Les variations spatiales de l'effort d'approvisionnement du bison des plaines soumis à la prédation par le loup gris : mémoire

Cette étude visait à expliquer les variations spatiales de l'effort d'approvisionnement du bison des plaines. J’ai caractérisé les cratères d'alimentation des bisons dans la neige et j’ai suivi des bisons et des loups munis de colliers émetteurs en hiver. Les bisons s’alimentaient davantage dans les prés où le couvert de neige était mince et peu dense, de même que dans les parcelles les plus profitables (énergie digestible/temps de manipulation) du paysage. La cooccurrence spatiale entre les loups et les bisons indique que le prédateur gagne le jeu spatial. Aussi, les bisons laissaient plus de végétation hautement profitable dans les grands prés que dans les petits, une décision concordant avec la notion que les bisons se déplacent fréquemment afin d'éviter que les prédateurs connaissent leurs localisations. L'étude de l'approvisionnement du bison dans son milieu naturel révèle comment des patrons spatiaux d'herbivorie émergent dans un paysage caractérisé par plusieurs niveaux d’hétérogénéité.The aim of this study was to explain spatial variation in the feeding effort of plains bison. I characterized feeding craters of bison in snow, and I radio-tracked bison and wolves in winter. Bison foraged more intensively in meadows with shallow and light snow, and in the most profitable (digestible energy / handling time) patches available in the landscape. Bison and wolves intensively used the same meadows, a co-occurrence indicating that wolves are ahead in the spatial game they play with bison. Also, bison left more vegetation of higher-than-average profitability in large than in small meadows. This decision is consistent with the notion that bison move frequently to prevent wolves from knowing their location. The assessment of bison foraging in a natural setting reveals how spatial patterns of herbivory emerge in landscapes characterized by multiple levels of heterogeneity

Data from: Spatial heterogeneity in the strength of plant-herbivore interactions under predation risk: the tale of bison foraging in wolf country

Spatial heterogeneity in the strength of trophic interactions is a fundamental property of food web spatial dynamics. The feeding effort of herbivores should reflect adaptive decisions that only become rewarding when foraging gains exceed 1) the metabolic costs, 2) the missed opportunity costs of not foraging elsewhere, and 3) the foraging costs of anti-predator behaviour. Two aspects of these costs remain largely unexplored: the link between the strength of plant-herbivore interactions and the spatial scale of food-quality assessment, and the predator-prey spatial game. We modeled the foraging effort of free-ranging plains bison (Bison bison bison) in winter, within a mosaic of discrete meadows. Spatial patterns of bison herbivory were largely driven by a search for high net energy gains and, to a lesser degree, by the spatial game with grey wolves (Canis lupus). Bison decreased local feeding effort with increasing metabolic and missed opportunity costs. Bison herbivory was most consistent with a broad-scale assessment of food patch quality, i.e., bison grazed more intensively in patches with a low missed opportunity cost relative to other patches available in the landscape. Bison and wolves had a higher probability of using the same meadows than expected randomly. This co-occurrence indicates wolves are ahead in the spatial game they play with bison. Wolves influenced bison foraging at fine scale, as bison tended to consume less biomass at each feeding station when in meadows where the risk of a wolf's arrival was relatively high. Also, bison left more high-quality vegetation in large than small meadows. This behavior does not maximize their energy intake rate, but is consistent with bison playing a shell game with wolves. Our assessment of bison foraging in a natural setting clarifies the complex nature of plant-herbivore interactions under predation risk, and reveals how spatial patterns in herbivory emerge from multi-scale landscape heterogeneity

Spatial heterogeneity in the strength of plant-herbivore interactions under predation risk: the tale of bison foraging in wolf country.

Spatial heterogeneity in the strength of trophic interactions is a fundamental property of food web spatial dynamics. The feeding effort of herbivores should reflect adaptive decisions that only become rewarding when foraging gains exceed 1) the metabolic costs, 2) the missed opportunity costs of not foraging elsewhere, and 3) the foraging costs of anti-predator behaviour. Two aspects of these costs remain largely unexplored: the link between the strength of plant-herbivore interactions and the spatial scale of food-quality assessment, and the predator-prey spatial game. We modeled the foraging effort of free-ranging plains bison (Bison bison bison) in winter, within a mosaic of discrete meadows. Spatial patterns of bison herbivory were largely driven by a search for high net energy gains and, to a lesser degree, by the spatial game with grey wolves (Canis lupus). Bison decreased local feeding effort with increasing metabolic and missed opportunity costs. Bison herbivory was most consistent with a broad-scale assessment of food patch quality, i.e., bison grazed more intensively in patches with a low missed opportunity cost relative to other patches available in the landscape. Bison and wolves had a higher probability of using the same meadows than expected randomly. This co-occurrence indicates wolves are ahead in the spatial game they play with bison. Wolves influenced bison foraging at fine scale, as bison tended to consume less biomass at each feeding station when in meadows where the risk of a wolf's arrival was relatively high. Also, bison left more high-quality vegetation in large than small meadows. This behavior does not maximize their energy intake rate, but is consistent with bison playing a shell game with wolves. Our assessment of bison foraging in a natural setting clarifies the complex nature of plant-herbivore interactions under predation risk, and reveals how spatial patterns in herbivory emerge from multi-scale landscape heterogeneity

HarveyFortinDataset S1

Field dat

Coefficients and standard errors of a linear mixed effects model predicting the area (ha) of foraging crater in individual meadows in winter.

<p>Independent variables included snow water equivalent, missed opportunity costs of foraging in that meadow and not elsewhere in the landscape (), index of wolf presence (absence  = 0, presence  = 1) and log-transformed meadow area. A total of 144 foraging craters were recorded in 26 meadows in Prince Albert National Park (Saskatchewan, Canada) during the winters of 1997, 1998 and 2011. Pseudo R² = 0.31.</p

Coefficients and standard errors for a mixed-effects logistic regression model predicting the probability that bison foraged in a given meadow in winter.

<p>Independent variables included snow water equivalent, missed opportunity costs of foraging in that meadow and not elsewhere in the landscape (), index of wolf presence (absence  = 0, presence  = 1) and log-transformed meadow area. N = 221 surveys in 26 meadows in Prince Albert National Park (Saskatchewan, Canada) during the winters of 1997, 1998 and 2011.</p

Relative level of support by competing models explaining plant biomass consumed in foraging craters by plains bison in winter.

<p><b>Note:</b> E: Plant profitability (kJ/min) in the landscape, meadow or patch, SWE: Snow water equivalent (cm), MOC: Missed opportunity costs (kJ/min), Wolf: Index of wolf presence, ln(MS): log-transformed meadow size (ha), ΔAIC: difference in Akaike information criterion between the current model and the lowest AIC.</p

Coefficients and standard errors for the top-ranking mixed effects linear model predicting the plant biomass consumed (g/m²) in a foraging crater in winter.

<p>Independent variables included snow water equivalent, missed opportunity costs of foraging in that meadow and not elsewhere in the landscape (), index of wolf presence (absence  = 0, presence  = 1) and log-transformed meadow area. A total of 255 quadrats of plant biomass were assessed in individual craters comprised in 23 meadows in Prince Albert National Park (Saskatchewan, Canada) during the winters of 1998 and 2011. Pseudo R² = 0.66. </p

First In Situ Measurements of Electron Density and Temperature from Quasi-thermal Noise Spectroscopy with Parker Solar Probe /FIELDS

International audienceHeat transport in the solar corona and wind is still a major unsolved astrophysical problem. Because of the key role played by electrons, the electron density and temperature(s) are important prerequisites for understanding these plasmas. We present such in situ measurements along the two first solar encounters of the Parker Solar Probe, between 0.5 and 0.17 au from the Sun, revealing different states of the emerging solar wind near the solar activity minimum. These preliminary results are obtained from a simplified analysis of the plasma quasi-thermal noise (QTN) spectrum measured by the Radio Frequency Spectrometer (FIELDS). The local electron density is deduced from the tracking of the plasma line, which enables accurate measurements, independent of calibrations and spacecraft perturbations, whereas the temperatures of the thermal and suprathermal components of the velocity distribution, as well as the average kinetic temperature, are deduced from the shape of the plasma line. The temperature of the weakly collisional thermal population, similar for both encounters, decreases with the distance as R-0.74, which is much slower than adiabatic. In contrast, the temperature of the nearly collisionless suprathermal population exhibits a virtually flat radial variation. The 7 s resolution of the density measurements enables us to deduce the low-frequency spectrum of compressive fluctuations around perihelion, varying as f(-1.4). This is the first time that QTN spectroscopy is implemented with an electric antenna length not exceeding the plasma Debye length. As PSP will approach the Sun, the decrease in the Debye length is expected to considerably improve the accuracy of the temperature measurements

Relating Streamer Flows to Density and Magnetic Structures at the Parker Solar Probe

International audienceThe physical mechanisms that produce the slow solar wind are still highly debated. Parker Solar Probe's (PSP's) second solar encounter provided a new opportunity to relate in situ measurements of the nascent slow solar wind with white-light images of streamer flows. We exploit data taken by the Solar and Heliospheric Observatory, the Solar TErrestrial RElations Observatory (STEREO), and the Wide Imager on Solar Probe to reveal for the first time a close link between imaged streamer flows and the high-density plasma measured by the Solar Wind Electrons Alphas and Protons (SWEAP) experiment. We identify different types of slow winds measured by PSP that we relate to the spacecraft's magnetic connectivity (or not) to streamer flows. SWEAP measured high-density and highly variable plasma when PSP was well connected to streamers but more tenuous wind with much weaker density variations when it exited streamer flows. STEREO imaging of the release and propagation of small transients from the Sun to PSP reveals that the spacecraft was continually impacted by the southern edge of streamer transients. The impact of specific density structures is marked by a higher occurrence of magnetic field reversals measured by the FIELDS magnetometers. Magnetic reversals are associated with much stronger density variations inside than outside streamer flows. We tentatively interpret these findings in terms of magnetic reconnection between open magnetic fields and coronal loops with different properties, providing support for the formation of a subset of the slow wind by magnetic reconnection