La quantité et la qualité de la ressource alimentaire varient dans le temps et dans l'espace sous l'influence des conditions environnementales. Il est à présent reconnu qu'une diminution de cette ressource est à l'origine d'un déclin pour bon nombre d'espèces granivores inféodées aux milieux cultivés. Face à cette hétérogénéité, les individus mettent en place différentes stratégies individuelles ou collectives pour garantir leurs besoins énergétiques. Dans ce contexte, ce travail s'est attaché à comprendre les stratégies mise en place chez une espèce aviaire caractéristique des agrosystèmes. L'étude expérimentale du compromis alimentation/prédation révèle l'importance de variables environnementales, liées à la ressource, à la taille du groupe ou encore au risque de prédation sur leurs gains énergétiques des individus. Plus étonnamment, nos résultats ont mis en évidence des différences sexuelles dans les stratégies d'acquisitions d'énergie. Les choix effectués par les individus dans la réalisons de ce compromis ont des conséquences sur la sélection de l'habitat. L'étude de la réponse agrégative a ainsi souligné l'impact du type de culture ou de variables paysagères dans l'utilisation de l'habitat. Enfin, une étude sur le sex-ratio en fonction de la latitude en France a pu mettre en évidence une ségrégation spatiale entre les sexes, avec une forte proportion de mâles au nord, et un rééquilibrage vers le sud. Cette ségrégation sexuelle a pu être mise en relation avec des capacités physiologiques différentes (dépenses énergétiques et hypothermie), soulignant une plus forte tolérance des mâles aux faibles températures hivernales rencontrées dans les latitudes plus élevées. / Both quantity and quality of food vary in time and space under the influence of environmental conditions. It is now recognized that a decrease of food is involving in a decline for many species of seed-eating farmland birds. According to this heterogeneity, individuals promote strategies to ensure individual and collective energy needs. In this context, this work has focused on understanding the strategies developed in an avian species characteristic of farmland. The experimental study of the trade-off between foraging and predation reveals the importance of environmental variables related to the resource, the group-size and group density, or the predation risk on the intake rate of individuals. More surprisingly, our results have shown sexual differences in energy acquisition strategies. The choices made by individuals under this trade-off have strong consequences on habitat selection. The study of the aggregative response has highlighted the impact crop type and different landscape variables in habitat use, more specially for foraging habitat. Finally, a study on the sex ratio in relationships with latitude in France has been able to demonstrate a spatial segregation between the sexes, with a high proportion of males in the north. This sexual segregation could be related with different physiological capabilities (energy expenditure and hypothermia), emphasizing a greater tolerance of males at low winter temperatures encountered in the higher latitudes during wintering period.

Patterns in algal community structure and physiochemical characteristics of streams and one impounded subalpine lake in the mountainous North Alouette River watershed, British Columbia, were described for one year from June 1977 to June 1978. In this period, 266 algal taxa were recognized, of which 59 were previously unrecorded in the province. The streams were characterized by an epilithic flora consisting predominantly of. unbranched Chlorophyta and secondarily by both branched and unbranched Cyanophyta. Bacillariophyta (diatoms) were species rich (over 100 taxa), but were at all times relatively unimportant in the streams, although frequently dominant in the epipelon of Jacob's Lake. Species of Rhodophyta were locally abundant only in shaded habitats. Many epilithic and epiphytic species were "host" specific in their substrate preferences. Stream water in the North Alouette was slightly acid (pH 6-7) and nutrient poor, the relative order of anions being S0₄²⁻ > SiO₂ > CI⁻ >N0₃⁻ > PO₄³⁻ and cations Ca²⁺ = Na⁺ > Mg²⁺ > K > NH₄⁺ > Fe²⁺/³⁺, Mn²⁺, and Al³⁺ were not detected in the dissolved fraction. Other variables indicated this to be a rapidly flowing (often > 1m sec⁻¹), cool (2-18°C seasonally), poorly buffered (HCO₃⁻ = .06-.40 meq 1⁻¹), and highly heterogeneous environment. Stations along the stream gradient differed in conditions of slope, current velocity, degree of shading, and substrate size, but not in temperature, pH, and possibly nutrient chemistry. A principle coordinates analysis (P-Co-A) of seasonal succession at one station (Station 1) revealed a cyclic pattern characterized by sequences of gradual and abrupt changes in species composition. Temporal extinction of dominant species did not occur, as has been shown for phytoplankton populations in lakes. Current velocity, depth, temperature, CI⁻, and SO₄²⁻ were significantly correlated (P<0.05) with most of the seasonal variability in the algal community. A smaller amount of the seasonal change was correlated with the flux of dissolved cations. P-Co-A also exposed similarities between six stations within the watershed which were not consistent seasonally, and gave no evidence of distinct zones. Distribution of algal species within Station 1 in May shown by cluster analysis, occurred roughly in two groups, corresponding to near-shore and midstream habitats. The general heterogeneity of algal distribution and the occasional disturbance by flooding gave rise to periodic peaks in diversity, although many common species never became abundant. Hence, no clear-cut relation was realized between the physiochemical environment and species diversity. Hypotheses are generated, suggesting that (1) distribution of red algae was shade limited; (2) diatom dominance was limited by nutrient chemistry; (3) the even pattern of seasonal succession was interrupted by periodic events, such as nutrient pulses and floods; and (4) a large degree of species coexistence was provided by these periodic disturbances. / Science, Faculty of / Botany, Department of / Graduate