<p> Indirect interactions have increasingly been recognized as important forces influencing population dynamics and structuring communities. Associational susceptibility is a form of indirect effect in which a focal plant experiences greater herbivore damage due to neighboring plant identity or diversity. These interactions remain poorly understood in the context of invasion ecology, though they may be responsible for huge impacts of invasive species on native communities. This dissertation investigates the potential mechanisms and consequences of associational susceptibility of a native perennial shrub, <i> Atriplex canescens,</i> driven by an invasive annual forb, <i>Brassica tournefortii,</i> and an invasive herbivorous stinkbug, <i>Bagrada hilaris.</i> In Chapter 1, a potential associational effect is experimentally demonstrated and a phenologically-driven trait is identified as a potential mechanism for this interaction. In Chapter 2, relative host plant quality is explored for its role in mediating the numerical response of the shared herbivore, and the herbivore&rsquo;s damage impact on <i>A. canescens. </i> In Chapter 3, neighbor density, herbivore presence and herbivore density were manipulated to identify their impacts on spillover timing, extent, and fitness consequences for <i>A. canescens.</i> Overall, potential mechanisms of <i>A. canescens</i> associational susceptibility to <i> Br. tournefortii</i> and <i>Ba. hilaris</i> identified include: <i> Ba. hilaris</i> accumulation on <i>Br. tournefortii</i> followed by <i>Br. tournefortii</i> senescence and depletion, triggering <i> Ba. hilaris</i> alternative host-seeking. Associational susceptibility of <i>A. canescens</i> could not be re-created under experimental conditions, but further study is required to ascertain whether this interaction is due to experimental limitations or ecological implausibility.</p><p>

<p> Prey species defend themselves behaviorally and morphologically, and often utilize varied antipredator strategies against dissimilar predator types (i.e. terrestrial vs. aerial). Striped skunks <i>(Mephitis mephitis) </i> spray noxious secretions at predators and advertise this danger with deterrent behaviors and black-and-white aposematic coloration. Evidence suggests skunks are effective at deterring terrestrial mammalian predators but are vulnerable to aerial predators; how skunks assess the risk posed by different predator types, however, has not been examined empirically. I recorded the behavioral responses of skunks to audio playbacks of coyotes and great horned owls (the primary terrestrial and aerial predators of skunks, respectively), and peregrine falcons and white noise as controls, as well as to a visual remote controlled model. Skunks engaged in vigilance and running away more often in response to owl vocalizations, suggesting skunks perceive owls as more threatening relative to coyotes. Skunks were more likely to foot stomp and run away in response to the remote controlled model compared with coyote vocalizations, implying visual cues were perceived as more risky than audio cues. This study elucidates how a well-defended mammal can determine which perceived threat is most risky and alter its behavior when its main defense strategy is not successful against all predator types.</p><p>

Abstract not available.

The location of the hypothesized "missing carbon sink" has become a subject of open debate in the scientific community, with North America considered to be a prime location for such a sink. While forest ecosystems are considered to be the major candidate, Prairie ecosystems, with their large capacity to store carbon in their soils, may or may not be an ancillary candidate. The North and South Saskatchewan River watersheds are a prime example of such prairie ecosystems and, with this in mind, I attempted to quantify their role as potential sources or sinks of atmospheric CO2. The sequestration of atmospheric CO2 by plants via photosynthesis requires that water and CO2 be utilized at a specific ratio called the water utilization efficiency (WUE). Thus, knowing the hydrological budget of a watershed, specifically the transpiration component, it is possible to calculate its net primary productivity (NPP). This, combined with biological model estimates for heterotrophic soil respiration (Rh), enables evaluation of source and sink tendencies in the watershed. (Abstract shortened by UMI.)

Mercury is increasing to toxic levels in Arctic biota living at the top of food webs. The rapid bioaccumulation and biomagnification of methylmercury (MeHg) in food chains, and the subsistence lifestyle of northern populations, has resulted in high levels of Hg in their blood. No prior measurements of MeHg sources to Arctic ecosystems have been made. In southern latitudes wetlands are considered important sources of MeHg with sulfate-reducing bacteria (SRB) thought to be responsible. Thus, the production of MeHg in Arctic wetlands was evaluated as well as SRB presence. Arctic wetlands were further evaluated as sources of MeHg in Arctic ecosystems, as well since snowmelt water provides 60 to 80% of water to Arctic terrestrial systems it was also evaluated as a source of MeHg. This was the first study to evaluate sources of MeHg entering Arctic ecosystems, and showed that although wetlands produced MeHg, the export to downstream lakes was dependant on site characteristics such as DOC levels, furthermore snowmelt water was the most significant source of MeHg to Arctic ecosystems measured here. (Abstract shortened by UMI.)

Physical and chemical variables were measured in riffle zones of 20 Fraser River tributaries, British Columbia, to examine which factors explain variation in algal biomass and taxonomic composition. July epilithic periphyton chlorophyll a was weakly correlated with TN, while October periphyton was significantly related to DP and conductivity. The phytoplankton biomass in July was best predicted by TP (r2 = 0.70, p &le; 0.001, n = 19), while October phytoplankton was best predicted by a multiple regression with conductivity, current velocity and TN. In terms of taxonomic composition in October, Bacillariophyta was most abundant, followed by Chlorophyta, Cyanobacteria and Phaeophyta. DP and current velocity explained 25% of the taxonomic variation among the rivers in a CCA. While changes in the relative abundance of specific diatom taxa were observed, shifts in division dominance were not. River algal biomass could be predicted from environmental variables, however the empirical models differed between July and October.

The central goal of this study was to describe the habitat use and movement patterns of juvenile black ratsnakes and contrast them to those of adults to identify any ontogenetic changes that should be included in the management plan of the species. My results indicated a clear ontogenetic shift in the movement patterns and spatial ecology of black ratsnakes. I found that daily and seasonal movements, home range size and fidelity, behaviour, and fidelity to over-wintering sites all vary with size. The results suggested that juvenile black ratsnakes, especially males, may be in a dispersal stage. If juveniles are playing a significant role in maintaining gene flow through dispersal, then adequate protection requires sufficient protected habitat to allow juvenile dispersal. I also showed that juveniles rarely attend communal hibernacula and often switch between hibernacula. Therefore, habitat to be protected cannot simply be defined as a fixed radius around a communal hibernaculum because many juveniles do not attend these communal hibernacula. This study reinforces the importance of understanding ontogenetic changes in the ecology of species of conservation concern. Insight into an ontogenetic shift in habitat use would be essential for sound management of any species where juveniles have more specific structural needs than adults, due to intense predation risk, specific prey requirements, etc. Ontogenetic shifts in movement patterns and spatial ecology have to be considered in management decisions. (Abstract shortened by UMI.)

Farmland bird species are known to be declining. In Europe, studies conducted at the farm-level have attributed this decline in abundance, at least partly, to agricultural practices including pesticide use. In North America, few studies have attempted to investigate the relationship between pesticide use and bird abundance and richness. Lack of sufficiently comprehensive pesticide data often limits rigorous testing of hypotheses. I therefore used the best available records of pesticide use and bird distributions: those in California. I calculated estimates of area treated with pesticides and insecticide direct toxicity. I related these to the spatial and temporal variation in bird species richness and abundance (from the Breeding Bird Survey) over a ten-year period in 5.0 km2 plots distributed over the Central Valley of California. I found that the relationships between bird species richness and abundance and pesticide use are weak, albeit borderline significant. I suggest that, if pesticide use actually does affect breeding bird abundance, it must be at much broader spatial scales than used in this study.

Since combined nitrogen is a pollutant in streams, rivers and coastal areas, conditions that favour nitrogen removal were identified. The importance of stream depth as a predictor of a reach's capacity to remove nitrogen was tested in the South Nation watershed (3915 km2), an agricultural watershed in eastern Ontario. Combined nitrogen (N) retention was estimated in 48 reaches across the watershed which varied in discharge (0.0008--118 m3 s-1), length (350 m to 5.2 km) and size from headwaters to the outflow of the South Nation River at the Ottawa River. Retention dynamics were also investigated within a few sites over the summer season. Retention efficiency (expressed as a percentage of inputs) varied widely for nitrate from almost complete removal within a reach to reaches acting as a net source. A narrower range in percent retention and loading was observed for total nitrogen (TN) among the sites suggesting that some forms of combined nitrogen are relatively inert. (Abstract shortened by UMI.)

By the 1940s, fishers (Mustelidae, Martes pennanti) were extirpated in Ontario south of the French and Mattawa Rivers, probably as a result of overharvesting and habitat loss. However, during the last several decades fishers have recolonized much of their former range in Ontario. This recolonization, combined with (for the most part) conservative harvest management, has led to increases in abundance. Perhaps inevitably, these increases have resulted in requests by fur harvesters to increase fisher quotas. The question then arises as to what the effect of the current quota system is on fisher populations in eastern Ontario. Unfortunately, very little is known about fisher demographics in eastern Ontario; as a result, the current management system is based almost exclusively on information and data on well-studied fisher populations from other regions, notably Algonquin Park. The extent to which these data---and the inferences regarding effective management therefrom---reflect fisher population characteristics in eastern Ontario is unknown.