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.

An asymmetric similarity index, defined as the proportion of species that one island has in common with a second, was introduced to infer inter-island colonization patterns from species lists. The matrix comparison or Mantel test was used to analyze the index for spatial patterns of species composition. Directional inter-island colonization was inferred from the skew-symmetric component of the index. Four biogeographical hypotheses were tested for the inter-island colonization of plants among 17 Galapagos islands and among 15 islands for birds. Plant and bird species composition and their inter-island colonization patterns are distance-dependent. More specifically, species density gradients are a function of the configuration of islands in the archipelago. The net direction of bird and plant density gradients for the archipelago increases from the south. Plant density gradients are significantly correlated with the prevailing northwest flow of the Humboldt Current, but bird density gradients are uncorrelated with the Humboldt Current or with the gradients of plant species. These results corroborate the most popular model of colonization for the Galapagos Archipelago: an initial colonization of islands in the southeast, and subsequent colonization of islands to the north and northwest.

A matrix comparison method is demonstrated for measuring the relative dispersion of different classes of individuals (e.g., species, age classes, life forms) in a sample of plants whose distributions has been mapped. The method employs combinatorial data analysis procedures and a Monte Carlo approach to significance testing; it is particularly suited to the study of subsets of interplant distances in relatively small data sets. As an example, within—site tree dispersion patterns in a Jamaican wet limestone forest are analyzed. Dispersion patterns vary among species, between size classes of the same species, and for the same species in different stands.