence of certain key organisms such as pelagic forage fish and the crustacean Mysis relicta determined among-lake variation in mercury in lake trout, a top predator in the lakes. Mercury levels from the longest food chains where pelagic forage fish and Mysis were present were higher than those from the shortest food chains where these last two components were missing. This biogeographic variation in food chain length was correlated with variation in the $ delta sp{15}$N of trout. The use of $ delta sp{15}$N as a continuous, integrative measure of trophic position was further supported by its correlation to mercury in lake trout. However, such cross-system comparisons in $ delta sp{15}$N can be complicated by differences in $ delta sp{15}$N at the base of the food chain. Using large primary consumers (unionid mussels) as bio-indicators, I showed that $ delta sp{15}$N increases markedly with the human population in the lake watershed, an effect of the high $ delta sp{15}$N of human sewage. Correcting for this baseline variation in $ delta sp{15}$N, I reported that food chains leading up to nearshore fish species varied by about only one trophic level among the 35 lakes studied. A study of the $ delta sp{15}$N of coral reef and intertidal organisms collected along the coast of the the Carribean island of Barbados extended these patterns observed in lakes to coastal systems: baseline variation in $ delta sp{15}$N was related to human density on the adjacent watershed and within-site variation $ delta sp{15}$N

Temporal changes in the environmental conditions upon which life depend are ubiquitous in nature, acting at every level of organisation from cells to ecosystems. Although the actions themselves are often poorly understood, they strongly depend on the temporal and hierarchical (organisational) scales at which they are measured; ecosystems are relatively stable through time while their species composition may undergo vast changes. Likewise any hierarchical level may be relatively stable in the short-term, but undergo vast long-term changes. This thesis aims to better understand the importance of these scales for mediating the impact of environmental variability on ecological systems. / The approach used herein employs both mathematical models and empirical data which represent food webs responding to environmental variability at different hierarchical scales. Within each of these representative food webs, the influence of environmental variability on the stability of the food web is determined using an approach which accounts for the effects of temporal scale. This thesis demonstrates that the stability of simple model food webs (high hierarchical scale) is tightly linked to environmental variability and the temporal scales at which these changes occur dictate which species in the model are most affected. At lower scales of organisation, empirical data indicate that environmental variability generally has a lesser impact on stability and that only certain temporal scales are responsible for this trend. At these temporal scales some species respond differently to environmental variability, negative changes in one species (or group) are offset by positive changes in another - a process known as compensation. These results highlight the importance of both temporal and hierarchical scale in mediating the response of food webs to environmental variability. Ultimately, they will serve to better understand how models and experiments should scale-up from low to high hierarchical and temporal scales.

Thesis (Ph.D.)--University of Washington. / Bibliography: l. [202]-208.